Gas separation: processes – Liquid contacting – And deflection
Reexamination Certificate
2000-03-15
2001-09-04
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And deflection
C095S235000, C095S236000, C096S323000, C261SDIG003
Reexamination Certificate
active
06284023
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the removal acid gases such as CO
2
, NO
x
, H
2
S, oxides of sulphur etc. from natural gas.
BACKGROUND OF THE INVENTION
Conventional systems for the absorption of acid gases employ a liquid solvent; typical solvents include amines such as methyldiethanolamine (MDEA), monoethanolamine (MEA) or diethanolamine (DEA), and mixtures of solvents. These solvents absorb CO
2
, NO
x
, H
2
S and other acid gases. The solvent is contacted with the sour gas mixture (gas mixture including acid gases) in a column which may be a packed column, a plate column or a bubble-cap column, or a column with some other form of contact medium. In these systems, the gas and liquid streams flow countercurrently.
The prior art absorption systems suffer the disadvantage that in order to achieve a significant degree of gas/liquid contact, the columns have to be large and their operation is hampered by excessive foaming. In addition, the subsequent stripping section which removes the acid gas from solution must also be large, to handle the large volume of solvent used. Since the operation normally takes place under high pressure and the fluids involved are highly corrosive, the capital costs of the large columns and subsequent stripping section is high. Furthermore, operating costs and maintenance costs are high.
It is an object of the present invention to provide a system for removing acid gas from natural gas which does not suffer from the disadvantages of the prior art, preferably with a high degree of efficiency and more economically than in existing methods.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a method of removing carbon dioxide and other acid gas components from natural gas which comprises: bringing the natural gas into contact with a liquid including a solvent or reagent for the carbon dioxide and other acid gases; subjecting the natural gas and liquid to turbulent mixing conditions thereby causing the carbon dioxide and other acid gases to be absorbed by the solvent or reagent; and separating a gas phase and a liquid phase.
The invention also extends to the apparatus for carrying out this method.
The turbulent mixing is very intense and results in extremely efficient gas liquid contact. The mixing regime is preferably turbulent shear layer mixing. The liquid entrained in the gas may be in the form of droplets for gas continuous fluid phase distribution. is The efficient mixing means that absorption can take place very rapidly and in a relatively small amount of solvent compared to that required in conventional absorption columns. This in turn means that the liquid duty in the equipment is dramatically reduced resulting in a consequential reduction in the size of any downstream regeneration section. At the same time, the mixing system used is simple and inexpensive compared to prior art systems, leading to reduced costs. Finally, an efficiency of approaching 100% for the removal of acid gas can be achieved for certain applications.
In addition, conventional absorbtion methods involve the evolution of heat which must then be removed from the system. While the method of the invention is capable of operation with a relatively low pressure drop across the mixing means, when greater pressure drop is employed, a cooling effect is achieved and this may render the need for additional cooling unnecessary.
The absorption may be achieved by simply dissolving the gas or by way of a chemical reaction with the solvent.
Preferably, the method is carried out as a continuous process with the natural gas and liquid flowing co-currently. The co-current flow eliminates the problems associated with foaming, since separation can easily be effected downstream of the contactor. Preferably, the method includes the step of treating the liquid phase to remove the absorbed acid gas components.
The turbulent mixing may be achieved by any convenient means, such as an ejector or a jet pump or more preferably in a turbulent contactor including a gas inlet, a liquid inlet, an outlet leading to a venturi passage and a tube extending from the outlet back upstream, the tube being perforated and/or being spaced from the periphery of the outlet.
One suitable contactor is a mixer supplied by Framo Engineering A/S and is described in EP-B-379319.
Preferably, the tube is located in a vessel, the vessel including the gas inlet, the liquid inlet and the outlet. In one possible regime, the natural gas is supplied to the tube, optionally directly, and, the liquid is supplied to the vessel, and so the natural gas stream draws the liquid into the venturi and the two phases are mixed. In another regime, the natural as is supplied to the vessel and the liquid is supplied to the tube, optionally directly, whereby the natural gas is drawn into the venturi by the liquid and the two phases are mixed. In a third regime, the liquid and the natural gas are supplied to the vessel, the liquid being supplied to a level above the level of the outlet, whereby the natural gas is forced out through the outlet via the tube, thereby drawing the liquid into the venturi so that the two phases are mixed.
Preferably, a solvent absorbs the carbon dioxide and other acid gases. Alternatively, a reagent reacts chemically with the carbon dioxide and other acid gases. Conceivably, the reagent is a biological reagent which removes the carbon dioxide and other acid gases biologically. In one variant of the invention, a plurality of acid gas components are absorbed by a plurality of respective solvents or reagents.
Preferably, the natural gas and the liquid are formed into a homogeneous mixture in the contactor, the homogeneous mixtures being cooled prior to separation into a gas phase and a liquid phase. Preferably, the cooled homogeneous mixture is separated into a gas phase and a liquid phase in a hydrocyclone. Preferably, the solvent in the liquid phase is subjected to a regeneration treatment to remove the absorbed acid gases. Preferably, the regenerated solvent-containing liquid phase is recycled to the contactor. Preferably, the regeneration is carried out by heating and/or by flashing off the absorbed gas component in a flash tank. Preferably, the post-mixing cooling and the regenerative heating are achieved, at least in part by mutual heat exchange.
In one alternative arrangement, a portion of the solvent, after extraction, is recycled to the contactor directly, without regeneration. Thus, part of the CO
2
-loaded solvent by-passes the regeneration section. This serves to increase the CO
2
loading of the solvent. It should be noted that optimisation of the process may not necessarily relate to the removal efficiency in terms of mole fraction of CO
2
removed, but rather the energy consumption required per unit mass of CO
2
removed. By increasing the CO
s
loading of the solvent, it is possible to reduce the amount of solvent that needs to be handled by the regeneration section.
In the case of CO
2
, as the initial solvent loading level is increased, the CO
2
absorption efficiency drops. However, a considerable fraction of total liquid flow rate can be recirculated directly from the gas liquid separated before the drop in CO
2
removal becomes significant.
According to a more specific aspect of the invention, there is provided a method for removing acid gases from a natural gas which comprises: supplying the natural gas to a turbulent contactor; supplying a liquid including a solvent for the acid gases to the contactor; subjecting the natural gas and the liquid to turbulent mixing in the contactor to form a homogeneous mixture; allowing the acid gas to be absorbed by the solvent; cooling the homogeneous mixture; separating the cooled homogeneous mixture into a gas phase and a liquid phase in a hydrocyclone (or any other gas/liquid separator); removing the gas phase; subjecting the solvent in the liquid phase to a regeneration treatment to remove the absorbed acid gas; and recycling the regenerated solvent-containing liquid phase to the contactor.
Again, a portion
Hanssen Per H.
Linga Harald
Nilsen Finn P.
Sigmundstad Martin
Torkildsen Bernt H.
Den Norske Stats Oljeselskap A.S.
Patterson, Thuente, Skaar & Christenesen, P.
Smith Duane S.
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