Acid gas disposal

Abstract

acid gas is liquified by compression and cooling, mixed with water under pressure and flowed into a disposal well.

Description

P_min=284+21.55×CO₂%
Where

• • R_H2O is the minimum ratio by volume of water having at least a pH of 7.5 to the dense fluid acid gas. (i.e. R_H2O:1AG where AG is unit volume of acid gas).
  • P_min is the minimum pressure in the confluent line in psi.
  • CO₂% is the percent of CO₂ in a mixture of acid gas on a mole basis.

The above formulas are for temperatures in the range of 60° to 140° F. in the confluent line 25.

The upper temperature limit of acid gas dense fluid at check valve 22 and the mix in the confluent line 25 is approximately 140° F. The preferred temperature of the acid gas at the check valve 22 is below 130° F., and of the mix in the confluent line 25 is below 110° F.

The lower operation limit of temperatures is freezing of the fluid involved.

The above ratios of water to acid gas dense fluid are approximately the minimal amount of water required. If lesser water is used, difficulties may be experienced. If there is an excess of alkaline water to be disposed of in excess of the minimum requirements there is no problem in mixing the additional water into the Tee 20. Those with ordinary skill will know how to proportion the desired volume of water to the volume of acid gas dense fluid. If other compatible and suitable liquids require disposal, they too may be pumped into the confluent line 25.

The pressures are also approximately the minimal pressures to form a dense fluid at temperatures below 120° F. at the check valve 22. The maximum pressures are limited only by the higher cost of higher pressures.

There must be sufficient water to form a stable mixture at the injection pressure. By injection pressure it is meant that pressure at the well head, which is at the top of the disposal well 28. Injection pressure gage 34 in the injection tube 32 down stream from the control valve 26 indicates the injection pressure.

If less than 5% of the mixture of alkaline water and aqueous acid gas vaporizes at this point, normally satisfactory operation will be maintained. If no greater amount of gas than 5% is formed within the flow of the mixture; the gas will normally be in the form of small bubbles. These bubbles will be carried by the flow of fluid into the disposal well 28. As the mixture descends into the disposal well 28 there will be a pressure increase which will force the vapor back into a liquid or dense fluid phase.

If there is insufficient water to form a stable mixture such that more than 5% gas vaporizes; a gas pocket will often form within the injection tube 32 at about the top of the disposal well. This gas pocket will cause an increase in pressure and the operation will become unstable at that time.

The pressure in the injection tube at the pressure gage 34 will be responsive to different events. Obviously the pressure gage 34 will increase with an increased volume of liquids being pumped. For example, if pump 18 were to pump twice as much water, the hydraulic flow in the injection tube would result in a higher pressure at pressure gage 34. Also, as the disposal well is operated, different solids, for example, chemical precipitates or the like will begin to collect in the strata 30 which will reduce its porosity surrounding the disposal well 28. This build-up is to be expected and will result in a slow increase in injection pressure. Such an increase in pressure might normally be no more than 1 psi per day.

Also, under certain condition hydrates will form and adhere to the well surfaces, particularly the inside bore of the injection tube 32. If hydrates form they would be an indication that there was insufficient water being mixed with the aqueous acid gas. This would result in a pressure increase in the range of 5 or 10 pounds per square inch per hour until no flow could be achieved.

The preferred operation includes that manual process calculations or a computer process simulation be conducted with a complete analysis of the acid gas and water to be used. When this simulation is completed, the appropriate process pressures and temperatures can be determined for optimum performance. Such computer simulations can be made with HYSYS software available from Hyprotech, Inc., located in Houston, Tex. for example.

With these different operating criteria the operator can adjust the ratio of water to acid gas dense fluid to obtain stable and satisfactory operating conditions. Normally the unstable conditions will result from an insufficient amount of water for the amount of aqueous acid gas being injected.

From the above it may be seen that it is desirable to always have water flowing into the disposal well even if there is no acid gas being liquified and disposed of at the time.

It is desirable that the confluent line 25 be of a material which resists corrosion from the acid gas and alkaline water mixture. The suitable grades of stainless steel for this purpose are well known in the art and in certain cases it is necessary to solution treat and/or coat the stainless steel before it is put into service. Those skilled in the art will know of the materials of construction for the confluent line.

The embodiment shown and described above is only exemplary. I do not claim to have invented all the parts, elements or steps described. Various modifications can be made in the construction, material, arrangement, and operation, and still be within the scope of my invention.

The restrictive description and drawings of the specific examples above do not point out what an infringement of this patent would be, but are to enable one skilled in the art to make and use the invention. The limits of the invention and the bounds of the patent protection are measured by and defined in the following claims.

Claims

1. The method of disposing acid gas removed from hydrocarbon products comprising the steps of:

a) compressing the acid gas to a pressure wherein the acid gas will form a dense fluid,

b) forming a dense fluid by cooling the compressed acid gas,

c) mixing the acid gas as a dense fluid with sufficient water to form a stable mixture at injection pressure, and

d) injecting said stable mixture at injection pressure into a disposal well.

2. The method as defined in claim 1 wherein said compression pressure is at least 650 psi.

3. The method as defined in claim 1 wherein said water is alkaline and has a pH of at least 7.5.

4. The method as defined in claim 1 wherein a minimum pressure of step a) is P_min=284+21.55×CO₂% wherein P_min is said minimum pressure in psi and CO₂% is the percentage of carbon dioxide in the acid gas.

5. The method as defined in claim 4 wherein the compressed acid gas is cooled to a temperature of below 130° F.

6. The method as defined in claim 1 wherein insufficient water is a minimum of

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7. The method as defined in claim 6 wherein the mixed acid gas and water has a temperature of below 110° F.

8. The method as defined in claim 1 further comprising:

e) mixing the acid gas and water in a confluent line, and

f) maintaining the pressure in the confluent line to the compression pressure by

g) limiting the flow from the confluent line by a pressure control valve.

9. The method as defined in claim 8 wherein

h) determining that the mixture found is stable by less than a significant amount of any component of the acid gas flashing into the gas phase down stream from the control valve.

10. The method as defined in claim 9 wherein determining that the amount of any component of acid gas flashed is more than a significant amount by formation of hydrates adhering on surfaces of the disposal well.

11. The method as defined in claim 9 wherein determining that the amount of any component of acid gas flashed is more than a significant amount by formation of gas pockets down stream of the control valve as determined by rapid increase of pressure down stream of the control valve.

12. The method as defined in claim 9 wherein less than a significant amount of any component of acid gas is flashed is determined when less the 5 percent of said mixture is flashed.

13. The method as defined in claim 9 wherein less than a significant amount of any component of acid gas is flashed is determined by process simulation.

14. The method as defined in claim 9 wherein less than a significant amount of any component of acid gas is flashed is determined by manual process calculations.

15. A system for disposing of acid gas comprising:

a source of acid gas;

a source of water;

a series of at least one compressor and one cooler to compress and cool the acid gas to form a dense fluid acid gas;

a gas line to convey the dense fluid acid gas from its source to a confluent line;

a water line to convey the water from its source to the confluent line;

a mixer connecting the gas and water lines to the confluent line, the mixture for mixing water and acid gas into a stable mixture; and

an injection tube for injecting the stable mixture of water and acid gas of the confluent line into a disposal well.

16. The disposal system of claim 15 further comprising a pressure controller in the confluent line.

17. The disposal system of claim 16 wherein the pressure controller maintains the pressure at a level sufficient to prevent substantial vaporization of the mixed water and acid gas.

18. The disposal system of claim 15 wherein water is continuously pumped through the injection tube to the disposal well even when no dense fluid acid gas is being disposed.

19. A method of injecting a liquid containing acid gas into a disposal well comprising the steps of:

forming the liquid by mixing water and dense fluid acid gas to form a stable mixture at a pressure; and

maintaining the pressure on the liquid prior to injection at a level sufficient to prevent substantial vaporization of the liquid.

20. The method according to claim 19 wherein the pressure is maintained at about 650 psi or higher.

21. The system according to claim 15, wherein the mixer comprises:

a tee connection fluidly connecting the water and gas lines to the confluent line; and

a check valve in each of the water and gas lines to prevent backflow of the stable mixture into the gas and water lines.