Process and apparatus for the separation of a stream containing carbon dioxide, water and at least one light impurity

Abstract

A process for the separation of a stream containing carbon dioxide, water and at least one impurity lighter than carbon dioxide wherein the stream is purified in a purification unit and then separated in a separation unit including a distillation column, a first gas depleted in CO2 is removed from the separation apparatus and is sent to a permeation unit producing a permeate gas, a second gas depleted in CO2 as compared with the stream separated in the separation apparatus is removed from the separation apparatus, being removed from the top of the column and at least a part of the permeate gas is combined with at least a part of the second gas, to constitute the regeneration gas and the regeneration gas is sent to the purification unit.

Description

[0001] PROCESS AND APPARATUS FOR THE SEPARATION OF A STREAM CONTAINING CARBON DIOXIDE, WATER AND AT LEAST ONE LIGHT IMPURITY

[0002] Background

[0003] Typically, there are several streams available that may be employed to regenerate the dryers in a process of capture of CO2 by partial condensation and / or distillation. These may include, but are not limited to:

[0004] 1 . The light gases from the partial condensation that are afterwards vented to the atmosphere. This is a very well-known solution, similar to the regeneration of air dryers with the waste gas from the cold boxes of air separation units and is disclosed in CN115069057A, WO2015 / 148927.

[0005] 2. In a process described in WO2012 / 064938 where the light gasses are further processed through membranes to increase recovery, the residue is employed and vented to the atmosphere

[0006] 3. In processes described in EP2872841 where the light gases are further processed through membranes to increase recovery, the permeate is employed and vented to the atmosphere

[0007] 4. In a process where the light gases are further processed through membranes to increase recovery the permeate is employed and recycled to the inlet of the process. This is the configuration of EP2872841 .

[0008] 5. Another possibility is to employ a part of the dried gas exiting the dryer, and to recycle it back at the inlet of the dryer by means of a compression step, the water being evacuated from the system by partial condensation (cooling of the recycled regeneration gas).

[0009] Summary

[0010] A process for the separation of a stream containing carbon dioxide, water and at least one impurity lighter than carbon dioxide including a separation step at a temperature below 0°C, wherein the stream is purified in a purification unit and then separated in a separation unit including a distillation column, a first gas depleted in CO2 is removed from the separation apparatus and is sent to one of a permeation unit producing a permeate gas, a second gas depleted in CO2 as compared with the stream separated in the separation apparatus is removed from the separation apparatus and is removed from the top of the column and at least a part of the permeate gas is combined with at least a part of the second gas , to constitute the regeneration gas and the regeneration gas is sent to the purification unit.

[0011] Brief Description of the Figures

[0012] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0013] Figure 1 is a schematic representation of a system in accordance with one embodiment of the present invention.

[0014] Figure 2 is another schematic representation of a system in accordance with one embodiment of the present invention.

[0015] Description of Preferred Embodiments

[0016] Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

[0017] It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer’s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

[0018] The present invention relates to a process and apparatus for the separation of a stream containing carbon dioxide, water and at least one light impurity, having a lower boiling point than carbon dioxide. In particular, it relates to a process in which the carbon dioxide is separated from the at least one light impurity by partial condensation followed by distillation. The at least one light impurity is chosen from the group: oxygen, nitrogen, carbon monoxide, hydrogen, helium.

[0019] In a carbon capture process using partial condensation and distillation, the water in the feed gas has to be removed in order not to freeze at cold temperatures. When the water removal is performed by a Temperature Swing Adsorption process, a regeneration gas is needed to desorb the water from the beds of adsorbent forming the Temperature Swing Adsorber.

[0020] The dryers operating by Temperature Swing Adsorption are usually regenerated at a lower pressure than the pressure at which the adsorption phase takes place. Therefore, it is preferable to regenerate with a gas that is already available at a lower pressure or that would later be vented. The regeneration process has to ensure that the majority of the water is desorbed, whilst remaining cost effective, particularly in terms of energy.

[0021] When the feed gas contains dangerous impurities such as toxic impurities for example NOx, SOx or explosive impurities such as hydrogen etc, some of them are co-adsorbed on the dryers, thus regenerating them with a gas that is later vented to the atmosphere leads to emissions of these compounds to the atmosphere. In particular, peaks of emissions during the regeneration are even possible leading to very high instantaneous flow of impurities to the atmosphere, beyond regulatory limits ( or even safety limits). In such a case, solutions number 1 and 2 from the state of the art are inacceptable. With high CO2 content feed gas, (e.g. between 35% mol and 95% mol CO2), the permeate flowrate from the membranes (solution number 3 and 4 of the state of the art) can be too low to ensure the complete regeneration of the dryers. The solution number 5 from the state of the art could solve this issue but it requires either recompression energy in the compressor before the dryer an additional blower to be able to recycle the regeneration gas at the inlet of the dryer.

[0022] The present invention proposes a solution to complete the permeate stream with another stream for the regeneration of the dryer so as to have a sufficient flowrate for the regeneration. This is possible with a limited impact on the energy and the cost of the plant.

[0023] It is proposed to mix a part of the permeate gas from the membrane system with a part of the gas from the overhead of the stripping column.

[0024] The stripping column operates under pressure and its overhead is usually recycled to an interstage of the compressor. It is proposed that at least a part of the column overhead gas is mixed with the permeate to regenerate the dryers, another part of the overhead gas being possibly is sent to the interstage of the compressor. The regeneration flow rate accounts for around 5 to 15 % of the inlet flowrate to the dryers, depending on the adsorption and regeneration conditions.

[0025] According to one object of the invention, there is provided a process for the separation of a stream containing carbon dioxide, water and at least one impurity lighter than carbon dioxide including a separation step at a temperature below 0°C wherein: i) a feed stream is purified in a purification unit to produce a dried stream, the purification unit comprising a first adsorber and a second adsorber, the method comprising: regenerating the first adsorber while the second adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure P1 , wherein a regeneration gas is used to regenerate the first adsorber, ii) at least one of the dried stream and a stream derived therefrom by partial condensation and / or adsorption is cooled to a temperature below 0°C and separated by distillation and possibly partial condensation and / or solidification in a separation apparatus, iii) a liquid and / or a solid enriched in carbon dioxide is removed from the separation apparatus, iv) a first gas depleted in CO2 as compared with a stream separated in the separation apparatus is removed from the separation apparatus and is sent to either a permeation unit producing a permeate gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 and a residue gas or a further adsorption unit producing a tail gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 v) a second gas depleted in CO2 as compared with the stream separated in the separation apparatus is removed from the separation apparatus and is removed from the top of a column in which a distillation step takes place v) the purification unit is regenerated using a regeneration gas and vi) at least a part of one of the permeate gas and the tail gas is combined with at least a part of the second gas , to constitute the regeneration gas and the regeneration gas is sent to the purification unit without undergoing a step of partial condensation or vaporisation.

[0026] According to other optional features:

[0027] - the separation apparatus comprises at least one phase separator and at least one distillation column and in which liquid from at least one phase separator is sent to the top of the at least one distillation column from which the second gas is removed.

[0028] - the second gas is removed from the separation apparatus and is a top gas of a distillation column of the separation apparatus.

[0029] -the second gas is expanded in gaseous form in a turbine upstream of being mixed with the permeate gas.

[0030] -the mixture of at least a part of the permeate of the permeation unit or the tail gas and at least a part of the second gas stream is not condensed upstream of the purification unit. -part of the dried stream also constitutes part of the regeneration gas.

[0031] -the feed gas is compressed in a compressor upstream of the purification unit.

[0032] -after regenerating the first adsorber, the regeneration gas containing water from the feed gas is cooled, dried and then sent to the compressor and mixed with the feed gas to be compressed, preferably at an intermediate stage of the compressor.

[0033] -the purification unit adsorption pressure is between 10 and 20 bar abs, and the dryers regeneration pressure is between 1.1 and 10 bar abs.

[0034] -feed gas is produced by separating a synthesis gas from an autothermal reformer by adsorption to produce a hydrogen enriched gas and the feed gas containing hydrogen as at least one component lighter than carbon dioxide.

[0035] -the feed gas is produced by combustion and contains NOx and / or SOx.

[0036] -the feed gas is produced by partial or full oxycombustion.

[0037] - a distillation column of the separation apparatus operates at a pressure P4 lower than the adsorption pressure P1 and preferably higher than the regeneration pressure P2.

[0038] -the permeation unit comprises two permeation separation steps wherein in the first step, the first gas depleted in CO2 is separated to form a permeate and a retentate, the retentate is sent to a second step in which it is separated to form the permeate gas used for the regeneration and a further retentate.

[0039] - the permeate from the first step is sent to be separated by adsorption with the synthesis gas to produce the hydrogen enriched gas and the feed gas containing hydrogen as at least one component lighter than carbon dioxide.

[0040] -at least a part of one of the permeate gas and the tail gas is combined with between 25 and 100% of the second gas, to constitute the regeneration gas.

[0041] -the stream contains at least one component heavier than carbon dioxide

[0042] -the separation apparatus includes means for removing the at least one component heavier than carbon dioxide

[0043] According to one object of the invention, there is provided an apparatus for the separation of a stream containing carbon dioxide, water and at least one impurity lighter than carbon dioxide including a separation step at a temperature below 0°C comprising a purification unit, , the purification unit comprising at least a first adsorber and a second adsorber, a separation apparatus including a distillation column, means for sending a feed stream to be purified in the purification unit, means for sending a regeneration gas to the first adsorber while the second adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure P1 , wherein the regeneration gas is used to regenerate the first adsorber, means for cooling at least one of the dried stream and a stream derived therefrom by partial condensation and / or adsorption to a temperature below 0°C, means for sending the cooled stream to the separation apparatus to be separated by distillation and possibly partial condensation and / or solidification; a conduit for removing a liquid and / or a solid enriched in carbon dioxide from the separation apparatus, a conduit for removing a first gas depleted in CO2 as compared with the stream separated in the separation apparatus from the separation apparatus, said conduit being connected to one of a permeation unit producing a permeate gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 and a residue gas and a further adsorption unit producing a tail gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 , a conduit for removing a second gas depleted in CO2 as compared with the stream separated in the separation apparatus from the top of the distillation column in which a distillation step takes place, a connection for mixing at least a part of one of the permeate gas and the tail gas with at least a part of the second gas , to constitute the regeneration gas and a conduit linking the connection and the purification unit to send the regeneration gas is sent to the purification unit without passing through a partial condensation unit or a vaporiser.

[0044] The separation apparatus may include at least one phase separator and the conduit for removing the first gas depleted in CO2 being connected to the at least one phase separator.

[0045] In Figure 1 , the feed gas containing carbon dioxide, saturated or not in water and at least one component lighter than CO2 1 is compressed from around atmospheric pressure up to 16 to 30 bar a in compressors C1 , C2 cooled in a cooler R to condense part of the water it contains, separated in a separator S1 to remove the water 7 and to form a partially dried stream 5 containing carbon dioxide, water and at least one component lighter than CO2. The at least one component lighter than CO2 can be nitrogen, oxygen, hydrogen, carbon monoxide. Typically the gas 1 does not contain methane.

[0046] The partially dried stream 5 is dried by temperature swing adsorption in a purification unit A.

[0047] The purification unit A comprises a first adsorber and a second adsorber, the method comprising: regenerating the first adsorber while the second adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure.

[0048] A regeneration gas is used to regenerate the first adsorber.

[0049] The adsorption unit A operates cyclically so that the second adsorber is regenerated while the first adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure.

[0050] The dried compressed gas 9 is cooled down and CO2 is partially condensed in a heat exchanger E and is separated from the remaining gaseous phase 11 (light noncondensable gases) in a phase separator S2. The liquid 13 from the phase separator S2 is fed to the top of a distillation column operating between 7 to 20 bar a . A liquid 17enriched in CO2 and depleted in the at least one component lighter than CO2 is removed from the bottom of the distillation column K and a gas 19 enriched at least one component lighter than CO2 and depleted in CO2 is removed from the top of column K.

[0051] At least part of the gas 11 enriched at least one component lighter than CO2 and depleted in CO2 is separated through a membrane system M producing a CO2 enriched low pressure permeate 23, between the inlet pressure of the wet flue gas compressor and 7 bar a, or possibly up to 4 bar a, and a low pressure CO2 depleted residue 21 . The membrane system M could be replaced by a pressure swing adsorption system.

[0052] In the case where the flue gas is obtained from an oxycombustion, the permeate 23 may contain 60 to 80% mol CO2, the remainder being essentially oxygen and nitrogen. The residue 21 contains around 70 to 80% mol nitrogen. The membrane system M could be replaced by a further adsorption unit producing a tail gas containing proportionally more CO2 than the gas 11 separated in the further adsorption unit.

[0053] The permeate 23 of the membrane M is mixed with at least a part 25 of the overhead 19 of the stripping column K, to regenerate the dryers A between the inlet pressure of the wet flue gas compressor C1 and 4 bar a and recycled upstream the dryers A , preferably upstream the dryers for example at the inlet of the compressor C1 . The overhead gas 19 is divided to form a gas 25 which is expanded in a valve down to the regeneration pressure of the dryers A. It is then mixed with permeate stream 23. The column overhead 19 is at a higher pressure than the permeate 23 so has to be expanded upstream of the mixing point.

[0054] By using the overhead 25 of the column K to complete the permeate flow 23, the energy cost is limited to the expansion from the available pressure of the overhead to the inlet of the compressor. If the permeate had been completed by a part of the dried gas at the outlet of the dryer, the energy cost would have been that induced by the expansion from the outlet of the dryer to the inlet of the compressor.

[0055] Numerical example

[0056] In a process not according to the invention, for a given flue gas wet flow 1 compressed from 0.9 bar a up to 22.5 bar a, with the stripping column K operating at 11 bar a, it is proposed to employ 10% of the inlet gas 5 sent to the dryers A to regenerate them. No part of the column overhead is sent to the dryers A.

[0057] The available permeate flow accounts to about 2% of the inlet flow to the dryers and the remaining 8% is completed with the dried outlet of the dryers. In this case, the totality of the overhead of the column is recycled to the interstage of the compressor, which compresses it from 11 up to 22.5 bar a. The energy of this reference is 100%.

[0058] To compare with the process according to the invention, the overhead 19 of the column K accounts to about 9.5% of the needed flowrate 29 for regeneration. By mixing the totality of the permeate gas 23 with at least a part 25 of the overhead 19 of the column K to regenerate the dryers A and sending the remaining part 27 to the interstage of the compressor (between C1 and C2), the energy is 97% of the reference case. From 25 to 100% of the available overhead 19 of the column flow rate is used as stream 25 to complete the permeate flow rate 23 to reach the targeted regeneration flow 29.

[0059] The feed 1 may come from a combustion unit and may contain NOx and / or SOx.

[0060] The feed gas 1 may be produced by partial or full oxycombustion.

[0061] Figure 2 shows an alternative process where the gas separated is the tail gas of a pressure swing adsorption unit producing a hydrogen enriched gas.

[0062] Autothermal Reforming (ATR) is a process for producing syngas from hydrocarbons, a typical feedstock being natural gas. When high purity hydrogen is the desired end product from the ATR, the final H2 purification step is typically performed with a Pressure Swing Adsorption Unit (or PSA). Nowadays, hydrogen is a potential vector of decarbonization if CO2 capture and storage is implemented. Separation by partial condensation and distillation is a potential solution for capturing CC from the PSA tail gas (low pressure stream). It consists of a combination of compression, temperature swing adsorption, cryogenic processing, and membrane separation. The invention is particularly capable of enhancing the efficiency of such process and lowering the energy intensity for capturing CO2.

[0063] Here follows definitions of key parameters:

[0064] - P1 : dryers adsorption pressure

[0065] - P2: dryers regeneration pressure

[0066] - P3: second membrane permeate pressure

[0067] - P4: distillation column pressure

[0068] In order to purify the PSA offgas from an ATR setup, the separation step involves partial condensation and distillation and is coupled with two stages of membrane separation in series acting on a first partially CO2 depleted stream removed from the partial condensation part of the separation apparatus to enhance the recovery of H2 and CO2 . Depending on the specific process conditions, the pressure of the gas sent to the membranes could vary from between 30 and 70 bar abs before entering the second stage of the membrane, and the permeate of the second stage of this membrane is leaving at a pressure P3 typically around 10 to 20 bar abs and is being sent to the inlet of the dry section of the tail gas compressor (downstream the dryers).

[0069] The distillation column pressure P4 is typically set at around 7 to 20 bar abs as the overhead gas exiting this column is intended to be injected back into downstream of the TSA before entering the dry section of the tail gas compressor.

[0070] In the case of a separation system for the synthesis gas generated by an ATR, the regeneration gas of the TSA is a slip stream from downstream of the TSA at a pressure P1 typically between 7 to 20 bar abs which is ultimately recycled back to the tail gas compressor. This configuration was selected for two major reasons. Firstly, in the case of an ATR, the flow of the residue of the second membrane) is recycled under pressure (typically > 30 bara) back to the inlet of the ATR to boost production, thus if this stream is used as regeneration gas, it implies that either the spent regeneration gas has to be compressed back up to the same pressure level of the inlet of ATR or the TSA systems will need to be at a higher pressure than P1 which is between 7 to 20 bar abs. Secondly, this residue stream flow is too low to ensure the regeneration of the TSA.

[0071] The regeneration is typically carried out at a pressure P2 lower than the driers adsorption pressure P1 , and after the spent regeneration gas goes through cooling and chilling, the vapor phase is separated in a separation drum S4 before being injected as gas 62 back to the tail gas compressor (here between stages C2 and C3) at an optimized pressure.

[0072] By the term “cooling” in the previous sentence, we mean cooling in which the final temperature produced is a direct result of the ambient conditions, for example cooling using cooling water. By the term “chilling” in the previous sentence, we mean cooling the final temperature produced is not a direct result of the ambient conditions, for example in which refrigeration is produced by mechanical or electrical means, e.g. by using a refrigerant cycle (propane, ammonia), the same CO2 from the cold section and / or chilled water (~2°C). In the case of a separation by partial condensation and / or distillation treating gas from a Steam Methane Reformer, there is no need to use dry gas recycled from downstream the dryers to perform the dryer’s regeneration (retentate of the second membrane is used instead). Therefore, P3 and P4 are typically equal or higher than P1 so that the second membrane permeate and the distillation overhead from the cryogenic section can be recycled downstream the dryers.

[0073] For an ATR application, if dry gas recycled from downstream the dryers is used to perform the dryers regeneration, this allows some process optimization. The second membrane permeate pressure P3 can be set between P2 and P1 in order to leverage the separation efficiency gain by having a larger differential pressure around the second membrane stage. By reducing the permeate pressure from 16 bar abs to 6 bar abs for example, it could boost the CO2 recovery of the second stage membrane block by ~12% or 0.5% of the global CO2 recovery with the same membrane counts. However this process also does not provide an efficient solution to manage distillation processes at elevated pressure, since the part of the liquid exiting the bottom of the distillation column would need to be let down from ~16 bar abs to ~10 bar abs before entering the main heat exchanger. By lowering the set pressure of the distillation column to a pressure at P4 between P1 and P2, the inefficiencies of this letdown step can be eliminated and the distillation column overhead can be used for the regeneration of the dryers.

[0074] Figure 2 shows a process flow diagram showing the invention in the context of removal of CO2 from a synthesis gas generated by autothermal reforming.

[0075] An autothermal reformer produces a synthesis gas 41 containing carbon dioxide, water and hydrogen which is cooled using cooling water CW and chilled by a refrigerant, sent to a phase separator S3 in which condensates 46, such as water, are removed and sent as gas 43 to a pressure swing adsorber PSA producing a stream 45 enriched in hydrogen and with a reduced CO2 content as compared to stream 43 (optionally as compared to stream 43 mixed with stream 18) and stream 1 enriched in CO2 and with a reduced hydrogen content as compared with stream 43(optionally as compared to stream 43 mixed with stream 18). Stream 1 contains carbon dioxide, water and at least one light impurity as compared to carbon dioxide, in particular hydrogen. At least one other component lighter than CO2 can be at least one of nitrogen, oxygen , carbon monoxide, argon, helium, methane and methanol.

[0076] Stream 1 is compressed in compression steps C1 , C2, C3, C4 with intercooling stages to 10 - 20 bar abs and then sent to be dried by temperature swing adsorption in purification unit A.

[0077] The purification unit A comprises a first adsorber and a second adsorber, the method comprising: regenerating the first adsorber while the second adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure P1 .

[0078] A regeneration gas is used to regenerate the first adsorber at a regeneration pressure P2 lower than the adsorption pressure P1 .

[0079] The adsorption unit operated cyclically so that the second adsorber is regenerated while the first adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure.

[0080] At least part of the dried feed gas is then further compressed in compression stages C5, C6 to 30 to 70 bar abs and then cooled and partially condensed in heat exchanger E. The partially condensed feed is separated in phase separator S2, the carbon dioxide enriched liquid 13 from phase separator S2 being sent as feed to the top of stripper column K and the carbon dioxide depleted gas 11 from phase separator S2 being separated by permeation.

[0081] Stream 13 is expanded to 7 to 20 bars, preferably 10 -11 bars, in a valve and sent to the distillation column K operating at a pressure P4 to be separated forming a carbon dioxide depleted top gas 19 enriched in the at least one light impurity and depleted in carbon dioxide and a bottom liquid 17 enriched in carbon dioxide and depleted in the at least one light impurity. The distillation column K operates at between 7 and 20 bars abs.

[0082] Bottom liquid 17 is divided in three parts. One part is pressurized in pump P1 typically to a pressure around 22 bars and divided in two, forming stream 15 which is vaporized in heat exchanger E and sent back to the bottom of the column K as reboil. Part 16 of the pressurized liquid is vaporized in exchanger E and compressed in stages C9, C10 of a product CO2 compressor comprising four stages C7, C8, C9, C10.

[0083] Another part of the bottom liquid is expanded in a Joule Thomson valve to typically 5 bars a to partially vaporize the liquid, the two-phase fluid is separated in separator S5 and the liquid formed 22 is vaporized in heat exchanger E and sent to stage C7 of the product CO2 compressor. The gas 20 is warmed in heat exchanger E and sent to stage C7 of the product CO2 compressor.

[0084] The third part 24 of the bottom liquid is vaporized in exchanger E at the column pressure and sent to stage C8 to be compressed.

[0085] The compressed product gas from stage C10 is cooled to condense the gas using water CW and pressurized using a pump P2 to form a pressurized liquid product stream before further cooling with water CW.

[0086] The gas 11 from separator S2 is warmed in exchanger E and preferably warmed in heat exchanger E1 before being separated in the permeation system which includes two membrane stages M1 , M2 in series. Gas 11 is separated in membrane stage M1 to form a permeate 20 which is sent to stage M2 and a residue 18 which is recycled to the feed 43 of the adsorption unit PSA after cooling in heat exchanger E1 .

[0087] The residue 23 of the second stage M2 is sent to the purification unit with its pressure P3 set at a pressure lower than the adsorption pressure P1 and higher than the regeneration pressure P2 and the residue stream is mixed with at least part 25 of the warmed gas 19 to form the regeneration gas 29. Optionally part 8 of the dried feed gas may also form part of the regeneration gas 29, after a compression step.

[0088] The regeneration gas 29 is warmed, used to regenerate the purification unit A and thereby becomes laden with the water contained previously in the feed gas 1 . To remove part of the water, the gas 29 is cooled and sent to separator vessel S4 where the condensed water 50 is removed and sent to the condensates 46. The dried regeneration gas is sent to a stage of the compressor upstream the purification unit (here stage C3 but potentially upstream any of stages C1 to C4) The key innovative element in this process is unlocking the efficiency of the membrane units by increasing the differential pressure around the membrane while cleverly utilizing the permeate and flash gas from distillation column together as TSA regeneration gas and optionally fitting the spent regeneration gas into the pressure profile of the tail gas compressor.

[0089] At least a part of the permeate gas 23 is combined with at least a part of the second gas 19 to constitute the regeneration gas 29 and the regeneration gas is sent to the adsorption unit A without undergoing a step of partial condensation or vaporization. The regeneration gas 29, having regenerated one of the adsorbers, contains water from the adsorber and is cooled and separated in separator S4 to form an aqueous liquid 50 and gas 52 which is sent back to the compressor. Optionally part 8 of the purified gas can form part of the regeneration gas 29, the purified gas being taken downstream of the purification unit A and upstream of compressor C5.

[0090] When no regeneration step is taking place, none of gas 19 is mixed with the stream 23 and all of gas 19 is sent to the compressor.

[0091] Preferably the purification unit adsorption pressure is between 10 and 20 bar abs, and the dryer’s regeneration pressure is between 1.1 and 10 bar abs.

[0092] It will be appreciated that both figures are schematic and may require additional sources of refrigeration or heat to function in an optimal way.

Claims

Claims1 . A process for the separation of a stream containing carbon dioxide, water and at least one impurity lighter than carbon dioxide including a separation step at a temperature below 0°C comprising: i) purifying a feed stream in a purification unit to produce a dried stream, the purification unit comprising a first adsorber and a second adsorber, regenerating the first adsorber while the second adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure P1 , wherein a regeneration gas is used to regenerate the first adsorber, ii) cooling at least one of the dried stream and a stream derived therefrom by partial condensation and / or adsorption to a temperature below 0°C and separated by distillation and possibly partial condensation and / or solidification in a separation apparatus, iii) removing a liquid and / or a solid enriched in carbon dioxide from the separation apparatus, iv) removing a first gas depleted in CO2 as compared with a stream separated in the separation apparatus from the separation apparatus and is sent to either a permeation unit producing a permeate gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 and a residue gas or a further adsorption unit producing a tail gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 , v) removing a second gas depleted in CO2 as compared with the stream separated in the separation apparatus from the separation apparatus and is removed from the top of a column in which a distillation step takes place, vi) regenerating the purification unit using a regeneration gas and vii) combining at least a part of one of the permeate gas and the tail gas with at least a part of the second gas, to constitute the regeneration gas and the regeneration gas is sent to the purification unit without undergoing a step of partial condensation or vaporization.

2. Process according to Claim 1 , wherein the separation apparatus comprises at least one phase separator and at least one distillation column and in which liquid from at least one phase separator is sent to the top of the at least one distillation column from which the second gas is removed.

3. Process according to Claim 1 or 2, in which the second gas is removed from the separation apparatus and is a top gas of a distillation column of the separation apparatus.

4. Process according to any preceding claim , in which the second gas is expanded in gaseous form in a turbine upstream of being mixed with the permeate gas.

5. Process according to any preceding claim, wherein a mixture of at least a part of the permeate of the permeation unit or the tail gas and at least a part of the second gas stream is not condensed upstream of the purification unit.

6. Process according to any preceding claim , wherein part of the dried stream also constitutes part of the regeneration gas.

7. Process according to any preceding claim, wherein the feed gas is compressed in a compressor upstream of the purification unit.

8. Process according to Claim 7, wherein after regenerating the first adsorber, the regeneration gas containing water from the feed gas is cooled, dried and then sent to the compressor and mixed with the feed gas to be compressed, preferably at an intermediate stage of the compressor.

9. Process according any preceding claim , wherein the purification unit adsorption pressure is between 10 and 20 bar abs, and the dryer’s regeneration pressure is between 1.1 and 10 bar abs.

10. Process according to any preceding claim, wherein feed gas is produced by separating a synthesis gas from an autothermal reformer by adsorption to produce a hydrogen enriched gas and the feed gas containing hydrogen as at least one component lighter than carbon dioxide.11 . Process according to any preceding claim , wherein the feed gas is produced by combustion and contains NOx and / or SOx.

12. Process according to any preceding claim, wherein a distillation column of the separation apparatus operates at a pressure P4 lower than the adsorption pressure P1.

13. Process according to any preceding claim , wherein the permeation unit comprises two permeation separation steps wherein in a first step, the first gas depleted in CO2 is separated to form a permeate and a retentate, the retentate is sent to a second step in which it is separated to form the permeate gas used for the regeneration and a further retentate.

14. Process according to Claim 13, wherein the permeate from the first step is sent to be separated by adsorption with the synthesis gas to produce the hydrogen enriched gas and the feed gas containing hydrogen as at least one component lighter than carbon dioxide.

15. Process according to any preceding claim, wherein at least a part of one of the permeate gas and the tail gas is combined with between 25 and 100% of the second gas, to constitute the regeneration gas.

16. Process according to any preceding claim, wherein the distillation column operates at between 7 and 20 bars abs.

17. Apparatus for the separation of a stream containing carbon dioxide, water and at least one impurity lighter than carbon dioxide including a separation step at a temperature below 0°C comprising a purification unit, the purification unit comprising at least a first adsorber and a second adsorber, a separation apparatus including a distillation column, a means for sending a feed stream to be purified in the purification unit, a means for sending a regeneration gas to the first adsorber while the second adsorber adsorbs water contained in the feed stream by adsorption at an adsorption pressure P1 , wherein the regeneration gas is used to regenerate the first adsorber, a means for cooling at least one of the dried stream and a stream derived therefrom by partial condensation and / or adsorption to a temperature below 0°C, a means for sending the cooled stream to the separation apparatus to be separated by distillation and possibly partial condensation and / or solidification; a conduit for removing a liquid and / or a solid enriched in carbon dioxide from the separation apparatus, a conduit for removing a first gas depleted in CO2 as compared with thestream separated in the separation apparatus from the separation apparatus, said conduit being connected to one of a permeation unit producing a permeate gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 and a residue gas and a further adsorption unit producing a tail gas containing proportionally more CO2 than the first gas and at a pressure P3 lower than the adsorption pressure P1 , a conduit for removing a second gas depleted in CO2 as compared with the stream separated in the separation apparatus from the top of the distillation column in which a distillation step takes place, a connection for mixing at least a part of one of the permeate gas and the tail gas with at least a part of the second gas , to constitute the regeneration gas and a conduit linking the connection and the purification unit to send the regeneration gas is sent to the purification unit without passing through a partial condensation unit or a vaporizer.

18. Apparatus according to claim 17, wherein the separation apparatus includes at least one phase separator and the conduit for removing the first gas depleted in CO2 being connected to the at least one phase separator.

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