EP3161399B1 - Cryogenic purification with heat uptake - Google Patents

Description

The present invention relates to a process for purifying a feed gas stream using an adsorption unit and a cryogenic distillation unit.

Adsorption is a phenomenon generally favored by a low temperature. For example, for an ASU (Air Separation Unit), stopping CO2 on a molecular sieve is up to 5 times greater than -100 ° C than at 20 ° C, about 3 times for stopping propane.

Regeneration requires additional heat which disturbs the refrigerant balance of the apparatus, if the adsorption took place at a negative temperature. Its energy cost can be all the more important as the temperature is low.

In the processes according to the state of the art, the adsorption is made at a positive temperature and the heat to regenerate (that in excess) is released into the atmosphere without impacting the cooling balance of the cryogenic part.

The document CN 1 873 357 A discloses a method for purifying a feed gas stream employing an adsorption unit comprising at least two adsorbers, a cryogenic distillation unit, an exchanger and a compressor operating at a temperature below -50 °, wherein the heat necessary for the regeneration of the adsorbers originates, at least in part, from at least a portion of the heat generated by the compressor, during the compression of a fluid.

Starting from there, a problem that arises is to provide a cryogenic purification in a cryogenic separation process that already knows how to manage at the refrigeration balance level a heat input at least equal to that required for the regeneration of the adsorbers.

A solution of the present invention is a method for purifying a feed gas stream using an adsorption unit (4) comprising at least 2 adsorbers, a cryogenic distillation unit (7), an exchanger (2) , and a compressor (10) operating at a temperature of less than or equal to -50 ° C, wherein the heat required for the regeneration of the adsorbers results, at least in part, from at least a portion of the heat generated by the compressor, during the compression of a fluid, and said process comprises an adsorption step carried out by the adsorption unit, with said process characterized in that the adsorption step is carried out at a negative temperature

• ledit procédé comprend selon une première alternative les étapes successives suivantes (figure 1):
  1. a) le flux gazeux d'alimentation 1 est refroidi dans l'échangeur 2 à une température inférieure à -50°C, de préférence inférieure à -100°C ;
  2. b) le flux gazeux refroidi 3 est envoyé à l'unité d'adsorption 4 où au moins une impureté X est au moins en partie adsorbée de manière à récupérer un flux gazeux 5 appauvri en impureté X ;
  3. c) le flux gazeux appauvri en impureté X 5 est introduit dans l'échangeur 2 pour être refroidi à une température inférieure à -50°C, de préférence -150°C ;
  4. d) le flux gazeux 5 appauvri en impureté X et refroidi est envoyé à l'unité de distillation cryogénique 7 où il est séparé en au moins 2 flux 8 et 9 ;
  5. e) une partie du flux 9 est introduit dans l'échangeur pour être réchauffée à une température supérieure à -150°C, de préférence supérieure à-100°C, plus préférentiellement supérieure à - 50°C, idéalement à une température proche de celle du flux gazeux d'alimentation 1 à l'issu de l'étape a)
     avant d'être comprimé dans le compresseur 10 avec un taux de compression supérieur à 1.2
  6. f) le flux 9 comprimé est envoyé à l'unité d'adsorption 4 pour régénérer un des deux adsorbeurs;
     avec la compression à l'étape e) entraînant une augmentation de température du flux 9 d'au moins 20°C et fournissant ainsi l'apport de chaleur nécessaire à la régénération d'au moins un des adsorbeurs ;
• ledit procédé comprend selon une deuxième alternative les étapes successives suivantes (figure 2):
  1. a) le flux gazeux d'alimentation 1 est refroidi dans l'échangeur 2 à une température inférieure à -50°C, de préférence inférieure à -100°C ;
  2. b) le flux gazeux refroidi 3 est envoyé à l'unité d'adsorption 4 où au moins une impureté X est au moins en partie adsorbée de manière à récupérer un premier flux appauvri en impureté X 5;
  3. c) le flux gazeux 5 appauvri en impureté X est comprimé dans le compresseur 10 avec un taux de compression supérieur à 1.2 avant d'être refroidi dans l'échangeur 2 à une température inférieure à -50°C, de préférence inférieure à -150°C ;
  4. d) le flux gazeux 5 appauvri en impureté X, comprimé et refroidi est envoyé à l'unité de distillation cryogénique 7 où il est séparé en au moins 2 flux 8 et 9 ;
  5. e) une partie du flux 9 est introduit dans l'échangeur pour être réchauffée à une température supérieure à -150°C, de préférence supérieure à -100°C, plus préférentiellement -50°C, idéalement à une température proche de celle du flux gazeux d'alimentation 5 à l'issue de la compression de l'étape c) ;
  6. f) le flux 9 réchauffé est envoyé à l'unité d'adsorption 4 pour régénérer au moins un des deux adsorbeurs;
     avec la compression à l'étape c) entraînant une augmentation de température du flux gazeux 5 appauvri en impureté X d'au moins 20°C et fournissant ainsi de façon indirecte via l'échangeur 2 l'apport de chaleur nécessaire au réchauffage d'une partie du flux 9 et donc à la régénération d'au moins un des deux adsorbeurs à l'étape f) ;
• les adsorbeurs comprennent un monolit, de préférence un tamis moléculaire.
• le flux gazeux d'alimentation est de l'air et l'impureté X est choisie parmi H₂O, CO₂, N₂O, C_nH_m, NOx;
• le flux gazeux d'alimentation comprend de l'eau et ledit procédé comprend avant l'étape a) une étape de pré-purification du flux gazeux d'alimentation permettant d'éliminer au moins une partie de l'eau ;
• l'étape de pré-purification se fait par adsorption à température ambiante ;
• l'adsorption de l'étape de pré-purification se fait sur monolit de type alumine, gel de silice ou tamis moléculaire.

Depending on the case, the method according to the invention may have one or more of the following characteristics:

• said method comprises according to a first alternative the following successive steps ( figure 1 ):
  1. a) the feed gas stream 1 is cooled in the exchanger 2 at a temperature below -50 ° C, preferably below -100 ° C;
  2. b) the cooled gas stream 3 is sent to the adsorption unit 4 where at least one impurity X is at least partially adsorbed so as to recover a gaseous stream 5 depleted in impurity X;
  3. c) the gas stream depleted in impurity X 5 is introduced into the exchanger 2 to be cooled to a temperature below -50 ° C, preferably -150 ° C;
  4. d) the cooled and impurity-degraded gaseous stream X is sent to the cryogenic distillation unit 7 where it is separated into at least 2 streams 8 and 9;
  5. e) a portion of the stream 9 is introduced into the exchanger to be heated to a temperature greater than -150 ° C, preferably greater than -100 ° C, more preferably greater than -50 ° C, ideally at a temperature close to that of the feed gas stream 1 at the end of step a)
     before being compressed in the compressor 10 with a compression ratio greater than 1.2
  6. f) the compressed stream 9 is sent to the adsorption unit 4 to regenerate one of the two adsorbers;
     with the compression in step e) resulting in a temperature increase of the flow 9 of at least 20 ° C and thus providing the heat input necessary for the regeneration of at least one of the adsorbers;
• said method comprises according to a second alternative the following successive steps ( figure 2 ):
  1. a) the feed gas stream 1 is cooled in the exchanger 2 at a temperature below -50 ° C, preferably below -100 ° C;
  2. b) the cooled gaseous flow 3 is sent to the adsorption unit 4 where at least one impurity X is at least partially adsorbed so as to recover a first stream impoverished in impurity X 5;
  3. c) the impurity-degraded gaseous stream X is compressed in the compressor 10 with a compression ratio greater than 1.2 before being cooled in the exchanger 2 to a temperature below -50 ° C, preferably below -150 ° C;
  4. d) the compressed and cooled impurity X impurity stream X is fed to the cryogenic distillation unit 7 where it is separated into at least 2 streams 8 and 9;
  5. e) part of the stream 9 is introduced into the exchanger to be heated to a temperature greater than -150 ° C, preferably greater than -100 ° C, more preferably -50 ° C, ideally at a temperature close to that of gaseous feed stream 5 at the end of the compression of step c);
  6. f) the heated stream 9 is sent to the adsorption unit 4 to regenerate at least one of the two adsorbers;
     with the compression in step c) resulting in an increase in the temperature of the impurity-impregnated gas stream X by at least 20 ° C. and thus indirectly supplying via the exchanger 2 the supply of heat required for reheating a part of the flow 9 and therefore the regeneration of at least one of the two adsorbers in step f);
• the adsorbers comprise a monolith, preferably a molecular sieve.
• the feed gas stream is air and the impurity X is selected from H ₂ O, CO ₂ , N ₂ O, C _n H _m , NOx;
• the feed gas stream comprises water and said process comprises before step a) a pre-purification step of the feed gas stream for removing at least a portion of the water;
• the pre-purification step is by adsorption at room temperature;
• the adsorption of the pre-purification step is done on monolith of alumina type, silica gel or molecular sieve.

The invention will be illustrated on an ASU with a cold compressor. The cold compressor introduces into the cold box a thermal input that heats the compressed gas. The natural refrigerant balance of the device makes it possible to manage this thermal input. Part of the hot gas will be used directly or indirectly via a heat exchange with another fluid to ensure the heating phase of the regeneration. This is done without real energy penalty, because it does not disturb (or little) the refrigeration balance of the device.

The figure 1 represents the first alternative of the solution according to the invention.

The air 1 is cooled in the exchange line 2 (for example, up to -120 ° C.), then passes into a bed of adsorbent 4 at a low temperature (-120 ° C.), and is then reintroduced (optionally slightly warmer, due to the adsorption) in the exchange line 2 for final cooling before being sent to the distillation part 7.

Part of the residual nitrogen 9 is drawn off at -120 ° C. from the exchange line, then compressed in a cold compressor 10 where it is heated to a temperature of, for example, -80 ° C. and then sent to an adsorbent bed in regeneration. The heat provided by the compression constitutes the heat input required for the heating phase of the regeneration. The nitrogen cools in the adsorbent bed 4, and is then sent at around -120 ° C towards the exchange line 2 for further heating to room temperature.

The adsorption temperature may be preferably close to the "natural temperature in the cold booster", that is to say that dictated by the process, as if there had been a conventional purification at room temperature.

It can be seen that the heating phase of the regeneration does not disturb (or little) the refrigeration balance of the apparatus, this being done on the natural heat input brought by the cold compression. There is no energy penalty to cryogenic purification. As regards the cooling phase of the regeneration of the process according to the first alternative, part of the residual nitrogen is withdrawn at about -120 ° C. from the exchange line, then is compressed first, before being passed through the bed in regeneration (cooling phase), then sent to the exchange line for further heating up to room temperature.

It is found that the heating and cooling phase is at a different pressure requiring an intermediate phase of adaptation of the bed to the correct pressure.

The figure 2 represents the second alternative of the solution according to the invention.

Air 1 is partially cooled to -120 ° C, then passes through the adsorbent bed 4 before being cold pressed where it heats to -80 ° C, and then returned to the exchange line 2 hotter, for final cooling before being sent to the distillation part 7.

Part of the waste nitrogen 9 heats up in the exchange line 2, to a temperature close to that of the cold compressed air, for example -80 ° C., thus indirectly recovering the heat introduced by the compression of the air. The nitrogen thus warmed to -80 ° C. ensures the heating phase of the regeneration by passing through a bed of adsorbent 4 where it cools to -120 ° C. and is then sent to the exchange line 2 for reheating. additional up to room temperature.

The adsorption temperature may be preferably close to the "natural" inlet temperature in the cold booster, typically around the temperature of the vaporization stage of oxygen, for example for the conventional single-machine cold booster -120 ° C for pressurized oxygen at 40 bar).

Again, it can be seen that the heating phase of the regeneration does not disturb (or little) the refrigeration balance of the apparatus, this being done on the natural heat input brought by the cold compression, indirectly in that case. There is no energy penalty to cryogenic purification.

With regard to the cooling phase of the regeneration of the process according to the second alternative, part of the residual nitrogen leaves the exchange line at a temperature close to the inlet of the cold compressor (around -120 ° C.), passes through the adsorbent bed to cool it, then is sent to the exchange line for additional heating up to room temperature. In this case, the heating and cooling phases are at the same pressure.

Claims (8)

1. Method for purifying a gaseous supply flow implementing an adsorption unit (4) comprising at least 2 adsorbers, a cryogenic distillation unit (7), an exchanger (2) and a compressor (10) functioning at a temperature lower than or equal to -50°C, wherein the heat necessary to regenerate the adsorbers, comes from, at least in part, at least one part of the heat generated by the compressor (10), during the compression of a fluid and said method comprises an adsorption step implemented by the adsorption unit, with said method characterised in that the adsorption step is carried out at a negative temperature.
2. Purification method according to claim 1, characterised in that said method comprises the following successive steps:

   a) the gaseous supply flow (1) is cooled in the exchanger (2) to a temperature lower than -50°C;

   b) the cooled gaseous flow (3) is sent to the adsorption unit (4) where at least one impurity X is at least in part adsorbed so as to recover the gaseous flow (5) depleted in impurity X;

   c) the gaseous flow depleted in impurity X (5) is inserted in the exchanger (2) to be cooled to a temperature lower than -50°C;

   d) the gaseous flow (5) depleted in impurity X and cooled is sent to the cryogenic distillation unit (7) where it is separated into at least 2 flows (8) and (9);

   e) a part of the flow (9) is inserted in the exchanger to be reheated to a temperature higher than -150°C before being compressed in the compressor (10) with a compression rate higher than 1.2;

   f) the compressed flow (9) is sent to the adsorption unit (4) to regenerate one of the two adsorbers;

   with the compression in step e) leading to an increase in temperature of the flow (9) by at least 20°C and thus providing the inflow of heat necessary to regenerate at least one of the adsorbers.
3. Purification method according to claim 1, characterised in that said method comprises the following successive steps:

   a) the gaseous supply flow (1) is cooled in the exchanger (2) to a temperature lower than -50°C;

   b) the cooled gaseous flow (3) is sent to the adsorption unit (4) where at least one impurity X is at least in part adsorbed so as to recover the gaseous flow (5) depleted in impurity X;

   c) the gaseous flow (5) depleted in impurity X is compressed in the compressor (10) with a compression rate higher than 1.2 before being cooled in the exchanger (2) to a temperature lower than -50°C;

   d) the gaseous flow (5) depleted in impurity X and cooled is sent to the cryogenic distillation unit (7) where it is separated into at least 2 flows (8) and (9);

   e) a part of the flow (9) is inserted in the exchanger to be reheated to a temperature higher than -150°C;

   f) the reheated flow (9) is sent to the adsorption unit (4) to regenerate one of the two adsorbers;

   with the compression in step c) leading to an increase in temperature of the gaseous flow (5) depleted in compound X by 20°C and thus indirectly providing, via the exchanger (2), the inflow of heat necessary to reheat a part of the flow (9) and therefore to regenerate at least one of the two adsorbers in step f).
4. Method according to one of claims 1 to 3, characterised in that the adsorbers comprise a monolith, preferably a molecular sieve.
5. Method according to one of claims 2, 3 or 4, when it refers to one of claims 2 or 3, characterised in that the gaseous supply flow is air and the impurity X is chosen among H₂O, CO₂, N₂O, C_nH_m, NOx.
6. Method according to one of claims 1 to 5, characterised in that the gaseous supply flow comprises water and said method comprises, before step a), a step for pre-purifying the gaseous supply flow enabling to eliminate at least one part of the water.
7. Method according to claim 6, characterised in that the pre-purification step is carried out by adsorption at room temperature.
8. Method according to claim 7, characterised in that the adsorption in the pre-purification step is carried out an alumina, silica gel or molecular sieve-type monolith.

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