1 Nitrogen expansion refrigeration liquefaction process Nitrogen expansion liquefaction process has been applied in China in recent years. Especially for small-scale liquefaction plants, it has the characteristics of simple process, compact structure, low cost, flexible operation, mature technology, easy operation and control. The refrigerant uses a single-component nitrogen gas, which is safe and does not cause fire or explosion hazard. The disadvantage is higher power consumption, which is about 40% higher than the mixed refrigerant liquefaction process.
(1) Coalbed methane process Coalbed methane feedstock gas enters this section after compression, desulfurization, decarburization, drying and mercury removal. First enter the cold box, the cold box is mainly composed of heat exchanger No.1, after pre-cooling through the heat exchanger No.1, the separated heavy hydrocarbons are discharged from point 25 to the heavy hydrocarbon storage tank, and the separated gas phase passes through 2, 3 The heat exchanger No. 4 is liquefied and deep-cooled separately, and is depressurized by a point of 6 to a point 24. 24. 24 is the product liquefied coalbed methane, which is transported to the storage tank by the residual pressure of the pipeline and stored under normal pressure or pressure.
(2) Refrigerant nitrogen process After the compressor is compressed by the compressor and cooled by water, it enters the low pressure supercharger, the supercharger of the medium pressure supercharger, and the cooler is cooled, and then enters the cold box from point 7. After pre-cooling by the No. 1 heat exchanger, it enters the medium-pressure expander. After expansion, the temperature and pressure are reduced, and the expansion power is driven to drive the medium-pressure supercharger. The expanded nitrogen enters the No. 3 heat exchanger from point 9 to continue cooling, and enters the low-pressure expander from point 10. After the expansion, the temperature and pressure are further reduced, and the expansion power is driven to drive the low-pressure supercharger. The refrigerant expanded by the low pressure expander enters each heat exchanger step by step from the coldest end point 11 of the heat exchanger to provide cooling capacity for the coal bed gas and the refrigerant.
2 Typical nitrogen expansion refrigeration liquefaction process calculation 2.1 Process calculation The purpose of process calculation is to determine the temperature, pressure and refrigerant flow at each point in the process and calculate the compressor power.
2. 2 Known conditions to be determined before calculation
(1) CBM temperature, pressure, flow and composition of the cold box inlet.
The calculation is based on coalbed methane in a certain area of ​​Shanxi Jincheng. The gas mole fraction is: methane 0.001, nitrogen 0. 024, oxygen 0. 005; daily coal bed gas treatment capacity is 25% 10 4 m 3 /d, average hour The flow rate is 10 417 m 3 /h, and the CBM pressure is 0. 5M Pa (the pressure refers to the absolute pressure), and the temperature is 34! .
(2) The storage temperature and pressure of the LCBM storage tank are determined by the type of LCBM storage tank.
(3) The temperature difference between the cold and hot fluid of the heat exchanger end face, take 3! .
(4) The efficiency of mechanical equipment is provided by the equipment manufacturer.
(5) Heat exchanger resistance, the total resistance of the cold box is taken to be 0.1 M Pa, and the average heat exchanger is distributed. The influence of the error caused by the calculation on the calculation result is within the error tolerance.
2. 3 Process calculation process (1) Point 1 calculates the resistance of the CBM compressor outlet pressure minus the absorption tower, dehydration, mercury removal device and pipeline by iteratively calculating the sum of the sum of the feed gas compressor power and the refrigerant compressor power. As the pressure into the cold box.
(2) Point 7:7 is the refrigerant inlet and outlet pressure, generally 4. 4 4. 7 M Pa. After iterative calculation to find the corresponding power consumption of the compressor is the minimum p 7. The temperature of the point T 7 = T 1.
(3) At 24:24, the LCBM is throttled in the cold box. The pressure is the storage pressure of the storage tank plus the local resistance of the conveying process, the resistance along the path and the static pressure of the liquid level difference, and considering a certain margin. The temperature is calculated according to the working temperature of the tank (considering the Joule-Thomson effect caused by the pressure drop from the cold box to the tank will cause a certain temperature drop, which will offset some of the temperature rise caused by the heat of the pipeline, so the calculation Calculated by equal temperature, the error caused by this is within the allowable range).
(4) The pressure at 6:6 is the inlet pressure of the CBM cold box minus the resistance of each heat exchanger. According to ç„“h 6 at the pre-throttle point 6 and ç„“h 24 at the post-throttle point 24, the point 6 temperature T is calculated by the software.
(5) Point 11:11 is in a state in which nitrogen is expanded by a low-pressure expander, p 11 is taken as 0. 4 MPa. The heat exchange temperature difference between the point 6 and the point 11 is taken as 3! Is the minimum heat transfer temperature difference of the plate-fin heat exchanger. If the temperature difference is too large, the heat exchanger has a large loss of fire and the compressor consumes a high amount of power. At the time of engineering design, the pressure of the point 10 is 1.49M Pa. According to the point 11 parameter and p 10 , the software can calculate T 10.
(6) Point 15 T 15 = T 1 - 3(1)p 15 = p 11 - p(2) where T i - point i temperature, K pi - point i absolute pressure, M Pa p - refrigeration The total resistance of the agent through the four heat exchangers, M Pa, takes 0.1 M Pa (7) point 8 and point 2 p 8 = p 7 - p 1 (3) T 2 = T 8 (4) p 2 = p 1 - p 1 (5) where p 1 - the resistance of the refrigerant or coalbed methane through the heat exchanger No. 1, M Pa, taking 0. 025 M Pa T 8 is the main factor affecting the power consumption of the refrigerant compressor One, iterative calculation is needed to find the minimum power consumption corresponding to T 8 and the iteration temperature range is 230 245 K.
(8) Point 9 and point 4 p 9 = p 10 + p 3(6)T 4 = T 9(7)p 4 = p 1 - p 1 - p 2 (8) where p 3 - refrigerant or The resistance of the coalbed methane through the No. 3 heat exchanger, MPa, taking 0. 025 MPa p 2 - the resistance of the refrigerant or coalbed methane through the heat exchanger No. 2, MPa, taking 0. 025 MPa according to the point 8 temperature T 8 The isentropic expansion is calculated by the software.
(9) Determine the circulation quantity qm of the refrigerant, and the heat balance equation of the heat exchangers of the 7th, 1st, 2nd, 3rd, and 4th.
Heat balance equation of heat exchanger No.1: qm,1(h 1 - h 2)+ qm,7(h 7 - h 8)= qm,7(h 15 - h 14)(9) No.2 heat exchanger heat balance equation :qm,1(h 3 - h 4)= qm,7(h 14 - h 13)(10) Heat exchanger heat balance equation No.3: qm,1(h 4 - h 5)+ qm,7(h 9 - h 10)= qm,7(h 13 - h 12)(11) Heat exchanger heat balance equation No. 4: qm,1(h 5 - h 6)= qm,7(h 12 - h 11)(12) In the formula, qm, i - point i coalbed methane or refrigerant mass flow, kg /hhi - point i coalbed methane or refrigerant ratio ç„“, kJ / (kg? K) is obtained by equation (9) (12) Qm, 7.
(10) Find T 14 from equation (9) and find h 14, for flash calculation.
(11) Find T 13 and obtain h13 from equation (10) for flash calculation.
(12) Find T 12 from equation (12) and find h 12 for flash calculation.
(13) After determining the outlet pressure of the low pressure supercharger p 20 to determine p 20 , the inlet pressure of the medium pressure supercharger is: p 21 = p 20 - pc (13) where pc - single cooler resistance, M Pa, Take 0. 01 M Pa to determine the inlet pressure, pressure and outlet pressure of the medium pressure booster, calculate the required power P s of the medium pressure booster, 2. Calculate the different p 20 to make the output of the medium pressure expander P e, 2 meets the power requirements of the medium voltage booster. Determining the compressor outlet pressure p 17 to determine p 17 is the same as above, that is, the low-pressure expander output power P e,1 can meet the low-pressure booster power P s, 1.? Calculate the compressor power P c from the compressor inlet pressure, temperature and outlet pressure to calculate the compressor power P c.
2. Constraint of results The negative temperature difference between the end faces of each heat exchanger cannot occur, and the design temperature difference between the hot and cold fluids is not less than 3 degrees Celsius.
In the nitrogen expansion refrigeration liquefaction process, when the pressure of coalbed methane is about 5 MPa after compression, it has been completely liquefied at 190 K. Since the coalbed methane is relatively small from the beginning of liquefaction to the full liquefaction temperature range, it is released in this temperature range. A large amount of latent heat is generated, so the temperature interval has a large proportion of the total load of the heat exchanger. In the nitrogen expansion refrigeration liquefaction system, the coalbed methane is mainly liquefied in the No. 2 heat exchanger.
If the liquefaction temperature range is at the bottom of the No. 2 heat exchanger, then the cooling load required for the No. 2 heat exchanger CBM is concentrated in the lower temperature range at the bottom. Since the refrigerant has no phase change, the provided cold load is relatively uniform with temperature distribution, and the refrigerant temperature is required to be lower than the temperature of the coalbed methane, and it is difficult for the refrigerant to provide a sufficient cold load in this small temperature range. Therefore, in the process calculation, each heat exchanger should be inspected for cold load from the low temperature section to the high temperature section by checking every 5 degrees Celsius temperature range from the bottom of the heat exchanger to ensure that the accumulated cold load from the low temperature to the high temperature temperature range is sufficient. .
3 Optimization results and analysis 3.1 Optimization results The liquefied unit volume of coalbed methane power consumption is 0. 623 kW? h / m 3. Since the coalbed methane component of this project does not contain heavy hydrocarbons, no heavy hydrocarbons are precipitated at point 25.
3. 2 compressor power consumption analysis (1) medium pressure expander inlet temperature under the condition that the pressure of the fixed feed gas after compression and the refrigerant enter the cold box pressure, the total power of the feed gas compressor and the refrigerant compressor with the medium pressure The variation curve of the inlet temperature of the expander can be seen, as the inlet temperature of the medium pressure expander increases, the total power consumption of the system decreases. This is because as the inlet temperature of the medium pressure expander increases, the heat load of the heat exchanger No. 1 decreases, and the total heat load of the system decreases, so the power consumption of the refrigerant compressor decreases. However, the inlet temperature of the medium pressure expander should not be too high, otherwise the heat exchanger of No. 2 will have a negative temperature difference and will not work properly.
(2) The pressure of the raw material gas after compression increases with the pressure of the coalbed methane compression, and the refrigerant compressor power decreases, the power of the feed gas compressor increases, and the compressor totals when the heat exchanger has no negative temperature difference. The power is increased. This is because as the pressure of the CBM compression increases, the coal bed gas enters the cold box and is lower than ç„“h1. Since the LCBM storage temperature and pressure are constant, the end point 24 of the cold box does not change. At this time, the difference between the inlet and the liquefaction of the coalbed methane is reduced, the total load required for the cold box is reduced, and the power of the refrigerant compressor is reduced.
Since the power increase rate of the raw material gas compressor is greater than the power reduction speed of the refrigerant compressor, the total power is slightly increased. In addition, as the pressure of the CBM compression increases, a higher intermediate pressure expander inlet temperature can be employed. After the comprehensive optimization calculation, the optimal coalbed methane enters the cold box pressure is 5. 3 MPa, and the medium pressure expander inlet temperature is 238 K.
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