WO2012092984A1

WO2012092984A1 - Rotating vacuum stripper

Info

Publication number: WO2012092984A1
Application number: PCT/EP2011/050179
Authority: WO
Inventors: Torbjørn FIVELAND
Original assignee: Statoil Petroleum As
Priority date: 2011-01-07
Filing date: 2011-01-07
Publication date: 2012-07-12

Abstract

A system for desorption of CO₂ from an absorption fluid comprising a cylinder with an open inner core and comprising a stripping unit between the open core and, where the packing (17) is rotatable arranged around an axis through the core, a conduit (2) for supplying CO₂ rich absorption fluid to the core of the packing, a lean absorbent (18) outlet at the perimeter of the cylinder of packing material, means for indirect heat supply (4, 6) to at least a periphery part of the packing material, and a vapour outlet (20) arranged at the core at a first end of cylinder is disclosed. According to the present invention, at least one enzyme is dispersed in the absorbtion fluid further a method for desorbing CO₂ is disclosed.

Description

Rotating Vacuum Stripper

The present invention relates to an apparatus and a method for removing and recovering C0₂ from exhaust gases. Furthermore the present invention relates to an apparatus and method for desorption of C0₂ from a liquid absorbent.

During the later years there has been an increased focus on C0₂ capture due to the environmental aspects associated with the release of C0₂ to the atmosphere.

The conventional method for removing C0₂ from exhaust gas is by use of a standard absorption-desorption process. In this process the gas has its pressure boosted by a blower either before or after an indirect or direct contact cooler. Then the gas is fed to an absorption tower where the gas is contacted counter-currently with an absorbent flowing downwards. In the top of the column a wash section is fitted to remove, essentially with water, remnants of absorbent following the gas from the C0₂ removal section. The absorbent, rich in C0₂ from the absorber bottom is pumped to the top of a desorption column via a heat recovery heat exchanger rendering the rich absorbent preheated before entering the desorption tower. In the desorption tower the C0₂ is stripped by steam, generated in a reboiler positioned at the column bottom. The steam moves up the tower serving as a diluent to the C0₂ although some of the steam condenses to provide desorption heat for the C0₂. Water and absorbent following C0₂ over the top is recovered in the condenser over the desorber top. Vapour is formed in the reboiler from where the absorbent lean in C0₂ is pumped via the heat recovery heat exchanger and a cooler to the top of the absorption column.

EP 0 020 055 Al teaches how e.g. a gas and a liquid can be contacted counter-currently in a rotating packed bed by introducing the liquid at the core of the bed and the gas from the perimeter. It is further known from Ramshaw (Heat Recovery Systems & CHP, vol 13, no 6, pages 493-513, 1993) that a rotating packed bed could also be fitted with a heat exchanger at the outer perimeter, and that this heat exchanger could be used as a reboiler.

JP1066420 disclose a system for separation C0₂ from a working fluid employing an absorption fluid. The system comprises two rotating cylinders and injection nozzles arranged there between. A desorption system is not disclosed. The aim of the present invention is to provide a compact desorption system, which is cost efficient both to construct, operate and maintain. It is an aim of the present invention to reduce the thermal degradation of the absorption solution by limiting the residence time of the absorption fluid in the desorber. Furthermore, the present invention aims to increase the efficency of the compact desorption system set forth in the Norwegian patent application NO 20092629 by using one or more enzymes in the process in order to increase the C0₂ transfer rate from a gas comprising C0₂, to a liquid C0₂ absorbent.

According to the present invention, the abovementioned aim is reached by means of an apparatus and a method according to the enclosed independent claims. Further advantageous features and embodiments are mentioned in the dependent claims.

The present invention can be utilized in connection with gases coming from different kind of facilities. These facilities could be combined cycle gas fired power plants; coal fired power plants, boilers, cement factories, refineries, the heating furnaces of endothermic processes such as steam reforming of natural gas or similar sources of flue gas containing C0₂.

The present invention can be utilized with any type of liquid C0₂ absorbent, comprising an absorbent and a liquid diluent. Examples of applicable absorbents comprise amine based absorbents such as primary, secondary and tertiary amines; one well known example of applicable amines is mono ethanol amine (MEA). An other example of an applicable absorbent is potassium carbonate (K₂CO₃) . The liquid diluent is selected among diluents that have a suitable boiling point, are stable and inert towards the absorbent in the suitable temperature and pressure interval. An example of an applicable diluent is water. Furthermore, the enzymes used to increase the C0₂ transfer rate from a gas comprising C0₂, to a liquid C0₂ absorbent can comprise a carbonic anhydrase (CA).

A advantageous aspect of the present invention is that it is possible to combine several process equipment items, e.g. five process equipment units, or unit functions, into fewer, possibly one, compact units. The reduced size of the unit or units allows a very compact construction, and the unit or units could be assembled on one skid.

In regard to rotating packed beds the present invention represents a solution to the problem of space in the radial direction and difference in centrifugal acceleration between the inner and outer perimeters, that the present invention also provides integrated condensers at a level right next to or above/below the mass transfer and reboiler zones. The method further involves returning the liquid stream from the main condenser to the core of a cylinder comprising a heated stripper unit, and that the main condenser is rotated around the same axis as the cylinder.

The present invention invention may provide solutions for the following problems associated with existing technology:

The compact technology uses less material, strongly reduces the piping needs, and removes the need to work high above the ground as is needed for a conventional column. This is expected to strongly reduce the cost of the desorption unit.

By allowing much smaller, compact equipment units to be made and through its compactness, the customary receiving vessel and reflux pump may be eliminated. These are traditionally standard and thus on the order of 5 conventional units are replaced.

The rotating desorber according to the present invention has a very short residence time with little backmixing. Due to this, thermal degradation of the absorbent solution is expected to be significantly reduced.

The present invention will now be disclosed in further detail with reference to the enclosed figures, wherein:

Figure 1 illustrates a rotating desorber according to a first embodiment of the present invention;

Figure 2 illustrates a rotating assembly according to a second embodiment of the present invention, the rotating assembly comprising an integrated rotating reboiler and desorber packing and stationary condenser;

Figure 3 illustrates a reboil desorber according to a third embodiment of the present invention;

Figure 4 illustrates the use of an absorbent reflux condenser according to a forth embodiment of the present invention; Figure 5 illustrates an embodiment of a rotating desorber according to a fifth embodiment of the present invention.

In conventional technology on the order of five pieces of equipment are needed in the desorption section, namely the column, a reboiler, a condenser, a condensate receiver vessel, and a reflux pump. According to the present invention these can all be incorporated in one or two pieces of equipment, thus eliminating significant piping connections and process control functions. This simplification leads to direct cost savings, but also significant cost savings with respect to erection, piping and process control can be expected.

With conventional rotating packed beds it is difficult to find enough space in the core area to allow the incorporation of an integrated condenser. According to the present invention these limitations are alleviated, allowing the provision of an integrated condenser at a level above/below or next to the mass transfer and reboiler zones. The present invention thus largely solves the problem of space in the radial direction and the difference in centrifugal acceleration between the inner and outer perimeters.

A further improvement to the process equipment in the desorption process is the reduction in size. Hence less material is used, less area is needed, and erection is further eased.

A first embodiment of the present invention is illustrated on figure 1 showing a cross sectional view along a vertical axis of rotation. The equipment comprises a rotating assembly with two levels. At the lower level there is a stripper unit comprising a rotating packed bed 12 next to the inner core. In this desorber packing 12 C0₂ is desorbed from the rich absorbent which is entered through conduit 2 and distributed at the core via nozzles 3. The desorption is achieved mainly by water vapour flowing in a counter-current fashion from the perimeter, and by part of this water vapour condensing thus providing heat for the endothermic desorption of C0₂. The inward vapour flow 13 is created in a reboiler section 14 forming a periphery part of the stripper unit. A part of the liquid 15, which is lean on C0₂ and moving radial outwards due to the rotation, is evaporated caused by condensing steam on the warm side of this heat

exchanger/reboiler section. The steam 4 referred to is entered at the core and leaves as condensate 6, also at the core after it has supplied heat to reboiler section 14. The liquid 18 is significantly stripped of C0₂ and is allowed to leave the rotating assembly at the outer periphery of the reboiler section 14. The vapour stream 20 reaching the core from the rotating packed bed rises to the upper level where this vapour stream flows outwards in a condenser 16, and where diluent vapour is condensed by a coolant 8 in indirect contact. The heated coolant leaves the condenser at the core as stream 10. At the outer periphery the gas stream 24 leaving the condenser 16 is mainly C0₂ and the stream 24 is fit for drying and compression if needed for sequestration. The liquid stream 22 leaving the outer periphery comprises condensed diluent and absorbent and this stream is returned to the core at the lower level via nozzles 5.

The liquids 2, 22 introduced at the core in the illustrated embodiment are distributed via nozzles. However, other means of feeding liquids may also be envisaged, such as perforated pipes or similar.

Firgure 2 shows a second embodiment of the present invention. Here equal reference numbers are utilized for those parts that are unchanged compared to the first

embodiment illustrated in figure 1. In the second embodiment the lower level is unchanged compared to the first embodiment in figure 1, except for a housing 30 that is added illustrating that the upper level is not part of the rotating assembly. The desorber overhead 20 comprising C0₂, diluent and absorbent is fed to a conventional condenser 116 and brought into indirect contact with a coolant 108. The coolant absorbs heat and leaves through conduit 110. The coolant may be cooling water or another suitable cooling liquid. Liquid condensed in the condenser 116 is returned to the lower level as reflux 22 comprising diluent and absorbent. The vapour stream 124 out of the condenser will contain the desorbed C0₂ fit for drying and compression if needed for

sequestration.

A third embodiment of the present invention is shown in figure 3. A desorption section 17 is constructed as a reboiler only without splitting the mass transfer stripping section and the formal reboiler. The reboiler heat transfer area thus doubles as mass transfer area along with the surface of droplets in the section, and all desorption of C0₂ is performed in the reboiler. Since the reboiler design in this invention is by nature a liquid flow through a stripping unit with limited back mixing, the liquid flows radially outwards counter-current to the vapour being created continuously on the reboiler walls. The advantage of this embodiment is a simpler construction compared to the second embodiment illustrated on figure 2.

In figure 4, a fourth embodiment of the invention, which could be used with either of the embodiments illustrated on figure 2 or 3, is shown. The further development consists of a reflux condenser 21 positioned between the rotating entity within the housing 30 and the stationary condenser 116. By applying a limited condensation at this point, it is possible to condense the least volatile vapour component, usually the valuable absorbent, and this separation of absorbent is aided by some water also condensing thus creating a refluxed wetted wall column or equivalent. The desorber overhead 20 is fed into the reflux condenser, and the non-condensed parts of this stream are fed into the main condenser 116. The condensate from the main condenser 116 is fed as stream 25 into the top of the reflux condenser 21. The combined liquid condensate streams are returned to the lower level via conduit 22. This leads to a small distillation taking place.

In another embodiment, not shown, the cold condensate from the main condenser 116 may be routed to some other point of advantage in the process thus reducing the need for heat supply to the reboiler equivalent to heating said condensate to the lean absorbent temperature.

In yet another embodiment the reflux condenser described could be fitted into the core of the rotating entity on the lower level, and rotating with the entity and some condensate from the condenser could be used for reflux.

Although the axis in most of the illustrated embodiments is vertically aligned the rotating axis could also be horizontally aligned. The speed of rotation will make the liquids travel radially thereby forcing the vapour phase to move towards the axis of rotation.

Figure 5 shows another embodiment of the present invention where the axis of rotation is horizontally aligned. The embodiment has many similarities with the embodiments shown on figure 1 and 3. Figure 5 illustrates the directions of flow in this embodiment. Similar elements are referred to with similar reference numbers with an addition of 300 for the reference numbers to be distinctive.

Figure 5 shows an integrated tubular reboiler and stripper. In the illustrated embodiment the reboiler unit 317 is designed with a number of small diameter tubes for heat supply. Steam is supplied trough conduit 304 and passed trough the tubes running in parallel with the axis of rotation. The tubes are in communication with a conduit 306 for removing the condensate. For the purpose of illustration three tubes are shown on each side of the axis of rotation, however the reboiler may comprise any number of tubes. In this embodiment the stripper is integrated in the reboiler. The C0₂ rich absorbent is introduced via conduit 302 and the stripping will take place when the absorbent solution is introduced to unit 317. Depleted absorbent solution leaves the reboiler unit 317 at the circumference as stream 318. Whereas the vapour phase including the C0₂ leaves the reboiler near the centre into conduit 320 and directed into a first condenser 316 at the perimeter. In order to create additional surface area for the mass transfer, it is proposed in one aspect of the invention to include layers of thin metal mesh between the rows of reboiler tubes, e.g 6 mm tubes in 9 mm centre diameter will give a reboiler specific surface of 233 m 2 /m 3. A fine metal mesh with wire diameter 0.5-1 mm diameter gives specific surface areas above 1000 m 2 /m 3 depending on mesh spacing. The small tubes can be fixed to the end plates using conventional roller expander techniques. In this embodiment it is proposed to use horizontal tubes in the reboiler and omit the slope. This is mainly because of design and manufacturing considerations. This solution requires that the tubes are open in both ends with condensate drainage in the end closest to the condenser section 316. The steam that flows from 304 to 306, from left to right and is gradually converted to condensate and drained to the right through 306. The condensate may be removed in a fluid mechanical seal located on the stator cylinder at the same axial position, instead of using special return channels to the stator end cover.

In one aspect of the present invention sieve trays or perforated plates are included between the rows of tubes for heat supply instead of thin metal mesh , the sieve trays/perforated plates will increase the area of liquid gas contact and also contribute to enhanced distribution of the liquid phase.

In another aspect of the present invention small spherical "pellets" are included between the rows of tubes.

Due to steam consumption considerations it is preferred to use a design with gas flow towards the rotation centre and absorbent flow towards the periphery. Subsequently the gas must be guided from the centre to the condenser section 316. This can be achieved by including radial flow channels with rigid steel plates.

The embodiment illustrated on figure 5 comprises a two stage condenser 326 and 346. Cooling liquid is entered at the centre through conduit 308 and supplied first to the second condenser 346 and thereafter onto the first condenser 316 before the cooling liquid leaves through conduit 310 arranged at the centre. In another aspect of the present invention the cooling liquid is supplied through conduits along the centre but with inlet and outlet from the reboiler side. In the first condenser 316 diluent and absorbent is condensed and will due to the rotation be transported to the perimeter where it leaves the condenser 316 as stream 322. Stream 322 may be returned to the reboiler 317 as reflux. The reflux of condensed vapour, which becomes stream 322, over the gas mixture in the first condenser, is considered to contribute to the elimination of absorbent vapour in the recovered C0₂. In the second condenser 346 mainly diluent free of absorbent is condensed and leaves the condenser as stream 342. If water is used as diluent the obtained water stream from the second condenser may in one aspect of the present invention be utilized as washing liquid in the absorption process to remove traces of the absorbent from the C0₂ depleted gas stream. The stream 324 out of the condenser will contain the desorbed C0₂ fit for drying and compression if needed for sequestration.

According to the present invention, one or more enzymes can be utilized to increase the C0₂ transfer rate from a gas comprising C0₂, to a liquid C0₂ absorbent. By adding one or more enzymes, e.g. carbonic anhydrase (CA), to the liquid C0₂ absorbent, the conversion reaction rate of dissolved carbon dioxide to bicarbonate can be increased by orders of magnitude:

C0₂ + H₂0 <-> HC0₃ ^" + H⁺

CA exists in a number of variants. The turnover rate (number of reactions at a single active site) for the most active variants of CA is about 10⁶ C0₂ molecules per second. A rough estimate is that in a process CA may speed up the hydration rate of dissolved C0₂ 10-50 times compared to chemical hydration rate under the same conditions. The process temperature may be a limitation, but the use of for example modified mesophilic enzymes and/or enymes from thermophilic prokaryotes may increases the possible process temperatures to at least 80-100 °C.

The enzymes may be dispersed in the liquid C0₂ absorbent. The process stability of immobilized CA is on the order of weeks to months, inter alia depending on residence times and temperature in the stripper.

One benefit of the present apparatus is the short residence time in the stripper. The absorbent (which contains enzymes) is heated for only 10-20 seconds before it is cooled. In the conventional process used for stripping, the absorbent is heated for 3-5 minutes. The short residence times that are possible according to the present invention, increases the lifetime of the enzymes used in the process considerably and thereby increases the feasability of using enzymes.

According to one embodiment of the present invention, the stripper operates at a pressure in the range of 0.6 - 0.9 bars and at a temperature in the range of 40 - 100 °C, and preferably in a range of 70 - 95 °C. The low pressure will also result in an enhanced flashing of C0₂. A lower temperature will tend to increase the duration of the process stability of CA

Claims

C l a i m s 1.

System for desorption of C0₂ from an absorption fluid comprising a cylinder with an open inner core and comprising a stripping unit arranged between the inner core and the circumference of the cylinder, where the stripping unit (12; 17; 317) comprises a rotating packed bed arranged around an axis through the core, a conduit for supplying C0₂ rich absorption fluid to the inner core, a lean absorbent outlet at the perimeter of the cylinder, means for heat supply to at least a periphery part of the stripping unit (12; 17; 317), and a vapour outlet arranged at the inner core at a first end of the cylinder, c h a r a c t e r i z e d i n that the system further comprises a main condenser (16; 116; 316, 346) arranged in proximity of the first end of the cylinder with an inlet (20; 320) in fluid communication with the vapour outlet from the core, a liquid outlet (22; 322) in fluid communication with the core of the cylinder and a C0₂ outlet (24; 124; 324), and where the condenser is cooled by indirect contact with a coolant (8; 108; 308), that the system further comprises a reflux condenser (21; 316) arranged with an inlet in fluid communication with the vapour outlet from the core and a top vapour outlet in fluid communication with the inlet to the main condenser, a liquid inlet in fluid communication with the liquid outlet from the main condenser and a liquid outlet in fluid communication with the core, that the main condenser is rotatable arranged in proximity of the cylinder and rotatable around the same axis, and the where the inlet to the main condenser is arranged in proximity of the axis and the liquid outlet form the main condenser is arranged at the perimeter, and that the absorption fluid comprises one or more dispersed enzymes

2.

System for desorption of C0₂ according to claim 1, c h a r a c t e r i z e d i n that the means for heat supply comprises an inlet (304) and an outlet (306) for a heating medium arranged at the core, said inlet and outlet being in fluid communication with a circulation system in heat transferable contact with the stripping unit (12; 17; 317).

3.

System for desorption according to claim 1, c h a r a c t e r i z e d i n that heat is supplied throughout the whole stripping unit (12; 17; 317).

4.

System for desorption according to claim 1 or 3, characterized in that the inner part of the stripper unit (12) near the axis is a desorber part without external heat supply whereas the periphery part of the stripper unit is heated as a reboiler (14).

5.

System according to any one of the previous claims, characterized in that the nozzles (5) are arranged in the core in fluid communication with the rich absorbent.

7.

System according to any one of the previous claims, characterized in that the at least one dispersed enzyme is a carbonic anhydrase.

8.

System according to any one of the previous claims, characterized in that the stripping unit (12; 17; 317) operates at a pressure in the range of 0.6 to 0.9 bars.

9.

System according to any one of the previous claims, characterized in that the stripping unit (12; 17; 317) operates at a temperature in the range of 40 - 100 °C, preferably in a range of 70 - 95 °C.

10.

Method for desorbing C0₂ from a C0₂ rich absorption fluid comprising C0₂, absorbent and a diluent, comprising the steps of feeding the C0₂ rich absorbent to a core of a rotating cylinder comprising a stripper unit (12; 17; 317), supplying heat to at least the periphery part of the stripper unit, removing lean absorbent and diluent form the periphery of the cylinder, removing vapour comprising C0₂, diluent, and absorbent from the core, feeding the vapour to a main condenser(16; 116; 316, 346), condensing the main part of the diluent and the absorbent in the vapour, obtaining a C0₂ rich vapour stream and a liquid diluent and absorbent stream characterized in that the method further comprises returning the liquid stream from the main condenser to the core of the cylinder, and rotating the main condenser around the same axis as the cylinder, wherein at least one enzyme is dispersed in the absorbtion fluid.

11.

Method according to claim 10, characterized in that the at least one dispersed enzyme is a carbonic anhydrase.

12.

Method according to claim 10 or 11, characterized in that the stripping unit (12; 17; 317) operates at a pressure in the range of 0.6 to 0.9 bars.

13.

Method according to claim 10, 11 or 12, characterized in that the stripping unit (12; 17; 317) operates at a temperature in the range of 40 - 100 °C, preferably in a range of 70-95 °C.