FR2612418A1 - LIQUID DEGAZIFICATION PROCESS - Google Patents

Abstract

IN A PROCESS FOR ELIMINATING DISSOLVED GASES, FOR EXAMPLE OXYGEN, LIQUIDS, FOR EXAMPLE WATER, THE IMPROVEMENT CONSISTING OF AN EXTRACTION GAS 27, FOR EXAMPLE NATURAL GAS, WITH RESPECT TO SUCH A LIQUID IS INJECTED THROUGH INPUT 27 AND FLOWS CO-CURRENTLY WITH THE LIQUID INTRODUCED THROUGH LINE 23 THROUGH A STATIC MIXING DEVICE 26. THE EXTRACTION GAS ALLOWS THE GAS DISSOLVED TO BE FREE OF THE SOLUTION SO IT CAN BE SEPARATED FROM THE LIQUID BY RESTATION IN A SEPARATION TANK 30.

Description

The present invention relates to a method for removing

  dissolved gases, such as oxygen, in liquids.

  It has long been known that water can contain certain gases in the dissolved state which can cause severe corrosion problems. For example, in the production of hydrocarbons, it is common to inject water into the producing formation while the hydrocarbons are drawn off to maintain the production pressure in the formation. In offshore locations, raw seawater is routinely used for this purpose. It is known that seawater normally contains dissolved gases, including oxygen, which lead to severe corrosion of the injection system which, at its

  tower, can possibly block the production formation itself

  even. As a result, raw seawater must be treated, that is,

  i.e. deaerated, to remove dissolved oxygen before the water

either injected into the training.

  A wide variety of techniques are known for removing corrosive gases dissolved in water. For example, there are chemical deaeration systems in which reducing agents are combined with dissolved corrosive gases to chemically reduce said gases. However, due to the large volume of seawater that must be treated in a routine injection operation, the logistics and costs of these chemical deaeration techniques make them unattractive for most offshore applications.

sides.

  Several mechanical deaeration techniques have been

  proposed as effective variants of chemical deaeration.

  They include vacuum extraction, thermal extraction, and gas extraction, the latter being most commonly

  used in locations off the coast. In the extrac-

  gas, the partial pressure of dissolved oxygen in seawater is reduced by replacing the ambient atmosphere around the water with an oxygen-free gas, for example methane or nitrogen, allowing so oxygen comes out

  of the solution and being transported elsewhere by the extraction gas.

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  Gas extraction systems successfully used at offshore sites normally require smaller and less expensive equipment than thermal deaeration or under

  empty. In addition, for a given easy supply of extractive gas

  tion, the cost of daily operation is lower than that

  with other known deaeration systems.

  However, in known systems of deaeration by

  gas extraction, the extraction gas always flows against

  current of the water stream to be treated. This flow against

  flow normally takes place inside a large column which contains trays, linings, and sprayers which, in turn, substantially increases construction and installation costs. For example, a column used in a typical offshore deaeration operation may

  reach 21 m in height and more than 4.3 m in diameter.

  Due to the size and weight of the column required for counterflow flow deaeration, the column is

  normally arranged in the vicinity of the outer edge of the platform

  form of production and takes up considerable space on this platform. In fact, in a large number of cases, an additional casing is cantilevered on the side of the platform on which the column is installed. In addition, even in still or calm waters, one or more floating cranes with booms up to 49 m are required to install one

  of these columns at its location off the coast.

  For a more complete description and discussions

  more complete, counter-current deaeration columns, see "recommended practice - drawing and operation of extraction columns to remove oxygen from the water", NACE Standard RP-02-78, national association of engineers corrosion, Houston, Texas 1978; "deaeration and its adaptation to the water injection system", the chemical engineer, November 1984, p. 44-48; "seawater injection systems from the Forties fields", OE-77.1.SPE 2.6677.5, Offshore conference, Europe 77, Aberdeen, Scotland, September 1977; and "oxygen desorption from water using natural gas for counter-current extraction"; journal of

  petroleum technology, May 1963, p. 51 and following.

  It can be seen from the above that although the known gas extraction deaeration systems are generally smaller and less expensive than other known deaeration systems, they are nevertheless still too large and too expensive to build and assemble. in offshore sites

sides.

  The present invention relates to a cocurrent gas extraction method which makes it possible to reduce the quantity of dissolved gas, for example oxygen, in liquids, such as water, to a level comparable to that achieved by known methods. against the current but which is implemented using equipment that is much smaller and more

easy to install.

  More particularly, the present invention resides in a method for removing gases dissolved in a liquid, said method comprising: causing a current of a liquid containing a dissolved gas to flow through a static mixing device; injecting an extraction gas inert with respect to said liquid into the liquid stream so that the extraction gas flows cocurrently with this stream of liquid through this static mixing device to reduce the pressure partial of said dissolved gas thus allowing at least a portion of this dissolved gas to be released from the solution; passing this liquid from said static mixing device into a separation tank; allowing this liquid to stand in the separation tank to allow the gases to separate from the Liquid; and remove these gases and liquid from this tank by

separate outlet openings.

  According to a modification of the present invention, a second step is added to further reduce the amount of dissolved oxygen in the water. This consists of connecting a second static mixing device and a second separation tank

  with the water outlet opening of the first separation tank.

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  In the present co-current process, a relatively small static mixer and a separation tank replace

  the large extraction column from previous processes against

  current. The static mixer and the separation tank of the present invention are easier and much less expensive

  to build and install as the columns of the prior art.

  The invention will now be described more particularly-

  with reference to the attached drawings, in which: FIG. 1 is a schematic view of a system of

  typical deaeration of the prior art which uses an extraction gas

  tion which flows against the current of the water to be treated; Figure 2 is a schematic view of the cocurrent flow deaeration system according to the present invention; and Figure 3 is a side view of a modification of the

Figure 2 system.

  Referring to Figure 1, in the deaeration system of the prior art shown, the water containing a corrosive gas, for example oxygen, flows in a column against the current 10 by a water inlet 11 and flows to the

  down through trays, linings and / or sprays

  12 in column 10. Extraction gas, inert to

  screw water, free of oxygen, for example natural gas or nitrogen, is supplied by a gas inlet 13 and flows upward through Column 10 against the current screw downward flowing water stream. As is well known in the art, the extraction gas reduces the partial pressure of the oxygen in the water thus allowing an appreciable portion of the dissolved gas to leave the solution and to be transported by the gas. extraction from column 10 through a gas outlet 14. The water from which the greatest amount of dissolved gas is now removed flows from column 10 through the water outlet 15. A purifying agent of oxygen, for example uncatalyzed ammonium bisulfite, is added to the treated water in column 10 through inlet 16 to further reduce the dissolved oxygen in the water. The process of FIG. 1 is well known and

  for a more detailed description of a counter-current column

  typical 10, see "deaeration and its adaptation to the water injection system", Mangnall, the chemical engineer, November 1984,

p. 44 and following.

  Referring now to Figure 2, the system

  co-current flow deaeration system 20 of your present invention

  tion is illustrated as it should be used to deaerate seawater at a location off the coast. A pump

  elevation 21 is submerged in a volume of water 22 at a depth

  Deur, for example 21 m or more, below which solids and animal matter, for example plankton, are normally present in substantial quantities. Raw seawater is pumped by pump 21 through line 23 and is chlorinated to inhibit bacteria, etc., by injecting a chlorination chemical, or by passing water through

  an electrochemical chlorination device 24.

  The chlorinated water is then sent through one or

  several filters 25 to reduce the solids contained in the water.

  The filtered water then passes through static mixing devices 26 comprising an elongated envelope containing a material which provides a tortuous path for the water which flows therein to produce a turbulent flow inside the device 26. The material static mixing may be permeable packing, granular material, or the like, but preferably comprises one or more sections of a commercially available static mixing element which are assembled by stacking and fixing the corrugated sheets together to form channels open intersection; for example, the static mixing element, model SMV, sold commercially by the company Sulzer Brothers Limited,

Winterhur, Switzerland may agree.

  An extraction gas, inert with respect to water, free of oxygen, for example natural gas (mainly methane) or nitrogen, is injected through the gas inlet pipe 27 in the stream of water in the pipe 23 just upstream of the static mixing material in the casing 26. As the water and the gas co-flow through the tortuous circuit in the static mixing device 26 , the extraction gas replaces the atmosphere surrounding the water, thereby reducing the partial pressure of the oxygen dissolved in the water. This allows dissolved oxygen to escape from the solution. The water, the extraction gas and the free oxygen then flow from the static mixing device 26 to the separation tank 30 via the water and gas inlet 29. The extraction gas and the free oxygen separate

  by gravity only water in the tank 30 and are put to the atmosphere-

  sphere through the gas outlet 31. If natural gas is used as

  extraction gas, the released gas can be used as combustion gas

  tible on board the platform offshore or for a similar use which adds to the efficiency of the deaeration process. A chemical agent for purifying oxygen, for example ammonium bisulphite, is added to the water in the tank 30 via line 32 to further reduce the quantity of oxygen dissolved in the water before this water s flows through the outlet 33 to the injection pump 34 which, in turn, pumps the deaerated seawater in a production formation to maintain it

pressure.

  To better appreciate the advantages that the present co-current deaeration method provided compared to the known method

  against the flow of the prior art, a brief example will now be

  described to compare the size of the equipment needed to treat 15,000 barrels of seawater per day containing

  8-10 parts per million (ppm) of dissolved oxygen.

  In the prior art counterflow process of Figure 1, column 10 should be approximately 18m high and 1.2m in diameter and should include multiple flow trays from edge to center and from center to edge. The column must be constructed of carbon steel with

  a corrosion resistant shirt.

  In the present process of Figure 2, the static mixing device 26 has an envelope having a length of only 2.6 m and an outside diameter of only 15 cm and is filled with commercially available static mixing elements as described previously. Extraction gas, for example natural gas, flows cocurrently with seawater at a rate of 0.7 Nm3 / m3 of seawater. The separation tank 30 has a height of 4.6 m and a diameter of 1.8 m. The tank 30 fulfills three functions: to allow the separation of the extraction gas and the free oxygen from the water; allow a stay of 5 min for the chemical reaction between the dissolved oxygen remaining in the water and the oxygen scavenger; and ensure that a quantity

  sufficient water is always available for the injection pump

  tion 34. Due to the nature of these functions, the tank 30 is made of glass fibers or an equivalent material which substantially reduces the cost compared to a counter-current column.

carbon steel 10.

  The other associated equipment, for example pumps, filters, etc., is identical for the two processes. The final dissolved oxygen content of the water after the initial deaeration by the present cocurrent deaeration is slightly higher than that of the deaerated water in column 10, for example 1000 compared to 50 parts per billion (ppb). However, by slightly increasing the oxygen scavenger in the present co-current deaeration process, the final dissolved oxygen content of the deaerated water will be substantially equal to that of the

  prior art. Substantial savings in construction

  tion and in the cost of the installation carried out thanks to the present invention largely compensate for any slight increase in the cost of the operation which could occur due to the use of

additional chemicals.

  FIG. 3 illustrates a modification of the method of FIG. 2 which includes a second co-current deaeration stage which is used when the oxygen dissolved in the water leaving by the outlet 33 of the tank 30 remains too high. In Figure 3,

  the water coming from the tank 30 flows through a second dispo-

  static mixing device 26a which has the same construction as the static mixing device 26. Extraction gas is injected into the water just upstream from the device 26a. Again, the extraction gas flows cocurrently with the water through the static mixing device 26a and the two flow into the second separation tank 30a where they separate. Gas and oxygen

  free flow from the tank 30a through the outlet 31a. The addi-

  oxygen purification can be added via line 32a if desired. The deaerated water flows from the tank 30a through the

  outlet 33b to the injection pump 34 (Figure 2).

  We can see from the previous description that the

  present invention provides a method of deaeration by extrac-

  tion of gas effective for water which requires much smaller and less expensive equipment than the gas extraction deaeration methods of the prior art. Equipment requires less space and is much easier to install, especially

  at a location off the coast.

  Although the present invention has been described as used for deaerating raw seawater at a location off the coast, it should be recognized that the co-current flow deaeration method of the present

  invention can also be used in extrac-

  tion of gases or other volatile substances dissolved in other water streams, for example the treatment of injecting water into

  onshore sites, wastewater treatment before disposal

  mining, water treatment for aquifer restoration,

  cooling water treatment, etc., for operations

  refining and the like and the treatment of acidic water for the removal of hydrogen sulfide, carbon dioxide,

  etc., for oil fields, refineries and operations

analogous tions.

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Claims (6)

  1. A method for removing gases dissolved in a liquid, said method comprising: flowing a stream of a liquid containing a dissolved gas through a static mixing device; injecting an inert extraction gas with respect to said   liquid in this liquid stream so that said extraction gas   tion flows cocurrently with said stream of liquid through said static mixing device to reduce the partial pressure of the dissolved gas thereby allowing at least a portion of the dissolved gas to be released from the solution; passing said liquid from this static mixing device into a separation tank; leaving this liquid at rest in the separation tank to allow the gases to separate from the liquid; and remove these gases and liquid from this tank by separate exits.   2. Method according to claim 1, characterized in that that the liquid is water.   3. Method according to claim 1 or 2, characterized   in that the dissolved gas is oxygen.   4. Method according to any one of the claims   previous, characterized in that the extraction gas is gas natural or nitrogen.   5. Method according to any one of the claims   previous, characterized in that an oxygen scavenger is added to this water while it is at rest in the tank of seperation.   6. Method according to any one of the claims   previous, characterized in that it includes other steps consisting:   to cause said liquid to flow from this separation tank   tion through a second static mixing device; injecting extraction gas inert with respect to said liquid into this liquid from the separation tank so that the extraction gas flows cocurrently with said liquid through this second static separation device for reducing the partial pressure of said dissolved gas remaining in this water thus allowing at least a portion of this dissolved gas to be released from the solution; passing this liquid from said second static mixing device into a second separation tank; leaving this liquid at rest in this second separation tank to allow these gases to separate from said liquid; and to remove these gases and this liquid from this second tank by separate exits.