Research and experiment of natural gas separation scheme
To this end, the author of this paper has established a new process plan. By adding a stable gas compressor system, the stable gas is divided into NGL products and dry gas products, which not only solves the problem of stable gas products, but also improves the recovery rate of C 3 and C 4 in NGL. The process is simulated and optimized using advanced simulation software PRO/II, which saves energy and reduces investment. On this basis, the owner's plan and the proposed plan were compared and analyzed from the aspects of energy consumption and product yield of the whole plant, and economic analysis was made.
1 Process introduction and simulation
The process was simulated using the advanced process simulation software PRO/II 8.1. The physical method selects the SRK equation suitable for natural gas separation.
1.1 Owner Program
1.1.1 Separation unit
The main function of the unit is to separate the oil, gas and water produced at the wellhead. Natural gas from the wellhead at 14.48 MPa and 25 °C was first depressurized to 10.3 MPa (gauge pressure). In this process, in order to prevent the natural gas from decompressing and cooling, it causes agglomeration and blockage, and must be heated before decompression. Adjust the temperature of the heater E-A101A/B in the simulation to ensure that the temperature after decompression is 30 °C. The decompressed natural gas enters the three-phase separator S-A101A/B, and the separated water is discharged to the sewage treatment unit, and the separated liquid phase is sent to the condensate stabilization tower, and the separated gas phase is introduced into the downstream separator.
1.1.2 Dehydration drying unit
The function of the natural gas dehydration unit is to remove the moisture in the natural gas so that the water content is less than 0.1 × 10-6 to prevent the hydrate in the natural gas from freezing in the downstream heat exchanger. The project adopts a double-tower model with built-in molecular sieve packing, one tower is dry and the other tower is regenerated. The author did not perform a detailed simulation of the dehydration drying unit. In the simulation, the “Stream Calculator†unit was used instead of the drying tower to remove all moisture.
1.1.3 NGL recovery unit
The function of the NGL recovery unit is to use the expansion refrigeration technology to cool the natural gas to the required temperature, condense the light hydrocarbon components, recover the C 3 and above components, and pressurize the dry gas for external transport. The gas phase from the dehydration unit is divided into two parts, one part is exchanged with the gas phase of the deethanizer T-A102 in the gas heat exchanger E-A105, and the other part is in the liquid phase of the second stage separator S-A105. Heat exchange in heat exchanger E-A106. After the heat exchange, the gas phase is mixed and then cooled to -83 °C through the primary separator S-A104 into the expansion end of the turboexpander/booster unit EC-A101-1A/B. After the expanded gas phase passes through the secondary separator S-A105, the gas phase is mixed with the deethanizer top gas phase and then enters the turbocharged end of the turboexpansion/supercharger unit to pressurize the EC-A101-2A/B, and then dry gas. Compressor C-A101A~D is pressurized to 7.5 MPa, which is a dry gas product. The liquid phase of the secondary separator S-A105 enters the deethanizer T-A102, the gas phase at the top of the column is a partial dry gas product, and the liquid phase at the bottom of the column is an NGL product.
1.1.4 condensate stabilization unit
The purpose of the condensate stabilization unit is to reduce the saturated vapor pressure of the condensate, reduce the volatilization loss of the condensate during storage and transportation, and recover the light hydrocarbons in the condensate. The liquid phase from the three-phase separator S-A101 enters the condensate stabilization tower T-A101 and is separated into a stabilized gas (used as a fuel) and a condensate product.
1.2 Suggested solutions
1.2.1 Separation unit
The natural gas from the wellhead was first depressurized to 6.1 MPa. Compared with the owner's plan of 10.3 MPa, although the partial energy was lost, the expansion ratio remained basically unchanged, and the temperature drop requirement was still met, and the design pressure of the subsequent system was reduced, and the system was increased. The safety factor, especially for the expansion/supercharger, can only be achieved at 6.1 MPa, but the price is less than half of that of the imported model, which reduces investment.
The decompressed natural gas enters the three-phase separator S-B101A/B, and the separated water is discharged to the sewage treatment unit; the separated liquid phase is further depressurized to 3 MPa, and then exchanged with the condensate product in E-B102 After the temperature is raised, the first-stage flash separator S-B102 of the condensate stabilization unit is removed; the separated gas phase enters the drying tower T-B103A/B through the downstream cyclone separator S-B103.
1.2.2 Dehydration drying unit
This part of the process is the same as the owner's plan, using the “Stream Calculator†unit instead of the drying tower to remove all moisture.
1.2.3 NGL recovery unit drying tower T-B103A/B The top gas passes through the cold box E-B105 and the cold oil heat exchanger E-B107 comes from the natural gas and the low temperature separator S-B104. Separator S-B104 completes gas-liquid separation. The separated condensate is throttled into the cold box E-B105 to recover the cooling capacity, and then enters the deethanizer T-B102 from the middle. The gas phase separated by the cryogenic separator S-B104 enters the expansion of the turboexpansion/charger unit. End EC-B101-1A/B, expanded to 2.2 MPa from the bottom into the heavy contact tower T-B104.
The deethanizer separates the feed into dry gas and NGL products. The gas phase at the top of the column is transferred from the top of the heavy oil heat exchanger E-B107 to the top of the heavy contact tower T-B104, and then enters the heavy contact tower from the top. The condensed liquid phase is in countercurrent contact with the natural gas entering from the bottom of the tower, and the natural gas is fully recovered. For C 3 and above components, CH 4 and C 2 H 6 in the natural gas are stripped to reduce the load on the deethanizer.
The heavy-handed gas at the top of the tower is exchanged by the cold oil heat exchanger E-B107, and then recovered by the cold box E-B105 and then enters the booster end of the turboexpansion/supercharger unit EC-B101-2A/B. The dry gas compressor C-B101A~D is pressurized to 7.5 MPa as a dry gas product. The condensate obtained by heavy contact with the bottom of the tower passes through the cryopump P-B101 from the top into the deethanizer tower T-B102.
In contrast to the owner's plan, the proposed solution is optimized in two aspects in the NGL recovery unit. First, the cold box is added instead of the gas-gas heat exchanger and the gas-liquid heat exchanger. On the basis of satisfying the heat transfer requirements of each stream, the pressure drop of the system is relatively small because the cold box allows operation under a small temperature difference. This reduces the consumption of utility projects; in addition, the heat transfer effect of the cold box is better, the recovery of the cold amount is more thorough, and the heat exchanger efficiency is improved. Second, the heavy contact tower and the cold oil heat exchanger are added, and the secondary cryogenic separator, the deethanizer reflux tank, and the condenser are eliminated. In this way, the liquid phase of the cryogenic separator (corresponding to the primary cryogenic separator in the owner's scheme) directly enters the deethanizer tower, and after the gas phase expansion, it enters the heavy contact tower, and more CH 4 and C 2 than the owner scheme. H 6 is raised by the gas, thereby reducing the load on the deethanizer. Under the same conditions of NGL production, it is recommended that the light components such as CH 4 and C 2 H 6 entering the deethanizer tower are less than 60% of the owner's scheme, and the energy consumption of the deethanizer reboiler is only 50 of the owner's scheme. %, saving energy.
1.2.4 condensate stabilization unit
The proposed scheme adopts the stripping stabilization method. The condensate is separated by two stages of dewatering and oil and gas separation before entering the tower. In order to increase the flashing effect, a condensate heat exchanger is added before the primary flash separator. The condensate obtained by the three-phase separator S-B101A/B is exchanged with the high-temperature condensate of the condensate heat exchanger E-B102 and the bottom of the condensate stabilization tower T-B101, and then enters the first-stage flashing. Separator S-B102, secondary flash separator S-B105 complete the separation of oil, gas and water, wherein the water phase is discharged to the sewage treatment, and the gas phase enters the three-stage and two-stage compressor inlet buffer tank S- of the stabilized gas. B107, S-B106. The dehydrated condensate enters the condensate stabilization tower T-B101; the top gas phase enters the stabilized gas compressor C-B102-1; the bottom liquid phase is in the condensate heat exchanger E-B102 Condensate heat exchange, the temperature drop to 32 °C is the condensate product.
The condensate stabilized overhead gas phase, the secondary flash separator gas phase and the primary flash separator gas phase are pressurized into the outlet separator S-B108 by the stabilized gas compressor C-102-1~3, and the gas phase is rich in C The components 1 to C 4 are returned to the cyclone separator S-B103 in front of the drying tower, and the C 3 and C 4 are recovered in the NGL recovery unit, and the remaining light components are proposed as dry gas products. The S-B108 liquid phase is returned to S-B102 as a recycle feed to increase natural gas utilization. The liquid phase of the stabilized gas compressor stage gas-liquid separator (S-B106, S-B107) is mixed with the deethanizer T-102 bottom product to form an NGL product.
By adding a stable gas compressor system, the stable gas in the owner's plan is divided into NGL products and dry gas products. It solves the problem that the original stable gas product has too much heavy component, and needs to be mixed with a certain amount of dry gas to be used as combustion gas, and the excess stable gas has no other way to be treated and can only be burned by the torch, resulting in a great waste of resources. The problem is that the content of C 3 and C 4 in the NGL product is increased and the recovery rate is met.
The two-stage oil and gas separation is used in front of the condensate stabilization tower, and the light components affecting the stability of the condensate are flashed to the gas phase and directly into the stabilized gas compressor system, which improves the stability of the condensate and reduces the load of the stabilization tower. The energy consumption of the reboiler is only 60% of the owner's plan. The hot product from the bottom of the tower is cooled by the heat exchange between the condensate heat exchanger and the incoming oil. The increased condensate heat exchanger improves the heat utilization rate, reduces the load of the stabilizer reboiler, increases the flashing effect of the oil and gas, and replaces the air cooler of the owner's solution condensate product. Reduced engineering investment.
2 Results and discussion
2.1 Energy consumption analysis
The project has its own gas generator for the whole plant; there is no cooling water system, all the coolers are air-cooled; the self-contained thermal oil furnace is used for the whole plant heater.
2.1.1 Power consumption
Since the compressors are all gas-fired, it is recommended that the newly added equipment such as the stabilized gas compressor system consumes little power. The main power-consuming equipments of the two schemes are air compressor stations, pumps and plant-wide control and lighting.
The capacity of the generator is considered at 480 kW.
2.1.2 Thermal oil furnace capacity
Although the proposed scheme has a relatively large temperature drop due to the decompression of natural gas to 3 MPa, the heat consumption is relatively large. It is recommended that the reboiler heat consumption of the deethanizer tower and the condensate stabilization tower is only 50% of the owner's scheme and 60%. Therefore, the total heat consumption is relatively small.
2.1.3 Compressor power
In addition to the original two-stage dry gas compressor system, a three-stage stabilized gas compressor system has been added to the proposed scheme. The total power is 60% more than the owner's plan. However, due to the low processing capacity, the stabilized gas compressor power is only It is 76 kW, which is less than 4% of the total compressor power. The recommended increase in compressor power is mainly due to the decompression of natural gas to 3 MPa and the loss of partial static pressure.
2.1.4 Total gas consumption
The generator, the heat transfer oil furnace and the compressor are all gas-fired, so the energy consumption of the entire process can be unified to the gas consumption. The natural gas combustion value is considered at 8800 kcal/m 3 (1 kcal = 4.18 kJ), so the gas consumption Q can be calculated according to formula (1).
3600 24 0.25 8800 4.18 10000 PQ×=×(1) where Q is the gas consumption, 10 4 m 3 /d; P is the power, kW.
2.2 Product Analysis
2.2.1 NGL products
The comparison of NGL products shows that the composition of NGL products of the two schemes is not much different, mainly based on propane and butane.
However, the output of the proposed scheme is 17 tons more per day than the owner's scheme, and the recovery rates of propane and butane are 90.0% and 99.6%, respectively, which not only meet the recovery requirements, but also increase the number of owners and schemes by 8 and 20 respectively. percentage point. This is mainly due to the separation of NGL products from the stabilized gas, that is, the heavy components such as propane and butane in the stabilized gas are separated into the NGL product of the deethanizer bottom by the three-stage stabilized gas compressor system.
2.2.2 Condensate products
Condensate product comparison can be seen that the owner's program condensate light component content is more, the saturated vapor pressure is 203.8 kPa. Compared with the owner's program, the proposed scheme will separate the light components that affect the stability of the condensate to the NGL product. Medium, so the condensate production is less than the owner's plan, but it reduces the saturated vapor pressure of the product, avoiding the safety hazard and respiratory loss caused by the volatilization of light components during product storage.
2.2.3 Dry gas products
Due to the high steady-state propane content, the owner's program needs to be mixed with dry gas to be used. Considering that the blending ratio is 1:2, the owner's plan still consumes 0.8×10 4 m 3 /d of dry gas.
With the proposed scheme, the dry gas consumption is 3.11×10 4 m 3 /d, so the final dry gas product is 127.9×10 4 m 3 /d, which is slightly more than the owner's plan.
2.2.4 The stable gas volume in the stable gas owner scheme is 3.25×10 4 m 3 /d, which is not a product, but is used as the combustion gas of the generator, the heat transfer oil furnace and the dry gas compressor in the whole project. However, due to the high content of propane (mass fraction 11.30%) and butane (mass fraction 7.96%) in the stabilized gas, it is not suitable for combustion gas. In actual production, a certain amount of dry gas must be mixed to meet the requirements of combustion gas. In addition, the output of the stabilized gas is much larger than the gas consumption of the combustion gas, and the excess stable gas (about 1.51 × 10 4 m 3 /d) can only be burned off and wasted by the torch.
The proposed scheme divides the stabilized gas logistics into NGL products and dry gas products by adding a stable gas compressor system, and solves the biggest problem of the owner's scheme. This is also the main purpose of the proposed scheme optimization and the biggest advantage of the scheme.
3 Economic analysis
According to the market conditions and the project itself, the prices of raw materials and products are determined. The prices of natural gas and dry gas at the wellhead are 1.0 yuan/m 3 and 1.8 yuan/m 3 respectively, and the prices of NGL and condensate are 4,500 yuan/t and 3,500 yuan/t, respectively. Based on this, the two schemes are Economic analysis.
3.1 Raw material cost
For the wellhead natural gas of 1.3×10 6 m 3 /d, it is separated and utilized, and the annual raw material consumption is 433.333 million yuan.
3.2 Utility consumption
The public works of the entire process can be unified into the consumption of gas, and the gas is sourced from the natural gas at the wellhead, so there is no need to consider the cost of public works.
3.3 Equipment fee
Although the proposed scheme adds a set of equipment for stable gas compressor systems and heavy contact towers, the localization of the expander and the replacement of the gas heat exchanger by the cold box have reduced the equipment cost of the whole plant.
3.4 Product Benefits
By optimizing the original process, the proposed solution increases product flow and increases the output-to-input ratio. Among them, the condensate product considers the influence of component concentration on the price based on the benchmark price.
3.5 Economic benefits
Through the analysis of the investment and output of the owner's plan and the proposed plan, combined with the formula (2), the economic benefits of the two programs are 728.863 million yuan and 76.70 million yuan respectively. The proposed scheme can generate more than 38.32 million yuan per year, which is equivalent to an increase of 5% on the original basis.
4 Conclusion
In summary, the proposed scheme divides the stabilized gas into dry gas and NGL products by adding a stable gas compressor system, which solves the problem that the product can be used as combustion gas due to too many heavy components and need to be mixed with a certain amount of dry gas. And the problem that the excess stable gas is burned by the torch causes waste of resources.
On this basis, the two schemes were compared from the aspects of full-field energy consumption and product yield. Among them, 1.3×10 6 m 3 /d natural gas was separated by the proposed scheme, and NGL products, condensate products and dry gas products were obtained at 216.37 t/d, 99.13 t/d and 127.9×10 4 m 3 /d, respectively. The recovery of propane and butane in the NGL product was greatly improved, reaching 90.0% and 99.6%, respectively, meeting the recovery requirements. The obtained condensate product has less propane butane content, and the product is relatively stable and less volatile. In addition, through economic analysis, the proposed program annual economic benefit is 76.70 million yuan, which is 38.32 million yuan more than the owner's plan, which is equivalent to an increase of 5%.
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