WO2013164752A2

WO2013164752A2 - Method for the reduction of the regeneration energy of acidic gas loaded solvents

Info

Publication number: WO2013164752A2
Application number: PCT/IB2013/053374
Authority: WO
Inventors: Geert Frederic Versteeg; Glenn Rexwinkel; Nick Antonius Maria Ten ASBROEK; Patrick Johannes Gerhardus HUTTENHUIS; R. Arendsen
Original assignee: Procede Holding Bv; Gerntholtz, Otto, Carl
Priority date: 2012-05-01
Filing date: 2013-04-29
Publication date: 2013-11-07
Also published as: WO2013164752A3

Abstract

The invention discloses a method for the reduction of the regeneration energy of acidic gas loaded solvents in a stripper, includes the step of reversing the pressure gradient in the stripper. The method may be applied in a method for the removal of acidic gas from industrial gases which includes the step of reducing the regeneration energy of acidic gas loaded solvents in a stripper by means of reversing the pressure gradient in the stripper. The invention also extends to an arrangement for the reduction of the regeneration energy of acidic gas loaded solvents includes a stripper in which the pressure gradient in the stripper is reversed.

Description

METHOD FOR THE REDUCTION OF THE REGENERATION ENERGY

OF ACIDIC GAS LOADED SOLVENTS

FIELD OF INVENTION

The present invention relates to a method for the reduction of the regeneration energy of acidic gas loaded solvents.

More particularly, the present invention relates to a method for the reduction of the regeneration energy of acidic gas, like hydrogen sulphide, cabonyl sulphide, mercaptans and carbon disulphide, loaded solvents by reversing the pressure gradient in the regeneration units.

BACKGROUND TO INVENTION

The removal of C0₂ from industrial gases, e.g. flue gas, natural gas and biogas respectively, is usually carried out via the absorption of the C0₂ in a solvent that contains a compound that reacts with the C0₂. These solvents are frequently aqueous solutions of alkanol / amines, aqueous solutions of carbonates / bicarbonates or amino acids but also other aqueous solvents may be used. More generally it can be stated that these solvents are aqueous solutions with one or more basic compounds.

The absorption solvent, loaded with C0₂, is usually regenerated in a so-called stripper in which the temperature is increased. In the stripper the conditions are such that the solvent is close to its boiling point and the reverse reaction of the C0₂ absorber takes place. In this way C0₂ is produced in the gas phase. Owing to the high temperatures a vast amount of water is evaporated (usually the added basic compound has a substantially higher boiling point and its concentration in the stripper outlet gas is negligible) that is condensed and fed back to the stripper in order to maintain the water balance.

In principle the minimum net energy required in these C0₂ capture processes would be equal to the heat of the chemical reaction but this is more a theoretical asymptotic number. However, one can state that basic compounds with a lower heat of reaction (or absorption) usually show a somewhat more favorable total heat of regeneration. As mentioned above vast amounts of water are also evaporated in the stripper and subsequently condensed again. This continuous cycle of evaporation and condensation of water increases substantially the energy consumption of the regeneration process.

It is disclosed in WO 2004/080573 Rochelle the regeneration of acid gas loaded solvents was carried out in three flash vessels operated at different pressures and varying temperatures. It is claimed by Rochelle that this leads to a lower energy consumption in the regeneration process.

It is an object of the invention to suggest a method for the reduction of the regeneration energy of acidic gas loaded solvents. SUMMARY OF INVENTION

According to the invention, a method for the reduction of the regeneration energy of acidic gas loaded solvents in a stripper, includes the step of reversing the pressure gradient in the stripper.

Also according to the invention, a method for the removal of acidic gas from industrial gases includes the step of reducing the regeneration energy of acidic gas loaded solvents in a stripper by means of reversing the pressure gradient in the stripper.

Yet further according to the invention, arrangement for the reduction of the regeneration energy of acidic gas loaded solvents includes a stripper in which the pressure gradient in the stripper is reversed. Yet further according to the invention, arrangement for the removal of acidic gas from industrial gases includes a stripper in which the pressure gradient in the stripper is reversed.

Yet further according to the invention, an acid gas treatment process includes the step of reducing of the regeneration energy of acidic gas loaded solvents in an aqueous solvent solution by replacing water with an organic compound in the solvent. The stripper may thus have the lowest pressure at the bottom and the highest pressure at the top. The pressure at the top of the stripper is thus higher than the pressure at the bottom of the stripper.

Additional compression energy may be introduced to keep the vapour/gas flowing from the bottom to the top of the stripper. The additional compression energy introduced may be largely compensated by the energy reduction of the stripping process.

The stripper may have various pressure stages.

The number of pressure stages may mainly depends on the characteristics of the solvent used. The method may include the step of preventing the liquid, that is transported from the top to the bottom of the stripper (down pressure) from severe flashing to prevent circulating too much vapour.

The invention also extends to and is applicable in acid gas treating processes.

The acid gas treatment process may include the removal of hydrogen sulphide, carbon dioxide, carbonyl sulphide, mercaptans, etc.

The stripper may be a mass transfer unit like e.g. packed column, tray column, bubble column, etc.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example with reference to the accompanying schematic drawings.

In the drawing there is shown in:

Figure 1: the C0₂ capture efficiency as a function of the desorber (packing) length in a typical C0₂ capture process using an aqueous 30 wt.% MEA solvent; a flow scheme showing the standard scheme for C02 capture in accordance with the invention; the decrease in reboiler duty as a function of the pressure in the top of the desorber (30 wt.% MDEA); the effect of increase pressure on the temperature profile in the desorber; the decrease in reboiler duty as a function of the pressure in the top of the desorber (50 wt.% MDEA); the temperature as a function of the desorber depth (50 wt.% MDEA); the flow scheme of WO 2004/080573 Rochelle; the flow scheme according to the present invention; the reboiler duty with flash vessels being higher for all pressures compared to packed strippers; flow diagram with a certain amount of strippers in series on which several simulations were performed; the reboiler duty as a function of the maximum stripper pressure for 30 wt% MEA; the additional compressor work as a function of the maximum stripper pressure for 30 wt% MEA; the reboiler duty as a function of the maximum stripper pressure for 50 wt% MDEA; the additional compressor work as a function of the maximum stripper pressure for 50 wt% MDEA; and

Energy (the reboiler duty and compressor work) as a function of the amount of strippers. DETAILED DESCRIPTION OF DRAWINGS

Referring to the drawings, there is shown a method for the reduction of the regeneration energy of acidic gas loaded solvents in accordance with the invention.

The method for the reduction of the regeneration energy of acidic gas loaded solvents in a stripper includes the step of reversing the pressure gradient in the stripper.

The stripper thus has the lowest pressure at the bottom and the highest pressure at the top.

The pressure at the top of the stripper is thus higher than the pressure at the bottom of the stripper.

Additional compression energy is introduced to keep the vapour/gas flowing from the bottom to the top of the stripper.

The additional compression energy introduced is largely compensated by the energy reduction of the stripping process. The stripper has various pressure stages.

The number of pressure stages mainly depends on the characteristics of the solvent used.

The method includes the step of preventing the liquid, that is transported from the top to the bottom of the stripper (down pressure) from severe flashing to prevent circulating too much vapour. In the traditional operation, the pressure in the stripper is the highest in the bottom and the lowest in the top cq. condenser. From process simulation studies it turned out to be very attractive with respect to the overall energy consumption of the stripping process to reverse the pressure gradient.

This means the top of the stripper has a higher pressure than the bottom (the reboiler). The invention also extends to a method for the removal of acidic gas from industrial gases includes the step of reducing the regeneration energy of acidic gas loaded solvents in a stripper by means of reversing the pressure gradient in the stripper.

The invention thus provides for an arrangement for the reduction of the regeneration energy of acidic gas loaded solvents includes a stripper in which the pressure gradient in the stripper is reversed.

The invention also provides for an arrangement for the removal of acidic gas from industrial gases includes a stripper in which the pressure gradient in the stripper is reversed.

The invention thus provides techniques to reduce the amount of water evaporated or to integrate the produced water vapor in the regeneration process and prevent it from condensation. In this way the energy consumption of the regeneration process is reduced.

The invention also extends to and is applicable in acid gas treating processes includes the step of reducing of the regeneration energy of acidic gas loaded solvents in an aqueous solvent solution by replacing water with an organic compound in the solvent. The acid gas treatment process may include the removal of hydrogen sulphide, carbon dioxide, carbonyl sulphide, mercaptans, etc.

As mentioned before, it is disclosed in WO 2004/080573 Rochelle the regeneration of acid gas loaded solvents was carried out in three flash vessels operated at different pressures and varying temperatures. It is claimed by Rochelle that this leads to a lower energy consumption in the regeneration process. However flash vessels are not efficient enough, the process is mass transfer controlled and therefore mass transfer equipment like, packed columns, etc. is required to obtain an efficient stripping process. This leads to lower energy requirements. From rate based process simulations for a packed desorber (see Figure 1), it can be concluded that the regeneration process cannot be regarded as an equilibrium controlled unit operation. In Figure 1 the C0₂ capture efficiency is presented as function of the desorber (packing) length in a typical C0₂ capture process using an aqueous 30 wt.% MEA solvent. Aqueous MEA is a well-known acid-gas treating solvent and for other frequently used solvents similar results are obtained. In the simulations a complete C0₂ capture process is modelled containing absorber, heat exchanger, pumps, solvent cooler and regenerator.

From Figure 1 it can be concluded that a desorber length of approximately 2-3 meter is needed to obtain 90 % C0₂ capture efficiency. When the desorber length is further increased, no additional increase in C0₂ capture is seen. However at lower column lengths the desorption of C0₂ will be limited by mass transfer effects in the desorber, because a decreasing C0₂ capture is noticed. One, who is skilled in the art, can conclude from the figure that the regeneration process requires, depending on the conditions, several meters of packing height. This clearly illustrates that the process is mass transfer controlled for C0₂ desorption. For other acidic gases similar trends are obtained.

The present invention also claims and shows that beside an inversed pressure gradient also a continuous temperature gradient is required, meaning that the bottom temperature is the highest and continuously decreasing to the top.

The flow scheme shown in Figure 2 shows the standard scheme for C0₂ capture. In the standard scheme the desorber operates at constant pressure, due to hydrostatic pressure the pressure increases from the top down. If this pressure gradient is inversed, decreased from the top down, the reboiler duty is decreased. This effect is caused by the condensing steam from the bottom up in the desorber.

Figure 3 shows the decrease in reboiler duty as a function of the pressure in the top of the desorber. The graph in Figure 3 does not include the work needed for compression.

Referring to Figure 4, the increase pressure has an effect on the temperature profile in the desorber. Without an inverse pressure gradient the temperature increases from the top down. Due to the higher pressure in the top the temperature in the top of the desorber increases, but the top temperature stays below the reboiler temperature. Therefore, at higher pressure also, the temperature in the top of the desorber is lower than the temperature in the reboiler. At 8 bar there is a minimum and maximum temperature outside the temperature range of the top and reboiler temperature. Referring to Figures 5 and 6, for 50 wt% MDEA the trends are comparable. The temperature in the top of the desorber is lower than the temperature in the reboiler, also.

According the following is evident:

(a) From the temperature gradients, both for MDEA and MEA, one can conclude that over the length of the desorber (stripper) no equilibrium is attained. Therefore one needs mass transfer equipment as desorber.

(b) Bottom temperature is higher than top temperature. So it seems important to have the bottom temperature above the temperature of the top. When compression steps are included, in real life an inversed pressure gradient can only be realized by compressing the gas phase, it will be demonstrated that this temperature drop from top to bottom must be realized.

(c) For other acidic gases similar results have been obtained.

In the section below the invention of WO 2004/080573 Rochelle (flow scheme shown in Figure 7), three flash vessels/flash drums (equilibrium stages), is compared to the present invention, the regeneration process performed in three packed columns and simulated rate base (flow scheme shown in Figure 8).

As first comparison the reboiler duty is calculated. Two different flow schemes with flash vessels and with packed strippers are shown in Figure 7 and 8. Figure 9 clearly shows that the reboiler duty with flash vessels is higher for all pressures compared to packed strippers. The rate based mass transfer is needed to condense the steam.

It is thus clear that according to the present invention the reboiler duty is lower compared to the duties required according the Rochelle patent.

Several simulations have been performed with a certain amount of strippers in series as shown in Figure 10. The compressor ration between the strippers in one simulation is always held constant, for instance in case of three compressors and maximum pressure of 8 bar, the pressures of the strippers are 2, 4 and 8 bar. The last compressor always increases the C0₂ pressure up to 8 bar. The reboiler duty is calculated as a function of the pressure in Desorber 1. The compressor duty is based on the difference between the work of all compressor in case all strippers work at 2 bar and the work of all compressors in the work at a higher pressure. In this case the work represents the extra work needed to compress the addition water between the strippers minus the work for compressing C0₂ from 2 to 8 bar.

The Figures 11 to 14 show the reboiler duty and additional compressor work as a function of the maximum stripper pressure for 30 wt% MEA and 50 wt% MDEA. They clearly show that the reboiler duty decreases and the compressor work increases. How these to add up and if this gives an optimal maximum pressure depends on the local situation (cost of heat and electricity). The reboiler duty for 3 strippers in lower than for 5 strippers. This is due to the fact that the higher work of the compressors for 3 strippers is converted to heat (high compressed steam temperature), and therefore gives a lower reboiler duty.

The graph in Figure 15 shows the reboiler duty and compressor work as a function of the amount of strippers. The maximum pressure in desorber 1 is always 8 bar. Two strippers gives a high amount of compressor work and therefore a lower reboiler duty. More than 5 strippers does not give much profit and probably 5 strippers in series is enough.

Claims

PATENT CLAIMS

1. A method for the reduction of the regeneration energy of acidic gas loaded solvents in a stripper, includes the step of reversing the pressure gradient in the stripper.

2. A method as claimed in claim 1, in which the stripper has the lowest pressure at the bottom and the highest pressure at the top.

3. A method as claimed in claim 1 or claim 2, in which the pressure at the top of the stripper is higher than the pressure at the bottom of the stripper.

4. A method as claimed in any one of the preceding claims, in which additional compression energy is introduced to keep vapour/gas flowing from the bottom to the top of the stripper.

5. A method as claimed in claim 4, in which the additional compression energy introduced is largely compensated by the energy reduction of the stripping process.

6. A method as claimed in any one of the preceding claims, in which the stripper has various pressure stages.

7. A method as claimed in claim 6, in which the number of pressure stages mainly depends on the characteristics of solvent(s) used.

8. A method as claimed in any one of the preceding claims, which includes the step of preventing the liquid, that is transported from the top to the bottom of the stripper (down pressure) from severe flashing to prevent circulating too much vapour.

9. A method as claimed in any one of the preceding claims, which is applied in an acid gas treating process.

10. A method as claimed in claim 9, in which the acid gas treatment process includes the step of removal of hydrogen sulphide, carbon dioxide, carbonyl sulphide and/or mercaptans.

11. A method for the removal of acidic gas from industrial gases, which includes the step of reducing the regeneration energy of acidic gas loaded solvents in a stripper by means of reversing the pressure gradient in the stripper.

12. A method as claimed in claim 11, in which the stripper has the lowest pressure at the bottom and the highest pressure at the top.

13. A method as claimed in claim 11 or claim 12, in which the pressure at the top of the stripper is higher than the pressure at the bottom of the stripper.

14. A method as claimed in any one of claims 11 to 13, in which additional compression energy is introduced to keep vapour/gas flowing from the bottom to the top of the stripper.

15. A method as claimed in claim 14, in which the additional compression energy introduced is largely compensated by the energy reduction of the stripping process.

16. A method as claimed in any one of claims 11 to 15, in which the stripper has various pressure stages.

17. A method as claimed in claim 16, in which the number of pressure stages mainly depends on the characteristics of solvent(s) used.

18. A method as claimed in any one of claims 11 to 17, which includes the step of preventing the liquid, that is transported from the top to the bottom of the stripper (down pressure) from severe flashing to prevent circulating too much vapour.

19. A method as claimed in any one of claims 11 to 18, which is applied in an acid gas treating process.

20. A method as claimed in claim 19, in which the acid gas treatment process includes the step of removal of hydrogen sulphide, carbon dioxide, carbonyl sulphide and/or mercaptans.

An arrangement for the reduction of the regeneration energy of acidic gas loaded solvents, which includes a stripper in which the pressure gradient in the stripper is reversed.

An arrangement as claimed in claim 21, in which the stripper has the lowest pressure at the bottom and the highest pressure at the top.

An arrangement as claimed in claim 21 or claim 22, in which the pressure at the top of the stripper is higher than the pressure at the bottom of the stripper.

An arrangement as claimed in any one of claims 21 to 23, in which additional compression energy is introduced to keep vapour/gas flowing from the bottom to the top of the stripper.

An arrangement as claimed in claim 24, in which the additional compression energy introduced is largely compensated by the energy reduction of the stripping process.

An arrangement as claimed in any one of claims 21 to 25, in which the stripper has various pressure stages.

An arrangement as claimed in claim 26, in which the number of pressure stages mainly depends on the characteristics of solvent(s) used.

An arrangement as claimed in any one of claims 21 to 27, which means for preventing the liquid, that is transported from the top to the bottom of the stripper (down pressure) from severe flashing to prevent circulating too much vapour.

An arrangement as claimed in any one of claims 21 to 28, which is applied in an acid gas treating process.

An arrangement as claimed in claim 29, in which the acid gas treatment process includes the step of removal of hydrogen sulphide, carbon dioxide, carbonyl sulphide and/or mercaptans.

An arrangement for the removal of acidic gas from industrial gases, which includes a stripper in which the pressure gradient in the stripper is reversed.

An arrangement as claimed in claim 31, in which the stripper has the lowest pressure at the bottom and the highest pressure at the top.

An arrangement as claimed in claim 31 or claim 32, in which the pressure at the top of the stripper is higher than the pressure at the bottom of the stripper.

An arrangement as claimed in any one of claims 31 to 33, in which additional compression energy is introduced to keep vapour/gas flowing from the bottom to the top of the stripper.

An arrangement as claimed in claim 34, in which the additional compression energy introduced is largely compensated by the energy reduction of the stripping process.

An arrangement as claimed in any one of claims 31 to 35, in which the stripper has various pressure stages.

An arrangement as claimed in claim 36, in which the number of pressure stages mainly depends on the characteristics of solvent(s) used.

An arrangement as claimed in any one of claims 31 to 37, which means for preventing the liquid, that is transported from the top to the bottom of the stripper (down pressure) from severe flashing to prevent circulating too much vapour.

An arrangement as claimed in any one of claims 31 to 38, which is applied in an acid gas treating process.

An arrangement as claimed in claim 39, in which the acid gas treatment process includes the step of removal of hydrogen sulphide, carbon dioxide, carbonyl sulphide and/or mercaptans.

An acid gas treatment process includes the step of reducing of the regeneration energy of acidic gas loaded solvents in an aqueous solvent solution by replacing water with an organic compound in the solvent.

A method for the reduction of the regeneration energy of acidic gas loaded solvents in a stripper substantially as hereinbefore described.

43. A method for the removal of acidic gas from industrial gases substantially as hereinbefore described.

44. An arrangement for the reduction of the regeneration energy of acidic gas loaded solvents substantially as hereinbefore described.

45. An arrangement for the removal of acidic gas from industrial gases substantially as hereinbefore described.

46. An acid gas treatment process substantially as hereinbefore described.