FIELD OF THE INVENTION

The present invention relates to gas treatment systems including the apparatus thereof, the processes thereof and any combination thereof. The present invention in a more particular aspect, but not to sole aspect, is to reduce methane slip in a biogas treatment regime without any significant adverse affect on throughput.

BACKGROUND TO THE INVENTION

We through subsidiary Greenlane Biogas Limited, and affiliated Flotech companies, support systems for upgrading hydrocarbon gases or biogas. See for example our disclosures in respect of biogas upgrading from raw biogas in New Zealand patent specification 553992 and our New Zealand patent specification 566845/571948.

The Greenlane™ biogas upgrading process upgrades methane content. Such a system simplistically can be described in sequence as a gas compression/water

scrubbing/ flashing/ ecovery gas type upgrading system.

Our New Zealand patent 566845/571948 mentioned above discloses the synergy to be obtained from using a water flooded compressor. In such a process in New Zealand patent 566845/571948 we describe the process in more detail as follows.

Feedstock of raw biogas is normally supplied to the plant at just above atmospheric pressure and water saturated (RH 100%). Moisture and particulates are removed at the inlet separator then the gas may be compressed in two stages up to ~ 9 bar(g) and cooled. Raw gas enters the scrubber at the bottom, contacting the process water in counter-flow towards the cleaned gas exit at the top. The scrubber has specially designed internals, which force the gas to be exposed as much as possible against the process water. C0 ₂ and H ₂ S are absorbed into the water, so the gas leaving the scrubber contains 97— 98% CH ₄ at RH 100%. The gas is then dried in a twin column pressure swing adsorption (PSA)/ temperature swing adsorption (TSA) drier to control the dew point below -80°C. Product gas is analysed; if it does not meet quality criteria, it is recycled back to the compressor inlet for reprocessing.

The process water absorbs some CH ₄ during the scrubbing process; this CH ₄ is recovered at intermediate pressure in the flashing tank and returned to the compressor to minimise CH ₄ losses. The process water then enters the stripping system where the C0 ₂ is removed at slighdy above atmospheric using air as the stripping medium— clean water is pumped from the stripper back into the scrubber process. The process water is heated (mainly by pump and compressor energy input) - hence it must be cooled and ideally to less than 7°C. Where copious cold, fresh water is available, cooling can be easily achieved however for sites where water is not freely available, or where improved process efficiency is desired, a water chilling system will be installed. The synergistic effect of using a water flooded screw ("WFS") type compressor or other water cooled/lubricated type compressor, where the compression is in the presence of water within the compressor, was the focus of our New Zealand patent 566845/571948. Such WFS compression, quite apart from using water of the compressor as a seal, for cooling, and for lubricating also ensures some partial scrubbing activity prior to passing of the compressed gases to a scrubber. With no introduction of organic lubricants that require removal downstream there is that further synergy.

It is an object of the present invention in a system, whether using a WFS type compressor or not, to provide a gas purification process, plant or system able to reduce methane slip and/or which will go at least some way towards addressing the foregoing problems or which will at least provide the industry with a useful choice.

A further or alternative object of the present invention to provide a gas purification process and/or method with at least two flashing steps following scrubbing, each flashing step creating a gas flow able to be recycled back into a compression stage and/or which will go at least some way towards addressing the foregoing problems or which will at least provide the industry with a useful choice.

It is a further object of the present invention to provide a process or plant for treating a methane including gas stream to return to a compression sequence of a gas purification process a recycle flow from a flashing step that better matches the pressurisation of the inflow to the particular pressure stage and/or which will go at least some way towards addressing the foregoing problems or which will at least provide the industry with a useful choice.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

SUMMARY OF THE INVENTION

In a first aspect, there is provided a gas purification process comprising: feeding a gas stream to be purified for to an upstream side of a compression stage and compressing the gas,

supplying a liquid stream and the compressed gas stream to a liquid scrubbing stage, and contacting said compressed gas stream with said liquid stream, said liquid scrubbing stage providing at least one downstream gas stream output and at least one downstream liquid stream output, and following said liquid scrubbing stage,

directing the at least one liquid stream output to two flashers operating in series, a first flasher and a second flasher, each of said flashers generating a recyclable flashed gas output stream from the liquid stream, and

recycling the respective flashed gas output streams into the gas stream to be purified, either upstream or downstream, or both, of the compression stage, and wherein said flashed gas is recycled into the gas stream at the respective flashed gas pressures of the respective flashers.

In one aspect the invention is, in a methane enrichment procedure involving gas compression and water scrubbing, the use of two flashers in series post scrubbing to retrieve methane from the liquid stream from the scrubber and to return, at the respective flashed gas pressures, the flashed gas into the gas stream either side of the compressor or first compressor.

Preferably the compressor is a WFS compressor.

Preferably the higher pressure flashed gas off the first flasher is fed inter-stage of the compressor.

Preferably the lower pressure flash gas off the second flasher is fed to inlet of the compressor.

Preferably the higher pressure flashed gas off the first flasher is fed inter-stage of the compressor and the lower pressure flash gas off the second flasher is fed to inlet of the compressor.

Preferably there are at least two compressors in series.

Preferably the higher pressure flashed gas off the first flasher is fed to the inlet of the downstream more of the two compressors.

Preferably the lower pressure flash gas off the second flasher is fed to inlet of the upstream more of the two compressors.

Preferably the higher pressure flashed gas off the first flasher is fed to the inlet of the downstream more of the two compressors and the lower pressure flash gas off the second flasher is fed to inlet of the or an upstream more of the two compressors.

In another aspect the invention is a gas purification process of a kind having at least a two stage compression to provide a pressurised gas flow for scrubbing, wherein there is at least a two stage flashing after scrubbing of the pressurised gas stream to provide the purified gas outfeed from the scrubber, and wherein the flashing steps are to different pressures, with that flashed stream of gas of lower pressure being recycled to the inflow of the first, or an earlier, compression stage and with that flashed stream of gas of higher pressure being recycled to the inflow of the second, or a later, compression stage.

Preferably the flashed stream of gas of lower pressure is from the second, or a later, flashing stage.

Preferably the flashed stream of gas of higher pressure is from the first, or an earlier, flashing stage.

Preferably there are only two flashing stages.

Preferably there are at least two flashing stages.

Preferably the gas being treated is a biogas or other gas.

Preferably the gas being treated includes a methane content

Preferably the gas being treaded includes a carbon dioxide content.

Preferably the gas being treated includes a methane and a carbon dioxide content. Preferably the scrubbing is with water.

Preferably there are only two compression stages.

Preferably there are at least two compression stages.

Preferably the compression is by one or more "WFS" type compressors or two or more rotary vane compressors.

Preferably the recycling on a normalised volume basis into the inflow of the first compression stage is in the range of from 15 to 20% of the raw gas flow and on a normalised volume basis into the inflow of the second compression stage is in the range of from 20 to 30% of the raw gas flow.

As an example - assuming a raw gas inflow of 2000 Nm3/h into the system there is additional flow of gas of about 600 Nm3/h from the recycled flash gas streams. Between 40 to 55% of this flash gas is from the first stage flash vessel and is recycled back after the first compressor (such as to the outlet of the first compressor) and the remainder is from the second stage flash which is recycled back before the first compressor (such as to inlet of the first compressor) (assuming an operating flash pressure of 3.5-4 bar(g) in the first flash vessel and 0.5- 2 bar(g) in the second flash vessel).

In another aspect the invention is a gas purification process of a kind having a two stage compression to provide a pressurised gas flow for scrubbing to provide the purified gas outstream, wherein there is a two stage flashing of recyclable gas(es) from the liquid stream after scrubbing of the pressurised gas stream, and wherein the flashed stream of gas of the later flashing stage is recycled to the inflow of the first compression stage and the flashed stream of gas of the first flashing stage is recycled to the inflow of the second compression stage.

Preferably the flashed stream of gas of the second flashing stage is at a lower pressure than that from the first flashing stage.

Preferably there are only two flashing stages.

Preferably the gas being treated is a biogas or other gas with a methane content.

Preferably the scrubbing is with water.

Preferably the compression is by one or more "WFS" type compressors or two or more rotary vane compressors.

Preferably the recycling on a normalised volume basis into the inflow of the first compression stage is in the range of from 15 to 20% of the raw gas flow and on a normalised volume basis into the inflow of the second compression stage is in the range of from 20 to 30% of the raw gas flow.

In another aspect the invention consists in a gas purification plant which comprises or includes

a first compressor to receive a gas inflow,

a second compressor to receive (optionally after heat exchange) the gas stream from the first compressor,

a scrubber or scrubber sequence ("scrubber") (optionally after heat exchange) to receive the gas flow stream from the second compressor and to provide a gas outflow stream (e.g. as the purified gas),

a first flasher to receive the liquid outflow with any entrained gas from the scrubber, and a second flasher to receive the liquid with some entrained gas from the first flasher; wherein the first flasher recycles at least some (and preferably all) its flashed gas stream as an additional infeed to the second compressor (optionally before heat exchange),

and wherein the second flasher feeds (at least some and preferably all) its flashed gas stream as an additional infeed to the first compressor.

In another aspect the invention consists in a gas purification plant which comprises or includes

a first compressor to receive a gas inflow,

a scrubber or scrubber sequence ("scrubber") (optionally after heat exchange) to receive the gas flow stream from the second compressor and to provide a gas outflow stream (e.g. as the purified gas),

a first flasher to receive the liquid outflow with any entrained gas from the scrubber, and a second flasher to receive the liquid with some entrained gas from the first flasher, wherein the second flasher feeds at least some (and preferably all) its flashed gas stream as an additional infeed to the first compressor,

and wherein the first flasher recycles at least some (and preferably all) its flashed gas stream as an additional infeed inter-stage to said first compressor.

In another aspect the invention consists a method of treating a gas with a methane gas inclusion, said method comprising or including the steps of

providing a feed supply of the gas to be treated,

pressurising the gas in a first compressor or first compressor stage ("first compressor"), optionally heat exchanging the pressurised gas stream from the first compressor stage, feeding the gas pressurised by the first compressor (optionally after heat exchange) into a second compressor or second compressor stage ("second compressor") thereby to provide gas at a higher pressure,

optionally heat exchanging the higher pressure gas stream from the second compressor, subjecting the higher pressurised gas stream from the second compressor (optionally after heat exchange) to a scrubber or at least a series of scrubbers ("a scrubber") thereby to provide an outtake feed of a methane enriched gas and to provide a liquid outfeed that includes some entrained gas or gases (inclusive of methane),

subjecting the liquid outtake from the scrubber to a first flashing stage ("first flasher") thereby to provide a liquid outtake from the first flasher and a gas recycle feed from the first flasher back to the second compressor as a component of its gaseous intake,

subjecting the liquid outtake with some entrained gas(es) from the first flasher to a second flashing stage ("second flasher") thereby to provide a gas recycle feed from the second flasher as an infeed component of the feed gases to the first compressor and to provide a liquid outtake of the second flasher

Preferably the compressor of each compressor stage is a liquid containing compressor able to commence a scrubbing of the gas encountered in that compressor.

Preferably the liquid outtake from one or both compressors can be subjected to a methane recovery procedure.

In a further aspect the present invention may broadly be said to reside in, in a biogas processing plant that includes two or more in series feed supply gas compression stages advance of a scrubber, the provision of two downstream and in-series flashers, each providing a gas recycle feed at different pressure to allow:

the recycling of gas from the first and downstream more flasher, at a pressure more compatible with the inlet pressure of gas at the first compression stage, to the first compression stage, and the recycling of gas from the second and upstream more flasher, at a pressure more compatible with the inlet pressure of gas at a second compression stage, to a second compression stage.

Preferably the first of said compressions stages is upstream of a said second of said compression stages.

Preferably the first flasher is downstream of said second flasher.

Preferably the pressure difference between the gas fed from the first flasher to the first compression stage and the feed supply gas is less than the pressure difference between the gas of the second flasher and the feed supply gas.

In a further aspect the present invention may be said to be a lower operating pressure second flasher downstream of a higher operating pressure first flasher in a biogas upgrade system, a recycle gas output from which is at least partially fed back into the system for introduction into the flow of feed supply gas entering a first compression stage of the system.

Preferably a recycle gas output from the first flasher is fed back into the system at where the pressure of the feed supply gas is higher than at the inlet of the first compression stage.

Preferably a recycle gas from the first flasher is fed back into the system at a location downstream of the inlet of the first compression stage.

Preferably said first compression stage comprises a first compression

Preferably said second compression stage comprises a second compressor.

Preferably the first and second compressions stage occurs within a WFS type compression and said recycle gas output from the first flasher is fed back into the WFS compressor downstream of the inlet of the first compression stage of the WFS compressor.

In a further aspect the invention consists in a WFS type compressor that includes an additional intake for a recycle gas feed.

Preferably the additional intake is at a location downstream of the supply gas infeed.

In a further aspect the present invention may be said reside in, in a biogas upgrade system that includes a scrubber upstream of multi stage flashing taking place in at least two flashing vessels and a stripper downstream of said multistage flashing, wherein a first and second of said flashing vessels are in a vertical disposition relative to each other and/ or said stripper and/ or flasher to tend towards operating the system with minimal slip.

In a further aspect the present invention may be said reside in, in a biogas upgrade system that includes a scrubber upstream of multi stage flashing taking place in at least two flashing vessels and a stripper downstream of said multistage flashing, wherein a first and second of said flashing vessels are in a vertical disposition relative to each other and/or said stripper and/ or flasher to tend towards operating the last of the flash vessels at or greater to the pressure at the outlet of the preceding flasher less static pressure head defined by the said vertical disposition.

In a further aspect the present invention may be said reside in, in a biogas upgrade process, that includes a scrubber upstream of multi stage flashing taking place in at least two flashing vessels and a stripper downstream of said multistage flashing, wherein a first and second of said flashing vessels are in a vertical disposition relative to each other and/ or said stripper and/ or flasher to tend towards operating the system with minimal slip.

In a further aspect the present invention may be said reside in, in a biogas upgrade process that includes a scrubber upstream of multi stage flashing taking place in at least two flashing vessels and a stripper downstream of said multistage flashing, wherein a first and second of said flashing vessels are in a vertical disposition relative to each other and/or said stripper and/or flasher to tend towards operating the last of the flash vessels at or greater to the pressure at the outlet of the preceding flasher less static pressure head defined by the said vertical disposition.

In a further aspect the present invention may be said reside in, in a biogas upgrade system that includes a scrubber upstream of multi stage flashing taking place in at least two flashing vessels and a stripper downstream of said multistage flashing, wherein a first flashing vessel is located below a second said flashing vessels that is downstream of the first flashing vessel.

In a further aspect the present invention may be said reside in, in a biogas upgrade system that includes a scrubber upstream of multi stage flashing taking place in at least two flashing vessels and a stripper downstream of said multistage flashing, wherein a first flashing vessel is located at a vertical disposition relative a second said flashing vessels that is downstream of the first flashing vessel such that the operating pressure in the first flashing vessel is higher than in the second flashing vessel.

In a further aspect the present invention may be said to be a method of reducing the operating pressure, in a multi stage flash process of a biogas upgrade system that includes at least two in series flash vessels, in a downstream of said flash vessels by placing the or the upstream flash vessel(s) at an appropriate elevation relative to said downstream of said flash vessels.

Preferably the appropriate elevation where one of more of said upstream flash vessels are below the downstream flash vessel.

Preferably there are two flash vessels and the upstream flash vessel is positioned lower than the downstream flash vessel.

Preferably the elevation is such that the fluid pressure entering the downstream flash vessel is less than the fluid pressure exiting the upstream flash vessel. ci - In yet a further aspect the invention consists in plant embodying a procedure substantially as hereinafter described with reference to Figure 2.

In yet a further aspect the invention consists in a compressor substantially as herein described with reference to Figure 3.

In yet a further aspect the invention consists in plant that operates substantially as shown by the flow diagram of Figure 2.

As used herein the term "(s)" following a noun means one or both of the singular or plural forms.

As used herein the term "and/ or" means "and" or "or". In some circumstances it can mean both.

As used herein the term "purification" in respect of a methane including gas means or includes increasing the methane content by removal of other gas or gases.

As used herein "methane slip" means loss of methane into the environment.

The term "comprising" as used in this specification means "consisting at least in part of. When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

The invention consists in the foregoing and also envisages constructions of which the following gives examples only.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred form of the present invention will now be described with reference to the accompany drawings in which:

Figure 1 shows a process with a single flashing step after the scrubbing step, the recycle from that single flasher being to in advance of a single compressor or a first compressor of a two or multiple compressor compression of the methane containing gas stream prior to presentation to the scrubber, where solid lines indicate the flow of biogas and dotted lines indicate the flow of water or two phase gas water mixture,

Figure 2 shows the enhanced procedure of the present invention with its two flashers each with a recycle back to either side of the compressor or first compressor, where solid lines indicate the flow of biogas and dotted lines indicate the flow of water or two phase gas water mixture,

Figure 3 shows the scrubbing vessel, the two flashing vessels and the stripping vessel as they might be arranged for gas purification according to the present invention, and

Figure 4 shows how, for example, a modified water flooded screw compressor can accommodate flash gas recycle at a zone labelled "A" that can represent a recycled gas inlet port.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention, an improved gas processing system is provided, where a gas to be processed (or purified) is passed provided as a compressed gas stream to a liquid scrubber. Such a liquid scrubber provides for a gas output stream (e.g. as a gas outlet or purified gas outfeed or purified gas outstream or gas outflow stream). The liquid scrubber also provides for a liquid output (e.g. as a liquid entraining gaseous components not otherwise contained in the gas output stream from the scrubber). The liquid output stream (or liquid stream or liquid outflow) is sent or directed to the two flash units (a first flasher and a second flasher) operating in series.

The first flasher (FLASH1) located downstream of the scrubber (SCBR) is operational at pressures higher than the pressures of the second flasher (FLASH2). The second flasher (FLASH2) is located downstream from the scrubber (SCBR) and downstream of the first flasher (FLASH 1).

Flashed gas or the flashed stream of gas output (16) from the first flasher (FLASH1) is returned or recycled back to join the gas stream either inter-stage of the first compressor, or to join the gas stream being fed to the inlet or inflow to the or a second compressor (or second or a later compression stage of a multi-compression compressor). Such a mixing of the gas streams and recycled flashed gas stream is made possible and facilitated by the pressure of the flashed gas exiting the first flasher (FLASHl). Liquid output (15) from the first flasher (FLASH1) entraining gaseous components can be sent or directed to the second flasher (FLASH2).

Gaseous components in the liquid directed to the second flasher (FLASH2) are flashed and provided as a gas or the flashed stream of gas output (18) from the second flasher

(FLASH2). Such a stream of gas is returned or recycled back to join the gas stream to the inflow or inlet of the first compressor, or an earlier stage of a multi-compression compressor. Such a mixing of the gas streams and recycled flashed gas stream is made possible and facilitated by the pressure of the flashed gas exiting the second flasher (FLASH2). Liquid output (17) from the second flasher (FLASH2) is sent to waste or for optional further processing. In the manner described above, a recycled series of gas streams from the flashers (FLASH1, FLASH2) are achieved for re -processing. Additionally, utilisation of the pressures of the flashed gases from the flashers enables the re-injection of such gases at points in the process which minimise the need to completely re-compress the flashed gas to the same requirements as the raw gas inflow or input or raw supplied gas to the process (e.g. gas stream 1).

Therefore, a higher overall level of gas may be processed by the plant, whilst minimising the energy inputs for compression of gas, and at the same time reducing slip of desired gas from the plant. For example, the flash units enable retrieval of desirable gaseous components to be recovered and contribute to the overall yield of a purified or processed gas stream (14).

With reference to the figures, further details are as follows.

In Figure 1 the following numerals and abbreviations refer to the following:

1 - the primary gas intake

2 - the feed of the primary gas stream 1 and the recycle gas stream 15 into the first compressor COMPRl. The first compressor preferably is a WFS type compressor.

3 - the first pressurised gas stream from the first compressor to STG1SEP, a separation vessel, to provide a liquid outtake stream 4 (e.g. water with some entrained gas) and a gaseous outflow stream 5 to be passed to a heat exchanger HEXl on the flow 5 to 6

6 - The cooled flow of gas into the second compressor COMPR2

the second compressor outfeeds 7 the higher pressurised gaseous outflow from the second compressor, to STG2SEP, a separation vessel, to provide a liquid outflow stream 8 (e.g. of gas entrained liquid) and a gaseous stream outflow 9 to a heat exchanger HEX2, on the gaseous flow from 9 to 10, to cool the gas stream.

SCBR the scrubber or scrubber series to provide the enriched methane gas outflow 13, to receive the liquid inflow 11 (e.g. scrubbing water) and to provide a liquid outflow 12 with entrained gas to a flasher.

FLASH is the one and only flasher of the system to provide a liquid outflow with some entrained gas 14 and the recycled feed 15 to be mixed prior to the first compressor.

As explained hereinafter such a return of all of the flashed off gas in advance of a single compressor or the primary compressor is detrimental to throughput.

In the arrangement of Figure 2 the primary gaseous infeed is shown as 1 where it is mixed with recycle gas from FLASH2, prior to its feed as 2 to a first compressor COMPRl.

The liquid gas separation vessel STG1SEP provides a liquid outflow stream 5 that contains gaseous content whilst the gaseous stream 6 is passed, via a heat exchanger HEXl, to provide a cooled gas streamed feed 7 into the second compressor COMP2 (preferably also a WFS type compressor, and, for example, as modified as suggested in Figure 3). Then that higher pressurised gas stream 8 is passed to the separation vessel STG2SEP where there is a liquid outflow 9 and gaseous outflow. The gaseous flow passes via heat exchanger HEX2 into the scrubber SCBR which receives a liquid inflow (e.g. water) 12, provides the enriched gas outflow 14 and has a liquid outflow with entrained gas 13 to the first of a series of two flashers FLASH1 and FLASH2.The remaining liquid/gas mixture then proceeds to the stripper STPR.

Figure 3 shows the typical vertical positions of the scrubber inlet port 12, the scrubber outlet port 13, the stripper inlet port 22 and the stripper outlet port 23. While a pump or similar device could be employed to elevate the process liquid from the base of the scrubber to the top of the stripper, the required additional energy input would be detrimental to the overall efficiency of the process. Instead, the pressure differentials between subsequent points in the system can be used to "pump" liquid around the process loop.

To achieve this, the minimum operating pressure in each flashing vessel will need to be sufficient to overcome the static head pressure and frictional losses incurred between each pair of inlet and outlet points. For example, the minimum pressure in FLASH1 will depend on the relative difference in height between the outlet port 19 and the inlet port 20. Similarly, the minimum pressure in FLASH2 will depend on the relative difference in height between the outlet port 21 and the stripper inlet port 22.

However, a lower flash pressure allows a greater amount of the CH4 entrained in the process liquid to be recovered.

For this reason a preferred positioning of the flashing vessels is as shown in Figure 3, with FLASH1 situated essentially at ground level while FLASH2 is positioned atop the stripping column. Because there is no static head pressure to overcome between outlet port 21 and inlet port 22, this arrangement allows FLASH2 to operate at minimal flash pressure. It will be appreciated that the two flashing vessels may be positioned at a range of vertical heights with respect to one another and with respect to the outlet port of the scrubber and the inlet port of the stripper. The relative positioning to achieve optimal reduction in methane slip may vary from plant to plant. The dashed line is an indication of a horizontal level or datum with respect to the relative heights or vertical positioning of the particular operational units in this process, e.g. FLASH1 is shown as optionally being positioned relatively at a vertically lower position that FLASH2.

The first flasher FLASH1 provides a recycle feed 16 to be mixed into the outtake feed from the first compressor COMPR1 as shown at MIX2 in Figure 2. The liquid outfeed from FLASH1, still with entrained gas 15, is passed to flasher FLASH2 thereby to provide a liquid outfeed 17 with some entrained gas and to provide a flashed or gas recycle feed 18 to be mixed at MIX in the flow 1 to 2 prior to the first compressor COMPR1.

COMPR1 may be a WFS type compressor. It may also be a rotary vane compressor.

COMPR2 may be a WFS type compressor. It may also be a rotary vane compressor. Preferably the recycle is such as to provide a normalised volume basis recycle to the first compressor of from 15 % to 20 % and to the second compressor of 20% to 30%, as percentage of the raw gas infeed.

Preferably the operating pressure differential of the recycle feed 16 is about 3.5 to 4 bar(g) and feed 18 is 0.5 to 2 bar(g).

As an example - assuming a raw gas inflow of 2000 Nm3/h into the system there is additional flow of gas of about 600 Nm3/h from the recycled flash gas streams. Between 40 to 55% of this flash gas is from the first stage flash vessel and is recycled back to the outlet of the first compressor and the remainder is from the second stage flash which is recycled back to inlet of the first compressor (assuming an operating flash pressure of 3.5-4 bar(g) in the first flash vessel and 0.5-2 bar(g) in the second flash vessel.

Application to Water Flooded Screw (WFS) Compressor Technology

Current WFS technology is a single stage screw design that uses injected water for rotor sealing and direct gas cooling. The two stage flash vessel design in a plant using this type of compressor may be realized by injecting the first stage flash vessel recycle gas 16 into another port along the length of the rotors of the WFS compressor where the pressure is lower than the final discharge pressure (Figure 4). The second stage flash vessel recycle gas 18 would be recycled back to flow into the inlet of the WFS compressor as per Figure 2.

COMPR 2 in such an arrangement may or may not be present. Neither may the components in the box as shown in figure 2 and gas feed 7 may be straight to the SCBR.

Some Advantages Explained

Previous Design (Figure 1):

One of the main limitations of the previous model was the flash vessel design.

The flash vessel is a key component of the design which controls the methane slip of the system. Reducing the pressure in the flash vessel reduces the methane slip; however it also results in a simultaneous decrease in the production capacity of the plant.

- The limitation of this flash vessel design is that it recycles all the flash gas (stream 2 in Figure 1) to the inlet separator and at low flash pressures the recycle gas flow rate is too high and this compromises throughput. This flash vessel design is also limited by its minimum operating pressure— a minimum pressure of at least 1.3 bar(g) is required to overcome to static head and frictional losses between the bottom of the flash vessel and the top of the stripper. This ultimately places a limitation on the methane slip achievable in this configuration.

New Design (Figure 2):

The new design consists of two flashing vessels. The first stage flash vessel is similar to the previous design however the flash gas recycle stream is fed back to the discharge stream of the first compressor where the pressure is nominally between 3-4 bar(g). Feeding the first stage flash gas to the outlet stream of the first compressor does not limit the production capacity of the biogas plant.

The second stage flash vessel sits on top of the stripping column which allows the flash pressure to be reduced further than the previous design and therefore also the methane slip - there is no static water head and the frictional losses are minimal. The operating pressure of this vessel would be between 0-2 bar(g).

This design update enables the biogas upgrading plant to achieve significantly lower methane slip without compromising production capacity further.

The foregoing description of the invention includes preferred forms thereof.

Modifications may be made thereto without departing from the scope of the invention.