The present invention is related to a method and a system for the optimization of the part load capabilities of a biogas based PtX (power-to-X) conversion.

Biogas based PtX conversion processes are widely used. These processes will form part in the replacement of fossil fuel for the energy and/or heat generation. For the optimization of the operating conditions in processes for the synthesis of hydrocarbons (e.g. methane, methanol, Diesel, kerosene) from the C02 in the biogas, an appropriate part load of the biogas with hydrogen is desired.

Thus, the objective of the present invention is to provide a method and a system for the optimization of the part load capabilities of a biogas based PtX conversion that offer a better capability of using hydrogen that is generated or has not been consumed in the hydrocarbon synthesis process (e.g. methanation) for further process steps, such as a recycling into the synthesis process.

This objective is achieved according to the present invention by a method for the optimization of the part load capabilities of a biogas based PtX conversion, comprising the steps of: a) converting the CO2 fraction of the biogas at least partially with hydrogen in a hydrocarbon synthesis reactor, into a CH4-rich product gas stream, preferably using a condensation unit to separate a fluid comprising water and/or condensable hydrocarbons, such as methanol, gas oil or kerosene, from the CH4 enrichted product gas stream; b) treating the CH4-enriched product gas stream by using a regular membrane upgrading unit that at least comprises a main membrane separating C02 from methane to yield injectable CH4 enriched gas and a C02 cleaning membrane recovering permeated methane downstream of the main membrane from C02 ; and c) leading the CH4-enriched product gas stream over a polymeric gas separation membrane thereby positioning the polymeric gas separation membrane in addition to the regular membrane upgrading unit downstream of the hydrocarbon synthesis reactor and upstream of the regular membrane upgrading unit.

This objective is also achieved according to the present invention by a system for the optimization of the part load capabilities of a biogas based PtX conversion, comprising : a) a hydrocarbon synthesis reactor, enabled to convert a gas feed, such as the biogas (3) , into a CH4-rich product gas stream, and preferably a condensation unit to separate a fluid comprising water and/or condensable hydrocarbons, such as methanol, gas oil or kerosene, from the CH4-enriched product gas stream; b) a regular membrane upgrading unit enabled to treat the CH4-enriched product gas stream wherein the regular membrane upgrading unit at least comprises a main membrane being enabled to separate C02 from methane to yield injectable CH4 enriched gas and a C02 cleaning membrane being enabled to recover permeated methane downstream of the main membrane from C02 ; and c) a polymeric gas separation membrane being enabled to treat the CH4-enriched product gas stream wherein the polymeric gas separation membrane is in addition to the regular membrane upgrading unit and is positioned downstream of the fluidized bed reactor and upstream of the regular membrane upgrading unit.

By the provision of the additional polymeric gas separation membrane, the method and the system allow to increase the recycle rate of hydrogen in the process and therefore reduce its loss through the off gas stream. At the same time, the present invention also allows to decrease the slip of the generated hydrocarbon through the same path and reduce the energy loss through the off gas stream.

Preferred embodiment of the present invention are listed below and can be also combined: i) the permeate side of the polymeric gas separation membrane can be flushed with a retentate gas stream of the CO2 cleaning membrane; ii) the permeate side of the polymeric gas separation membrane can be flushed with raw feed gas, e.g. biogas; iii) a gas feed can be to the hydrocarbon synthesis reactor comprising hydrogen, carbon oxides (CO and/or CO2 ) and methane, stemming e.g. from a gasification or a pyrolysis step; and/or. iv) a gas feed, preferably stemming from a direct gasification or a pyrolysis step with air as oxidation mean, can hydrogen, carbon oxides (CO and/or CO2 ) and nitrogen, wherein the nitrogen content can be extracted as retentate of the regular membrane upgrading unit. Preferred embodiments of the present invention are hereinafter described in more detail with reference to the drawing which depict in the figure a schematic view on a system 2 for biogas based PtX conversion. In order to further improve the part load capabilities of a biogas-based PtX process, the figure shows the introduction of a polymeric gas separation membrane 4 in addition to a regular membrane upgrading unit 6 that usually comprises a main membrane 8 (that separates C02 from the hydrocarbon, such as methane, to yield injectable gas for a gas grid 14) and a C02 cleaning membrane 10 (that recovers permeated hydrocarbons from the CO2 before it is vented with the off gas stream 12) . The off gas can also be used for sequestration, this way allowing for negative CO2 emissions.

The present invention allows thus to increase the recycle rate of hydrogen in the process at an early stage and therefore reduce its loss through an off gas stream 12. At the same time, the present invention also allows to decrease the slip of the generated hydrocarbons, such as CH4, through the same path and reduce the energy loss due to recompression .

The polymeric gas separation membrane 4 - also called hereinafter sweep membrane - can be applied in a raw product gas stream 16 coming from the hydrocarbon synthesis reactor 18 optionally followed by a condensation unit 19 to separate a fluid 21 comprising water and/or condensable hydrocarbons, such as methanol, Diesel or kerosene, and being disposed upstream of the regular membrane upgrading unit 6, optionally also mixed with unreacted raw gas (due to the part load) . Under favorable conditions, the polymeric gas separation membrane 4 separates at least a part of the hydrogen already before the raw product gas stream 16 enters the regular membrane upgrading system 6. In the present example, the hydrocarbon synthesis reactor can be for example a methanation reactor in fix bed or fluidized bed layout .

Furthermore, CO2 may be transferred from the recycle stream to the retentate side. This leads to an accumulation of CO2 and therefore a replacement of H2 with CO2 in the membrane upgrading unit 6.

A scheme for the sweep membrane process indication the desired and undesired permeating flow in the sweep membrane is illustrated in the figure. The left arrow over the sweep membrane 4 represents the desired H2 permeation (light-grey color) while the right arrow over the sweep membrane represents the blocking function of the sweep membrane 4 for the undesired permeation of CO2 and the hydrocarbons, such as CH4 (dark-grey color) .

This additional sweep membrane 4 is equipped with an inlet at the permeate side for a so-called sweep stream (see Figure) . The recycle stream of the plant, corresponding to the retentate of the second upgrading stage in the membrane upgrading unit, directly forms the sweep-stream 20. The advantage of feeding the recycle stream as a sweep stream 20 to the permeate side is, that it lowers the partial pressure difference of CO2 and the hydrocarbon, such as CH4.

Therefore, the permeation of these components in the sweep membrane 4 can be inhibited. On the other hand, as the H2 content in the recycle stream is already decreased by the second membrane stage 10, the permeation of H2 is favored. Therefore, under favorable conditions, it is possible to increase the selective recycling of H2 without changing the membrane type .

Alternatively, instead of using the gas stream 20 from the retentate side of membrane 10, the raw feedgas (e.g. biogas) could be used as sweep stream, too.

In order to investigate the feasibility of this selective separation of H2 and the benefits of a sweep stream on the membrane' s performance, simulations as well as field tests were performed on the commercial biogas upgrading membrane. Both delivered promising results in this regard.

The polymeric gas separation membrane 4 can be made from polymers, such as poly-imide, poly-propylene etc. Ceramic and metallic membranes are often a bit more selective than polymeric membranes but they require significantly higher temperatures (as compared to ambient temperatures or slightly elevated temperatures above ambient temperature) and often also higher pressure gradients over the membrane. The polymeric membranes further enables the operator of the system 2 to support the adjustment of specific selectivity of the permeation conditions by an appropriate adjustment of the content in the sweep gas stream 20 without losing any permeability for the desired H2 permeation in the respective sweep membrane 4.