1 device status
1.1 device process
Daqing Oilfield Natural Gas Branch Company's apricot shallow cooling device process principle. The raw material gas (moisture) enters the separator to separate the condensate, and then enters the compressor. After a period of two stages, the pressure is compressed to 1.65 MPa, and then passed through a water cooler. After cooling to 18 °C, enter the first-stage three-phase separator, separate free water and light hydrocarbons, enter the lean-rich gas heat exchanger, pre-cool to 5 ° C, and then cool to -25 ° C by ammonia evaporator, The light hydrocarbon is recovered in the two-stage three-phase separator, and the purified low-temperature natural gas is directly sent out at 5 ° C after being recovered by the lean gas-rich gas heat exchanger; the separated from the second-stage three-phase separator -25 The low temperature light hydrocarbons of °C directly enter the light hydrocarbon stabilization tower, and the low temperature light hydrocarbons are heated and stabilized by the second stage compressed gas (155 ° C).
1.2 Problems with the device
After being separated by refrigeration in an ammonia evaporator, the light hydrocarbon at -25 °C is mixed with the light hydrocarbon separated by the three-phase separator, and then exchanged with the natural gas at 155 ° C of the compressor outlet, and stabilized as a product for external transportation. In this process, the recovery of light hydrocarbon cooling is not considered, and a large number of low-temperature light hydrocarbons are not properly used for cascade utilization. In actual production, the process medium needs to be cooled to 30 ° C, generally by air cooling or water cooling, the process medium needs to be cooled to -20 ~ -35 ° C, generally using refrigerant cooling. In the case of obtaining the same amount of cooling, the lower the temperature is, the more energy is consumed. Therefore, the light hydrocarbon at -25 °C exchanges heat with the natural gas at 155 °C, and the light hydrocarbon cooling at low temperature is not well utilized. In actual operation, due to the large temperature difference between the heat and cold medium of the heat exchanger, the heat exchanger leaks. There are two sets of light cooling towers of the No. 500 shallow cooling unit that are shut down. The low temperature light hydrocarbons are directly piped without being stabilized, which is not only wasteful. A large amount of light hydrocarbons are cooled, and the temperature of light hydrocarbons is gradually increased during the pipeline transportation process, and some light hydrocarbons are gasified, which increases the light hydrocarbon loss and the pipeline energy consumption during the pipeline transportation process. In addition, the dry gas at 5 °C is directly sent out, and the amount of cold is not recovered.
2 cold recovery recovery program
2.1 Light hydrocarbon cold recovery plan
According to the principle of cold cascade utilization, the temperature of light hydrocarbon at -25 °C is 35 °C lower than the temperature of cooling water. The cooling capacity should be placed after the water cooler, and the hydrocarbon can be set at the appropriate position after the primary three-phase separator. A gas heat exchanger to recover light hydrocarbon cooling. After the recovery of the light hydrocarbon cold recovery, the cold amount of light hydrocarbons of -25 to -10 ° C can be recovered for cooling the natural gas after the water cooler. Recycling this part of the cooling capacity can reduce the ammonia compressor load and save energy.
2.1.1 The hydrocarbon gas heat exchanger is connected in series with the lean gas-rich gas heat exchanger. The lean gas-rich gas heat exchanger is connected in series with one hydrocarbon gas heat exchanger to recover the light hydrocarbon cooling capacity. There are two types in series: 1 The hydrocarbon gas heat exchanger is in the front, and the lean-rich gas heat exchanger is in the rear. The advantage of this scheme is that the light hydrocarbon cooling capacity is fully recovered. The disadvantage is that the cold volume of the external gas is not fully recovered; 2 lean gas-rich The gas heat exchanger is in the front and the hydrocarbon gas heat exchanger is in the rear. The advantage of this scheme is that the external gas cooling capacity is fully recovered. The disadvantage is that the light hydrocarbon cooling capacity is not fully recovered. Since the light hydrocarbon cooling capacity is less than the external cooling gas, in order to make reasonable use of the cooling capacity, the lean gas-rich gas heat exchanger is selected in the front, and the hydrocarbon gas heat exchanger is in the latter mode.
2.1.2 The hydrocarbon gas heat exchanger is connected in parallel with the lean gas-rich gas heat exchanger. The lean gas-rich gas heat exchanger and the hydrocarbon gas heat exchanger are operated in parallel. The hydrocarbon gas heat exchanger is divided into a lean gas-rich gas. The load of the heat exchanger is very important. If the temperature of the rich gas at the outlet of the two heat exchangers after heat exchange is basically matched, and then mixed into the ammonia evaporator, the effect will be better.
2.2 External dry gas cold recovery plan
The external dry gas temperature is 5 ° C. If the device is directly discharged according to the process, this part of the cooling capacity is not utilized. For the recycling of this part of the cold, only one gas-rich gas heat exchanger of the same type is added, which is realized by connecting the original lean-rich gas heat exchanger in series.
3 cold recovery scheme simulation calculation and comparison
According to the actual daily processing gas volume (2.911×105 m3) of apricot three-shrinking device, the composition of raw material gas (see Table 1) and the actual production operation parameters of the plant, the simulation calculation was carried out by HYSIM software, and the natural gas volume of the first-stage separator was obtained. 12257kg / h, the secondary separator light hydrocarbon and external dry gas volume are 2087kg / h and 10162kg / h respectively. All calculations below are based on this.
3.1 hydrocarbon gas heat exchanger and lean gas-rich gas heat exchanger in series
If the light hydrocarbon recovery temperature is 0 ° C, the calculation result of the hydrocarbon gas heat exchanger in series with the lean gas-rich gas heat exchanger. When the light hydrocarbon recovery temperature is 0 ° C, the hydrocarbon gas heat exchanger will have the natural gas temperature from 5 ° C. It was lowered to 1.7 ° C, the temperature drop was 3.3 ° C, and the cooling amount of 130.9 MJ / h was recovered. In the original process, the load of the ammonia evaporator was 1293 MJ/h. Through the series of hydrocarbon gas heat exchangers, the load of the ammonia evaporator was reduced by 10.1%.
3.2 Hydrocarbon gas heat exchanger in parallel with lean gas-rich gas heat exchanger
If the light hydrocarbon recovery temperature is 5 ° C, the calculation results of the hydrocarbon gas heat exchanger and the lean gas-rich gas heat exchanger in parallel. When the light hydrocarbon recovery temperature is 5 ° C, the hydrocarbon gas heat exchanger will enrich the natural gas temperature by 5 The °C was lowered to 0.76 ° C, the temperature drop was 4.24 ° C, and the cooling capacity of 159.1 MJ / h was recovered. In the original process, the load of the ammonia evaporator was 1293 MJ/h. By paralleling the hydrocarbon gas heat exchanger, the load of the ammonia evaporator was reduced by 12.2%.
3.3 Comparison of series and parallel schemes for hydrocarbon gas heat exchangers and lean gas-rich gas heat exchangers
The comparison between the series scheme and the parallel scheme is shown in Table 4. As can be seen from Table 4, in the series scheme, the light hydrocarbons at -25 °C exchange heat with the natural gas at 5 °C, and the recovery temperature of light hydrocarbons can not be too low, which is set at 0 °C. The light hydrocarbon cooling capacity is not fully recovered. At the same time, the pressure drop of the system is increased by about 50 kPa after the original process is connected in series with a heat exchanger. In order not to affect the subsequent light hydrocarbon production, this part of the pressure drop needs to be increased by the compressor outlet pressure. To compensate, so that the compressor load increased by 22.9kW. In the parallel scheme, the light hydrocarbon of -25 °C and the natural gas of 24 °C heat exchange, the recovery temperature is lower than the series scheme, set to 5 ° C, light hydrocarbon refrigeration recovery Better than the series solution, in addition, the parallel solution does not increase the system pressure drop. Therefore, the parallel scheme is better than the series scheme.
3.4 External dry gas cooling recovery plan
When the recovery temperature of the external dry gas reaches 10 °C, the calculation result obtained by connecting a lean gas-rich gas heat exchanger of the same type in series can reduce the rich gas temperature to 2.3 °C after the reform, which is 2.7 °C lower than before the reform. The recovery cooling capacity is 104.5MJ/h. In the original process, the ammonia evaporator load is 1293MJ/h, by adding one of the same type of lean-rich gas heat exchanger to the original lean-rich gas heat exchanger in series, ammonia The evaporator load was reduced by 8.1%.
4 cold recovery process optimization and benefit estimation
4.1 Cold recovery process optimization
Compared with the above single modification scheme, if the light hydrocarbons are optimally combined with the external gas cooling recovery scheme, the heat exchange between the external gas heat exchanger and the lean gas-rich gas heat exchanger is adopted. In parallel with it, the effect will be better. At this time, the light hydrocarbon at -25 °C and the external gas at 5 °C need to be exchanged with the rich gas to recover the cooling capacity, so that two heat exchangers need to be added, and three heat exchangers are added to the original heat exchanger. The scheme is complicated in process and the equipment footprint is also increased. It is also inconvenient to rationally mix the rich gas and the antifreeze glycol in the operation. In addition, in the external dry gas cold recovery process, the pressure drop of the rich gas entering the ammonia evaporator is increased due to the series of lean gas-rich gas heat exchangers in series. In order to ensure the subsequent condensation pressure, the compressor must be increased accordingly. Outlet pressure. If the compressor pressure is increased, the natural gas temperature at the compressor outlet will increase, and the subsequent cooling load needs to be increased, so the extra pressure drop during the cold recovery process should be minimized.
In summary, the original process is improved, and the new high-efficiency multi-stream heat exchanger is used to replace the original lean-rich gas heat exchanger. The heat exchanger is an important equipment in the utilization of energy systems. The multi-stream heat exchanger is a heat exchange device that performs heat exchange at the same time. It can realize special process and is widely used in petrochemical, aviation, power machinery, Vehicles, electronics and many other fields. The use of multi-stream heat exchangers in the reforming process is more suitable. The main reason is that the initial temperatures of the external dry gas and light hydrocarbons are both -25 °C, and the temperature after heat exchange does not need to be specially controlled, because there is no follow-up process. The heat transfer temperature is required, and the portion of the heat that is not recovered by the heat exchanger can be further utilized by subsequent systems. For example, a shell-and-tube multi-stream heat exchanger is used, which consists of a front end pipe box, a casing, an array heat exchange tube, a baffle plate and a rear end structure. A plurality of tube-flow fluid inlets and outlets are disposed on the front end tube box and the rear end structure to achieve the purpose of heat exchange between the shell-side fluid and the plurality of tube-flow fluids in the same heat exchanger. If you are rich, you can take the shell, and send dry gas and light hydrocarbons to the pipe. The use of the heat exchanger can reduce the number of heat exchangers, improve the efficiency of the heat exchanger, save space, save energy, and optimize the process.
4.2 Benefit Estimation
After the modified optimization process, when the recovery temperature of light hydrocarbons and external dry gas reaches 10 °C, the heat recovery is 263.6 MJ/h, the temperature of rich natural gas is reduced from 5 °C to -1.94 °C, and the load of ammonia evaporator is reduced by 20.4%. Before the transformation, the power of the refrigeration unit was 381.7kW, the power consumption of refrigeration was 2.927×10 -2 kWh/m3, the power of the refrigeration unit after transformation was 303.8kW, and the power consumption of refrigeration was 2.330×10 -2 kWh/ m3. The amount decreased by 5.97×10 -3 kWh/m3. In 2008, the total amount of moisture treatment of the apricot shallow cooling unit was 1.0151×108 m3, which was reformed by cold recovery technology. The annual electricity saving was 6.061×105 kWh, and the annual production cost was 477,000. yuan.
5 Conclusion
Taking the apricot shallow cooling device of Daqing Oilfield Gas Company as an example to analyze the current status of light hydrocarbons and dry gas cooling in shallow cooling units, it is proposed to add one hydrocarbon gas heat exchanger to recover light hydrocarbon cooling capacity and one series of the same type. The lean-rich gas heat exchanger further recovers the cooling scheme of the dry gas from the outside, and on the basis of the simulation calculation by HYSIM software, analyzes and compares various reform schemes, and finally proposes the use of multi-stream heat exchange. The process of cold recovery.
Using this optimized process, the recovery temperature of light hydrocarbons and external gas reaches 10 °C, the heat recovery is 263.6 MJ/h, the temperature of rich natural gas is reduced from 5 °C to -1.94 °C, and the load of ammonia evaporator is reduced by 20.4%. 6.061×105 kWh, annual production cost savings of 477,000 yuan.
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