WO2011083242A1 - Method for purifying gas containing carbon dioxide - Google Patents

Abstract

The invention relates to a method for totally or partially purifying a gas of the CO₂ thereof. According to said method, a gas, for example a synthetic gas, undergoes a step of purifying by means of countercurrent backwashing with an absorption agent, thus producing a purified gas, a loaded solvent, and at least CO₂. The solvent is then regenerated. For said regeneration, a nonzero proportion of the CO₂-loaded solvent is rid of at least a portion of the CO₂ thereof so as to produce at least one CO₂-depleted solvent liquid flow and at least one gas flow that is more CO₂-rich. Said operation includes at least the following steps: at least one step of heating said nonzero proportion of the solvent to a boiling point by means of indirect heat exchange; and at least one step of separating the loaded and heated solvent so as to produce a CO₂-depleted solvent liquid flow and a gas flow S_1G that essentially contains gaseous CO₂ but also at least one nonzero proportion of solvent.

Description

 Process for purifying a gas containing carbon dioxide

The present invention relates to a process for purifying a gas in whole or in part of its CO ₂ by backwashing a solvent thus producing a purified gas and a solvent loaded with CO ₂ and in which the solvent loaded is subsequently regenerated.

 Among the purification processes concerned, there may be mentioned, for example, the amine processes, but also the processes of the Selexol or Rectisol type. These methods are similar in that the main steps are:

· Washing a gas containing CO ₂ with a solvent in an adsorption column,

• flash and reboil the solvent to extract the CO ₂ .

The invention will apply in the same manner to any other process for extracting CO ₂ by washing and then regenerating the solvent (purisol, etc.).

In the usual processes, the regeneration of the solvent is carried out by expansion and / or by entrainment by a gas (stripping in English language) - in general nitrogen - which will degas the CO ₂ of the solvent. These two methods have the disadvantage of producing CO ₂ at a relatively low pressure, or of flowing in a nitrogen stream, which is not conducive to subsequent recovery of CO ₂ .

Most of the time, the more the solvent is relaxed, and the greater the amount of CO ₂ released. The solvent is then purer, and therefore more likely to be recycled to the adsorption column: in fact, the less it contains CO ₂ , the more it will be able to extract the fluid to be treated. In some applications, the solvent is even drawn to vacuum to extract the maximum amount of CO ₂ .

The disadvantage of these known solvent regeneration solutions is - as we have pointed out above - that CO ₂ is then available at atmospheric pressure, regardless of the subsequent use of CO _2. is made (underground storage after purification, use in the process or other), it will require compression of CO ₂ . The compression unit, the same applies to the eventual final purification, is expensive in investments, and / or in operation because the equipment located upstream of this compression or purification unit must deal with very large volume flow rates. CO2 is available at relatively low pressure and may be charged with nitrogen.

 It is known from J. Menzel EP1606041 to recover at least a portion of the CO 2 at an intermediate pressure between the gas washing pressure from which it is desired to extract the CO 2 and the solvent regeneration pressure. This solution reduces the disadvantages mentioned above, but these, although diminished, remain.

 The present invention aims to further reduce the costs of solvent regeneration when it can be accompanied by a recovery of CO2 at relatively high pressure or liquefied form.

 The invention consists in regenerating at least a portion of the solvent charged with CO2, not by lowering the pressure, but by heating, by indirect heating, the solvent. The CO2 is thus recovered substantially at the pressure of the solvent. The liquid phase still contains CO2 and can be relaxed later to extract more. The gaseous phase contains solvent and can be cooled to condense it. Thus, various heating, cooling, vapor-liquid separation and expansion operations make it possible to separate the CO2 from the solvent while maintaining the maximum pressure in the CO2.

 The solvent can then be finely regenerated in a recooling column with reflux and condenser at the top, as is conventionally used in a conventional scheme.

 The CO2 can be treated in a compression and purification unit to be made available under the conditions required for future use - for example transport for underground sequestration. During this CO2 purification operation, entrained gases can be extracted from the CO2 to be usefully recycled upstream of the process and minimize losses of the gas that is to be purified of its CO2.

Another essential advantage of the invention - compared to conventional solutions of the state of the art where CO2 _{, for} lack of recovery, is put in the air in a large part of the existing installations - lies in the possibility of making during the different stages of heat exchange, thermal integration of the process. Extraction at reduced pressure, by expansion of the solvent, ensures self-cooling of the solvent. This is particularly useful in the methanol washing process.

 In the method according to the invention, thermal integrations have been found which ensure an economically optimized operation of the installation implementing the method. Thus, the compression energy gain achieved by extracting most of the CO2 at a pressure close to the pressure of the gas to be purified is partly spent, heating the solvent to extract the CO2, and then cooling the solvent so that it can again absorb CO2.

 Thus, according to the invention, there is provided a method of cleaning a gas in all or part of its CO2 comprising at least the following steps:

 a step (a) of providing a gas containing at least CO2 at a pressure greater than or equal to 9 bar abs,

a step (b) of purifying the gas of step (a) of at least a portion of the CO 2 by backwashing with a solvent (absorption agent) SO 5, thereby producing a purified gas at less CO2 and at least one solvent S1 loaded with at least CO2,

a step (c) of regeneration of said solvent S0 during which a non-zero proportion of the solvent S1 charged with CO2 is freed from at least a portion of its CO2 so as to produce at least one liquid flow Su _. of solvent depleted in CO2 with respect to S1 and at least one gas stream G G richer in CO2 than S1,

characterized in that said step c) comprises at least the following steps:

 at least one step (c1) of indirectly heating said non-zero proportion of the solvent Si to boiling, and

- At least one step (c2) liquid / vapor separation of the charged and heated solvent from step (c1) for the production of liquid flow Su _. of solvent depleted in CO2 and at least one gas flow G G essentially containing gaseous CO2, but also at least a non-zero proportion of solvent.

Advantageously, the pressure of the flow S1 G is less than the pressure of the gas of the step (a) of less than 5 bar, preferably less than 2 bar, and more preferably less than 0.5 bar, while being at a pressure greater than or equal to 8 bar abs.

 In addition to CO 2, the process of the invention will thus make it possible to purify the gas of step (a) in other acid gases possibly present, among which, in particular, H 2 S and COS.

Fraction S1 G causes a non-zero fraction of solvent. A second aspect of the invention aims to avoid the loss of this solvent in CO ₂ . For this, according to an advantageous variant, the method of the invention may also contain an additional step (d) of purifying the gas flow S1 G from step (c2) to produce at least one richer liquid flow S2L in a solvent that the flow S1 G and a gaseous flow S2G richer in CO ₂ than the flow S1 G, said step (d) comprising at least one step (d1) for cooling said flow S1 G until partial condensation, and minus a step (d2) for separating the cooled stream from step (d1) into a gas stream S2G that is richer in CO ₂ than the stream S1 G and a liquid S2L that is richer in solvent than the stream S1 G.

 Advantageously, the pressure of the flow S2G is less than the pressure of the gas of step (a) of less than 5 bar, preferably less than 2 bar and even more preferably less than 0.5 bar.

The fraction S 1 L results in a non-zero fraction of CO ₂ . According to a preferred variant of the invention which aims at recovering the CO ₂ contained in the fraction S1 L, the method additionally comprises a step of relaxation during which the liquid flow Su. is expanded to an intermediate pressure between the first separation pressure (step c1 -c2) and the final purification pressure of the solvent before it is sent to the top of the absorption column-so as to produce after separation a new gas stream S3G plus rich in CO ₂ than S1 L and a new S3L liquid stream richer in solvent than Sn_. This technique differs from the state of the art in that the higher temperature is higher than is commonly practiced.

Alternatively, it is possible to heat the liquid S1 L again to extract a portion of the CO ₂ by repeating steps c) and d). An interest in heating in two steps is to avoid heating all the solvent S1 to the maximum temperature, thus reducing the thermal power necessary to reach the highest temperature. The phases obtained according to one of the above variants can be separated again, and the gaseous fraction then obtained is at a pressure lower than the pressure of the gas of step (a) of less than 5 bar, preferably 2 bar. and still more preferably 0.5 bar.

 The above processes can also be repeated on the gaseous phases.

The invention will be better understood with the aid of the attached figure which presents a diagram of a process according to the invention applied to a nonlimiting example of purification of a synthesis gas in its CO2 using of methanol according to the example described below which refers to it.

 Most fluids and equipment are designated by numbers, but some are designated by combinations of numbers and letters. This is particularly the case of the different fluids involved or resulting from the treatment of the solvent which are identified by the letter S in combination with other letters and / or numbers. Some equipment may also be designated by combinations formed by a letter followed by numbers, in order to facilitate the reading of the figure.

 Thus, according to the figure, a synthetic gas 1 available at a pressure greater than 9 bar abs (it can be much higher, and reach a pressure of the order of 80 bar) that it is desired to purify its CO2 by washing with methanol (solvent S).

Note: It should be noted that washing will remove water and various impurities such as carbonyl groups or some heavy metals in a lower section KO of an absorption column K, for this it is not necessary to have a very clean solvent, a solvent of intermediate quality is sufficient; in the upper section K1 of the column, which is the part concerned by the invention since it is the one where the CO2 absorption takes place, the solvent must be clean (it is solvent S "new" or regenerated S0 solvent), the "clean" solvent will ensure good purity acid gases (CO2 but also other acidic gases, and in particular H ₂ S) outgoing purified gas 2 at section K1 of the absorption column); in the rest of the description, we will only talk about CO2 _; : if CO2 contains sulfur in the form of H ₂ S or COS, it will be separated when necessary as described below.

 After regeneration of the methanol - the different stages of which are detailed below - CO2 S2G is fully compressed, purified and conditioned for transport by dense phase pipe (> 150 bar in general) in the form of product 3. The purification and conditioning treatment (essentially a pressurization in our example, this could be a liquefaction) T1 CO2 S2G extracted from methanol can extract the recoverable gas 4 (typically hydrogen and carbon monoxide for a synthesis gas as in our example) and recycle them in syngas 1. Impurities other than CO2 extracted by the methanol wash are removed from the methanol in one T0 treatment unit. It is essentially water, carbonyl group, and other impurities soluble in water.

 When the methanol S1 leaves section K1 at a temperature of the order of -40 ° C. and a pressure of 37 bar, it can contain about 18% vol of CO2. It is then heated to 100 ° C., in particular via exchangers E1 and E2, and passes into a phase separator V1. A gas phase S1 G and a liquid phase L1 G are recovered at the separator outlet. The gas phase S1 G contains almost half of the CO2 in gaseous form and under pressure. The liquid fraction S1 L at the separator outlet is expanded in the expansion element 10 at an intermediate pressure, the fluid thus expanded can be heated (possibly and not indicated in the figure). Cooling of the CO2-rich gas phase S1 G (or of the S3G gas phase) will allow the condensation recovery of most of the methanol entrained in the gas phase by the CO2 during the first high-temperature separations, in the phase separators. respectively V2 (treatment of S1 G) and V4 (treatment of S3G resulting from the separation of S1 L).

The heat required to heat S1 from -40 ° C to 100 ° C in the example may come from any origin, but the invention provides a particularly interesting solution which consists of thermally integrating different hot and / or cold sources available on or at near the site. It will of course be a priority to recover the heat of the regenerated solvent. S0 to heat the solvent still charged with CO2 S1. These heat exchanges are schematically represented by the exchange lines E1 and E2.

 According to the example of the figure, the flow S1 L, expanded in the member 10 is separated in the phase separator V3 in the gas phase S3G and the liquid phase S3L. The S3G gas phase is richer in CO2 than S1 L and the S3L liquid phase is richer in solvent than S1 L.

 The flow S3G can in turn be cooled in E2 and partially condensed, generating in the separator V4 the gas phases S4G and liquid S4L.

 The flow S3L is expanded to 1 1 and led into a distillation column 12 where the solvent will be regenerated to produce clean solvent S0 and able to purify - after cooling - the gas 1 (synthesis gas or gas to be purified from the step (a)). The distillation column 12 is equipped with a reboiler in the tank 12R and a top condenser 12W providing reflux in the distillation column.

 The purified solvent S0 can thus be cooled (E2, E1, 13) and then pumped into

14 and sent to the section K1 of the absorption column K.

Note: A part of the solvent S3L could possibly be purified only by a step of expansion, without reinforcement to be sent to an intermediate level in the absorption column KO or K1 as soon as intermediate purity is sufficient. ; the solvent can also be used partly in another column where one will seek to wash other acid gases, H ₂ S or sulfur-containing organic compounds.

 The gas provided in step a) is here a synthesis gas produced for example by steam reforming, it can also be derived from the gasification of hydrocarbons, it can also be gas recovered from the sub- soil or landfill gas or any other gas capable of being treated according to the invention.

With regard to the gas streams rich in CO2 (S2G, S4G, and others if other stages of heating, expansion, cooling are envisaged), it will be appropriate in a large number of applications to bring them all to a higher pressure. . Thus, for example to ensure the condensation of CO2, this pressure will be between 50 and 1 10 bar depending on the temperature of the cold source which is typically available refrigeration water; it may also be the pressure of a pipe, which may exceed 200 bar absolute.

 Note that if the CO2 streams are not intended to be valued at high pressure, we can consider relaxing them by recovering energy.

 The S5G gas fraction resulting from the purification of the solvent via the 12W overhead condenser is also recyclable to the CO2 compression and treatment unit T1.

 The compressions necessary to equalize the pressures of the streams S2G, S4G and S5G in our example are carried out in the compressors 15, 16.

 Upon cooling after compression of the CO2-rich gas phase (s) in the coolers 17 and 18, the entrained solvent will again be condensed. This solvent can be separated in the already mentioned phase separators V4 and V2, then recycled to S2L and S4L directly in the column 12 of final purification of the solvent by reboiling.

 As mentioned above, the compressed CO2 may be purified by cryogenic condensation (less than 0 ° C.) in the treatment unit T1. It will then be possible to extract the lighter gases 4 that may be entrained, such as CO or hydrogen. A distillation step of these elements is possible, the CO2 being recovered in the tank. These light elements, often valued as present in the gas 1 to be treated in step a) can be recycled.

Another potential interest of the invention is thus demonstrated: such a recycling would not require a compressor dedicated to recycling (small compressor), indeed, the compression of these gases entrained in the solvent is ensured, in the same way that of CO2 by the compressors 16 and 16 and the compressors integrated in the purification step T1. This therefore constitutes an investment gain on the recovery of the valorized gases (H ₂ , CO, etc.).

 Finally, it should be noted that in the event that sulfur elements are present in the gas 1 of step a), two different strategies can be adopted:

- if these elements are not harmful, that is to say if it is acceptable for the sulfur elements to remain in the CO2 - for a sequestration for example - the simplest will then be not to envisage a separation step additional. - if it is necessary, or simply desirable to separate the sulfur elements, several methods can be proposed:

 an absorption with a part of the solvent (as presented in EP1606041

 - Distillation of sulfur elements because they are generally heavier than CO2.

 adsorption on suitable adsorbents (activated carbons, zeolites, etc.)

 The invention is also particularly advantageous if different arrangements are used to ensure an effective thermal integration with steps of the purification process or steps of the main process implemented, or steps of other processes, related to previous processes considered or implemented in the vicinity -being understood that by main process means the process producing said gas containing at least CO2, and the provision of which constitutes step a) of the purification process according to the invention, but also the downstream process for treating said gas, after purification.

 Thus, among the thermal integration variants, there may be mentioned an integration by heat exchange between the hot fluids constituted by the regenerated solvent S0 and / or the flows S1 G, S3G and following (s) (when there is other stage (s) of relaxation envisaged (s) on the liquid flow) and the cold fluid consisting of the solvent S1 rich in CO2 from step (b) at the exit of the adsorption column. The fluid S1 can thus heat up from the operating temperature of the column K1 (approximately -40 ° C.) (stream S1) to ambient temperature in the exchanger E 1 while cooling the regenerated solvent S0. The CO2-rich solvent will then be heated in the exchanger E2 against the gas phases S1G, S3G (and any subsequent ones) and the regenerated solvent S0 at a temperature of about 80.degree. C., preferably 100.degree. vs. Finally, it will be necessary to complete the heating of S1 in a heater. It will also be necessary to complete the refrigeration of the purified solvent S0 in a refrigerant for use in step (b).

Another interesting variation is the use of a CO2 cycle to provide the necessary cold for refrigeration of the purified solvent for use in step (b). The latter is traditionally brought by the expansion of the solvent and the extraction of CO2 at low pressure and by supplementing with ammonia refrigeration units in general. The presence of a compression of CO2 - in case of production of CO2 extracted at a supercritical pressure or in liquid form (equipment 16, 17 and other compressor possibly included in stage T1) - will indeed make it possible to integrate a refrigeration unit running on CO2 at a lower cost since it will only be necessary to enlarge an existing compressor to include the molecules of the CO2 refrigeration cycle.

 It may also be advantageous to use low level heat for boiling the solvent in step c1 as well as reheating steps, for example:

 the heat of the low pressure steam taken from the synthesis gas generation process, in the case where the gas to be purified containing CO2 is synthesis gas,

 the heat of the synthesis gas itself before being washed with the solvent according to step (b) of the process according to the invention,

 - CO2 heat at compression inter-stages when the CO2 extracted from the solvent is compressed (before its possible fine purification or during the compression of CO2 to a pipe).

 the integration, via an appropriate cycle, of the reboiling and condensation energies (to reduce the fraction of methanol in gaseous CO2) at the level of the column K2. etc.

Claims

1. Process for the purification of a gas in all or part of its CO2, comprising at least the following steps:  - A step (a) of providing a gas containing at least CO2, at a pressure greater than or equal to 9 bar.  a step (b) of purifying the gas of step (a) of at least part of the CO2 by backwashing with a solvent (absorption agent) S0, thus producing a purified gas at less CO2 and at least one solvent S1 loaded with at least CO2, a step (c) of regeneration of said solvent S0 during which a non-zero proportion of the solvent S1 charged with CO2 is freed from at least a portion of its CO2 so as to produce at least one liquid stream S1 L of a solvent depleted in CO2 with respect to S1 and at least one gas stream S1 G which is more CO ₂ rich than S1, characterized in that said step c) comprises at least the following steps:  at least one step (c1) of indirectly heating said non-zero proportion of solvent S1 to boiling, and  at least one liquid / vapor separation step (c2) of the charged and heated solvent resulting from step (d) for the production of the liquid stream S1 L of solvent depleted in CO2 and at least one gas stream S1 G containing essentially CO2 gas, but also at least a non-zero proportion of solvent. 2. Method according to claim 1 characterized in that the pressure of the flow S1 G is less than the pressure of the gas of step (a) of less than 5 bar, preferably less than 2 bar, and more preferably less of 0.5 bar, and is greater than 8 bar abs 3. Method according to one of claims 1 or 2 characterized in that it contains an additional step (d) of purifying the gas stream S1 G from step (c2) to produce at least one liquid stream S2L richer in solvent that the flow S1 G and a gaseous flow S2G richer in CO2 than the flow S1 G, said step (d) comprising:  at least one step (d1) of cooling said stream S1 G until partial condensation, and  at least one step (d2) for separating the liquid / vapor type of the cooled stream from step (d1) into a gas stream S2G which is richer in CO2 than the stream S1 G and a liquid S2L which is richer in solvent than the flow S1 G. 4. Process according to claim 3, characterized in that the pressure of the flow S2G is less than the pressure of the gas of step (a) of less than 5 bar, preferably less than 2 bar, and more preferably less than 0.5 bar. 5. Method according to one of claims 1 to 4 wherein the liquid flow S1 L is expanded to an intermediate pressure between the first separation pressure (step c1-c2) and the final purification pressure of the solvent before being sent to the top of the absorption column so as to produce, after separation, a new S3G gas stream richer in CO2 than S1 L and a new S3L liquid stream which is more solvent rich than S1 L. 6. Method according to one of claims 1 to 5 wherein the gas to be purified is a synthesis gas, characterized in that it implements at least one thermal integration between steps of the purification process, or with steps of the main process implemented, or steps of other processes related to the main or purification processes, or carried out in the vicinity - it being understood that by a main process is meant the process producing said CO2-containing gas, the production of which provision constitutes step a), but also the downstream process for treating said gas, after its purification. 7. Process according to claim 6 in which at least one thermal integration implemented is a heat exchange between the hot fluids constituted by the regenerated solvent S0 and / or the streams S1 G, S3G and following (s) (when there are other (s) relaxation stage (s) envisaged (s) on the liquid flow) and the cold fluid consisting of the solvent S1 rich in CO2 from step (b) 8. Method according to one of claims 6 or 7 wherein the use of a CO2 cycle provides the cold necessary for the refrigeration of the purified solvent for use in step (b). 9. Process according to one of claims 6 to 8 wherein low-level heat is used for heating to boiling of the solvent in step c1, said heat coming from:  the low pressure steam taken on the process for generating the synthesis gas, when the gas of step (a) is a synthesis gas, and / or synthesis gas itself before being washed with the solvent according to step b), and / or  - CO2 at inter-stages of compression when the CO2 extracted from the solvent is compressed (before its possible fine purification or during the compression of CO2 to a pipe). 10. Method according to one of the preceding claims wherein the solvent is selected from methanol, amines or the solvent used in the Selexol process.