Large-temperature-span cascade adjustable heat recovery rectification system

Abstract

The invention discloses a large-temperature-span cascade adjustable heat recovery rectification system, which belongs to the technical field of rectification systems, and comprises a rectification tower, an evaporator group, a compressor, a tower kettle heat exchanger, an ejector, a gas-liquid separator, a first liquid storage tank and a second liquid storage tank, the evaporator set comprises a first evaporator and a second evaporator which are connected in series, one end of the rectifying tower is connected with the first evaporator, the second evaporator is connected with the second liquid storage tank, the first evaporator and the second evaporator are both connected with the ejector, and one end of the gas-liquid separator is connected with one end of the ejector. And the other end of the gas-liquid separator is respectively connected with the first liquid storage tank, the first evaporator and the second evaporator. According to the invention, compression and injection modes are adopted, liquid supply flash gas is used as power to inject heat pump working medium gas of the second evaporator, cascade recovery of tower top gas condensation heat is realized, the evaporation temperature is increased, the heat pump rectification efficiency is improved, and the operation cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of distillation systems, and in particular to a large-temperature, cross-step, adjustable heat recovery distillation system. Background Art

[0002] In the petrochemical and chemical industries, heat pump distillation technology is being promoted and applied as a means of energy conservation and consumption reduction. Among them, closed heat pump distillation systems are characterized by high safety, wide application range, and high system stability.

[0003] For the tower top gas which is a non-azeotropic medium gas with a large temperature span, there is a problem of large temperature glide span during condensation. The temperature glide span is related to the composition and proportion of the multi-component gas.

[0004] When a high-temperature cross-medium distillation tower uses a heat pump distillation system, the following problems may occur: One method is to reduce the evaporation temperature of the heat pump distillation system so that the heat of the tower top gas can be fully recovered. However, the evaporation temperature of this method is low, which will seriously affect the energy efficiency of the overall heat pump distillation. Another approach is to use a heat pump distillation system for the high evaporation temperature portion and a water-cooled chiller for the low evaporation temperature portion. This dissipates heat into the environment, wasting cooling water and overhead gas heat. If the overhead medium temperature glide requires cooling water below 30°C, and high summer ambient temperatures prevent the cooling water from meeting the requirement, adding a chiller system is the only option. This increases costs, and because some heat cannot be fully recovered, the overall energy efficiency of the heat pump distillation system is also low.

[0005] At the same time, the height difference between the top condenser and the bottom heat exchanger of the distillation tower is generally large, which leads to stability problems in the liquid supply of the closed heat pump distillation. Summary of the Invention

[0006] In response to the deficiencies in the prior art, the present invention provides a large-temperature-span step-adjustable heat recovery distillation system. By adopting an ejector to use the depressurized flashed working fluid gas in the liquid supply pipeline of the heat pump system as power, the condensation heat of the large-temperature-span medium is recovered in steps, the evaporation temperature of the heat pump distillation system is increased, and the heat of the top gas is completely recovered without the use of cooling water and ice machines, thereby improving the operating efficiency of the distillation system, reducing equipment operating costs and investment costs, and solving the stability problem of liquid supply with large height differences. Depressurization flash refers to the phenomenon that when a high-pressure liquid flows through a valve, orifice plate, pressure reducer or other throttling element in a pipeline, the pressure suddenly drops below the saturated vapor pressure of the liquid at its current temperature, causing part of the liquid to instantly vaporize into steam. This is also called flash steam.

[0007] The technical solution of the present invention to solve the above technical problems is as follows: A large temperature-span step-adjustable heat recovery distillation system comprises a distillation tower, an evaporator group, a compressor, a tower bottom heat exchanger, an ejector, a first liquid storage tank, a second liquid storage tank, and a gas-liquid separator; the evaporator group comprises a first evaporator and a second evaporator, the first evaporator and the second evaporator tube sides are connected in series, one end of the first evaporator tube side is connected to the distillation tower, the other end of the first evaporator tube side is connected to one end of the second evaporator tube side, and the other end of the second evaporator tube side is connected to the second liquid storage tank; the first evaporator and the second evaporator tube sides flow through the tower top medium; the first evaporator and the second evaporator shell sides flow through the heat pump medium, one end of the first evaporator shell side is connected to the ejector outlet and then connected to the compressor inlet; the second evaporator shell side outlet is connected to the ejector outlet; the first evaporator shell side and the second evaporator shell side are arranged in parallel; The two ends of the tower bottom heat exchanger are respectively connected to the distillation tower, and the top gas of the distillation tower passes through the first evaporator and the second evaporator in sequence, and is condensed in sections and steps therein; the other end of the compressor is connected to the tower bottom heat exchanger; one end of the first liquid storage tank is connected to the tower bottom heat exchanger, and the other end of the first liquid storage tank is connected to the gas-liquid separator; the gas phase outlet of the gas-liquid separator is connected to the air inlet of the ejector, and the gas phase of the gas-liquid separator serves as the power source of the ejector. The gas-liquid separator is also respectively connected to the first liquid storage tank, the first evaporator and the second evaporator.

[0008] By adopting the above technical solution, the top gas of the distillation tower passes through the first evaporator and the second evaporator respectively, and then realizes segmented cascade condensation in the evaporator. The heat pump working fluid in the evaporator recovers the heat in the top gas, so that the heat pump working fluid is vaporized, and enters the tower bottom heat exchanger after being heated and pressurized by the compressor to provide energy for the tower bottom heat exchanger; wherein, the top gas generally exchanges heat in the tube side, and the heat pump working fluid generally exchanges heat in the shell side; the high-temperature and high-pressure heat pump working fluid releases heat and condenses into liquid in the tower bottom heat exchanger and then enters the first liquid storage tank; the heat pump working fluid condensed into liquid due to gravitational potential energy and pipeline pressure loss or regulating valve, the heat pump working fluid in the tower bottom heat exchanger is heated and pressurized by the compressor. Isoenthalpic pressure reduction during the liquid supply process refers to a thermodynamic process in which the enthalpy value (h) of a fluid, usually gas or steam, remains constant during the process of pressure reduction. A portion of gas, namely medium-pressure gas, will flash out. The pressure of this portion of gas is higher than the evaporation pressure. The medium-pressure gas separated in the gas-liquid separator enters the ejector and serves as power to recover the condensation heat of the high-temperature cross-medium in a step-by-step manner, thereby increasing the evaporation temperature of the heat pump distillation system and completely recovering the heat of the tower top gas. There is no need to use cooling water and ice machines, which improves the operating efficiency of the distillation system, reduces equipment operating costs and investment costs, and solves the stability problem of liquid supply with large height differences.

[0009] Furthermore, a second regulating valve is provided between the gas-liquid separator and the first evaporator; and a third regulating valve is provided between the gas-liquid separator and the second evaporator.

[0010] Furthermore, a first regulating valve is provided between the first liquid storage tank and the gas-liquid separator.

[0011] Furthermore, a fourth regulating valve is provided between the gas-liquid separator and the ejector.

[0012] Furthermore, a fifth regulating valve is provided between the injector and the second liquid storage tank.

[0013] Furthermore, the first evaporator is a shell and tube evaporator.

[0014] Furthermore, the second evaporator is a shell and tube evaporator or a plate evaporator.

[0015] In summary, compared with the prior art, the above technical solution has the following beneficial effects: (1) The present invention provides a large temperature-spanning step-adjustable heat recovery distillation system, which adopts compression and injection methods, uses the liquid supply flash gas as power, and injects the heat pump working fluid gas of the second evaporator to achieve step-by-step recovery of the condensation heat of the tower top gas, while increasing the evaporation temperature, improving the heat pump distillation efficiency, and reducing the operating cost; (2) The present invention provides a large temperature span step-adjustable heat recovery distillation system, which completely recovers or subcools the condensation heat of the large temperature span medium without using an ice machine or cooling water, thereby reducing cooling water and power consumption, and lowering investment and operating costs; (3) The present invention provides a large temperature span step adjustable heat recovery distillation system, which solves the problem of poor liquid supply stability in distillation towers with large height differences and avoids the influence of flash gas on the liquid supply regulating valve during the climbing process; (4) The present invention provides a large temperature span step adjustable heat recovery distillation system. When the distillation system fluctuates slightly, the fourth regulating valve and the fifth regulating valve can be used to fine-tune the system, so that the system can operate stably without adjusting the load of the compressor. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of the overall structure of Example 1; Figure 2 This is a schematic diagram of the overall structure of Example 2.

[0017] Explanation of the accompanying symbols: 1. Distillation tower; 2. Compressor; 3. Ejector; 4. First evaporator; 5. Bottom heat exchanger; 6. First liquid storage tank; 7. Gas-liquid separator; 8. Second evaporator; 9. Second liquid storage tank; 10. First regulating valve; 11. Second regulating valve; 12. Third regulating valve; 13. Fourth regulating valve; 14. Fifth regulating valve. DETAILED DESCRIPTION

[0018] The principles and features of the present invention are described below in conjunction with all the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0019] The embodiment of the present invention discloses a large-temperature, step-span, and adjustable heat recovery distillation system.

[0020] Example 1 Reference Figure 1 A large temperature-span step-adjustable heat recovery distillation system comprises a distillation tower 1, an evaporator group, a compressor 2, a tower bottom heat exchanger 5, an ejector 3, a gas-liquid separator 7, a first liquid storage tank 6, and a second liquid storage tank 9. The evaporator group comprises a first evaporator 4 and a second evaporator 8. Both ends of the tower bottom heat exchanger 5 are respectively connected to the distillation tower 1. One end of the distillation tower 1 is connected to the first evaporator 4. The overhead gas of the distillation tower 1 passes through the first evaporator 4 and the second evaporator 8 in sequence and is condensed in stages in the evaporators. One end of the compressor 2 is connected to the first evaporator 4 and the ejector 3, and the other end of the compressor 2 is connected to the tower bottom heat exchanger 5. The heat pump working fluid vaporized in the evaporator is heated and pressurized by the compressor 2 and enters the tower bottom heat exchanger 5. One end of the first liquid storage tank 6 is connected to the tower bottom heat exchanger 5, and the other end of the first liquid storage tank 6 is connected to the gas-liquid separator 7. One end of the second liquid storage tank 9 is connected to the second evaporator 8. One end of the first evaporator 4 is connected to the distillation tower 1 and the ejector 3, respectively. The other end of the first evaporator 4 is connected to one end of the second evaporator 8, and the other end of the second evaporator 8 is connected to the second liquid storage tank 9 and the ejector 3. The overhead medium flows through the tube side of the first and second evaporators 4 and 8; the heat pump medium flows through the shell side of the first and second evaporators 4 and 8. One end of the shell side of the first evaporator 4 is connected to the outlet of the ejector 3 and then connected to the inlet of the compressor 2; the shell side outlet of the second evaporator 8 is connected to the ejection port of the ejector 3; the shell side of the first evaporator 4 and the shell side of the second evaporator 8 are arranged in parallel.

[0021] Specifically, the distillation tower 1 is connected to a first evaporator 4, which is in turn connected to a second evaporator 8. That is, the first evaporator 4 and the second evaporator 8 are connected in series. The overhead gas from the distillation tower 1 passes through the first and second evaporators 4, 8, in sequence, where it undergoes staged condensation. The heat pump working fluid in the first and second evaporators 4, 8, recovers heat from the overhead gas medium. The liquid-phase heat pump working fluid vaporizes, is heated and pressurized by the compressor 2, and then enters the bottom heat exchanger 5, providing heat for the bottom heat exchanger 5. The overhead gas exchanges heat in the tube side of the first and second evaporators 4, 8, while the heat pump working fluid exchanges heat in the shell side. The high-temperature, high-pressure heat pump working fluid releases heat in the bottom heat exchanger 5, condensing into a liquid that enters the first liquid storage tank 6. The first liquid storage tank 6 is placed on the ground. A first regulating valve 10 is added between the first liquid storage tank 6 and the gas-liquid separator 7 to control the liquid level in the first liquid storage tank 6.

[0022] The gas-liquid separator 7 is placed at the same height as the first evaporator 4 and the second evaporator 8, typically at the top of the tower. Due to gravitational potential energy and pipeline pressure loss, the liquid heat pump working fluid undergoes isenthalpic decompression during the liquid supply process, causing a portion of gas, i.e., intermediate-pressure gas, to flash over. This gas has a pressure higher than the evaporation pressure. The separated intermediate-pressure gas enters the ejector 3 as power. The ejector 3 outlet is connected to the compressor 2 suction line, and the ejector 3 inlet is connected to the outlet of the second evaporator 8.

[0023] A second regulating valve 11 is installed between the first evaporator 4 and the gas-liquid separator 7, and a third regulating valve 12 is installed between the second evaporator 8 and the gas-liquid separator 7. The second regulating valve 11 regulates the liquid level in the first evaporator 4, which is typically a shell-and-tube evaporator. The third regulating valve 12 controls the liquid level or outlet gas superheat in the second evaporator 8, which is typically a shell-and-tube evaporator or a plate evaporator.

[0024] A fourth regulating valve 13 is provided between the gas-liquid separator 7 and the ejector 3 . The fourth regulating valve 13 controls the pressure of the gas-liquid separator 7 , thereby controlling the air intake of the ejector port 3 and further controlling the heat exchange amount of the second evaporator 8 .

[0025] According to the operating principle of the ejector 3, the air intake volume of the ejector port is mainly related to the inlet pressure of the ejector 3, the inlet flow rate of the ejector 3, the outlet pressure of the ejector 3, and the ejector port pressure.

[0026] Furthermore, the flash gas volume is related to the operating conditions of the heat pump system and the pressure of the gas-liquid separator 7. According to theoretical calculations and the performance of the ejector 3, the mass flow rate of the general ejector port is about 0.4-0.5 times the flow rate of the ejector power source; according to the theoretical calculation of isenthalpic throttling, the flow rate of the gas in the gas-liquid separator 7 is actually about 30% of the total flow rate, that is, the liquid flow rate accounts for 70%; further, the load of the second evaporator 8 is about 17%-21% of the load of the first evaporator 4.

[0027] A fifth regulating valve 14 is provided between the ejector port of the ejector 3 and the heat pump working medium outlet of the second evaporator 8. The fifth regulating valve 14 is used to adjust the outlet flow rate of the medium on the top of the second evaporator 8. Specifically, when the outlet temperature of the medium on the top of the second evaporator 8 is higher than the target temperature, the opening of the fifth regulating valve 14 is increased; when the outlet temperature of the medium on the top of the second evaporator 8 is lower than the target temperature, the opening of the fifth regulating valve 14 is decreased.

[0028] According to the injection principle of the ejector 3 , the pressure at the outlet of the ejector 3 is higher than the pressure at the injection port of the ejector 3 , that is, the evaporation pressure of the second evaporator 8 can be lower than the evaporation pressure of the first evaporator 4 .

[0029] Furthermore, the present invention is most applicable when the overhead medium is a non-azeotropic gas mixture with a wide temperature range. This gas experiences a wide temperature glide during the condensation process, and as the overhead mixed gas continues to condense, the condensation temperature gradually decreases, resulting in a lower evaporation temperature required by the heat pump working medium. Using the system of the present invention, the overhead gas condensation heat can be recovered in a staged manner, with the higher temperature section being recovered in the first overhead evaporator 4 and the lower temperature section being recovered in the second overhead evaporator 8, thereby increasing the evaporation temperature.

[0030] Furthermore, the present invention is also applicable to the following scenarios: the overhead medium is a single component or an azeotropic mixture, and the overhead medium is completely condensed in the first overhead evaporator 4 to fully recover the gas latent heat. In the second overhead evaporator 8, the overhead medium is supercooled to enable better extraction and reflux.

[0031] The specific example calculation is as follows: The condensation temperature of the non-azeotropic mixed component gas at the top of the distillation tower 1 of a certain project slides from 45-30℃; the boiling point of the tower bottom is 65℃, the condensation load of the tower top gas is completely recovered, the heat is about 2000kw, and the heat demand of the tower bottom is 2850kw.

[0032] User electricity price: 0.7 yuan / kwh, external steam price: 250 yuan / t steam.

[0033] A closed heat pump distillation system is introduced. The following is a parameter comparison of several schemes, as shown in Table 1.

[0034] Table 1 Comparison of scheme parameters

[0035] By adopting the technical solution of the present invention, the evaporation temperature can be increased by 3°C, the power consumption can be reduced by 69kw per hour, and the operating cost can be saved by 390,000 yuan per year, which is about 10% lower than the operating cost.

[0036] The working principle of the present invention is: The top gas of the distillation tower 1 passes through the first evaporator 4 and the second evaporator 8 respectively, and realizes segmented cascade condensation in the two evaporators. The heat pump working fluid in the first evaporator 4 and the second evaporator 8 recovers the heat in the top gas, so that the heat pump working fluid is vaporized, and enters the tower bottom heat exchanger after being heated and pressurized by the compressor 2, providing energy for the tower bottom heat exchanger 5; wherein, the top gas generally exchanges heat in the tube side, and the heat pump working fluid generally exchanges heat in the shell side; the high-temperature and high-pressure heat pump working fluid releases heat in the tower bottom heat exchanger 5 and condenses into liquid and then enters the first liquid storage tank 6; the condensed liquid Due to the gravitational potential energy and pipeline pressure loss, the heat pump working fluid will flash out a part of gas, namely medium-pressure gas, during the liquid supply process due to isenthalpic decompression. The pressure of this part of gas is higher than the evaporation pressure. The separated medium-pressure gas enters the ejector 3 and is used as power to recover the condensation heat of the large temperature cross medium in a step-by-step manner, thereby increasing the evaporation temperature of the heat pump distillation system and completely recovering the heat of the tower top gas. There is no need to use cooling water and ice machine, which improves the operating efficiency of the distillation system, reduces the equipment operating cost and investment cost, and solves the stability problem of liquid supply with large height difference.

[0037] Example 2 Reference Figure 2 The main difference between Example 2 and Example 1 is that the technical solution of Example 2 cancels the setting of the gas-liquid separator 7. At the same time, a second regulating valve 11 is provided between the ejector 3 and the first liquid storage tank 6; a third regulating valve 12 is provided between the first liquid storage tank 6 and the second evaporator 8.

[0038] High-pressure working fluid is used as the injection power source, the gas-liquid separator 7 is cancelled, the second regulating valve 11 controls the liquid level of the first evaporator 4, and the third regulating valve 12 controls the liquid level of the second evaporator 8 or the superheat degree of the heat pump working fluid outlet.

[0039] The fifth regulating valve 14 regulates the outlet temperature of the medium side of the top of the second evaporator 8 .

[0040] Since the embodiment 2 of the present invention does not solve the stability problem of liquid supply with large height difference, it is more suitable for the occasions where the distillation tower 1 is not high or the first evaporator 4 and the second evaporator 8 are not placed at a high height.

[0041] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A high-temperature, step-span, adjustable heat recovery distillation system, comprising a distillation tower (1), characterized in that: The invention also comprises an evaporator group, a compressor (2), a tower bottom heat exchanger (5), an ejector (3), a first liquid storage tank (6), a second liquid storage tank (9) and a gas-liquid separator (7); the evaporator group comprises a first evaporator (4) and a second evaporator (8), the first evaporator (4) and the second evaporator (8) are connected in series in terms of tube passage, one end of the tube passage of the first evaporator (4) is connected to the distillation tower (1), the other end of the tube passage of the first evaporator (4) is connected to one end of the tube passage of the second evaporator (8), and the The other end of the tube side of the second evaporator (8) is connected to the second liquid storage tank (9); the tower top medium circulates through the tube side of the first evaporator (4) and the second evaporator (8); the heat pump medium circulates through the shell side of the first evaporator (4) and the second evaporator (8); one end of the shell side of the first evaporator (4) is connected to the outlet of the ejector (3) and then connected to the inlet of the compressor (2); the shell side outlet of the second evaporator (8) is connected to the ejection port of the ejector (3); the shell side of the first evaporator (4) and the shell side of the second evaporator (8) are arranged in parallel; The two ends of the tower bottom heat exchanger (5) are respectively connected to the distillation tower (1), and the top gas of the distillation tower (1) passes through the first evaporator (4) and the second evaporator (8) in sequence and is condensed in stages therein; the other end of the compressor (2) is connected to the tower bottom heat exchanger (5); one end of the first liquid storage tank (6) is connected to the tower bottom heat exchanger (5), and the other end of the first liquid storage tank (6) is connected to the gas-liquid separator (7); the gas phase outlet of the gas-liquid separator (7) is connected to the air inlet of the ejector (3), and the gas phase of the gas-liquid separator (7) serves as the power source of the ejector (3). The gas-liquid separator (7) is also respectively connected to the first liquid storage tank (6), the first evaporator (4) and the second evaporator (8).

2. A large temperature span step adjustable heat recovery distillation system according to claim 1, characterized in that: A second regulating valve (11) is provided between the gas-liquid separator (7) and the first evaporator (4); and a third regulating valve (12) is provided between the gas-liquid separator (7) and the second evaporator (8).

3. The large temperature span step adjustable heat recovery distillation system according to claim 1, characterized in that: A first regulating valve (10) is provided between the first liquid storage tank (6) and the gas-liquid separator (7).

4. The large temperature span step adjustable heat recovery distillation system according to claim 1, characterized in that: A fourth regulating valve (13) is provided between the gas-liquid separator (7) and the ejector (3).

5. The large temperature span step adjustable heat recovery distillation system according to claim 1, characterized in that: A fifth regulating valve (14) is provided between the injector (3) and the second liquid storage tank (9).

6. A large temperature span step-adjustable heat recovery distillation system according to any one of claims 1 to 5, characterized in that: The first evaporator (4) is a shell and tube evaporator.

7. A large temperature span step-adjustable heat recovery distillation system according to any one of claims 1 to 5, characterized in that: The second evaporator (8) is a shell and tube evaporator or a plate evaporator.

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