Gas separation: processes – Liquid contacting – And filtration of gas
Reexamination Certificate
2002-10-25
2004-10-05
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And filtration of gas
C095S218000, C095S229000, C096S286000, C096S359000
Reexamination Certificate
active
06800115
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and a device for treatment of a gas. The invention is particularly applicable for energy recovery and/or removal of CO
2
from exhaust or flue gas.
When using gas turbines for converting thermal energy into mechanical energy, it is common to produce steam from the waste heat and use this steam to drive a steam turbine, thus increasing the overall energy efficiency. Adding steam production and a steam turbine to a gas turbine energy conversion process today involves very bulky process equipment. The boiler in particular is voluminous. The piping is also very substantial and complex. In an offshore environment as on an oil/gas platform it is therefore normal practice to install the gas turbine without any device for energy recovery from the exhaust gas since gas is cheap in such an environment while the space is at a minimum.
With the increasing focus on carbon induced global warming and introduction of carbon dioxide or energy taxes, this situation is being revaluated. Increasing the energy efficiency of the process means that less climate gases are emitted. There is also focus on the possibility of sequestering carbon dioxide from the flue gases from such energy conversion plants.
Carbon dioxide recovery demands that the temperature of the exhaust gas is reduced to lower levels than the typical gas turbine exhaust gas temperature of 500° C. Highest temperature allowable for separating carbon dioxide from gas with today's technology is in the region of 100-150° C., but more typical for exhaust gas treatment would be 20-50° C.
When treating the exhaust gas with liquid based methods (e.g. absorption), droplets will usually become present in the gas phase. Droplets may also arise when condensation occurs. Thus it is desirable to remove such droplets either to recover the liquid or to avoid an extra effluent to the atmosphere.
A conventional design for a plant to achieve heat recovery, cooling, and carbon dioxide removal from exhaust gas will embrace a separate boiler, followed by a cooler for the gas before the carbon dioxide is removed in an absorption column with a demister unit downstream, and probably a blower to overcome the ensuing pressure drop. These process units are bulky and require requiring instrumentation and control, in addition to a complex piping system to connect the various units.
It is known from the literature (e.g. Sawyer's Gas Turbine Engineering Handbook, 3rd ed, vol 11, chapters 7 and 14) how the energy recovery problems in gas turbine cycles may be solved technically, but the known solutions are costly in general, and too bulky for applying offshore. Furthermore, these technical solutions are associated with finite pressure drops that will lead to higher gas turbine exit pressure with reduced energy efficiency as a result. Alternatively some kind of blower can be installed to overcome the pressure drop.
It is further known from the literature (e.g. W. W. Bathie, Fundamentals of Gas Turbines, 2nd ed., Wiley, 1996, chapter 8) that gas turbine blades are cooled by internal water flow. The technique as it stands is designed to keep the blade surfaces cool enough to avoid material failure, and is not suitable for efficient heat transfer or energy recovery.
Also known from the literature (e.g. Kohl and Nielsen, Gas Purification, 5th ed., Gulf Publishing, 1997) is how carbon dioxide may be removed from gas by various means. Removal of carbon dioxide from flue gas represents a special problem due to lack of available pressure and the presence of contaminants like nitrogen oxides and oxygen in the gas. One such application is described in the literature (Pauley et al., Oil & Gas Journal, May 14, 1984, pp 84-92). Many developments to improve such technology have in recent years been invested in (see e.g. Greenhouse Gas Control Technologies, edited by Eliasson, Riemer and Wokaun, Pergamon, 1999).
GB Patent No.1,332,684 teaches how heat exchangers may be attached to the rotating assembly of an air compressor combined with combustion chamber and gas turbine. It is shown how heat from the gas turbine exhaust may be used to preheat combustion air via an intermediate heat transfer medium operating in a closed loop including heat exchanger matrices placed in the exhaust gas stream and the air, respectively. The disadvantages of this invention include the creation of a pressure drop in the exhaust gas through using said gas to hydraulically drive the heat exchange assembly, and a limited potential for heat recovery. The physical placement of the heat transfer facility and the created pressure drop will also interfere with the design of the gas turbine assembly. Furthermore, the heat exchanger can not be mounted as a retrofit.
The patents DE 33 26 992, EP 262 295, and EP 556 568 deal with various features of what is essentially the same invention or developments thereof. They teach how an apparatus may be placed in the exhaust duct from any combustion engine, and how that apparatus can be formed to recover heat and kinetic energy from the exhaust gas. Furthermore, they teach how steam may be produced to feed a steam turbine, and how this apparatus may be designed to sit on one rotating axle from which power is transferred to the combustion engine's crankshaft. This invention is intended to increase the efficiency or power output from a combustion engine. The pressure drop induced in the exhaust gas stream is a disadvantage, but less so for a combustion engine than a gas turbine. The narrow flow path of the exhaust gas with or without turbine blades leads to considerable pressure drop. A further disadvantage is that steam is formed in a single chamber such that only one temperature level is possible, thus effectively configuring the heat transfer as cross-flow with the limitation this gives with respect to heat recovery. It is yet a further disadvantage that the temperature of the exhaust gas can not be reduced to a level where carbon dioxide removal can be performed.
WO 98/30486 describes how a compact heat pump may be constructed by combining transport facilitating features like compression and pumping on the same axle. This patent application also describes a heat exchanger in the shape of a spiral wound in a tight annular space where one medium flows inside the spiral tube and the other medium flows in the annular space. This solution will have difficulty in coping with the large gas flows normally associated with handling exhaust gas or large gas based processes.
U.S. Pat. No. 5,363,909 teaches how a gas can be piped into a chamber in which a rotating packing is placed, and where the gas is forced to flow radially towards the center of said packing while being in contact with a liquid moving radially outwards. Cooling or heating can take place by direct heat transfer, but the direct contact would make it impossible to produce useful steam or any other useful heat medium. And it represents separate equipment and a significant pressure drop since the said packing will impart some fan action on the gas trying to force it outwards, and this action must be overcome by extra pressure drop.
SU 1189473 teaches how a gas flow through a column in which concentric discs sloping towards the axis are fixed on the inside wall, and in the center is placed a rotating entity with discs sloping outwards. Liquid flows over the rotating discs and is flung to the wall from where it flows over the static discs back on to the rotating disc below. The apparatus is intended for mass transfer, but by letting the liquid evaporate, cooling of the gas may be effected. Again it is impossible to make any use.
SU 359040 teaches how a gas and a liquid can be contacted by letting the gas flow through a horizontal tank at the bottom of which there is a liquid pool. Liquid is splashed into the gas phase by rotating entities dipping into the liquid and flinging it out in the gas. And the rotating entities are driven by the flow of the gas. Mass transfer may be effected, as above, but the device is of little use in recover
Norsk Hydro ASA
Smith Duane S.
Wenderoth , Lind & Ponack, L.L.P.
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