Gas separation: processes – Liquid contacting – And deflection
Reexamination Certificate
2000-03-15
2001-08-28
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And deflection
C095S235000, C095S236000, C096S323000, C261SDIG003
Reexamination Certificate
active
06280505
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fluid separation systems. It is particularly concerned with the selective removal of a component or components from a mixture of gases using liquid solvent and is more particularly concerned with the absorption of acid gases such as CO
2
, NO
x
, H
2
S, oxides of sulphur etc. from natural gas and from combustion gases.
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 a sour gas mixture which does not suffer from the disadvantages of the prior art.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a method of removing acid gas components from a gas mixture which comprises: bringing the gas mixture into contact with sea water; subjecting the gas mixture and sea water to turbulent mixing conditions thereby causing the acid gas to be absorbed by the sea water; and separating a gas phase and a liquid phase.
There may also be no need to treat the liquid phase to remove the absorbed acid gas components, since sea water is plentiful and does not need to be recovered for re-use.
This would mean that no downstream regeneration section is necessary. The presence of carbon dioxide in sea water does not represent an environmental hazard in the same way as atmospheric carbon dioxide and since the concentrations are relatively low, the carbon dioxide remains below the saturation capacity of sea water. The carbon dioxide-containing sea water can therefore be conveniently disposed of offshore where it will quickly be dispersed and will therefore have no detrimental effect on the environment.
If the sea water does absorb harmful acid gas components, they can be neutralised with suitably selected reagents. Again then, the gas-containing sea water can be disposed of offshore.
The method is particularly applicable to the removal of acid gases, especially carbon dioxide from combustion gas and from natural gas.
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. The efficient mixing means that absorption can take place very rapidly. The mixing system used is simple and inexpensive compared to prior art systems, and requires no solvent regeneration.
Preferably, the method is carried out as a continuous process with the gas mixture and sea water flowing co-currently. The co-current flow eliminates the problems associated with foaming, since separation can easily be effected downstream of the contactor.
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 gas mixture is supplied to the tube, optionally directly, and the sea water is supplied to the vessel, and so the gas stream draws the sea water into the venturi and the two phases are mixed. In another regime, the gas mixture is supplied to the vessel and the sea water is supplied to the tube, optionally directly whereby the gas mixture is drawn into the venturi by the sea water and the two phases are mixed. In a third regime, the sea water and the gas mixture are supplied to the vessel, the sea water being supplied to a level above the level of the outlet, whereby the gas mixture is forced out through the outlet via the tube, thereby drawing the sea water into the venturi so that the two phases are mixed. In a fourth regime, the gas and liquid are supplied via separate pipelines into a common mixing point which can be made up as an ordinary pipe junction or as a venturi device as explained in regimes 1 to 3, or as a pipe junction followed by a static mixer device. In the contact pipeline, one or several secondary mixer stages can be installed to maintain the gas/liquid mixing efficiency. In all cases, the sea water absorbs the carbon dioxide and other acid gases.
Preferably, the gas mixture and the sea water are formed into a homogeneous mixture in the contactor, and the homogeneous mixture may be 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 or any suitable gas-liquid separator.
In an alternative arrangement, a portion of the sea water, after extraction of CO
2
, is recycled to the contractor. This serves to increase the CO
2
loading of the sea water. 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
2
loading of the sea water solvent, it is possible to reduce the amount of sea water that needs to be pumped out to sea.
According to a more specific aspect of the invention, there is provided a method for removing carbon dioxide from a combustion gas or natural gas which comprises: supplying the gas to a turbulent contactor; supplying sea water to the contactor; subjecting the gas and the sea water to turbulent mixing in the contactor to form a homogeneous mixture; allowing carbon dioxide from the gas to be absorbed by the sea water; 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; and disposing of the liquid phase offshore.
Again, a portion of the sea water, after extraction of CO
2
may be recycled directly to the contactor.
In instances involving combustion gas which might be at a low pressure, the sea water is pumped to the contactor and thereby draws the combustion gas with it through the contactor. The system may include a pump arranged to supply sea water to the liquid inlet of the contactor. In instances involving natural gas which might be at a high pressure, the gas is conveyed to the contactor at a high pressure and thereby draws the sea water with it through the contactor.
The invention may be considered to extend to the use of sea water as an absorbent for acid gas components from natural gas and combustion gas. This is preferably effected by forming a homogeneous mixture of the gas mixture with the sea water in a turbulent contactor, allowing the acid gas to be absorbed by the sea water, and subsequently separating a gas phase and
Hanssen Per H.
Linga Harald
Nilsen Finn P.
Sigmundstad Martin
Torkildsen Bernt H.
Den Norske Stats Oljeselskap A.S.
Patterson, Thuente, Skaar & Christensen, P.
Smith Duane S.
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