The present invention relates to an apparatus and a method for removing and recovering CO2 from flue gases. Furthermore the present invention relates to an apparatus and method for desorption of CO2 from a liquid absorbent.
During the later years there has been an increased focus on CO2 capture due to the environmental aspects associated with the release of CO2 to the atmosphere.
The conventional method for removing CO2 from flue gas is by use of a standard absorption-desorption process. In this process the flue gas has its pressure boosted by a blower either before or after an indirect or direct contact cooler. Then the flue gas is fed to an absorption tower where it 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 flue gas from the CO2 removal section. The absorbent, rich in CO2 from the absorber bottom is pumped to the top of a desorption column via a heat recovery heat exchanger rendering the rich absorbent pre-heated before entering the desorption tower. In the desorption tower the CO2 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 CO2 although some of the steam condenses to provide desorption heat for the CO2. Water and absorbent following CO2 over the top is recovered in the condenser over the desorber top. Vapour is formed in the reboiler from where the absorbent lean in CO2 is pumped via the heat recovery heat exchanger and a cooler to the top of the absorption column.
EP 0 020 055 A1 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 CO2 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. Further the present invention aim to reduce the thermal degradation of the absorption solution by limiting the residence time of the absorption fluid in the desorber.
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 gasses 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 CO2.
The present invention can be utilized with any type of liquid CO2 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). 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.
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 present 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.
According to the present invention, the absorption liquid has a very short residence time in the rotating desorber wheel. Due to this, thermal degradation of the absorbent solution is expected to be significantly reduced as compared to conventional solutions.
These and other objectives are obtained by an apparatus according to claim 1 and a method according to claim 9. Other advantageous embodiments and features are set forth in the dependent claims.
The present invention will now be disclosed in further detail with reference to the enclosed figures, wherein:
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
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.
A third embodiment of the present invention is shown in
In
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.
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 CO2 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. The vapour phase including the CO2 leaves the reboiler near the centre into conduit 320 and is then 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 m2/m3. Other dimentions and configurations may of course equally well be used. A fine metal mesh with wire diameter 0.5-1 mm diameter gives specific surface areas above 1000 m2/m3 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 to 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 elements 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
The configuration of the rotating desorber wheel with two mirrored desorber and condenser sections on each side of the axial centre plane shown in
Splitting the reboiler in two sections makes it possible to handle large volumes of absorbent, more than 250 liter per second, which is considered to be a very large volume.
Yet another advantage is that the desorber section is the compact part of the rotor with respect to the mass of steel per unit volume. Splitting the reboiler in two sections and installing them as close as possible to the main bearings of the shaft reduces the mechanical loads of the rotating equipment significantly.
Still another advantage of providing symmetry according to the present invention is that the rotating desorber easily can handle varying volumes of absorbents. A gas power plant or a coal power plant does not operate at 100% all the time and the flue gas volume that needs to be cleaned for CO2 will vary. The volume of liquid absorbent will thus vary. Since the liquid absorbent is equally distributed to the two reboiler sections, the problems with weight balance is not an issue.
Number | Date | Country | Kind |
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20092629 | Jul 2009 | NO | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/NO2010/000283 | 7/12/2010 | WO | 00 | 3/26/2012 |