IMPROVEMENT TO SOUR WATER STRIPPING PROCESS

Field of the Invention

The present invention is directed to a process and a system to produce ammonia salt and hydrogen sulphide from sour water.

Background to the Invention

Sour water produced in refineries mainly contains ammonia and hydrogen sulphide (H2S) with a variety of contaminants such as cyanide and phenol, depending on the upstream processes. Typically a sour water stripper (SWS) is used to treat the sour water so that the water can be reused or safely disposed of.

A conventional sour water stripping (SWS) unit includes a sour water stripper column or stripper and operates by partially condensing the stripper gases and returning the condensed liquid to the top of the sour water stripper column (as “reflux”), or in some cases the condensed liquid is mixed with the sour water feed. The gas from the SWS unit is mainly a mixture of H2S, ammonia and water vapour. Light components, such as hydrogen, CO2, hydrocarbons and hydrogen cyanide, if present in the sour water are also present in the gas stream. The heavier impurities, such as phenol, largely leave the column bottom with the stripped water.

The gases (SWS off-gas) from sour water stripping processes are normally sent to a Claus unit where the hydrogen sulphide (H2S) is converted to sulphur. The ammonia in the gases (SWS off-gas) from the SWS process must be destroyed or decomposed if the Claus unit is to operate without blockages caused by ammonium salts.

Another effect of ammonia being present in the gases (SWS off-gas) feeding the Claus unit is a reduction in Claus plant sulphur processing capacity and a reduction in the sulphur recovery efficiency due to dilution and additional water vapour in the gases. The ammonia sent to the Claus unit adds to the Claus unit air requirement as each mole of ammonia requires approximately 0.75 moles of oxygen (as O2) for decomposition. This compares with approximately 0.5 moles of oxygen (as O2) needed to react with hydrogen sulphide to make sulphur. The ratios are approximate because of disassociation reactions in the furnace. This means that 1 kg of ammonia is equivalent to approximately 3 kg of H2S based on the Claus unit’s air requirement.

Because of this it is a great benefit in terms of sulphur processing capacity to exclude ammonia from the Claus unit’s feed gas. Furthermore production of a usable product from recovered ammonia is environmentally beneficial as the processes to manufacture ammonia are energy intensive and commonly produce CO2.

The additional water vapour in the process gases from the decomposition of ammonia acts adversely to the Claus equilibrium to form sulphur (2H2S + SO2 = 3S + 2H2O). The dilution of the process gases leads to an increase in the sulphur vapour loss from the final Claus condenser.

If ammonia is removed from the gases from sour water stripping unit (SWS off-gas) then the temperature of this gas stream can be reduced because there is no risk of ammonia salt formation. This is a benefit when the stream is fed to a Claus unit as it contains less water vapour than a stream at higher temperature.

A common process to destroy ammonia from the SWS off gas is based on variations of US patent number 4,038,036 (Beavon) which uses a two section reaction furnace upstream of the waste heat boiler and Claus catalytic stages (two section furnace process).

Other known processes to remove ammonia from a Claus unit feed are based on chemical or physical separation of ammonia from hydrogen sulphide.

A process using physical separation is the Chevron WWT process (US patent 3,335,071) which uses high pressure and low pressure strippers. The energy used for this stripping process is relatively high, typically medium pressure steam (about 19 barg) is used for heating the high pressure stripper and low pressure steam (about 3.5 barg) is used for heating the low pressure stripper. A conventional SWS unit would only use low pressure steam. An alternative to using a high pressure stripper to produce a separate ammonia rich stream is to “wash” the overhead vapour product from a low pressure stripper. This captures the ammonia in a liquid stream so that the gas stream contains H2S with very little ammonia. The liquid stream is then processed in a second stripper to remove the ammonia from the water. The wash water could be fresh water, but is more usually recycled stripped water from the sour water stripper. Clearly this process uses significantly more energy than a conventional SWS unit because two sour water stripper columns are used and the first column processes the full water flow plus the recycle or additional wash flow. Two such systems are described in the Sulphur Magazine (published by BCInsight) issue 397, November- December 2021, pages 38 and 44.

Therefore, there is a need for a process which allows efficient recovery of ammonia salt and H2S from sour water.

Summary of the Invention

In a first aspect, the present invention provides a process comprising:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and a gas stream; and

(iii) treating said condensed liquid to separate hydrogen sulphide as a gaseous product substantially free of ammonia.

In one embodiment, the process of the present invention comprises:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and a gas stream; wherein the condensed liquid is not recirculated or returned to be used in steam stripping the sour water or is not added to the sour water; and

(iii) treating said condensed liquid to separate hydrogen sulphide as a gaseous product substantially free of ammonia.

In one embodiment, the process of the present invention comprises: (i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and a gas stream;

(iii) treating said condensed liquid with a mineral acid; and

(iv) separating hydrogen sulphide as a gaseous product substantially free of ammonia from the condensed liquid treated with the mineral acid.

In one embodiment, separating the H2S as a gaseous product substantially free of ammonia from the condensed liquid treated with a mineral acid comprises:

(i) steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream; and

(ii) cooling the vapour stream to obtain H2S and an ammonia salt solution.

In one embodiment, separating the hydrogen sulphide as a gaseous product substantially free of ammonia from the condensed liquid treated with a mineral acid comprises:

(i) heating or evaporating the condensed liquid treated with a mineral acid to give an ammonium salt solution and a vapour stream; and

(ii) cooling or condensing the vapour stream to recover hydrogen sulphide and an aqueous product.

In a second aspect, the present invention provides a system for recovering hydrogen sulphide and ammonia from sour water, the system comprising: a) a sour water stripping unit; b) a cooling system; and c) a separation system.

In one embodiment, the system for recovering H2S and ammonia from sour water, comprises: a) a sour water stripping unit; b) a cooling system; c) an acid addition system; and d) a separation system to obtain hydrogen sulphide as a gaseous product substantially free of ammonia.

Description of the Drawings

A number of embodiments of the invention will now be further described, by means of example only, with reference to the drawings, in which:

Figure 1 is a schematic flow diagram illustrating a known sour water stripping process.

Figure 2 is a schematic flow diagram illustrating a known sour water stripping process.

Figure 3 is a schematic flow diagram illustrating a process according to the invention. Figure 4 is a schematic flow diagram illustrating a process according to the invention. Figure 5 is a schematic flow diagram illustrating an example of a separation system than may be used in the process illustrated in figures 3 and 4.

Figure 6 is a schematic flow diagram illustrating an example of a separation system than may be used in the process illustrated in figures 3 and 4.

Figure 7 is a schematic flow diagram illustrating a process according to the invention Figure 8 is a schematic flow diagram illustrating a process according to the invention Figure 9 is a schematic flow diagram illustrating an example of a separation system than may be used in the process illustrated in figures 7 and 8.

Detailed Description of the Invention

Known sour water stripping processes are represented in figures 1 and 2.

In figure 1 sour water (1) to be treated is fed to the top of the stripping section of a sour water stripping (SWS) unit. Typical SWS units have a sour water stripper column or stripper (3), a heat exchanger (4) and a reboiler (5). Sour water stripper columns can contain 20 to 40 contact trays or equivalent packing.

The lighter impurities contained in the sour water (1), such as ammonia and hydrogen sulphide, are removed from the sour water (1) by steam which flows up the sour water stripper column, counter-current to the sour water (steam stripping). The steam can be provided by reboiler (5). The vapour stream (6) reaching the top of the sour water stripper column (3) is cooled or partially condensed by an overhead cooling system comprising a condenser (7) followed by a reflux or separation drum (9). The condenser (7) cooling the vapour stream (6) can be an external condenser. The condensed vapours are passed to a reflux drum (9) which separates condensed liquid from gas.

The gas stream (SWS off-gas) (10) from the reflux drum (9) (figure 1) is passed to a Claus unit in normal refinery practice. These gases (10) are mainly a mixture of H2S, ammonia, volatile non-polar components from the sour water and water vapour.

The condensed liquid (11) passes through a liquid return pump (12) and is returned to the sour water stripper column (3) typically two or more trays (or equivalent) above the sour water (1) feed point. In some operations the condensed liquid is mixed with the sour water (1) feed.

The stripped water (13) (treated water) is typically cooled and re-used in the refinery.

In figure 2 sour water (1) to be treated is fed to the top of the stripping section of the sour water stripping (SWS) unit having a column (3) and a heat exchanger (4). The lighter impurities contained in the sour water (1), such as ammonia and hydrogen sulphide, are removed (stripped) from the sour water (1) by steam which flows up the column, counter-current to the sour water. The steam is provided by live steam injection (2). The vapour stream (6) reaching the top of the stripping section of the sour water stripper column (3) is cooled and partially condensed by a circulating cooling system comprising a condensing section (8) with a chimney tray at the top of the sour water stripper column (3) which delivers circulating cooling water containing condensed liquid to a circulating water pump (15) and circulating water cooler (16).

The gas stream (SWS off-gas) (10) from the condensing section (8) at the top of the column (3) are passed to a Claus unit in normal refinery practice. These gases (10) are mainly a mixture of H2S, ammonia, volatile non-polar components and water vapour.

The condensed liquid (11) from the condensing section (8) passes through a level control (indicated as a dashed line in figure 2) from chimney tray (14) and is returned to the sour water stripper column (3). The stripped water (13) (treated water) is typically cooled and re-used in the refinery.

It was surprisingly found that in SWS units with an overhead cooling system having a condenser (7), if the condensed liquid (11) is not returned to the sour water stripper column or to the sour water feed then unexpectedly the amount of ammonia in the gas stream 10 is significantly reduced. Similar results were found for SWS units with a circulating cooling system having a condensing section (8), as in figure 2.

The inventors also observed that if the condensed liquid is not returned to the SWS unit or to the sour water feed, less energy is required than with the conventional SWS processes to achieve a given specification for the stripped sour water.

In the process of the present invention the condensed liquid from the cooling step is not returned to the SWS unit or to the sour water feed, for example as “reflux”, but is treated to separate a H2S stream with negligible ammonia content and a solution containing ammonia (ammonia salt solution). A productive and efficient way of treating the condensed liquid stream is to treat it with a mineral acid so that the ammonia is “fixed” as an ammonium salt, for example a non-volatile ammonium salt, from which remaining H2S can be separated. The ammonium salt has potential uses, for instance, as a fertiliser.

As explained above sour water produced in refineries mainly contains ammonia and hydrogen sulphide. The process of the present invention produces separate streams of ammonia and H2S from sour water. Furthermore, the process of the present invention produces H2S with minimal, or no, ammonia from sour water. The process of the present invention further produces an ammonia salt solution or an ammonia salt product with minimal, or no, H2S.

In a first aspect, the present invention provides a process comprising:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and a gas stream; and (iii) treating said condensed liquid to separate hydrogen sulphide as a gaseous product substantially free of ammonia.

In one embodiment, the process of the present invention comprises:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and a gas stream;

(iii) treating said condensed liquid with a mineral acid;

(iv) separating hydrogen sulphide as a gaseous product substantially free of ammonia from the condensed liquid treated with a mineral acid.

In the process of the present invention, the condensed liquid obtained from cooling the vapour stream, is not recirculated or returned to the steam stripping step or is not added to the sour water feed.

In the process of the present invention steam stripping comprises passing the sour water comprising hydrogen sulphide and ammonia through a sour water stripping unit.

The sour water comprising hydrogen sulphide and ammonia, may further comprise non-polar volatile components, such as hydrogen and/or hydrocarbons.

The sour water stripping unit may comprise a sour water stripper column or stripper and a heat exchanger. Steam may be fed into the sour water stripping unit. Steam may be provided by a reboiler or by live steam injection. The sour water stripping unit may further comprise a reboiler, a live steam injection or a combination of both. The sour water stripping unit may comprise a sour water stripper column, a heat exchanger and a reboiler.

Steam stripping removes the non-polar volatile components, ammonia and hydrogen sulphide from the sour water, by use of steam, in the form of vapour stream and produces residual stripped water.

The vapour stream produced by steam stripping the sour water may comprise ammonia and hydrogen sulphide. The vapour stream produced by steam stripping the sour water may also comprise non-polar volatile components, such as hydrogen and/or hydrocarbons.

The cooling of the vapour stream from steam stripping the sour water may involve condensing said vapour stream to obtain a condensed liquid and a gas stream.

The cooling may be a partial condensation of the vapour stream reaching the top of the SWS unit (e.g. the top of the sour water stripper column).

The cooling of the vapour stream from steam stripping the sour water may be done by a suitable cooling system. The cooling system may comprise an overhead cooling system or a circulating cooling system. The overhead cooling system may comprise a condenser. The overhead cooling system may comprise a condenser, such as an external condenser, and a reflux drum.

The cooling system may comprise an overhead cooling system, or a circulating cooling system comprising a condensing section in the sour water stripper unit. The cooling system may comprise a condenser and reflux drum or a condensing section in the sour water stripper unit.

The process of the present invention may comprise passing the sour water comprising hydrogen sulphide and ammonia through a sour water stripping unit to produce a vapour stream and cooling said vapour stream by a condenser, such as an external condenser, and a reflux drum to obtain a condensed liquid and a gas stream.

The cooling system may comprise a pump-around or circulation cooling system. The circulating cooling system may comprise a condensing section in the sour water stripper unit. The condensing section in the sour water stripper unit may be at the top of the sour water stripping unit. The condensing section may be at the top of the sour water stripper column of the sour water stripper unit. The condensing section may have a chimney tray. The circulating cooling system may comprise a condensing section at the top of the sour water stripping unit with chimney tray.

The process of the present invention may comprise passing the sour water comprising hydrogen sulphide and ammonia through a sour water stripping unit to produce a vapour stream and cooling said vapour stream by a condensing section at the top of the sour water stripper unit, such as a condensing section at the top of the sour water stripper column of the SWS unit with chimney tray, to obtain a condensed liquid and a gas stream.

The process of the present invention may comprise passing the sour water comprising hydrogen sulphide and ammonia through a sour water stripping unit to produce a vapour stream and cooling said vapour stream by a circulating cooling system. The vapour stream from the sour water stripper unit may be cooled by a condensing section in the sour water stripper unit.

Cooling the vapour stream may comprise passing said vapour stream through a condensing section with chimney tray at the top of the sour water stripping unit (for example at the top of the sour water stripper column), a circulating water pump and circulating water cooler to obtain a condensed liquid and a gas stream. The circulating cooling system may further comprise a level control.

The vapour stream from steam stripping the sour water may be cooled at a temperature of 90°C or less, or 85°C or less, or 80°C or less, or 70°C or less, or 60°C or less to obtain a condensed liquid and a gas stream. The vapour stream from steam stripping the sour water may be cooled at a temperature from 30 °C to 90°C, from 30 °C to 85°C, or 30 °C to 80°C, such as from 30 °C to 75°C, or from 30 °C to 70°C, or from 30 °C to 65°C, or from 30 °C to 60°C, or from 35 °C to 90 °C, or from 35 °C to 85 °C, or from 35 °C to 75 °C, or from 35 °C to 70 °C, or from 35 °C to 65 °C, or from 35 °C to 60 °C, or from 40 °C to 90 °C, or from 40 °C to 80 °C, or from 40 °C to 70 °C, or from 40 °C to 65 °C, or from 40 °C to 60 °C, or from 45 °C to 90 °C, or from 45 °C to 80 °C, or from 45 °C to 70 °C, or from 45 °C to 65 °C, or from 45 °C to 60 °C, or from 50 °C to 90 °C, or from 50 °C to 80 °C, or from 50 °C to 70 °C, or from 50 °C to 65 °C, or from 50 °C to 60 °C, or from 55 °C to 65 °C, or from 55 °C to 60 °C.

The gas stream obtained from cooling the vapour stream may comprise the non-polar volatile components from the sour water. The gas stream may comprise non-polar volatile gases, such as hydrogen or hydrocarbons. The gas stream may comprise volatile non-polar gases and H2S. The gas stream may further comprise volatile nonpolar gases, H2S and negligible or a small amount of ammonia. This gas stream may be passed to a Claus unit. It was found that, although with the process of the present invention the amount of ammonia in the gas stream may be small or negligible, the composition of the gas stream (SWS off-gas) and condensed liquid, obtained from cooling the vapour stream, may vary with the ratio of ammonia and hydrogen sulphide in the sour water feed and the cooling temperature.

For a typical refinery operation, the normal molar ratio of H2S to ammonia in a refinery sour water feed is between 0.5 and 0.8 (from a paper presented at the Laurence Reid Gas Conditioning Conference, Norman, Oklahoma, February 28, 2012 “SOUR WATER STRIPPERS EXPOSED” Ralph H. Weiland, Optimized Gas Treating, Inc., Clarita, OK Nathan A. Hatcher, Optimized Gas Treating, Inc., Buda, TX).

In the process of invention, where the condensed liquid is not returned to the steam stripping step or to the sour water feed, if the ratio of ammonia to hydrogen sulphide in the sour water is more than needed to form NH4HS or when there is a stoichiometric excess of ammonia in the sour water, then most of the ammonia and hydrogen sulphide from the feed passes to the condensed liquid. For example, in this scenario, approximately 99% or more of the ammonia in the sour water and 99% or more of hydrogen sulphide from the sour water feed may pass to the condensed liquid and the gas stream may mainly comprise the non-polar volatile components, such as hydrogen or hydrocarbons contained in the sour water. For example, approximately 99.98% of the ammonia and 99.9% of hydrogen sulphide from the sour water feed may pass to the condensed liquid.

If the gas stream carries a small amount of residual ammonia and H2S these may be reduced further by reducing the cooling temperature, for example by reducing the condensing temperature.

In cases with an excess of H2S over ammonia in the sour water, the gas stream may have some H2S and ammonia. In this case, for example, 97% or more of the ammonia in the sour water and approximately 80% or more of the H2S in the sour water may pass to the condensed liquid and the gas stream may comprise the non-polar volatile components, H2S and some residual ammonia. For example, more than 99.8% of the ammonia and approximately 80% of the H2S in the sour water may pass to the condensed liquid and the gas stream may comprise the non-polar volatile components, H2S and some residual ammonia. The amount of residual ammonia in the gas stream may further by reducing the cooling temperature, for example by reducing the condensing temperature.

As used herein in relation to the gas stream, the term “residual ammonia” is intended to mean the amount or proportion of ammonia from the sour water that may pass to the gas stream.

In any case, in the process of invention, where the condensed liquid is not returned to the steam stripping step or to the sour water feed, the amount of residual ammonia in the gas stream is small. The gas stream may comprise 3% (vol.) or less of ammonia, with respect to the total amount of ammonia in the sour water. This means that 3% (vol.) or less of the ammonia from the sour water may pass to the gas stream. In one embodiment the gas stream may comprise ammonia in an amount of 2.5% (vol.) or less, or 2% (vol.) or less, or 1% (vol.) or less, such as or 0.7% (vol.) or less, or 0.5% (vol.) or less, or 0.3% (vol.) or less, or 0.1 (vol.) or less, with respect to the total amount of ammonia in the sour water. In one embodiment the gas stream may comprise between 3% (vol.) and 0.001% (vol.) of ammonia, such as between 2.8% (vol.) and 0.001% (vol.) of ammonia, or between 2.5% (vol.) and 0.001% (vol.), or between 2% (vol.) and 0.001% (vol.), or between 1% (vol.) and 0.001% (vol.), or between 0.5% (vol.) and 0.001% (vol.), or between 0.1% (vol.) and 0.001% (vol.) of ammonia, with respect to the total amount of ammonia in the sour water. In one embodiment the gas stream comprises ammonia in an amount between 3% (vol.) and 0.005% (vol.), such as between 2.5% (vol.) and 0.005% (vol.), or between 2% (vol.) and 0.005% (vol.), or between 1% (vol.) and 0.005% (vol.), or between 0.1% (vol.) and 0.005% (vol.), with respect to the total amount of ammonia in the sour water.

In one embodiment of the process of the present invention, the gas stream may comprise 3% (vol.) or less of ammonia, with respect to the total amount of ammonia in the sour water, when the temperature used for cooling the vapour stream is from 30 °C to 90 °C. In one embodiment the gas stream may comprise between 2% (vol.) and 0.001% (vol.) of ammonia, with respect to the total amount of ammonia in the sour water, when the temperature used for cooling the vapour stream is from 30 °C to 70 °C. In one embodiment the gas stream may comprise between 1.5% (vol.) and 0.001% (vol.) of ammonia, with respect to the total amount of ammonia in the sour water, when the temperature used for cooling the vapour stream is from 30 °C to 65 °C. In one embodiment the gas stream may comprise between 0.1% (vol.) and 0.001% (vol.) of ammonia, with respect to the total amount of ammonia in the sour water, when the temperature used for cooling the vapour stream is from 30 °C to 50 °C.

This means that by cooling the vapour stream at temperature of 60°C or less to obtain a condensed liquid and a gas stream, the gas stream may be passed to a Claus unit without special processing.

The controlling or reducing the cooling temperature does not affect the operation of the process of the invention, but reducing the cooling temperatures may reduce the amount of ammonia in the gas stream. This might be done by reducing the condensing temperature.

In the present invention, the gas stream may comprise some residual ammonia and this amount of ammonia in the gas stream may be reduced by controlling or reducing the cooling temperature. As described above cooling the vapour stream from steam stripping the sour water may involve condensing said vapour stream, so the amount of ammonia in the gas stream may be reduced by controlling or reducing the condensing temperature.

The ability to operate at reduced cooling temperatures is of great utility as it is always desirable to reduce the amount of ammonia in the Claus unit feed and recover more of the ammonia as a usable product.

As described above the cooling of the vapour stream from steam stripping the sour water may be done by a suitable cooling system, for example by an overhead cooling system comprising a condenser and a reflux drum or by a circulating cooling system comprising a condensing section in a sour water stripper unit. In one embodiment, the amount of ammonia in the gas stream may be reduced by controlling or reducing the condenser outlet temperature. In one embodiment the amount of ammonia in the gas stream may be reduced by controlling or reducing the condensing section outlet temperature of the sour water stripper unit.

In one embodiment the process of the present invention may further comprise: (i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and a gas stream comprising ammonia;

(iii) controlling or reducing the cooling temperature to reduce the amount of ammonia in the gas stream; and

(iv) treating said condensed liquid to separate hydrogen sulphide as a gaseous product virtually free of ammonia.

In one embodiment the process of the present invention may further comprise:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream at a temperature of 90°C or less, or of 80°C or less, or of 70°C or less, or of 60°C or less to obtain a condensed liquid and a gas stream; and

(iii) treating said condensed liquid to separate hydrogen sulphide as a gaseous product virtually free of ammonia.

In the process of the present invention, the ammonia in the gas stream from cooling the vapour stream may be further reduced when said gas stream is mixed with the gaseous product comprising H2S from the separating step (namely from separating hydrogen as a gaseous product substantially free of ammonia sulphide from the condensed liquid treated with the mineral acid).

In the process of the present invention, the ammonia in the gas stream from cooling the vapour stream may be further reduced by reducing the pH in the cooling step (e.g reducing the pH of the reflux or separation drum or the circulating cooling system) to approximately 4, for instance by adding sulphuric acid. Manipulation of the overhead temperature to suppress ammonia in the gas stream is preferred.

The condensed liquid from cooling the vapour stream obtained by steam stripping sour water may comprise hydrogen sulphide and ammonia. In the process of the present invention the condensed liquid obtained from cooling the vapour stream is treated with a mineral acid.

Adjusting the pH of a sour water stripper feed to “fix” the acid or basic components is known. Adding acid to the sour water to “fix” the ammonia in the feed to a conventional SWS could produce a gas containing H2S with virtually no ammonia and a liquid stream containing ammonium salts with very little H2S. However, the liquid would also contain all the “heavy” impurities in the sour water (such as phenol) and the ammonium salt solution would be very dilute as a refinery sour water typically contains only in the region of 4000 ppm(w) ammonia. This stripped liquid would require significant further processing to provide a usable product.

In the process of the present invention, mineral acid is added to the condensed liquid, so ammonia is “fixed” as a, ideally non-volatile, ammonium salt. Treating with a mineral acid “fixes” the ammonia so the remaining H2S can be separated from the ammonia salt contained in the condensed liquid treated with a mineral acid.

The mineral acid used in the process of the present invention may be selected from sulphuric acid, phosphoric acid, nitric acid and combinations thereof.

The mineral acid may be added so that the pH of the condensed liquid treated with the mineral acid is approximately between 1 and 5.5, so that the ammonia is present as ammonium ions and not ammonia gas. In oner embodiment, the pH of the condensed liquid treated with the mineral acid is between 1 and 5.4, or between 2 and 5.1, or between 3 and 5.5, or between 4 and 5.3, or between 4.5 and 5.1 or between 4.8 and 5.1. In one embodiment, the mineral acid may be added so that the pH of the condensed liquid treated with the mineral acid is approximately 5.

The mineral acid may be at any commercially available concentration, such as from 30% (battery acid) to 98% (concentrated acid) for sulphuric acid. The acid may be injected upstream of a mixing device (such as an inline mixer or pump) with the pH measured downstream of the mixing device so that the rate of acid injection can be varied to control the pH of the treated solution. The process of the present invention may further comprise pH control of the condensed liquid treated with a mineral acid. This pH control allows knowledge of the minimum amount of mineral acid needed so there is no excess free acid in the treated condensed liquid.

In the process of the present invention hydrogen sulphide as a gaseous product virtually free of ammonia is separated from the condensed liquid.

In the process of the present invention separating the hydrogen sulphide (H2S) from the condensed liquid treated with a mineral acid may comprise passing the condensed liquid treated with a mineral acid through a separating system. This step removes H2S, and any CO2 or HCN if present, from ammonia salt contained in the condensed liquid treated with a mineral acid.

In the process of the present invention an ammonia salt solution is recovered from the separation of H2S from the condensed liquid treated with a mineral acid.

The H2S removed from the condensed liquid treated with a mineral acid may be in the form of a gas stream or gaseous product. This gaseous product may comprise H2S, and CO2 or HCN. This gaseous product removed from the condensed liquid treated with a mineral acid may comprise H2S and negligible, or no, ammonia. This gaseous product comprising H2S can be fed to a Claus plant or unit without special precautions as it is substantially free of ammonia. In one embodiment the gaseous product comprising H2S removed from the condensed liquid treated with a mineral acid does not have ammonia. As used herein, when referring to the gaseous product comprising negligible, or no, ammonia or the gaseous product being substantially free of ammonia is intended to mean that the ammonia content in the gaseous product may be lOOOppm or less, or 700 ppm or less, or 500 ppm or less, or 200ppm or less, or 100 ppm or less. The ammonia content in the gaseous product may be from lOOOppm to Oppm, or from 700ppm to Oppm, or from 500ppm to Oppm, or from 200ppm to Oppm, or from lOOppm to Oppm.

The gaseous product comprising H2S may be also added to or mixed with the gas stream from cooling the vapour stream obtained from steam stripping the sour water.

In the process of the present invention separating the H2S from the condensed liquid treated with a mineral acid may comprise passing the condensed liquid treated with a mineral acid through a separation system. This step may obtain H2S, which may be in the form of a gaseous product or gas stream, and an ammonia salt, which may be in the form of a solution.

The separating system may comprise a) a sour water stripper unit and a cooling system, or b) a heater or evaporator and a cooler or condenser.

In one embodiment of the process of the present invention separating the H2S from the condensed liquid treated with a mineral acid may comprise: (i) steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream and (ii) cooling the vapour stream to obtain an ammonia salt solution and H2S as a gaseous product substantially free of ammonia. The cooling of the vapour stream from steam stripping the condensed liquid treated with a mineral acid may involve condensing said vapour stream to obtain an ammonia salt solution and the H2S.

Separating the H2S from the condensed liquid treated with a mineral acid may comprise passing the condensed liquid treated with a mineral acid through a separation system comprising a sour water stripping unit and a cooling system. By passing the condensed liquid treated with a mineral acid through a sour water stripping unit and a cooling system the H2S is stripped from the condensed liquid treated with a mineral acid and an ammonium salt solution is obtained.

The sour water stripping unit of the separation system may comprise a sour water stripper column and a heat exchanger. The sour water stripping unit of the separation system may further comprise a reboiler, live steam injection or a combination of both. The sour water stripping unit of the separation system may comprise a sour water stripper column, a heat exchanger and a reboiler.

Separating the H2S from the condensed liquid treated with a mineral acid may comprise passing the condensed liquid treated with a mineral acid through a separation system comprising a sour water stripper unit and a cooling system, such as a circulating cooling system, or an overhead cooling system. The overhead cooling system may comprise a condenser, such as an external condenser.

The condensed liquid treated with a mineral acid may pass through the sour water stripper unit of the separation system producing a vapour stream and said vapour stream may be cooled, for example, by a cooling system, such as a circulating cooling system or an overhead cooling system, to obtain the ammonia salt solution and the H ₂ S.

Separating the H ₂ S from the condensed liquid treated with a mineral acid may comprise: (i) steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream; and (ii) cooling the vapour stream to obtain an ammonia salt solution and H ₂ S as a gaseous product substantially free of ammonia, wherein cooling the vapour stream comprises passing said vapour stream through an overhead cooling system, such as a condenser. Cooling the vapour stream steam from stripping the condensed liquid treated with a mineral acid, may comprise passing the vapour stream through a condenser, such as an external condenser. The cooling, for example by the external condenser may be with reflux. So the vapour stream may be cooled by passing it through a condenser, such as an external condenser, and a reflux drum to obtain an ammonia salt solution and the hydrogen sulphide.

Separating the H ₂ S from the condensed liquid treated with a mineral acid may comprise: (i) steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream; and (ii) cooling the vapour stream to obtain an ammonia salt solution and H ₂ S as a gaseous product substantially free of ammonia, wherein cooling the vapour stream comprises passing said vapour stream through a circulating cooling system, such as a condensing section at the top of the sour water striping unit. The circulating cooling system of the separation system may comprise a condensing section at the top of the sour water stripper column of the SWS unit with chimney tray, a circulating water pump and a circulating water cooler. The circulating cooling system of the separation system may further comprise a level control. The vapour stream may be cooled by passing it through a circulating cooling system, such as a condensing section at the top of the sour water striping unit, to obtain an ammonia salt solution and the hydrogen sulphide.

The H ₂ S recovered from steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream and cooling said vapour stream, may be in the form of a gaseous product or gas stream as described herein. This gaseous product may comprise H ₂ S, and CO ₂ or HCN. This gaseous product removed from the condensed liquid treated with a mineral acid may comprise H ₂ S and negligible, or no, ammonia. The gaseous product comprising H ₂ S can be fed to a Claus plant or unit without special precautions as it is substantially free of ammonia. The gaseous product comprising H2S may be added to or mixed with the gas stream from cooling the vapour stream obtained from steam stripping the sour water. In one embodiment the gaseous product comprising H2S removed from the condensed liquid treated with a mineral acid does not have ammonia. As used herein, when referring to the gaseous product comprising negligible, or no, ammonia or the gaseous product being substantially free of ammonia is intended to mean that the ammonia content in the gaseous product may be lOOOppm or less, or 700 ppm or less, or 500 ppm or less, or 200ppm or less, or 100 ppm or less. The ammonia content in the gaseous product may be from lOOOppm to Oppm, or from 700ppm to Oppm, or from 500ppm to Oppm, or from 200ppm to Oppm, or from lOOppm to Oppm.

The ammonium salt solution obtained from separating the H2S from the condensed liquid treated with a mineral acid by steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream and cooling the vapour stream, may be a solution of an ammonium salt and substantially free, or free, of H2S.

The process of the present invention may further comprise concentrating or crystallising the ammonium salt solution obtained from separating the H2S from the condensed liquid treated with a mineral acid.

In the process of the present invention separating the H2S from the condensed liquid treated with a mineral acid may comprise: (i) steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream; (ii) cooling the vapour stream to obtain hydrogen sulphide and an ammonia salt solution; and (iii) concentrating or crystallising the ammonium salt solution to obtain water vapour and an ammonium salt product.

Concentrating or crystallising the ammonium salt solution may be done by evaporating. An evaporator, for example an ammonia salt evaporator, may be used to concentrate the ammonium salt solution. The evaporator may further comprise a boiler and optionally a separator vessel. The evaporator may comprise a boiler and a separator vessel.

Concentrating or crystallising the ammonium salt solution may produce a water vapour (steam) and an ammonia salt product. The water vapour from concentrating or crystallising the ammonium salt solution may be recirculated and used for steam stripping the sour water or may be used for steam stripping the condensed liquid treated with a mineral acid in the separation step. Thus the water vapour from concentrating or crystallising the ammonium salt solution may be recirculated and used in the first or upstream SWS unit or in the SWS unit of the separating system (second or downstream SWS unit). The water vapour may be injected into the sour water stripping unit for use as stripping steam. The water vapour may be used in a SWS reboiler to generate stripping steam. This can act as part of the energy needed for stripping the sour water, namely for the sour water stripping unit, so there is little increase in the overall energy needed for the system if an evaporator is used.

Thus in the process of the present invention some of the steam needed for steam stripping or in the SWS column may be provided by recirculating the water vapour produced by separating the hydrogen sulphide from the condensed liquid treated with a mineral acid.

The process of the present invention may comprise:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and a gas stream;

(iii) treating said condensed liquid with a mineral acid;

(iv) separating H2S as a gaseous product substantially free of ammonia from the condensed liquid treated with a mineral acid by steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream and cooling said vapour stream to obtain an ammonia salt solution and H2S;

(v) concentrating or crystallising the ammonium salt solution to obtain water vapour and an ammonium salt product;

(vi) using the water vapour for steam stripping the sour water or for steam stripping the condensed liquid treated with a mineral acid.

The process of the present invention may comprise:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water; (ii) cooling the vapour stream at a temperature of 90°C or less, such as between 90°C and 30°C, or between 80°C and 30°C, or between 70°C and 30°C, or between 60°C and 30°C ,to obtain a condensed liquid and a gas stream; and

(iii) treating said condensed liquid with a mineral acid;

(iv) separating H2S as a gaseous product substantially free of ammonia from the condensed liquid treated with a mineral acid by steam stripping the condensed liquid treated with a mineral acid to produce a vapour stream and cooling said vapour stream to obtain an ammonia salt solution and H2S;

(v) concentrating or crystallising the ammonium salt solution to obtain water vapour and an ammonium salt product;

(vi) using the water vapour for steam stripping the sour water or for steam stripping the condensed liquid treated with a mineral acid.

In one embodiment of the process of the present invention separating the H2S from the condensed liquid treated with a mineral acid may comprise: (i) heating or evaporating the condensed liquid treated with a mineral acid to give an ammonium salt solution and vapour stream; and (ii) cooling or condensing the vapour stream to obtain H2S as a gaseous product substantially free of ammonia and an aqueous product.

Separating the H2S from the condensed liquid treated with a mineral acid may comprise passing the condensed liquid treated with a mineral acid through a separation system comprising a) a heater or evaporator and b) a cooler or condenser. By passing the condensed liquid treated with a mineral acid through a) a heater or evaporator and b) a cooler or condenser, the H2S is removed from the condensed liquid treated with a mineral acid and an ammonium salt solution is obtained. The H2S may be in the form of a gaseous product or gas stream.

In the process of the present invention the condensed liquid treated with a mineral acid may pass through a heater or evaporator of the separation system to obtain an ammonium salt solution and vapour stream; and said vapour stream may be condensed or cooled by a condenser or cooler of the separation system to separate or remove hydrogen sulphide and recover an aqueous product. The heater or evaporator could also function as a crystalliser to produce an ammonium salt product, such a solid ammonium salt.

The heater or evaporator of the separation system may further comprise a separation drum. The cooler or condenser of the separation system may further comprise a separation drum. So the process of the present invention may comprise (i) passing the condensed liquid treated with the mineral acid through the heater or evaporator and the separation drum to obtain an ammonium salt solution and a vapour stream; and (ii) cooling or condensing said vapour stream by the condenser or cooler with a separation drum to obtain hydrogen sulphide as a gaseous product substantially free of ammonia and an aqueous product. The aqueous product may be recirculated to the sour water.

The ammonia salt solution from heating or evaporating the condensed liquid treated with a mineral acid may be free or substantially free of H2S. The ammonia salt solution from heating or evaporating the condensed liquid treated with a mineral acid may be a relatively strong solution of ammonium salt which is free, or substantially free, of H2S. Ammonia salt solution from heating or evaporating the condensed liquid treated with a mineral acid substantially free of H2S means that the ammonia salt solution may have less than 10 ppm of H2S, such as less than 8 ppm of H2S, or less 6 ppm of H2S, or less than 4 ppm of H2S, or less than 2 ppm of H2S, or less than 1 ppm of H2S. The H2S content in the ammonia salt solution may be from 0 ppm to 10 ppm of TUS, such as from 0.001 ppm to 10 ppm of TUS, from 0.001 ppm to 8 ppm of TUS, from 0.001 ppm to 6 ppm of H2S, from 0.001 ppm to 4 ppm of H2S, from 0.001 ppm to 2 ppm of H2S.

The amount of H2S contained in the ammonium salt solution may be further reduced to less than 1 ppm by stripping with steam. This may be carried out by adding contact stages (e.g. trays or packing) to the separation drum of the separation system described herein.

The ammonium salt solution may comprise ammonium sulphate.

The vapour stream from heating or evaporating the condensed liquid treated with a mineral acid may comprise water vapour (steam) and H2S and may be cooled or condensed and fed to a separator drum to separate a condensed liquid (the aqueous product) from a gaseous product (the H2S). The vapour stream from heating or evaporating the condensed liquid treated with a mineral acid may be cooled or condensed at a temperature from 30 °C to 90 °C, or form 30 °C to 85 °C, from 30 °C to 70 °C, from 30 °C to 60 °C, or from 35 °C to 90

°C, or from 35 °C to 85 °C, or from 35 °C to 70 °C, or from 35 °C to 60 °C, or from

40 °C to 90 °C, or form 40 °C to 80 °C, or form 40 °C to 70 °C, or form 40 °C to 60

°C, or from 45 °C to 90 °C, or from 45 °C to 80 °C, or from 45°C to 70 °C, or from

45°C to 60 °C, preferable from 55 °C to 90 °C, or from 55 °C to 80 °C, or from 55 °C to 70 °C, or from 55 °C to 60 °C, or from 57 °C to 63 °C, or from 58 °C to 62 °C and fed to a separator drum. These temperatures can be achieved with air or water cooling.

The vapour stream from heating or evaporating the condensed liquid treated with a mineral acid may be cooled or condensed and then passed to a separator drum, which may operate at between 0.5 and 5 barg, such as between 0.5 and 4 barg, or between 0.5 and 3 barg, or between 0.5 and 2 barg. For example, 1 barg may be used as a practical pressure of the separation drum so that the gaseous product comprising H2S can be fed to a Claus plant without the need for compression.

The H2S recovered from cooling or condensing the vapour stream (namely the vapour stream from heating or evaporating the condensed liquid treated with a mineral acid) may be in the form of a gaseous product or gas stream as described herein. This gaseous product may comprise H2S, and CO2 or HCN. This gaseous product recovered from cooling or condensing the vapour stream may comprise H2S and negligible, or no, ammonia. The gaseous product comprising H2S can be fed to a Claus plant or unit without special precautions as it is substantially free of ammonia. The gaseous product comprising H2S may be added to or mixed with the gas stream from cooling the vapour stream obtained from steam stripping the sour water. In one embodiment the gaseous product comprising H2S recovered from cooling or condensing the vapour stream does not have ammonia. As used herein, when referring to the gaseous product comprising negligible, or no, ammonia or the gaseous product being substantially free of ammonia is intended to mean that the ammonia content in the gaseous product may be lOOOppm or less, or 700 ppm or less, or 500 ppm or less, or 200ppm or less, or 100 ppm or less. The ammonia content in the gaseous product may be from lOOOppm to Oppm, or from 700ppm to Oppm, or from 500ppm to Oppm, or from 200ppm to Oppm, or from lOOppm to Oppm. In addition, an aqueous product is obtained from cooling or condensing the vapour stream (the vapour stream from heating or evaporating the condensed liquid treated with a mineral acid). The aqueous product may be recirculated to the sour water feeding the sour water stripping unit. Because the sour stripping unit is not a refluxed SWS unit as conventional SWS unit, the reboiler duty needed to handle additional liquid is less than needed for the traditional SWS units operating with reflux. This recycle flow may be approximately 9% of the original sour water stripping unit feed rate and the heat needed for the sour water stripping unit, even with the recycle, is less than that used for a conventional sour water stripping unit with a refluxed column.

The aqueous product (liquid stream) from cooling or condensing the vapours stream of the separation step, may comprise a residual amount of H2S. For example, the aqueous product may comprise H2S in an amount of from 1 to 15 wt%, such as from 1 to 13 wt % or, 1 to 10 wt % or, 2 to 15 wt % or, 2 to 13 wt %% or, 2 to 10 wt % % or, 2 to 8 wt %. It may, for example, be from 3 to 15 wt %, such as from 3 to 13 wt % or, from 3 to 10 wt % or, from 3 to 8 wt %, or, from 4 to 15 wt % or, from 4 to 13 wt % or, from 4 to 10 wt %, based on the total weight of the aqueous product. The amount of H2S in the aqueous product may vary with the temperature used for cooling or condensing the vapour stream, for example at a temperature from 30 °C to 85 °C the aqueous product obtained may comprise between 11 and 4% of the H2S.

The process of the present invention may comprise:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream to obtain a condensed liquid and gas stream;

(iii) treating said condensed liquid with a mineral acid;

(iv) separating H2S as a gaseous product substantially free of ammonia from the condensed liquid treated with a mineral acid by heating or evaporating the condensed liquid treated with a mineral acid to give an ammonium salt solution and a vapour stream, and cooling or condensing the vapour stream to obtain hydrogen sulphide and an aqueous product; and

(v) adding the aqueous product to the sour water feed. The process of the present invention may comprise:

(i) steam stripping sour water comprising hydrogen sulphide and ammonia to produce a vapour stream and stripped water;

(ii) cooling the vapour stream at a temperature of 90°C or less, such as between 90°C and 30°C, or between 80°C and 30°C, or between 70°C and 30°C, or between 60°C and 30°C, to obtain a condensed liquid and gas stream;

(iii) treating said condensed liquid with a mineral acid;

(iv) separating H2S as a gaseous product substantially free of ammonia from the condensed liquid treated with a mineral acid by heating or evaporating the condensed liquid treated with a mineral acid to give an ammonium salt solution and a vapour stream, and cooling or condensing the vapour stream to obtain hydrogen sulphide and an aqueous product; and

(v) adding the aqueous product to the sour water feed.

Known refineries vary in the amounts of SWS off-gas processed from sour water striping processes, but as an example a refinery may feed a Claus unit with approximately 2200 kg/h of acid gas and 540 kg/h of SWS off-gas. Using the process of present invention, the tail gas flow from the Claus unit is reduced by about 22%. The Claus unit sulphur recovery efficiency from the 2 stage Claus unit is increased by about 1%, which significantly reduces the duty of the tail gas treatment unit. Compared with traditional operations with ammonia in the SWS off-gas the sulphur processing capacity is increased by approximately 27% using the process of present invention. This increase in capacity is similar to that achieved with low level oxygen enrichment (i.e. by adding oxygen to the Claus combustion air up to a concentration of about 28% oxygen).

Moreover, in the process of the present invention the combined energy needed for both the steam stripping of sour water and the steam stripping of the condensed liquid treated with a mineral acid is less than that needed for a single conventional sour water stripping process which does not separate H2S and ammonia, and far less than known systems needed to produce separate ammonia and H2S streams. The process of the present invention can be added to or used with existing conventional sour water stripping processes (e.g current SWS units) without modifying the existing equipment. The conventional operation of the sour water stripping units could be resumed if the ammonia salt production was not needed.

Considering the total or combined off-gas produced in the process of the invention, namely the gas stream (SWS off-gas) from cooling the vapour stream obtained from steam stripping sour water and the gaseous product (H2S) from the separation step, this total or combined off-gas may be processed in the Claus plant without special precautions.

An example of the process according to the invention wherein the vapour stream from the SWS unit is cooled by an external condenser and reflux drum is shown in Figure 3. Sour water (1) to be treated is fed to the top of the stripping section of a sour water stripping (SWS) unit (A) having a sour water stripper column (3), a heat exchanger (4) and a reboiler (5).

The lighter impurities contained in the sour water (1), such as ammonia and hydrogen sulphide, are removed (stripped) from the sour water (1) by steam which flows up the sour water stripper column (3). The steam is provided by a reboiler (5). The vapour stream (6) reaching the top of the sour water stripper column (3) is cooled or partially condensed by an overhead cooling system comprising an external condenser (7) and a reflux drum (9).

The gas stream (SWS off-gas) (10’) from the reflux drum (9) may comprise volatile non-polar gases, such as hydrogen or hydrocarbons. The gas stream (10’) may comprise volatile non-polar gases, and H2S and can be passed to a Claus unit.

The condensed liquid (11) from the reflux drum (9) is treated with a mineral acid. The mineral acid addition (17) to the condensed liquid (11) stream “fix” the ammonia as a non-volatile ammonium salt.

The condensed liquid treated with a mineral acid (18) passes through a pump (19) to provide mixing of the mineral acid and condensed liquid stream. An in-line mixer may be used if the pump is unsuitable. A pH control (21) monitors the pH of the acidified stream (the condensed liquid treated with a mineral acid (18)), so the minimum amount of mineral acid is added so there is no excess free acid.

The condensed liquid treated with a mineral acid (18) is then added or fed to a separation system (22) or (22’) to separate H2S (26), as a gaseous product, from the ammonia salt contained in the condensed liquid treated with a mineral acid (18). This gaseous product from the separation system (22) or (22’) comprises H2S (26) and is substantially free, or free, of ammonia. The H2S (26) can be added to or combined with the gas stream (SWS off-gas) (10’) so the total or combined off-gas (32) can be sent to a Claus unit.

Examples of separation systems (22) and (22’) are shown in figures 5 and 6.

An ammonia salt solution (25) is recovered from the separation system (22) or (22’). This solution (25) is a relatively strong solution of an ammonium salt substantially free, or free, of H2S. The ammonia solution (25) can be concentrated by evaporation (23) (e.g. in an evaporator) to make a commercial ammonia salt product (27). In figure 3 the water vapour (24) from the evaporation process (23) can be recirculated to the sour water stripping unit (A) for use as stripping steam.

An example of the process according to the invention wherein the vapour stream from the SWS unit is cooled by a circulating cooling system is shown in Figure 4.

In figure 4 sour water (1) to be treated is fed to the top of the stripping section of the sour water stripping (SWS) unit (B) having a sour water stripper column (3) and a heat exchanger (4). The lighter impurities contained in the sour water (1), such as ammonia and hydrogen sulphide, are removed (stripped) from the sour water (1) by steam which flows up the column, counter-current to the sour water. The steam is provided by live steam injection (2). The vapour stream reaching the top of the stripping section of the sour water stripper column (3) are cooled and partially condensed by a circulating cooling system comprising a condensing section (8) with a chimney tray at the top of the sour water stripper column (3), a circulating water pump (15) and a circulating water cooler (16). The gas stream (SWS off-gas) (10’) from the condensing section (8) at the top of the column (3) may comprise volatile non-polar gases, such as hydrogen or hydrocarbons. The gas stream (10’) may comprise volatile non-polar gases, and H2S and can be passed to a Claus unit.

The condensed liquid (11) from the chimney tray (14) is treated with a mineral acid. The mineral acid addition (17) to the condensed liquid (11) stream “fix” the ammonia as a non-volatile ammonium salt.

The condensed liquid treated with a mineral acid (18) passes through a mixing device (20) (such as an inline mixer or pump). A pH control (21) monitors the pH of the acidified stream (the condensed liquid treated with a mineral acid (18)), so the minimum amount of mineral acid is added so there is no excess free acid.

The condensed liquid treated with a mineral acid (18) is then added or fed to a separation system (22) or (22’) to separate H2S (26), as a gaseous product, from the ammonia salt contained in the condensed liquid treated with a mineral acid (18). This gaseous product from the separation system comprises H2S (26) and is substantially free, or free, of ammonia. The H2S (26) can be added to or combined with the gas stream (SWS off-gas) (10’) so the total or combined off-gas (32) can be sent to a Claus unit.

Examples of separation systems (22) and (22’) are shown in figures 5 and 6.

An ammonia salt solution (25) is recovered from the separation system (22) or (22’). This solution (25) is a relatively strong solution of an ammonium salt. The ammonia salt solution (25) can be concentrated by evaporation (23) (in an evaporator) to make a commercial ammonia salt product (27). In figure 4 the water vapour (24) (steam) from the evaporation process (23) can be recirculated to the steam injection (2) feeding the sour water stripping unit (B).

Figures 5 and 6 show separation systems (22) and (22’) that can be used in the process according to the present invention. In figure 5 the condensed liquid treated with a mineral acid (18) is added or fed to a separation system (22) to separate H2S (26), as a gaseous product, from the ammonia salt contained in the condensed liquid treated with a mineral acid (18). The condensed liquid treated with a mineral acid (18) is fed to a sour water stripping unit of the separation system (C) having a sour water stripper column (3 ’), a heat exchanger (4’) and a reboiler (5’). The steam is provided by a reboiler (5’). The vapour stream (6’) reaching the top of the sour water stripper column (3’) is collected and cooled or partially condensed by a cooling system comprising an external condenser (7’) and pass to reflux drum (9’) to recover an ammonia salt solution (25) and a hydrogen sulphide (26) as a gaseous product. This gaseous product mainly comprises H2S (26) and negligible ammonia, or no ammonia, and can be passed to a Claus unit. The gaseous product comprising H2S (26) can also be added to or combined with the gas stream (SWS off-gas) (10’) so the total or combined off-gas (32) can be sent to a Claus unit.

The ammonia salt solution (25) from the separation system (22) can be concentrated or crystallised by passing the ammonia salt solution (25) through an evaporator (23) with a boiler and optionally a separation vessel giving an ammonia salt product (27) and a water vapour (24) or steam that can be injected into the sour water stripping unit (A) for use as stripping steam.

In figure 6 the condensed liquid treated with a mineral acid (18) is added or fed to a separation system (22’) to separate H2S (26), as a gaseous product, from the ammonia salt contained in the condensed liquid treated with a mineral acid (18). The condensed liquid treated with a mineral acid (18) is fed to the top of a stripping section of the sour water stripping (SWS) unit (D) having a sour water stripper column (3’) and a heat exchanger (4’). The steam may be provided by live steam injection (2’). The vapour stream reaching the top of the stripping section of the sour water stripper column (3’) is cooled and partially condensed by a circulating cooling system comprising a condensing section (8’) with a chimney tray at the top of the sour water stripper column (3’) a circulating water pump (15’) and a circulating water cooler (16’).

The gaseous product (26) from the condensing section (8’) at the top of the sour water stripper column (3’) comprises H2S. This gaseous product mainly comprises H2S (26) and negligible ammonia, or no ammonia, and can be passed to a Claus unit. The gaseous product comprising H2S (26) can also be added to or combined with the gas stream (SWS off-gas) (10’) so the total or combined off-gas (32) can be sent to a Claus unit.

The ammonia salt solution (25) from the separation system (22’) can be concentrated or crystallised (23) by passing the ammonia salt solution (25) through an evaporator with boiler and optionally a separation vessel giving an ammonia salt product (27) and a water vapour (24) or steam that can be added to the live steam injection (2) of the sour water stripping unit (B).

Another example of the process according to the invention wherein the vapour stream from the SWS unit are cooled by an external condenser and pass to a reflux drum is shown in Figure 7. Sour water (1) to be treated is fed to the top of the stripping section of an sour water stripping (SWS) unit (A) having a sour water stripper column (3), a heat exchanger (4) and a reboiler (5).

The lighter impurities contained in the sour water (1), such as ammonia and hydrogen sulphide, are removed (stripped) from the sour water (1) by steam which flows up the column. The steam is provided by a reboiler (5). The vapour stream (6) reaching the top of the sour water stripper column (3) are cooled or partially condensed by an external condenser (7) with a reflux drum (9).

The gas stream (10’) from the reflux drum (9) may comprise volatile non-polar gases, such as hydrogen or hydrocarbons. The gas stream (10’) may comprise volatile nonpolar gases, and H2S and can be passed to a Claus unit.

The condensed liquid (11) from the reflux drum (9) is treated with a mineral acid. The mineral acid addition (17) to the condensed liquid (11) stream “fix” the ammonia as a non-volatile ammonium salt.

The condensed liquid treated with a mineral acid (18) passes through a pump (19). A pH control (21) monitors the pH of the acidified stream (the condensed liquid treated with a mineral acid (18)), so a minimum amount of mineral acid is added and there is no excess free acid. The condensed liquid treated with a mineral acid (18) is then added or fed to a separation system (22”) to separate H2S (26), as a gaseous product, from the ammonia salt contained in the condensed liquid treated with a mineral acid (18). This gaseous product obtained from the separation system (22”) comprises H2S (26) and is substantially free, or free, of ammonia. The H2S (26) can be added to or combined with the gas stream (SWS off-gas) (10’) so the total or combined off-gas (32) can be sent to a Claus unit.. Separation system (22”) is shown in figure 9.

An ammonia salt solution (25) and an aqueous product (28) (condensed water with residual H2S) are also recovered from the separation system (22”). This ammonia salt solution (25) is a relatively strong solution of an ammonium salt and is substantially free, or free, of H2S. The aqueous product (28) from the separation system (22”) can be added to the sour water (1) feeding the sour water stripping unit (A).

Another example of the process according to the invention wherein the vapour stream from the SWS unit is cooled by a circulating cooling system is shown in Figure 8.

In figure 8 sour water (1) to be treated is fed to the top of the stripping section of the sour water stripping (SWS) unit (B) having a sour water stripper column (3) and a heat exchanger (4). The lighter impurities contained in the sour water (1), such as ammonia and hydrogen sulphide, are removed (stripped) from the sour water (1) by steam which flows up the column, counter-current to the sour water. The steam is provided by live steam injection (2). The vapour stream reaching the top of the stripping section of the sour water stripper column (3) are cooled and partially condensed by a circulating cooling system comprising a condensing section (8) with a chimney tray at the top of the sour water stripper column (3), a circulating water pump (15) and a circulating water cooler (16).

The gas stream from the condensing section (8) at the top of the sour water stripper column (3) is a gas comprising volatile non-polar gases, such as hydrogen or hydrocarbons. The gas stream (10’) may comprise volatile non-polar gases and H2S and can be passed to a Claus unit. The condensed liquid (11) from level control from chimney tray (14) is treated with a mineral acid. The mineral acid addition (17) to the condensed liquid (11) stream “fix” the ammonia as a non-volatile ammonium salt.

The condensed liquid treated with a mineral acid (18) passes through a mixing device (20) (such as an inline mixer or pump). A pH control (21) monitors the pH of the acidified stream (the condensed liquid treated with a mineral acid (18)), so a minimum amount of mineral acid is added and there is no excess free acid.

The condensed liquid treated with a mineral acid (18) is then added or fed to a separation system (22”) to separate H2S (26), as a gaseous product, from the ammonia salt contained in the condensed liquid treated with a mineral acid (18). The gaseous product from the separation system (22”) comprises H2S (26) and substantially free, or free, of ammonia. The H2S (26) can be added to or combined with the gas stream (SWS off-gas) (10’) so the total or combined off-gas (32) can be sent to a Claus unit. Separation system (22”) is shown in figure 9.

An ammonia salt solution (25) and an aqueous product (28) (condensed water with residual H2S) are also recovered from the separation system (22”). This ammonia salt solution (25) is a relatively strong solution of an ammonium salt. The aqueous product (28) from the separation system (22”) can be recirculated and added to the sour water (1) feeding the sour water stripping unit (B).

Figure 9 shows the separation system (22”) that can be used in the process according to the present invention.

In figure 9 the condensed liquid treated with a mineral acid (18) is added or fed to a separation system (22”) to separate H2S (26), as a gaseous product, from the ammonia salt contained in the condensed liquid treated with a mineral acid (18). The condensed liquid treated with a mineral acid (18) is fed to a separation system (22”) having a heater or evaporator (29) and a condenser or cooler (31). The heater or evaporator (29) and the condenser or cooler (31), both or one of them may additionally have a separation drum. In the separation system (22”), the condensed liquid treated with a mineral acid (18) passes through a heater or evaporator (29) with a separation drum to remove the ammonium salt solution (25). This ammonia solution (25) is a relatively strong solution of an ammonium salt. The vapour stream (30) from evaporating the condensed liquid treated with a mineral acid (18) is cooled or condensed by a condenser or cooler (31) and a separation drum system to recover hydrogen sulphide (26), as a gaseous product, and an aqueous product (28). The gaseous product from the separation system (22”) comprises H2S (26) and is substantially free, or free, of ammonia and can be passed to a Claus unit. The gaseous product comprising H2S (26) can also be added to or combined with the gas stream (SWS off-gas) (10’) so the total or combined off-gas (32) can be sent to a Claus unit. The aqueous product (28) from the separation system (22”) can be recirculated and added to the sour water (1) feeding the sour water stripping units (A) or (B).

In a second aspect, the present invention provides a system for recovering hydrogen sulphide and ammonia from sour water, the system comprising: a) a sour water stripping unit; b) a cooling system; and c) a separation system.

The sour water stripping unit of the system of the present invention may comprise a sour water stripper column and a heat exchanger. The sour water stripping unit may further comprise a reboiler, live steam injection or a combination of both.

The cooling system of the system of the present invention may comprise an overhead cooling system or a circulating cooling system. The cooling system may comprise overhead cooling system. The overhead cooling system may comprise a condenser, such as external condenser, and a reflux drum. The cooling system may comprise a circulating cooling system. The circulating cooling system may comprise a condensing section in the sour water stripper unit. The condensing section in the sour water stripper unit may be at the top of the sour water stripping unit. The condensing section may be at the top of the sour water stripper column. The condensing section may have a chimney tray. The circulating cooling system may comprise a condensing section at the top of the sour water stripper column with chimney tray. The circulating cooling system may comprise a condensing section with a chimney tray at the top of the sour water stripper column, a circulating water pump and circulating water cooler.

In one embodiment, the system for recovering H2S and ammonia from sour water comprises: a) a sour water stripping unit, b) a cooling system c) an acid addition system; and d) a separation system.

The acid addition system adds a mineral acid. The mineral acid used may be selected from sulphuric acid, phosphoric acid, nitric acid and combination thereof.

The separation system produces hydrogen sulphide as a gaseous product substantially free of ammonia and an ammonium salt solution.

The system of the present invention may further comprise a pH control. The pH control may be between the acid addition system and the separation system.

In one embodiment, the separation system of the system of the present invention may comprise a) a sour water stripping unit and a cooling system or b) a heater or evaporator, and a condenser or cooler.

The separation system of the system of the present invention may comprise a sour water stripping unit and a cooling system, wherein the sour water stripping unit of the separation process may comprise a sour water stripper column and a heat exchanger. This sour water stripping unit may further comprise a reboiler, live steam injection or a combination of both.

The separation system of the system of the present invention may comprise a sour water stripping unit and a cooling system, wherein the cooling system may comprise an overhead cooling system or a circulating cooling system. The overhead cooling system of the separation system may comprise a condenser, such as an external condenser. The overhead cooling system of the separation system may comprise a condenser and a reflux drum. The cooling system may comprise a circulating cooling system. The circulating cooling system may comprise a condensing section in the sour water stripper unit of the separation system. The condensing section in the sour water stripper unit of the separation system may be at the top of the sour water stripping unit. The condensing section may be at the top of the sour water stripper column of the separation system. The condensing section may have a chimney tray. The circulating cooling system may comprise a condensing section at the top of the sour water stripper column with chimney tray. The circulating cooling system of the separation system may comprise a condensing section with a chimney tray at the top of the sour water stripper column, a circulating water pump and circulating water cooler.

The system of the present invention may comprise a) a sour water stripping unit; b) a cooling system; c) an acid addition system; d) a separation system comprising a sour water stripping unit and a cooling system; e) an evaporator; and f) means for connecting the evaporator to the sour water stripping unit or the sour water stripping unit feed.

The sour water stripping unit and cooling system may be as defined herein above.

The evaporator may comprise a boiler. The evaporator may further comprise a separation vessel.

The system of the present invention may comprise a sour water stripping unit (e,g. first sour water stripping unit or upstream sour water stripping unit) and a sour water stripping unit of the separation system (second sour water stripping unit or downstream sour water stripping unit).

The system of the present invention may comprise a) a sour water stripping unit (e,g. first sour water stripping unit or upstream sour water stripping unit); b) a cooling system (e.g. first cooling system or upstream cooling system); c) an acid addition system; d) a sour water stripping unit of the separation system (e.g. second sour water stripping unit or downstream sour water stripping unit) and cooling system of the separation system (e.g. second cooling system or downstream cooling system); e) an evaporator; and f) means for connecting the evaporator to the sour water stripping unit. In one embodiment, the separation system of the system of the present invention may comprise a) a heater or evaporator, and b) a condenser or cooler. The heater or evaporator may further comprise a separation drum. The condenser or cooler may further comprise a separation drum.

The system of the present invention may comprise a) a sour water stripping unit, b) a cooling system c) an acid addition system; d) a separation system comprising a heater or evaporator and a condenser or cooler; and f) means for connecting the separation system to the sour water stripping unit or to the sour water stripping unit feed (e.g. the sour water).

The separation system of the system of the present invention may comprise a) a heater or evaporator and a separation drum, and b) a condenser or cooler and a separation drum.

Examples:

The following examples show systems treating 65 m ³ /h of sour water.

In all examples the first step is to feed the sour water to the sour water stripper column of a sour water stripping unit (first or upstream SWS unit). For the simulation this sour water stripper column contains 12 theoretical trays, which is normally equivalent to between 30 and 40 actual trays.

In the examples the cooling system used is an overhead cooling system having an external condenser followed by a separation drum (this would be the reflux drum for a conventional SWS units). The results are similar if a circulating cooling system (e.g. a “pump-around” cooling system) is used for the cooling system.

For example 1, the condensing temperature is 85°C, which is conventionally used to avoid deposition of ammonia salts from the gas stream. The high condensation temperature means there is a significant amount of water vapour in the gas stream which would reduce a Claus unit efficiency and capacity.

In the examples 2 to 5, processes according to the present invention the outlet temperature of the external condenser, namely the condenser of the overhead cooling system after the first or upstream SWS unit, is 60° C, which can normally be achieved with either air or water cooling. Reducing this temperature reduces the amount of water vapour in the gas stream (SWS off-gas). There is only a negligible amount of ammonia in the gas stream (SWS off-gas) from the overhead cooling system and the multiple of the partial pressure of ammonia and H2S from the overhead cooling system is less than example 1, so the condensation temperature can be reduced without risk of blockages (the temperature at which solid NH4HS forms increases as the multiple of the partial pressure of ammonia and H2S increases).

The mineral acid used for the examples is sulphuric acid.

For the examples of the process of invention option 1 uses a separation system having a sour water stripper unit (second or downstream SWS unit) to treat the condensed liquid treated with a mineral acid, namely the acidified “reflux” stream from the first or upstream SWS unit and cooling system. Option 2 feeds the condensed liquid treated with a mineral acid (the acidified “reflux” stream) to a separation system comprising evaporation system.

Both options can be a simple “add-on” to existing sour water strippers, with only the “reflux” stream re-routed.

Example 1 is a known process where the sour water passes through a typical sour water stripping unit comprising a SWS column with a reboiler and the vapour stream produced is cooled by an overhead cooling system comprising an external condenser and reflux drum treating the sour water to produce stripped water and a gas stream (SWS off-gas) comprising H2S and ammonia. In a refinery the gas stream would normally be fed to a Claus unit.

Example 2 is an example of the process according of invention wherein the sour water passes through a sour water stripping unit (first or upstream SWS unit) comprising a sour water stripping column with a reboiler, the vapour stream produced is cooled by an overhead cooling system (first or upstream overhead cooling system) comprising an external condenser and a reflux drum as in example 1. The condensed liquid collected from the external condenser and reflux drum is treated with sulphuric acid. The condensed liquid treated with a mineral acid (acidified “reflux” stream) feeds a separation system having an additional sour water stripping unit (second or downstream SWS unit) which is modelled with 4 theoretical trays and a second or downstream overhead cooling system). As in example 1, stripped water is produced from the first or upstream sour water stripping unit. There is very little gas stream (first SWS off-gas) produced from the first or upstream overhead cooling system. The gaseous product (second off-gas) from the SWS unit and overhead cooling system of the separation system (second or downstream SWS unit and second or downstream overhead cooling system) contains mainly H2S with negligible or no ammonia and the ammonium salt solution recovered is an ammonium sulphate solution.

Example 3 is an example of the process according of invention wherein the sour water feed has a molar excess of H2S over ammonia. Conditions are similar to example 2, except that more SWS off-gas containing H2S is produced from the reflux drum after the first or upstream SWS unit and first or upstream overhead cooling system. This gas stream (first SWS off-gas) is predominately H2S with an amount of ammonia which depends on the condenser temperature.

For instance, when the cooling temperature of the first or upstream overhead cooling system is 70°C there is 1.56% ammonia in the gas stream (first SWS off-gas) from the reflux drum after the first or upstream external condenser, at 60°C there is 0.72% ammonia and at 50°C there is 0.3%. Considering the total or combined off-gas produced in the process of the invention, namely the gas stream (first SWS off-gas) from the first or upstream overhead cooling system and the gaseous product (H2S) (second off-gas) from the separation step (the gaseous product comprising H2S obtained from the second or downstream overhead cooling system) the ammonia content in this total or combined off-gas may be 0.41% ammonia at 70°C, 0.135% at 60°C and 0.04% at 50°C.

As in example 1, stripped water is produced from the first or upstream sour water stripping unit. Example 4 is an example of the process according of invention wherein the sour water passes through a sour water stripping unit (first or upstream SWS unit) and an overhead cooling system (first or upstream overhead cooling system) as in example 1. The condensed liquid collected from the external condenser and reflux drum is treated with sulphuric acid. The condensed liquid treated with the mineral acid (acidified “reflux” stream) is fed to a separation system having an evaporator and a separation drum. The evaporator is modelled as a double effect evaporator, such that steam from the first stage of evaporation is used to heat the liquid to the second stage of evaporation. Each evaporation stage contains a separation drum and the ammonium salt solution from the final separation drum is a 40% solution of ammonium sulphate with less than approximately 0.3 ppm(w) H2S. The vapour stream from the evaporators contains H2S and after cooling the aqueous product is recycled to the SWS feed and the gaseous product (second off-gas) comprising H2S is combined with gas stream (first SWS off-gas) from the first or upstream overhead cooling system. This or combined or total off-gas is sent to a Claus unit.

As in example 1, stripped water is produced from the first SWS unit. There is a very little gas stream (first SWS off-gas) produced from the first or upstream SWS unit and first or upstream overhead cooling system. The separation system having an evaporator and a separation drum further include a cooler or condenser, so the vapour stream from the evaporator with a separation drum is cooled and the condensed liquid is further separated in a further separation drum to give hydrogen sulphide as a gaseous product and an aqueous product. The gaseous product from this separation drum is largely H2S with no ammonia and the aqueous product is water with a fraction of a percent H2S. In this example (and example 5) this aqueous product is recycled to join the feed to the SWS unit.

Example 5 is an example of the process according of invention as in example 2, except the sour water feed contains 100 ppm(w) of HCN and 200 ppm(w) phenol in addition to the ammonia and H2S in the feed to examples 1 and 2.

Summary of simulation results

Feed characteristics All examples use 3500 ppm(w) of ammonia, 17 ppm(w) methane and 3 ppm(w) ethane in the sour water feed. The methane and ethane represent non-polar gases dissolved in the sour water.

For examples 1, 2, 4 and 5 the sour water feed contains 4000 ppm(w) H2S. This gives a molar ratio of H2S:NH3 of 0.57, which is fairly typical of refinery sour water feeds.

For example 5 the sour water also contains 100 ppm(w) HCN and 200 ppm(w) phenol.

Example 3 uses sour water containing 8000 ppm(w) H2S, which gives a molar ratio of H2S:NH3 of 1.14 (i.e. an excess of H2S over that needed to form NH4HS).

Treated water quality

In all examples the treated water (i.e. the stripped water (13) from the first SWS unit) contains between 14 ppm(w) and 15 ppm(w) ammonia and less than 1 ppm(w) H2S.

Ammonium sulphate product and sulphuric acid use

In all examples the amount of ammonia in the sour water feed is 227.5 kg/h (3500 ppm(w) of ammonia with a flow rate of 65,000 kg/h sour water).

For examples 2 to 5 virtually all the ammonia in the feed is converted to ammonium sulphate, so the ammonium sulphate production is approximately 880 kg/h (dry basis) and the theoretical sulphuric acid use is approximately 670 kg/h of 98% sulphuric acid.

The ammonium sulphate solution produced in all examples contains less than 0.1 ppm(w) H2S. In example 5 the ammonium sulphate solution contains approximately 40 ppm(w) phenol and 0.1 ppm(v) HCN. These must be removed chemically or physically before usable ammonium sulphate can be produced.

Example 5 uses direct steam injection into the second or downstream SWS unit instead of a reboiler. This reduced the equipment needed but produces a more dilute ammonium sulphate solution. The ammonium sulphate solution can be concentrated or crystallised. Multi-effect evaporators can be energy efficient and to avoid interaction with the first or upstream SWS unit a stand-alone system may be preferred.

Liquid products, energy use and cooling duty for the first or upstream SWS

Note 7: The ammonium sulphate solution can be concentrated to 40% (wt) in a two stage evaporator with a heat input of approximately 2,200 kw. The solution could be concentrated in a single flash system operating at (say) 3.5 barg so that the flash steam could be used in the SWS reboiler. A concentrated solution would then be produced with a total energy use less than used for the conventional SWS as shown in example 1.

Note 2: A two effect evaporator is used with the condensed water recycled to the first or upstream SWS unit feed and flashed vapours cooled to condense most of the water and fed to the Claus unit.

Note 3: When steam is used the heating value has been converted to kw for comparison purposes.

Note 4: Even though in examples 2 to 5 the vapour stream from the first or upstream SWS unit is cooled to 60 °C the cooling duty (in kw) is lower than the conventional SWS process where the vapour stream is cooled to 85 °C. When in examples 2 to 5 the vapour stream from the first or upstream SWS unit is cooled to 85 °C, the SWS cooling duty is reduced as follows: 4588 kw (example 2), 4603 kw (example 3), 4946 kw (example 4) and 4830 kw (example 5). There is no effect on the heat input to the first or upstream SWS unit. If in example 1 the cooling temperature is reduced to 60 °C the heat input to the SWS unit increases to 8750 kw and the overhead cooling duty increases to 6424 kw.

Vapour products - Total or combined off-gas to the Claus unit

Note 5: In example 1, the total or combined off-gas composition is only the composition of the gas stream (SWS off-gas) from the overhead cooling system.

In examples 2 to 5, the vapour stream from the first or upstream SWS unit is cooled to 60 °C by a first or upstream overhead cooling system, at this temperature the cooling duty is lower than for the conventional SWS process of example 1 with a cooling temperature of 85 °C, so existing equipment may be suitable. In examples 2 to 5, when the cooling temperature is increased to 85 °C the ammonia concentration in the total or combined off-gas is as follows: example 2: 0.132% (mol), example 3: 1.84% (mol), example 4: 0.124% (mol), and example 5: 0.36% (mol). For example 3, the ammonia concentration is higher and it is preferred to operate the first or upstream cooling system at a lower temperature than 85 °C.

If example 1 is operated at 60 °C there is a risk of fouling with deposited ammonium salts but the composition of the off-gas is similar to that shown above for example 1 at 85 °C except that the water content is reduced.

A comparison between example 1 and 2 shows the process of invention reduces energy use by 14% and the air needed to process the off gas in the Claus unit by 61%, which is typical of the examples where ammonia is not fed to the Claus unit. The ammonium sulphate solution may be useable to apply agriculturally or industrially, so some local use may be considered. Normally the solution would be concentrated to provide a commercial concentration (typically 40% (w)) or crystallised. The solution produced from example 2 or 3 (and example 5 after purification) can be concentrated in a multiple effect evaporator in order to reduce energy use.

Example 3 shows the effect of high H2S concentrations in that some ammonia is carried forward into the gas stream (first SWS off-gas) from the first or upstream overhead cooling system. However, the amount (0.13% (vol)) would not need special treatment in the Claus unit.

Example 4 uses a two effect evaporator to treat the condensed liquid from the SWS “reflux” drum. This reduces the equipment needed to produce 40% (wt) ammonium sulphate solution and used the SWS to process the condensed water from the evaporator.

Example 5 shows a sour water feed containing phenol and HCN. Approximately 98% of the phenol fed to the SWS is transferred to the stripped water. This can be done by conventional means, such as using activated carbon. The second or downstream SWS unit uses 8 theoretical trays instead of the 4 used for examples 2 and 3 and removes HCN to less than 0.02 ppm(w). This residual would also be reduced with (for example) activated carbon. Example 5 shows direct steam injection to the second or downstream SWS unit. This avoids the need for a reboiler but does dilute the produced ammonium sulphate solution.