1 Overview
Sulfur-containing natural gas contains hydrogen sulfide, organic sulfur (mercaptans), carbon dioxide, saturated water, and other impurities, so it is necessary to remove the harmful components to meet the requirements of factory production and commercial commodity gas use. The commodity natural gas standards vary from country to country, mainly due to the hydrocarbon dew point and water dew point that are required to meet the pipeline transportation requirements. At the same time, there are requirements for the maximum content and low combustion value of hydrogen sulfide, mercaptan, and carbon dioxide in natural gas. The natural gas composition of raw materials is different from that of commodity natural gas, and the selected natural gas purification process technology scheme is also different. This article will combine the composition of natural gas processed by a Kazakhstan oil and gas processing plant and the requirements for natural gas to be exported to international pipelines. The selection method for desulphurizing natural gas desulphurization natural gas containing sulfur gas was proposed.
2. Raw material gas conditions
The main conditions for associated natural gas in the oilfields handled by a Kazakhstan oil and gas processing plant are:
1) The processing volume is 600×104m3/d (standard state is 0°C, 101.325kPa, the same below);
2) The pressure is 0.7MPa, in order to meet the needs of pipeline pressure and purification process, the pressure of the booster after booster station is 6.8MPa;
3) Main components
Components
Composition (mol%)
C175.17C29.44C37.21C43.35C5+1.06CO20.71H2O0.51H2S36g/m3 mercaptan sulfur 500mg/m3
3, commodity natural gas technical indicators
The natural gas of the plant will be exported to international pipelines, and the requirements of the ОСТ51.40-93 standard shall be met. The main technical indicators to be achieved are:
1) factory pressure 6.3MPa;
2) water dew point ≤ -20 °C;
3) hydrocarbon dew point ≤ -10 °C;
4) Hydrogen sulfide (H2S) ≤ 7mg/m3;
5) mercaptan sulfur (based on sulfur) ≤ 16mg/m3;
6) Low combustion calorific value ≥ 32.5 MJ/m3.
4, preliminary selection of process routes
According to raw natural gas conditions and commercial natural gas technical indicators, the overall process flow diagram of the plant is shown in Figure 1.
After the associated natural gas of the oil field is pressurized by the booster station, it is processed to the natural gas desulfurization and dehydration unit, and most of the H2S and RSH in natural gas need to be removed to meet the technical indicators of the hydrogen sulfide and mercaptan sulfur content in the natural gas of the product; Remove most of the water in natural gas to meet the technical indicators of natural gas water dew point. At the same time, in order to recover more liquefied gas and light oil products, the dewatering depth needs to meet the subsequent water dew point of light hydrocarbon recovery unit. -35°C requirement. The CO2 content in the feed gas is relatively low, 0.71% (mol), and the low combustion heat value of commercial natural gas is ≥ 32.5 MJ/m3, which may be excluded from consideration.
The natural gas treated by natural gas desulfurization and dehydration unit recovers light hydrocarbons (above C3) in natural gas through a light hydrocarbon recovery unit, produces liquefied gas and light oil products, and makes commodity natural gas meet the technical index of hydrocarbon dew point ≤ -10°C.
The acid gas removed by the desulfurizer is mainly composed of H2S, RSH, CO2, H2O, etc., and sent to the sulfur recovery unit for sulfur recovery. The sulfur production facility produces sulfur products. The sulfur recovery unit tail gas is treated by the tail gas treatment unit and burned. atmosphere.
The following part of this article mainly discusses how to select a reasonable technological scheme for the desulfurization and dehydration device so that the hydrogen sulfide and mercaptan content in the product gas of the desulfurization and dehydration device is qualified, and the water dew point can meet the requirements of commodity natural gas and subsequent light hydrocarbon recovery devices.
5, preliminary selection of dehydration process plan
Dewatering processes commonly used include solvent dehydration and solid desiccant adsorption. The solvent absorption method has the advantages of lower equipment investment and operating costs, and is more suitable for the dehydration of high flow and high pressure natural gas. The most widely used method is the dehydration method of the triethylene glycol solution, but its dehydration depth is limited, and the dew point drop generally does not exceed 45°C. The dry gas after dehydration by the solid desiccant adsorption method can have a dew point lower than -50°C.
Since the dewatering device product natural gas demand dew point ≤ -35 ​​°C, the solvent dehydration method is difficult to achieve, so the need to use solid desiccant dehydration process, such as molecular sieve dehydration process.
6, preliminary selection of desulfurization and desulfurization alcohol process plan
The associated natural gas to be treated in the associated natural gas has an H2S content of 36g/m3, a mercaptan content of 500mg/m3, and a natural gas treatment capacity of 600×104m3/d, which is relatively large. At present, the domestic single set of desulfurization equipment has a maximum processing capacity of only 400 ×104m3/d.
The commonly used desulfurization methods for desulfurization of alcohols include liquid desulfurization and fixed bed desulfurization.
If a single fixed-bed desulfurization method is used, such as molecular sieve desulfurization and desulfurization, the flow rate and sulfur content of the natural gas to be treated according to the present plan are calculated and changed once in 10 days. The amount of hydrogen sulfide to be removed within 10 days is 2.16. ×106 kg, about 500 molecular sieve desulfurization towers of DN 3000 are required, which is obviously not feasible.
At present, the more mature and feasible liquid desulfurization process in China is the alkanolamine process. Because mercaptan exists in sulfur-containing natural gas, the sulfone amine method can be used to remove hydrogen sulfide and mercaptan. The process is relatively mature and can remove H2S in natural gas to ≤7mg/m3. At the same time, the average removal rate of mercaptans in natural gas is 75%, and the mercaptan sulfur content in product natural gas is 125mg/m3. It is still not possible to reach the technical specifications for mercaptan sulfur ≤ 16 mg/m3. At this time, a fixed bed sweetening process, such as a molecular sieve sweetening alcohol process, can be used to remove the remaining mercaptans in natural gas.
The solution can also use alkali elution mercaptan process to remove mercaptans in natural gas. In order to reduce the consumption of alkali in the production process and the amount of waste alkali produced, the previous alcohol amine desulfurization device needs to use a monoethanolamine process to remove Except for most of the hydrogen sulfide and carbon dioxide in natural gas.
7. Comparison and selection of dehydrated natural gas dehydration process solutions
From 5 and 6, the desulfurization and dehydration process solutions have the following two more feasible solutions:
1) Scheme 1: Sulfone amine desulfurization + molecular sieve dehydration sweetening alcohol
The technical block diagram of this program is shown in Fig. 2. The sulfur-containing natural gas pressurized by the booster station is sent to the sulfone amine desulfurization unit to remove almost all of the H2S and 75% of the mercaptan, and then enters the molecular sieve dehydration sweetening methanol plant to remove the water and the remaining Thiol, purification of natural gas through the light hydrocarbon recovery unit for the recovery of liquefied gas and light oil products. The molecular sieve regeneration gas of the dehydration and desulfurization alcohol device needs to be pressurized and then returned to the sulfone amine desulfurization device for desulfurization, which is a cycle process.
2) Option 2: Desulfurization with ethanolamine + alkali elution with mercaptan + molecular sieve dehydration
The technical block diagram of this program is shown in Fig. 3. After the supercharged sulfur-bearing natural gas from the booster station enters the monoethanolamine desulfurization unit, almost all H2S and CO2 are removed, and then the alkali elution mercaptan unit removes almost all the mercaptans. After the sulphide, the natural gas enters the dehydration unit of the molecular sieve dehydration device, and the purified natural gas is sent to the light hydrocarbon recovery device to recover the liquefied gas and light oil products. Dehydration device Molecular sieve regeneration gas needs to be pressurized and then returned to the dehydration device for dehydration. It is a cycle process.
7.1 Program-Process Features
1) The sulfone amine desulfurization device uses sulfolane and methyldiethanolamine aqueous solution as desulfurizing agent. The main components of the solution include methyldiethanolamine, sulfolane and water in a weight ratio of 45:40:15, both chemical absorption and physics. Absorption of both effects, but also partial removal of organic sulfides (average removal rate of mercaptan above 75%), the solution of methyl diethanolamine has better selectivity for H2S absorption, reduce the CO2 Absorption greatly reduces the amount of solution circulation, reduces the size of the equipment of the regenerative system such as the regeneration tower, the lean liquid heat exchanger, the solution filter, the sour air cooler, etc., thereby reducing the investment and reducing the regeneration. The amount of steam needed and the amount of circulating water required for solution cooling are more significant.
2) The molecular sieve dehydration sweetening alcohol device utilizes the adsorption characteristics of molecular sieves to selectively remove water and mercaptans in natural gas. Unlike traditional caustic washing processes, the molecular sieve process selectively removes hydrogen sulfide and mercaptans, but does not remove CO2, which can increase the amount of natural gas that is exported by 2× 104m3/d.
Zeolite dehydration and desulfurization use molecular sieves are different, the use of different two molecular sieve beds, are generally arranged in the same adsorption tower.
7.2 Scheme II Process Features
1)-Ethanolamine desulfurization is a typical chemical absorption process. This method can remove only trace amounts of organic sulfur. It has almost no selective absorption of H2S and CO2. It absorbs H2S and also absorbs CO2. Therefore, the content of CO2 in the purified gas is very high. Small, beneficial for the subsequent alkali elution mercaptan device to reduce the production of waste alkali, reducing alkali consumption. However, the acid gas load of the method is lower than that of the sulfone amine method, so the required amount of solution circulation is larger, the size of the solution regeneration system equipment is also larger, and the device energy consumption is also higher.
2) Alkali elution sulfur is a relatively traditional natural gas desulfurization process. The MEROX process is a widely used alkali elution sulfur technology. At present, the technology uses fiber membrane technology to enhance the mass transfer and separation process.
The lye was an aqueous NaOH solution that chemically reacted with H2S, CO2, and thiol (RSH), respectively. Na2S and Na2CO3 produced by the reaction of NaOH and H2S and CO2 can not be regenerated in the lye, which increases the consumption of lye. The sodium thiolate (RSNa) formed by the reaction between NaOH and RSH can be converted into disulfide and NaOH by reacting with oxygen and water under the action of the catalyst to complete the regeneration of the alkali liquor. The alkali liquor has only a small part of the loss. Therefore, in the previous desulfurization equipment, the content of H2S and CO2 in the natural gas should be removed to the minimum to reduce the load of the alkaline cleaning device. For this reason, the desulfurization device adopts the ethanolamine desulfurization method in this solution.
In addition, the caustic scrubber will have a large amount of spent caustic (about 250m3/a) that needs to be treated and is intended to be transported to a waste caustic treatment plant near the plant for processing. Otherwise, the factory will need to add a spent caustic soda treatment unit.
3) The molecular sieve dehydration device is mainly used for molecular sieve bed dehydration. Compared with the dehydration sweetening alcohol device, the adsorption tower reduces the molecular sieve desulfurization of the alcohol bed.
8. Conclusion
The desulfurization natural gas dehydration process plan is the core part of the overall process flow of the oil and gas processing plant. It must be based on the natural gas composition of the raw material and the technical specifications of the commodity natural gas, and the technical and economic indicators such as the characteristics of the process, the total process flow, and the project volume, investment, energy consumption, etc. Determined by comparison. Comparing scheme 1 with scheme 2, the total process flow is simpler, the project volume is smaller, the investment is less at one time, the main consumption index and the annual operating cost are lower, and the waste alkali liquor is not generated, which is beneficial to environmental protection. Therefore, this process technology scheme Using the first route of the program, namely sulfone amine desulfurization + molecular sieve dehydration sweetening alcohol.