REMOVAL OF CONDENSABLES FROM PROCESS GAS CARRIER STREAMS

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a method and system for condensing, collection and removal in gas process streams of condensables, mist and aerosols contained within those streams. More specifically it relates to a method and system for connecting a liquid separator such as a mist extractor to a setting tank using a vertical riser with a portion of the liquids from the tank residing in the bottom region of the riser to act as a liquid lock preventing circulation of the gas stream through the portion of the riser subject to the liquid lock and permitting liquids from the separator to mix with the liquids in the settling tank.

Background

A process gas carrier stream in a fast pyrolysis process will contain entrained condensables, mist and aerosols, including a multitude of organic compounds appearing in these forms as products from a fast pyrolysis reactor processing organic plant waste material for the purpose of producing biooil. The invention addresses in particular the absolute need in such processing plant to remove most effectively mist and aerosols to facilitate and ensure operational continuity and availability. A failure to remove these condensables from the process stream at the appropriate stage in the cycle can result in the fouling of downstream components, such as electrostatic precipitators used to remove fine condensables from the gas stream and compressors used to pump the gas stream through the system. A very effective aerosol and mist removal system is an absolute requirement and guard against compressor fouling and loss of process availability.

It is well known that if the gas stream is too heavily laden with condensables, the downstream electronic precipitator will foul and require repeated cleaning in order to restore its efficiency. Similarly relatively small amounts of liquids in a gas compressor can cause damage to components of the compressor, leading in some cases to catastrophic failures. Furthermore,

relatively small amounts of condensables in the gas stream entering these downstream components of the process can result in earlier than normal shut down of the system leading to limitations in operational efficiencies.

This invention may be linked to operate in tandem with and down stream of a prime quench cooler in which the majority of condensables (biooil) and solid particles from a fast pyrolysis reactor process gas stream is collected. While the quench cooler is very effective in cooling and condensing condensables it is also very effective in coalescing and collecting the majority of liquid droplets and aerosols present in the gas stream also containing the key product biooil. However a residual of fine droplets will escape from the quencher with the carrying gas stream of non condensable gases. Typically these will be collected downstream by an electrostatic precipitator though occasionally and operationally the liquid droplet load within the gas stream can overload the precipitator. The concentration of condensables in the gas stream can be sufficient to damage a downstream compressor used to circulate the gas stream through the system.

Consequently it is necessary to insert an auxiliary intermediate collecting stage to reduce the intake of condensables to the precipitator. This intermediate collecting stage conveniently can be a fully flooded spray system where the product biooil, cooled, is used as spray medium. The spent spay medium together with any collected liquid is then returned to a holding tank, also called a settling tank. The invention addresses the way the system functionally and mechanically links the intermediate collecting stage to the settling tank.

SUMMARY OF THE INVENTION

In one aspect of the invention a riser channel has its lower end submerged with biooil from the settling tank to form a liquid lock. The return stream from the intermediate collecting stage enters the riser channel at some suitable point sufficiently above the liquid level in the riser channel . This level may be significantly above the general level in the settling tank. The collected liquid from the intermediate collecting stage drains into the riser channel, typically by gravity, and discharges into the setting tank to mix with the liquid (biooil) in the tank. Simultaneously the gas stream will enter the riser channel and escapes upwardly with less

entrained condensables to enter the electrostatic precipitator, or similar separator, and compressor. The gas stream continues its flow as the carrier gas of the system to the compressor which circulates the gas through the system. The gas then enters to fast pyrolysis reactor in order to carry the condensables (the biooil) into the initial quencher to be separated, and then on to the downstream components discussed above.

In an embodiment of the invention, a system for removing condensables and liquid droplets entrained in a process carrier gas stream includes a tank for collection of condensables and liquid droplets extracted from the gas stream flowing through it, the tank containing a level of liquid collected at the bottom region of the tank; a gas outlet on said tank; a liquid separator connected to said gas outlet for separating and collecting condensables and liquid droplets from the gas stream; a downward tilting drain connected to the separator to act simultaneously as a discharge of collected condensables and liquid droplets from the separator and as an exhaust to permit the gas stream to exit the separator; and a vertical riser channel connected to the drain for receiving the gas stream and collected condensables and liquid droplets from the separator, the riser channel associated with the tank such that the level of liquid in the tank may be maintained at a level which fully submerges the lower open end of the channel in the liquid, thereby preventing the gas in the tank from entering the lower open end of the riser channel; the riser channel having an upper outlet end to permit the gas stream received from the drain to exit the riser channel.

The lower open end of the riser channel may be located within the tank.

The tank may include an outlet for the condensables and liquid droplets to exit the tank and all or a part of the discharged condensables and liquid droplets may be used to simultaneously cool the gas stream as well as coalesce condensables and liquid droplets from the gas stream before entering the tank. A quencher may be located upstream of the tank for receiving the condensables and liquid droplets from the tank and the gas stream from the system, the quencher includes a spray outlet for spraying the condensables and liquid droplets through the gas stream to cool the gas stream and to absorb any liquid droplets entrained in the gas stream, as well as the droplets formed by condensation of condensables in the gas stream.

The tank may include an outlet for the condensables and liquid droplets to exit the tank and all or a part of the discharged condensables and liquid droplets may be used to simultaneously cool the gas stream as well as coalesce condensables and liquid droplets from the gas stream flowing through the separator. The separator may include a spray outlet for spraying the condensables and liquid droplets from the tank through the gas stream to cool the stream and to absorb the liquid droplets entrained in the stream, as well as the droplets formed by condensation of condensables in the gas stream.

The gas outlet of the tank may include a device for collecting liquid droplets entrained in the gas stream. The device for collecting liquid droplets enhanced in the gas stream may include a demister.

The separator may include a device for coalescing liquid droplets entrained in the gas stream, the device having an outlet end connected to a sump vessel for collecting the separated liquid and the gas stream, the sump vessel connected to the drain. The device for coalescing liquid droplets may include a mist eliminator. The mist eliminator may comprise a venturi.

The system may include a liquid cooler for cooling the liquid received from the outlet of the tank.

The system may include a liquid flow regulator controlling the flow of liquid from the tank to maintain the level of liquid in the riser channel sufficiently high to prevent the gas stream from the drain from re-entering the tank.

The system may include a fluid-transport device to move the gas stream through the system. The fluid-transport device may comprise a compressor, fan or blower.

In a further embodiment of the invention a method for removing condensables and liquid droplets entrained in a process carrier gas stream, includes the steps of (a) flowing the gas stream through a tank; (b) separating in the tank a portion of the condensables and liquid droplets entrained in the gas stream from the gas stream; (c) retaining a level of the condensables and liquid droplets in the tank; (d) directing the gas stream from the tank into a

liquid separator for further separation of condensables and liquid droplets from the gas stream; (e) flowing the portion of condensables and liquid droplets separated from the gas stream in the liquid separator by gravity through a slanted conduit; (f) discharging the collected condensables and liquid droplets in the slanted conduit into a vertical riser channel located in the tank, said riser channel being open at its bottom end to receive liquid from the tank, the collected condensables and liquid droplets in the slanted conduit discharging through the lower end of the riser channel into the settling tank and the gas stream from the slanted conduit flowing to exit the riser channel through the upper end of the riser channel, and, (g) maintaining a level of liquid in the tank at an elevation high enough to submerge the lower end of the riser channel thereby creating a vapour lock preventing the gas stream in the tank from flowing into the lower end of the riser channel.

DESCRIPTION OF THE DRAWING

Figure 1 is a schematic flow diagram of the fast pyrolysis apparatus and method of the present invention.

DETAILED DESCRIPTION

Definitions:

Fast Pyrolvsis refers to the processing any kind of forestry, agricultural or other plant waste in whatever form it may be available as feedstock for the pyrolytic process. Fast Pyrolysis is exposing these waste materials to decomposition at high temperature (approximately 400- 500° C) with little or no ingress of atmospheric oxygen.

Biooil refers to the liquid product obtained by Fast Pyrolysis. In this context biooil will combine all condensable compounds as well liquid particles as they emanate from the pyrolytic reactor and are subsequently cooled to ambient temperature. Biooil has several uses, including as a fuel and as a precursor to the creation of various chemicals.

Char refers to the solid product obtained by Fast Pyrolysis as it emanates from the pyrolytic reactor and are subsequently cooled to ambient temperature.

Non-condensables refer to the gaseous products obtained by Fast Pyrolysis as they emanate from the pyrolytic reactor and are subsequently cooled to ambient temperature.

Recycle gas flow refers to the carrier stream of non-condensables that are driven around through the system by compressor(s) and function as fluidizing medium in the fast pyrolysis reactor and as transporting medium of products produced in the reactor.

The Figure depicts schematically the preferred embodiment of this invention in association with a fast pyrolysis system for production of biooil from organic waste material. This includes the preferred system and method for removal of a part of the condensables downstream of the fast pyrolysis reactor and upstream of an electrostatic separator and/or a gas compressor to provide more efficient recirculation of the gas stream as the carrier gas of the system and method.

Solid biomass such as wood waste, bagasse, switch grass, and other organic materials enter pyrolysis reactor 10 wherein they are thermally decomposed using a fast pyrolysis process. Gas borne pyrolysis products from reactor 10, consists of a high temperature (400-500 ⁰ C) carrier gas flow laden with a multitude of compounds in the form of condensable gases, non condensable gases, aerosol and mist particle and some solids particles, which are basically char. The gas flow flows from reactor 10 through conduit 12 into a separator such as cyclone 14 (which may be plurality of cyclones 14 in series), where the bulk of the solids (char) will be removed. The gas flow will then pass through a conduit 16 to a quencher 18, typically a venturi scrubber, where it is intercepted by a intense spray of liquid (in this embodiment preferably being recycled cooler biooil received from conduit 20). The spray of liquid within quencher 18 will perform at least the following three major functions: a) reduce the gas flow temperature received in the quencher from conduit 16 whereby the bulk of condensable compounds in the gas flow will be liquefied and b) unite with the so formed liquid as well as absorb the bulk of mist and aerosol particles by coalescence / impingement and c) remove residual stray solid particles by impact.

Following the quencher 18 the gas flow and separated liquids will discharge through conduit 22 and flow into the primary separator 24 (sometimes referred to as a settling tank) wherein the separated liquids 23 will collect at the lower region 25 of settling tank 24 by force of gravity. The gas flow, still containing smaller amounts of liquids (condensables) in the form of mist and aerosols, exit settling tank 24 at the top of settling tank 24 preferably through demister 26. Demister 26 collects by impingement some of the liquid droplets in the gas flow as it passes through the demister and the collected liquids fall into the settling tank 24 by gravity.

The gas flow, still laden with residual mist droplets, enters conduit 28 and flow into a mist extractor 30, typically a high energy venturi. The gas flow is intercepted by a spray of liquid within the mist extractor 30 which further separates the liquid entrained within the gas flow in the same manner as discussed above with reference to quencher 18. Preferably the spray of liquid consists of recycled biooil from conduit 32 at a cooler temperature as compared to the gas flow in extractor 30. The separated liquids and the gas flow enter a sump vessel 34 at the lower end of mist extractor 30.

The gas flow and separated liquids enter a downward tilting drain conduit 36 from sump vessel 34 through a wall of settling tank 24 into riser channel 38 positioned vertically within tank 24. In an alternative embodiment channel 38 could be located outside tank 24 when connected to tank 24 below the liquid level and achieve the same function. The separated liquid from sump vessel 34 flows by gravity flow due to downward tilt of conduit 36 and falls downwardly when the separated liquid exits conduit 36 into riser channel 38. The gas flow in conduit 36 travels upwardly in riser channel 38 when the gas flow enters riser channel 38 from conduit 36.

Settling tank 24 is depicted with vertical riser channel 38 shown within settling tank 24. Lower end 40 of riser channel 38 comprises lower opening 41 which is open into tank 24. Level of liquid 42 in tank 24 is above the lower end 40 of channel 38. It can be seen that liquid exiting conduit 36 will fall downwardly in channel 38 by force of gravity to mix with liquid 23 in tank 24. By using control valve 48 to discharge product liquid from separator tank 24 and thereby maintaining the level of liquid 44 in separator tank 24 (and consequently also in riser channel 38) high enough above the lower end 40 of channel 38 liquid lock 51 is formed. Liquid lock 51 prevents gas flow within area 50 from flowing directly into channel 38, but instead forces the gas

flow to exit through demister 26, conduit 28 and then through mist extractor 30 to then flow through downwardly tilting conduit 36, acting as a drain, into riser channel 38. Conduit 36 acts simultaneously as a discharge of collected liquid from extractor 30 flowing by gravity to channel 38, and as an exhaust to permit the gas stream to exit the extractor 30.

As the gas flow through quencher 18 and settling tank 24 is driven by the suction of compressor 62, the level 44 in the riser channel 38 typically will be above level 42 in the settling tank 24 further reinforcing the function of the liquid lock 51.

The gas flow in channel 38 will then exit through conduit 56 at the top end 54 of riser channel 38.

The gas flow through conduit 56 with a reduced amount (or concentration) of liquid droplets entrained therein enters electrostatic precipitator 58 for final removal of liquid entrainment. The gas stream then proceeds from precipitator 58 through conduit 60 to gas compressor 62. Compressor 62 acts as a pump forcing the gas stream through the system. The gas stream leaving the compressor 62 is directed through conduit 66 into pyrolysis reactor 10 to complete the circuit of the gas stream.

In the preferred embodiment, this invention provides a system for condensing, collection and removal of condensables and liquid particles in gas process streams is an important part of the pyrolysis process referred to herein and specifically enhances the reliability of the operation of compressor 62 and reactor 10.

The biooil produced by the pyrolysis reactor 10 and removed from the gas stream is removed for use through drain conduit 68 controlled by valve 48. The in-line filter 70 removes any solid particles remaining in the biooil received through conduit 68. The biooil is then directed through conduit 72 into liquid pump 74. A desired portion of the biooil leaving pump 74 travels through conduits 76 and 80 into biooil product cooler/heat exchanger 82 which cools the biooil. The cooled biooil then travels through conduit 84 either through conduit 20 to spray on the hot biooil in quencher 18 or through conduit 32 to spray on the hot biooil in mist extractor 30. The

balance of the biooil leaving pump 74 exits the system by travelling through conduit 76 and then conduit 78 to be received in an appropriate storage vessel (not shown).

By temporarily lowering the level 42 in the settling tank 24 and level 44 in riser channel 38 below lower end 40 of riser channel 38, and isolating extractor 30 and its associated components and system, this enables maintenance on-line of the isolated parts.

As will be apparent to those skilled in the art to which the invention is addressed, the present invention may be embodied in forms other than those specifically disclosed above, without departing from the spirit or essential characteristics of the invention. The particular embodiments of the invention described above and the particular details of the processes described are therefore to be considered in all respects as illustrative or exemplary only and not restrictive. Other configurations could be developed based on known systems as may in the future be developed. The scope of the present invention is as set forth in the complete disclosure rather than being limited to the examples set forth in the foregoing description.