Method and apparatus for treating liquid/gas mixtures

Provided is a system for treatment of and mass transfer in liquid/gas mixtures, especially removal of oxygen from seawater to be injected into deep structures to increase the production of underground hydrocarbon resources. The oxygen removal is performed by an inert stripping gas which is purified and regenerated before it is recirculated. The liquid and the gas are pumped in turbulent concurrent flow with the inert gas through one or more tube-like zones to one or more separation zones where the gas and liquid are separated. The liquid is led to the next treatment stage, while the stripping gas is returned to the prior treatment stage. The liquid flow and the gas flow are pumped and controlled independently of each other. This provides freedom to obtain optimal treatment conditions and correct amounts of transported liquid.

BACKGROUND OF THE INVENTION 
The present invention relates to the treatment of and mass transfer in 
mixtures of liquid and gases. 
In such methods liquid and gas are brought into contact with each other to 
provide mass transfer between the phases. Subsequently the phases are 
separated. 
From Norwegian Pat. No. 152,209 is known a method where an inert gas is 
employed to separate an unwanted gas from a gas/liquid mixture by 
stripping. The inert gas is separated from the liquid, regenerated and 
purified in the gaseous state by means of a catalyst and thereafter 
recirculated. The liquid is transported by simultaneously pumping and 
stripping utilizing a so-called "gas lift effect" through a vertical tube. 
Utilizing the gas lift principle for pumping and mass transfer in a 
vertical tube, a flow pattern for gas and liquid is created and this 
pattern will vary within the tube. By choosing an optimal gas volume to 
obtain satisfactory mass transfer in a tube of a length of 100 m, it was 
found that an increase in the amount of gas beyond the optimal functioning 
of the gas lift effect was not advantageous. A further increase of the gas 
volume resulted in ring flow in significant areas of the tube and reduced 
effectively the total liquid flow and the gas lift effect. Therefore there 
is no freedom available to choose gas/liquid ratios which results in both 
optimal mass transfer and efficient pumping simultaneously. 
SUMMARY OF THE INVENTION 
Thus, it is an object of the present invention to provide a method and 
apparatus for gas/liquid treatment whereby it is possible to establish 
conditions for gas/liquid contacts with freedom to choose the gas/liquid 
ratios giving a desired transported amount of liquid and simultaneously 
optimal mass transfer. 
It is a further object of the invention to provide a multistage method and 
apparatus utilizing a gasflow from one stage to the next in counterflow to 
the liquid, this reducing the circulating gas volume and increasing the 
relative amount of mass transfer in the gas. 
An additional object of the invention is to provide a gas recirculation 
stage with heat exchange, for thereby freely regulating and controlling 
the temperature in a catalyst which is used for regeneration and 
purification of the circulating gas. Hereby a greater freedom for choice 
of the temperature in the catalyst bed is obtained. 
These and additional objects of the invention are obtained with the method 
and apparatus which are described below, and the invention is 
characterized and defined by the accompanying claims.

DETAILED DESCRIPTION OF THE INVENTION 
Thus, the drawing shows a multi-stage system which is used to remove 
oxygen, or a first gas, from seawater and utilizing nitrogen as a 
treating, or a second, gas. Such an embodiment of a multi-stage system 
requires at least one compressor for gas circulation. Seawater containing 
oxygen is pumped by means of a conventional low pressure water pump or the 
like through a pipeline 1 to a first treatment stage 2,6. 
Depending upon design parameters the system may require the use of pumps 
for controlled supply of liquid in every treatment stage. Such pumps, 
however, are not shown in the drawing, which is a schematic flow sheet. 
Each stage includes a tube 2, 8, 13 of unobstructed interior and constant 
inner diameter, and formed as a hair pin with vertically extending legs. 
The gas/liquid mixture leaves the tube 2 and enters a subsequent 
gas/liquid separator 6 of traditional design. 
Gas from a subsequent treatment stage is pumped by means of a compressor 3 
under pressure into the tube 2 through a pipeline 4. The gas/liquid 
mixture from the tube 2 thereafter enters the gas/liquid separator 6, 
where gas is separated in the upper part and seawater is collected in the 
lower part. The gas is led through a tube 5 to a gas purification- and 
recirculation system, which will be described in detail below. The 
seawater which is treated in the first stage is led through a pipeline 7 
to the next treatment stage which consists of tube 8 and a gas/liquid 
separator 11. This treatment stage corresponds to the first, and the tube 
8 is supplied with gas through a pipeline 9 coming from a subsequent stage 
by a compressor 10. The liquid/gas mixture is passing through the tube 8 
to the subsequent liquid/gas separator 11. The separated gas is led, as 
mentioned above, back to the first treatment stage, while the liquid from 
the bottom of the separator 11 is led through a pipeline 12 to third tube 
13 which is designed similarly to the previous tubes. At this stage, 
purified or otherwise treated gas is supplied through a pipeline 15. The 
tube 13 is connected to a third liquid/gas separator 14 and the treated, 
purified liquid is led from the bottom of the separator 14 and through a 
pipeline 16. 
The positive pumping of the liquid through the different treatment stages 
and the circulation of gas by means of compressors 3, 10, makes it 
possible to return the gas to the tubes under pressure and also freely to 
control or regulate the ratio of liquid/gas. If the pressures in the 
different separators are conveniently chosen, a compressor between each of 
the stages will not be necessary. 
When the gas- and liquid streams are led through the tubes, they will mix 
efficiently because the liquid and gas are pumped independently of each 
other and one has freedom to choose liquid/gas ratios as well as optimal 
liquid/gas velocities. In practice this is done by regulating the 
liquid/gas ratios until near to ideal two-phase gas/liquid turbulent flow 
velocities are obtained. To ascertain that all the gas which is led into 
the last tube is completely purified and free of oxygen, the system is 
equipped with a gas recirculation system which includes a deoxidization 
unit with a precious metal catalyst. 
The impure stripping gas is led through the pipeline 5 via a compressor 26, 
a gas/liquid separator 27 and a pipeline 28 to a heat exchanger 18 
provided in a gas regenerating stage. This includes a catalyst unit 19 
with a chamber filled with dry, granular catalyst containing active 
palladium or platina. Purified, regenerated treating gas is led via a 
pipeline 20 through the heat exchanger 18 and will thus give off heat 
which is transferred to the incoming contaminated stripping gas which via 
the pipeline 28 is passed through the heat exchanger 18 to the catalyst 
unit 19. Purified gas can also be led directly to the tube 13 via a valve 
21 and a separate pipeline 22. This makes it possible to accurately 
regulate the temperature of the contaminated gas which is to be purified 
by deoxidization. Pure hydrogen gas may be used for reaction with oxygen 
in the catalyst unit. 
With the possibility for temperature regulation by means of the above 
mentioned heat exchanger, whereby it is possible to establish any desired 
temperature, there is an opportunity to employ other reduction means than 
hydrogen gas. Thus, the drawing shows a tank 23 with liquid methanol which 
via a pipeline 24 and a level indicator 25 is led directly into the 
catalyst unit 19, where it reacts with the oxygen on the catalyst. 
To remove salt containing vapour from the contaminated treatment gas, a 
scrubbing system is provided in front of the catalyst unit 19, wherein the 
gas, by the means of liquid ring compressor 26, is scrubbed and led to 
gas/liquid separator 27. The liquid is, via a liquid cooler 30 and a valve 
29, circulated back to the compressor 26. The gas/liquid separator 27 is 
provided with gas pipeline 28 which is connected to the heat exchanger 18. 
Vapour which is condensed due to compression or cooling is tapped off from 
the gas/liquid separator 27 through a valve 31. By means of a valve unit 
32, new fresh nitrogen or possibly air may be added to the system to 
replace gas absorbed by the treated water. 
EXAMPLE 
To illustrate the efficiency of this new stripping system, we have compared 
the prior known gaslift system according to Norwegian Pat. No. 152,209 
with the system according to the invention, which is described above and 
shown in the drawing. Both systems were used to remove oxygen from 
seawater. The diameters of the treatment tubes were identical, and each 
system was supplied with pure nitrogen from the same source. 
The following results were obtained with seawater: 
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Gaslift system 
Hair pin system (one-stage) 
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Zone length, m 
40 13 
Residence time, sec. 
12 4 
Result: 
O.sub.2 -content 
0.10 ppm 0.10 ppm 
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Thus, compared to the gaslift treatment, the length of the treatment zone 
as well as the residence time may be reduced to a third, and still the 
same efficiency is obtained. 
Compared to conventional stripping in filled towers or columns the 
improvements will be considerably greater, and in addition significant 
equipment weight savings will be obtained. 
The invention makes it possible to obtain and maintain conditions for 
gas/liquid treatment with practically unlimited interfacial interaction. 
Furthermore, liquid contaminants can be removed from a gas phase or gas 
contaminants removed from a liquid phase. 
Additionally the invention can be utilized both for the removal of oxygen 
from a liquid phase and for the enrichment of oxygen to a liquid phase. In 
the latter case, oxygen is circulated through the system instead of inert 
gas. Other gases, like CO.sub.2 and H.sub.2 S, can also be removed or can 
be added. 
Above is described a preferred embodiment of the apparatus. There are 
several alternatives to this. A number of parallel tubes can be placed 
side by side functioning as a tube set. The tubes can be curved or 
undulated resulting in a wave- or sinus-like configuration. Additional 
mechanical pumps can also be employed to maintain the liquid velocities at 
the desired levels. Because the system according to the invention makes it 
possible to increase the temperature in the gas/regeneration stage, also 
other reduction means than hydrogen, e.g. methanol or natural gas, can be 
employed.