There are many energy saving measures for distillation. A large number of theoretical analysis and experimental research as well as industrial applications show that a better solution for the rectification process is heat pump rectification, and its energy saving effect and economic benefit are very significant.
The source of heat pump rectification can be divided into two categories: one is direct heat pump rectification, the material in the tower is the working medium; the other is indirect heat pump rectification, with additional circulating substances (such as Refrigerant, water, etc.) are working fluids.
The choice of heat pump rectification process should be closely combined with specific conditions (such as local fuel price, grade and quantity of waste heat used, high-grade energy use, etc.) in order to give full play to the advantages of each heat pump distillation process and achieve maximum energy saving effect. And economic benefits.
1 direct heat pump distillation
111 tower top direct heat pump distillation
The tower top direct heat pump distillation process, the main components are: distillation column (1), compressor (2), drive (3), evaporator (4), auxiliary evaporator (5) and so on.
Tower top direct heat pump rectification flow chart In the compressor 2, the overhead steam is raised to a higher pressure and temperature, and then in the evaporator 4, the heat is transferred to the bottom product and condensed by itself, and the condensate is partially The reflux is carried out and the other part is discharged as a product.
The top-of-line heat pump rectification is characterized by (1) the required heat transfer medium is ready-made; (2) because only one heat exchanger is required, the compression ratio of the compressor is usually lower than the compression ratio with indirect circulation. (3) The system is simple, stable and reliable.
The top product may not be suitable as a heat transfer medium. The main reasons are: (1) a large amount of excess heat (for low molecular weight compounds); (2) a liquid phase may be generated during compression or cause polymerization or decomposition (high Molecular compound); (3) Inefficient system efficiency due to unfavorable thermodynamic properties and heat transfer characteristics; (4) Safety and corrosion problems involved; (5) High volume flow rate near critical pressure or very low pressure (At this time, a large, expensive compressor is required). In addition, the compressors in this process are demanding, non-leakable, non-contaminating, and sometimes resistant to corrosion, and different systems require different compressors.
The direct heat pump rectification at the top of the tower is suitable for application in the temperature difference between the top of the tower and the bottom of the tower, or the components of the separated system are difficult to separate due to the similar boiling point, and a large reflux ratio must be used, thereby consuming a large amount of heating steam (ie, high load). The reboiler), or the top condensate (ie, the fraction) is a rectification system that requires cryogenic cooling.
The direct heat pump distillation at the top of the tower is the most widely studied and applied. The propylene-propane separation system uses a heat pump, and its thermodynamic efficiency can be increased from 3161% to 811%. It can be seen from the table that when heat pump distillation is used, energy is used. The cost has dropped dramatically. At this point, the cooling water temperature is no longer a determining factor, and the tower can operate at lower pressures, which simplifies the separation process and reduces equipment costs.
112 split heat pump distillation
The split heat pump distillation process is shown in Figure (2). The main components are: upper tower (1), compressor (2), drive (3), upper tower evaporator (4), lower tower (5), lower tower. Boiling device (6).
Split heat pump rectification flow chart Split heat pump rectification process is divided into upper and lower two towers. The upper tower is similar to direct heat pump rectification, but only one feed port; for the lower tower, it is similar to conventional fine The stripping section of the distillation is the steaming tower, the feed liquid from the upper tower is fed, and the steam discharge enters the bottom of the upper tower. The energy saving effect of the split heat pump rectification is obvious, the investment cost is moderate, and the control is simple.
The characteristic of split heat pump rectification is that the temperature difference of the upper tower can be adjusted by controlling the concentration of the split point (ie, the feed concentration of the lower tower) to select a suitable compressor. In actual design, optimization of the concentration of the split point is necessary. .
The split heat pump rectification is suitable for the phase diagram of the separation system. There is a large temperature difference rectification in the constant concentration zone and the constant dilution zone, such as an aqueous solution of ethanol or an aqueous solution of isopropanol. The results of a conventional rectification, overhead direct heat pump rectification and split heat pump rectification process for the treatment of isopropanol aqueous solution, can be seen from the table, the split type can be selected as a single-stage compressor, and its power consumption is greatly reduced. And the top of the tower must choose an expensive multi-stage compressor, which consumes almost twice as much power as the split type.
In addition, direct heat pump rectification is also a tower-type, intermediate-type, SRV-type heat pump rectification, and their energy saving and economic effects are also very obvious.
2 indirect heat pump distillation
The main components are: rectification tower (1), compressor (2), actuator (3), evaporator (4), auxiliary evaporator (5), condenser (6), expansion valve (7).
In the closed cycle, the circulating medium absorbs the condensation heat of the overhead product in the condenser 6 and vaporizes itself. After being compressed by the compressor 2, it is raised to a higher pressure and temperature, and then evaporated at the bottom of the column. The working fluid in vessel 4 condenses again, transferring its heat to the vaporized bottom product. As a result, the working medium enters the condenser 6 through the expansion valve 7 for recycling, from which it can be seen that this makes full use of the heat released by the condensation system itself.
Indirect heat pump rectification is characterized by: (1) the product to be separated in the tower is completely isolated from the working fluid; (2) can be designed and controlled using a standard rectification system; (3) direct vapor compression phase with the top of the tower In comparison, there is one more heat exchanger, which means that the compressor needs to overcome the higher temperature difference and pressure difference, so its efficiency is lower.
The scope of indirect heat pump rectification is the rectification system that is not suitable for heat sensitive products, corrosive media or overhead products.
Considering the chemical stability of the working fluid, the temperature range of indirect heat pump rectification applications is limited to about 130 ° C, while many organic product towers operate at higher temperatures. Compared with ordinary refrigerants, water has many advantages as a high-temperature working medium. Water has a high degree of chemical and thermal stability (discharge temperature up to 246 ° C), rich in physical properties during engineering design, no negative effects on people, the environment and the ozone layer when leaking, and very cheap, almost no cost, the most important point It is an excellent heat transfer characteristic of water, and the heat exchange area required for heat exchange is small, and it is particularly suitable for a rectification system having a higher temperature at the bottom of the column. Using water as the working medium, the indirect heat pump distillation process is used to treat the energy saving and economic effects of an ethylbenzene-p-xylene solution.
It can be obtained from the table that both types of heat pump distillation are energy-saving, and the additional investment recovery period is less than one year. The core of the heat pump rectification system is the compressor. The indirect heat pump rectification compressor is very small (the impeller diameter is about 350mm, and the direct impeller diameter is 1100mm compared to the top of the tower). Moreover, compared with ethylbenzene, engineers are even more Familiar with the physical properties of water, design is more convenient and reliable.
The degree of matching between quality and cold source and heat source. Therefore, when the temperature of the cold source and the heat source changes greatly during the heat transfer process, it is preferable to use a ternary mixed working medium (the composition of the mixed working medium is too complicated if the components are too much).
3 Summary and outlook
(1) Under suitable conditions, heat pump rectification can obtain significant economic benefits. For example, a multi-downcomer rectification tower, a reboiler with high-efficiency components and a heat pump can be used, and the effect is more obvious.
(2) In the actual design, several typical heat pump rectification processes described above can be improved to expand the application range of heat pump rectification. Moreover, if the heat pump rectification is optimized, the energy saving effect and economic benefit are better.
(3) In the process integration technology, the process flow of heat pump rectification should be reasonably selected according to the actual conditions of the plant to ensure that the heat pump rectification crosses the defect.
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