CA1222200A

CA1222200A - Deaeration of water

Info

Publication number: CA1222200A
Application number: CA000436415A
Authority: CA
Inventors: Norolf Henriksen; Sven A. Kjýberg; Erik Fareid; Tor Christensen
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 1982-09-13
Filing date: 1983-09-09
Publication date: 1987-05-26
Also published as: AU1862183A; GB2127711B; NO152209B; NO152209C; AU573534B2; DK412283A; OA07529A; GB8324157D0; GB2127711A; EG17112A; DK412283D0; PH18841A; NO831561L; BR8304891A

Abstract

Abstract This invention concerns degassing of water, especially deoxygenation of seawater which is to be used as injection water in underground oil reservoirs to obtain a higher degree of hydrocarbon recovery.

A main objective with this invention is to achieve a new and improved process that eliminates the consumption of stripping gas and at the same time attains the highest possible degree of oxygen removal without polluting the water.

According to the invention this is obtained by pumping and degassing water in a system with recirculating inertgas.

Degassed water is separated from the inert gas; and the latter is routed through a zone for purification and regeneration, as hydrogen is introduced into the gas conaining oxygen from the gas-lift area, and the gas-mixture subsequently flows through a catalytical combustion zone where hydrogen and oxygen are burned to water, whereafter the purified gas is recirculated.

Description

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This invention concerns the removal of gases from water, in par-ticular the deaeration of seawater which will be injected into deep structures in order to increase the extractable fraction of subsurface hydrocarbon resources.

In this connection it is important to remove the oxygen from the seawater in order to reduce corrosion rates and prevent growth of aerobic bacteria which will impede the flow of hydrocarbons from the structure.

This problem has been known for a long time and different solu-tions have been proposed. The great majority are based on one of two principles, degassing by reduced pressure, degassing by gas-stripping or a combination of these two. Large quantities of seawater are injected, and the degassing has to be satisfactory at all times. Untreated seawater is usually saturaked with oxygen and it may cause severe corrosion if the deoxygenation treatment is not working satisfactory.

It is therefore necessary to put yreat demands on the quality of the deoxygenation process; large and expensive plants are required.

Vacuumdegassinq is often considered unsuitable because of the very compllcated and heavy equipment. Stripping with natural gas has therefore often been preferred in practice. Natural gas is easily accessible in great quantities offshore has so far been used as a stripping agent for deoxygenation. After the stripping process, however, the gas is not suitable for sale and has to be flashed.

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Pollutants in the natural gas, including CO2 and H2S, reduce the quality of the water and this is also a disadvantage.
Furthermore~ stripping towers using natural gas have a high gas consumption, and the weight and volume are almost the same as for vacuum towers.

A stripping plant employing recirculated nitrogen gas is de-scribed in U5-patent number 4~017.276 ~Norwegian patent Applica-tion No. 77 2185). According to this patent, deoxygenation is performed in a stripping tower using nitrogen gas. The oxygen gas is removed from the nitrogen usings low temperature frac-tionation. The gas consumption will hence be low.

The fractionation device however, is bulky and expensive, and the cooling device consumes large amounts of energy. Expensive precautions are required in order to obtain sufficiently low oxygen levels. Nitrogen stripping-gas from gas fractionting generally contains 10 ppm to about 100 ppm oxygen and must be further purified before recycling.

Seawater used for injection is usually lifted from below sea level by submersible centrifugal pumps. It has been proposed to use a gas-lift instead. This is a simple system. It normally has a higher reliability than mechanical pumps, and it is capable of pumping the water to elevations high above the surface.

The gas-lift pipes will under certain conditions have a stripping effect, causing removal of oxyyen from the water. Natural gas has been used in gaslift systems. A great drawback, however, is the demand for great amounts of natural gas. The consumption is at least three times higher than for stripping towers. Furthermore, H2S and CO2 will be transferred to the water and acidify it.

Thus, it is the main objective of this invention to obtain a new and improved process to remove oxygen by gas treatment, achieving highly efficient and more reliable oxygen removal without consump-tion of stripping gas.

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A further object of this invention i5 to obtain a nsw and improved procedure for water supply and at the same time to achieve an optimum oxygen removal.
Another object is to minimize the space requirements and the weight of the equipment. Finally, it is an object of -the invention to avoid -the acidification of the treated water.
In a process according to the invention, for deoxidizing water by intimately mixing an inert gas with said water, whereby said inert gas removes oxygen from said water, and separa-ting the thus deoxidized water and said inert gas thus enriched with oxygen, the improvement comprising: mixing hydrogen gas with said oxygen enriched inert gas' passing the resultant gas mixture into a catalytic reaction zone and therein catalytically reacting said hydrogen and oxygen at atmospheric pressure -to form water;
removing the thus purified inert gas and said water from said catalytic reaction zone and directly mixing said purified inert gas and said water wi-th additional water to be deoxidized; and conducting the entire said process without additional drying, cooling, purification or rec-tiEication of the inert gas.
A main feature of this invention is that water is degassed in a gas/water strippiny system using a recirculating inert gas, which is regenerated ancl purified in the gaseous state.
According to one preferred embodiment the water is pumped and degassed in a gas-lift system.
Hydrogen is introduced to react with the oxygen in the gas from the gas-lift. The reaction takes place on a catalyst in the recirculating system.

~.;2 i2~2~13 - 3a -Other features characteri~ing the invention are des-cribed in the following and also shown in the figures 1 to 6 where:
Figure 1 is a schematic drawing of the system when utiliæing a stripping tower.
Figure 2 shows a simple flowsheet with amounts (volume parts) for -the individual stream.
Figure 3 is a schematic drawing of the system when utilizing gas-lift for simultaneously pumping and degassing.
Figure 4 is an enlarged sketch of the bottom section of the gas-lift system.
Figure 5 shows a cut through the same section.
Figure 6 shows examples of different flow-patterns.
Figure 1 shows a stripping column or -tower 1 equipped with perforated plates 2, raschigrings or the like. Seawater containing oxygen is conducted through the pipe 3 via a low pressure pump 4 to the top of the stripping tower.

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Thus, the water passes downwards countercurrent to upwards streaminq stripping gas, which is introduced at the bottom throuqh the pipe 5. In the stripping tower the oxygen is removed by the stripping gas as well known, so that the sea water is subs~an~ially free of oxygen when leaving the stripping zone.
Oxygenrich stripping gas leaving the top of the tower is then led through the pipeline 6 and mixed with a stream of pure hydrogen from the pipe 8.

The pure hydrogen gas (99.9%) is added in measured stoichio-metric amounts or in excess o~ the oxygen content in the stripping gas from a water electrolyzer 7 or from another suitable hydrogen generator through the pipe 8. Care is taken to obtain adequate mixing of th~ gases. The gas mixture is then with a suitable pumping means in the form of a compressor 9 or the like, led into a catalyzing chamber 10 filled with dry granulated catalyst consisting of active palladium or platinum precipitated on an alumina base. This will spontaneously ignite the dry gas mixture at a higher temperature, depended on the water content in the gas. The combustion takes place at atmos-pheric pressure, and hydrogen and oxyqen burn completely gene-rating heat which will evaporate the generated water and heat the stripping gas. The warm steam will go with the oxygenfree stripping gas back to the tower~

The remaining stripping gas will now be substantially free of oxygen and may immediately be recirculated through the pipe 5, which is connected to the lowermost part of the stripping tower~

Due to the fact that some gas is consumed in the system being ab-sorbed in the oukgoing sea water, minor amounts of fresh stripping gas is added through pipe 11, so that it continously will be established an optimal ratio between stripping gas and sea water.
This stripping qas will also be led through the catalytic gas com-bustion chamber 10 before it is conducted to the stripping tower.

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This ensures that all the gas used is efficiently puri~ied and ree of oxygen, so that the stripping tower continously i5 supplied with stripping gas, which is containing less than 1 ppm oxyg~n. The gas mixture which is led to the combustion chamber should preferably be of a tempera~ure between 10-40C.

A purer stripping gas will have greater power to absorb oxygen from the water to be deoxydized. Due to the fact that the puri-fication step results in an end pollution product which is water, it will not be necessary to remove such product in a separate product stream (bleed), because this can be led into the main stream of the system, which is in fact water. Further, this also results in that energi generated by the catalytic combustion will be absorbed by the system and utilized.

The resulting purified sea water will, as mentioned above, be collected at the bottom of the tower 1 and will be injected into the hydrocarbon containing structure through the pipeline 12 by means of high pressure compressors 13, thereby to enhance the production of hydrocarbons from the deeply situated production wells.

Example:

Sea water containing lO ppm 2' corresponding to an 2-content of 70 l/h, is added to the top of a vertical stripping tower in amounts of 10 m3/h and at a temperature of 20C.
140 l/h H2 ls added through pipe 8 and mixed with 50 m3/h N2 stripping gas which circulates in the system.

Throuyh the pipe 11 is added 70 l/h nitrogen as fresh stripping gas to replace the gas consumed. The resulting gas mixture con-taininq 50 m /h N2 and 140 l/h H2 and ,0 l/h 2' is pre-heated to 2S C and is led to the catalyst chamber for cataly-tic combustion of the H2 ~ 2 components. The catalytic combustion and formation of steam will increase the temperature in the system without the introduction of external energy.

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The purified nitrogen containing < l ppm 2~ is led to the bottom of the stripping tower at a temperature of 45Co Deoxidized sea water from the bottom of the tower contains < l ppm 2~ and is without further trea~ment ready to be injected into the hydrocarbonrich structure.

With a simple flow sheet, Fig. 2, is illustrated the asnounts incorporated in the individual gas- and liquid streams of the system. The abbreviation 5W denotes sea water, while the refe-rence number 1 and 10 refer to the stripping tower and the deoxidizer respectively.

The embodiment of the invention using gas-lift will now be described. The advantages of "gas-lift" with circulating inertgas as compared to "gas-lift" with single flow at natural gas through the pipes can briefly be summarized as follows:

- Loss of natural gas is avoided.
- Contamination with H2S and C02 is avoided.
- Dissolution of natural gas in water is avoided.
- Amount oE gas to be flared and hence pollution is reduced.
- Possibilities for better oxygen removal and/or pumping performance is achieved by greater freedom in selecting the gas flow.
- Considerably higher degree of safety is achieved because of reduced quantities of flammable gas in the system.
- Reduced corrosion rates in the gas-lift pipes (avoiding H2S and C02 in combination with 2) Foaming is reduced - Assures a constant and known gas quality as well as molecular weight, composition, amount trace of elements and condensable compounds.
Fig. 3 shows a basic gas-lift design with circulating inertgas according to the invention.

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Inertgas containing stripped oxygen and small quantities of hydrogen from a hydrogensource 18 is compressed in a com-pressor 19. Hydrogen and oxygen are burned to water in a deoxo catalyst chamber 20. Inertgas, almost free from oxygen, i5 transferred through tube 23 and down to the bottom section 24 where gas and liquid are mixed. The gas/liquid mixture in the tube is lifted to a higher level than the water surface since the mixture in the tube has lower density than the liquid. The difference between the static pressure in the bottom section 24 and the static pressure caused by gas/liquid mixture provides tlle driving force.

The mixture rises up through a tube 25 which represents the "gas-lift" where the oxygen removal from the water takes place.
The gas-liquid mixture from tube 25 is separated in a separa-tor 26, for example a cyclone. Most of the remaining fluid in the gas can be separated in an additional separator 91, for example a demistor.

The oxygen is removed by means of the additional hydrogen supply l8 in the deoxodevice 20. Make~up gas, for example air, is in-troduced in inlet 71 in order to maintaion the pressure in the system. Instead of hydrogen other reducible gases may be used.

Water from aseparator 26 is passed to a possible further treat-ment 10 (for example ircorporation of a O2-scavenger) and thereafter to consumption.

The capacity of the gas-lift system can easily be made suffi-ciently large to cover any water consumption in addition to the water required for injection, without significant increases in the gas consumption.

The pure hydrogengas (99,9%) is added in stoichiometric amounts or in slight excess to the oxygen containing inertgas, from an electrolyzer 18 or another type of hydrogen generator through a pipe to be mixed into the gas from the gas-lift.

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The gas-mixture is r by means of the compressor r passed into the catalyst chamber which is filled with a dry catalyst consisting of active palladium or platinum precipitated on an aluminabaseO
This will spontaneously ignite the dry gas-mixturel The combustion occurs at a pressure which is sufficiently high to force the gas down to the bottom section 24. Hydrogen and oxygen are comp~etely burned generating heat that evaporates the produced water and also heats the inertgas.

The remaining inertgas is now almost completely free of oxygen and may directly be recirculated to the gas-lift.

Figures 4 and 5 show the bottom section of the gas-lift in more detail. The gas-lift technology is considered well-known and the design of the equipment will therefore not be described or showed on the drawing. The circulating inertgas is passed under pressure through the line 23 to the bottom section 24, which is a jacket 14' around the gas-lift tube 25. The gas-lift tube 25 is provided with a number small openings evenly distributed around the circumference. If chemical àddition is needed a further tube 13' can be provided below the bottom section.
Underneath the tube 25 is situated a straining device 21.

Fig. 6 is an example of flow patterns formed inside the gas-lift tube which depends on gas/liquid volume ratios.

Since the quantity of circulating inertgas may be chosen freely, it is always possible to choose the gas-volume that gives the best combinations of oxygen removal and pumping efficiency. A
possible flowpattern is shown in Fig. 6 for a lift-height of 40 m above the water surface with inlet 60 m below the surface and a tube diameter 0,3 m.

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Section 31 shows so-called annular 10w.

Section 41 shows so-called whispy annular flow.

Section 51 shows so-called churn flow and Section 61 shows so-called plug flow.

The special and important features of a "gas-lift" with circulating inertgas according to the invention are:

- Oxygen is removed in the pumping operation itself, pumping and oxygen removal achieved simultaneously~
- The low oxygen content in the water reduces the required amount of oxygen scavenger.
- The combustion of oxygen with hydrogen creates water which is already a part of the system.
- High reliability; the system has few moving parts.
- The system consists of well-known elements and technology.
- The system is not harmful to the environment, no pollution of air and water.
- The system requires little maintenance.
- Operational weight is approximately equal to dry weight and is substansially lower than Eor vacuum and for stripping towers.

Example The gas-lift system has been tested in a pilot-plant:

0,5 m seawater an hour was pumped by the gas-lift system which lifted water from 4 m below to 6 m above seawater level using 2~5 Nm nitrogengass an hour.

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The compressed and oxygenfree nitrogengas was fed into the bottom of a gas-lift tube consisting of a single tube with a diameter of 2,1 cm.

The resulting water/gas-mixture was lifted up through the gas-lift tube and at the same time the oxygen diffused from the water phase to the gas phase. The oxygen content in the water was reduced by approximately 90%.

In a separate pilot~plant nitrogengas with more than 1000 ppm oxygen and stoichiometric volumes hydrogen was supplied to a Pd-catalyst. Downstream of the catalysts the oxygen content was reduced to less than 1 ppm. The apparatus ran for three months without permanent reduction in the catalyst activity.

The experiments demonstrated that scale-up of the gas-lift and catalyst systems according to well-known principles will result in an operational system with acceptable efficiency.

Final results:
Sufficient quantities of seawater can be pumped up to for example 40 m above the surace and the oxygen content can be reduced to 0,1 ppm or less in one and the same operation.

The process according to the invention is especially advantage-ously due to the complete combustion of oxygen which is achieved in the regeneration step. The inert gas will contain < 1 ppm oxygen, while normally 10-100 ppm~ This improves the efficiency of the stripping resulting in nearly oxygenfree sea water after the treatment.

The above methods are only ~o be considered as preferred embodi-ments of the invention. It is therefore possible within the spirit and scope of this ivention to make use of another cataly-tical combustion than the Pd-based combustion discussed earlier.
Other inertgases than nitrogen and other methods for purifying the inertgas may also be taken into consideration.

1 ~ 1Jq ~3~3~

Furthermore, various designs of the ga~lift-tubes and bottomH
section are possible. Both the tube diameter, the height arld number of tubes are variables. Depending on the conditions required to create optimum flow patterns and pumping efficiency combined with optimum oxygen removal, one may freely change the volume of input gas without extra expenses.

Furthermore, the form of the stripping tower as well as the interior of the same will vary, and it is possible to use two or more strippin~ columns joined together~ '~ith the minor amounts of supplemental nitrogen which is required, it will normally be sufficient to use liquid nitrogen stored in vacuum isolated containers. Airfractionating plants or N2-generators may also be used if the need for fresh nitrogen gas is high, i.e. when great volumes of sea water have to be deoxidized. But also in these circumstances it is necessary to circulate the gas through the catalytic combustion zone before it is added to the bottom of the stripping tower~

Furthermore, other stripping gases than nitrogen, as used according to the examples, may also be employed. Natural gas e.g. is normally available and may be used for stripping. It is recirculated after leaving the catalytic regeneration and possibly a purification step, and this eliminates the need for flashing the gas off. Other inert gases may also be employed e.g. argon~

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
l. In a process for deoxidizing water by intimately mixing an inert gas with said water, whereby said inert gas removes oxygen from said water, and separating the thus deoxidized water and said inert gas thus enriched with oxygen, the improvement com-prising:
mixing hydrogen gas with said oxygen enriched inert gas;
passing the resultant gas mixture into a catalytic reaction zone and therein catalytically reacting said hydrogen and oxygen at atmospheric pressure to form water;
removing the thus purified inert gas and said water from said catalytic reaction zone and directly mixing said purified inert gas and said water with additional water to be deoxidized; and conducting the entire said process without additional drying, cooling, purification or rectification of the inert gas.
2. A process according to claim 1, wherein the water is simultanesously pumped and deoxidzed using a gas-lift system with circulating inert gas.
3. A process according to claim 1, wherein the water to be deoxidized is stripped by the circulating inert gas.
4. A process according to claim 1, wherein pure nitrogen gas is used as the circulating gas.
5. A process according to claim 1, wherein said hydrogen is added in stochiometric amounts and reacts stochio-metrically with the oxygen at atmospheric pressure in the presence of a catalyst.
6. A process according to claim 4, wherein new inert gas replacing the consumed gas is added to the inlet of the combustion section of the circulation system.
7. A process according to claim 1, wherein the oxygen containing inert gas is supplemented by new inert gas to replace gas dissolved in the water, before the resulting mixture is lead through the combustion zone.
8. A process according to claim 3, wherein the stripping is performed in a stripping tower with the gas passing countercurrent to the water.
9. A process according to claim 5, wherein the gas mix-ture is introduced into the reaction zone at a temperature of 10° - 40° under conditions for catalytic reaction.
10. A process according to claim 1, wherein the purified inert gas from the reaction zone has a oxygen content of < 0,1 ppm.