Refrigeration and liquefaction in cryogenic air separation process

Refrigeration in cryogenic air separation is an extremely important part of cryogenic air separation process and process plan, which is also known as cryogenic engineering R7 e9 I' k& r
1The core of the cryogenic air separation process and process plan is undoubtedly distillation. The cryogenic air separation is a fully self heating open heat pump distillation process. However, because the cryogenic air separation distillation process is in the cryogenic condition far away from the ambient temperature, the start of both the open heat pump and the distillation process must be realized under the coexistence of gas and liquid. The refrigeration and liquefaction of the cryogenic air separation become the prerequisite for the start of the open heat pump distillation. As the cryogenic air separation is a distillation process under cryogenic conditions, the cooling loss of the cryogenic air separation unit, the reheating normal temperature compression or cryogenic compression of the working medium of the open heat pump cycle, the temperature difference between the heat exchange of the rectification raw materials and the rectification products, and the liquid products drawn out will all produce cooling losses, which can only be achieved by refrigeration and liquefaction in order to achieve the so-called cooling capacity balance (in essence, the cryogenic liquid balance). Refrigeration and liquefaction in cryogenic air separation are very important components next to distillation process$ [7 ]) M- U1 }3 y
Refrigeration in deep cooling air separation has a specific meaning, specifically referring to the process of outputting heat to the environment. The open heat pump used for distillation cooling and heating in deep cooling air separation process and process plan also generates cold energy (of course, the effective energy of temperature difference is also a kind of cold energy), but does not produce cold energy. Its heat circulates between the bottom and top of the distillation tower, so it is not deep cooling air separation refrigeration- Q) P9 o' k$ ~( [( U
When discussing deep cooling air separation refrigeration, generally only the isothermal enthalpy difference and expander refrigeration are discussed, and only the refrigeration capacity balance, refrigeration coefficient and refrigeration efficiency are discussed. This is not comprehensive. The so-called refrigeration capacity balance only shows the quantitative relationship between the isothermal enthalpy difference, the expansion refrigeration capacity, the cooling loss and the diffuse cooling loss caused by the temperature difference at the hot end of the positive return air, and the cooling capacity taken away by liquid products. If we analyze from the perspective of cooling capacity balance, the higher the inlet temperature of the expander is at high temperature and high enthalpy drop, the greater the expansion refrigeration capacity is at the same expansion capacity and expansion ratio, which is more conducive to the cooling capacity balance. But just from the perspective of cooling capacity balance, we cannot know how to improve the inlet temperature of the expander, and what factors affect the inlet temperature of the expander! 4 o2 ~9 l0 ~1 z/ a
The cooling capacity balance (heat balance) is the universal principle of the first law of thermodynamics. As far as the cryogenic air separation unit is concerned, the so-called cooling capacity balance has a more specific and profound meaning, that is, the profit and loss of the cryogenic liquid. If the cryogenic liquid is surplus (specifically, the main cold liquid level rises), the cooling capacity balance is surplus, and if the cryogenic liquid is deficit (mainly, the main cold liquid level drops), the cooling capacity is insufficient, The so-called cooling capacity balance (heat balance) is the balance of cryogenic liquid (shown as the balance of main cold liquid level does not rise or rise). In this way, we will transform the refrigeration balance (heat balance) of the universal abstract first law of thermodynamics into a more specific and profound cryogenic liquid balance of the air separation unit! From the perspective of cryogenic liquid balance, we can see that the cryogenic liquid in the air separation unit is different from the perspective of cooling capacity balance. First, all the cryogenic liquid in the air separation unit comes from the liquid air formed by the liquefaction of the return gas cooling capacity below the condensation temperature of the positive flow pressure air absorbed by the positive flow pressure air. The cooling energy of liquid oxygen, liquid nitrogen and liquid argon is converted from the cooling energy of liquid air cooling energy. The other is the liquid air from the liquefaction of positive flow pressure air. In addition to a part of the cold energy converted into liquid products, most of the liquid air latent heat is used to compensate the cooling loss of the distillation system and the sensible heat between the air and the air condensation temperature after expansion refrigeration. Third, the inlet temperature of the expander is determined by the positive flow pressure air condensation temperature and the number of liquid products (the number of liquid products taken out increases, the number of return gas decreases, and the inlet temperature of the expander increases). G& B7 {( j" {) Z8 t7 R# D) _
From the perspective of cooling capacity balance, we only know that high temperature and high enthalpy drop, the higher the inlet temperature of the expander, the better! However, from the perspective of cryogenic liquid balance, we can see that the inlet temperature of the expander depends on the positive air pressure for liquefaction and the amount of cryogenic liquid products taken out! At the same time, it is also constrained by the cryogenic liquid balance. Under the same expansion ratio, the higher the inlet temperature of the expander, the higher the air temperature after the corresponding expansion refrigeration, and the greater the difference between the temperature and the air condensation temperature, the more liquid air is consumed!
From the perspective of universal abstract cooling capacity balance (heat balance), we can see that the air separation unit is composed of isothermal enthalpy difference and expansion refrigeration and open heat pump distillation, while from the perspective of more specific and profound cryogenic liquid balance, we can see that the cryogenic air separation unit is composed of open heat pump distillation and open heat pump - isothermal enthalpy difference expansion refrigeration liquefaction, It is a combined unit of cryogenic gas liquefaction unit and cryogenic air separation distillation unit. Isothermal enthalpy difference expansion refrigeration is only a part of open heat pump - isothermal enthalpy difference expansion refrigeration! However, there is a significant difference between the effective energy efficiency of isothermal enthalpy difference expansion refrigeration (subsystem effective energy efficiency) and the liquefaction efficiency of open heat pump isothermal enthalpy difference expansion refrigeration. Under the given equipment performance parameters of compressor isothermal efficiency 70% and expander adiabatic efficiency 85%, under the extreme engineering conditions (no diffuse cooling loss, no positive reflux resistance loss, and no heat exchange temperature difference), the ultimate effective energy efficiency of isothermal enthalpy difference expansion refrigeration is lower than 50%, while the liquefaction efficiency of open heat pump isothermal enthalpy difference expansion refrigeration is lower than 35%! The gap between the two is huge!
2There are three processes to realize refrigeration liquefaction. One is heat pump refrigeration liquefaction, which is the most widely used and most efficient refrigeration liquefaction process. Air conditioners and refrigerators in daily life use heat pump refrigeration. The working medium of the heat pump refrigeration cycle is compressed to a certain pressure, and heat is exchanged with the environment in the high-temperature and high-pressure heat exchanger connected with the environment. The heat is output to the environment, and the circulating working medium is liquefied by itself, After throttling and depressurization, the heat is absorbed in the low temperature and low pressure heat exchanger connected with the system (cold output) and vaporized into the next cycle. Heat pump refrigeration can only produce cold energy and cooling capacity with the temperature difference less than 50 degrees from ambient temperature. If the isothermal efficiency of the compression process of circulating working medium is about 70%, its refrigeration efficiency is about 60%! Of course, the cooling energy and cooling capacity with greater temperature difference between the environment and the heat pump relay can be obtained through the heat pump relay scheme. With the increase of heat pump relay times, the cooling efficiency drops rapidly. The cooling efficiency of the two heat pump relay has been below 40%! Both heat pump refrigeration and heat pump relay refrigeration are not suitable for cryogenic air separation refrigeration.
Isothermal enthalpy difference refrigeration and expansion refrigeration are used in cryogenic air separation. There is no isothermal enthalpy difference for ideal gas, and the isothermal enthalpy difference is actually a sign of deviation between actual gas and ideal gas. Although the internal energy of pressure gas at ambient temperature increases compared with that at ambient temperature and pressure, its enthalpy is smaller than that of atmospheric gas at ambient temperature. The difference between the two is the so-called isothermal enthalpy difference. Using this characteristic of actual gas, the so-called isothermal enthalpy difference refrigeration can be achieved through gas compression, but to generate sufficient cooling capacity, the gas needs to be compressed to a very high pressure! This is a very inefficient refrigeration scheme. The so-called expansion refrigeration is actually a thermal work conversion process. The thermal work conversion process above the ambient temperature is used for power generation. Its focus is on expansion output work. The thermal work conversion below the ambient temperature is the so-called expansion refrigeration. Its focus is on the enthalpy drop (refrigerating capacity) of the expansion working medium. In terms of quantity, the enthalpy drop of the expansion working medium during the adiabatic expansion process is equal to the expansion output work, which is relative to the isothermal enthalpy difference refrigeration, A cryogenic air separation refrigeration scheme with higher heat pump relay efficiency.
3The efficiency of thermal power conversion expansion refrigeration process has two indicators, one is the expansion refrigeration coefficient, and the other is the expansion refrigeration efficiency. The expansion refrigeration coefficient is the ratio of the enthalpy drop of the expansion refrigerant and the compression power consumption at room temperature (ambient temperature). The expansion refrigeration efficiency is the ratio of the cold energy (effective energy) in the enthalpy drop of expansion refrigeration and the compression power consumption of the expansion refrigerant at room temperature! Expansion refrigeration coefficient and expansion refrigeration efficiency are both useful! If the performance parameters of the equipment remain unchanged, the higher the inlet temperature of the expander is under the same pressure, the greater the enthalpy drop in the expansion refrigeration process, and the greater the corresponding refrigeration coefficient, which is the so-called high temperature high enthalpy drop! However, the expansion refrigeration efficiency is opposite. If the performance parameters of the equipment remain unchanged, the higher the inlet temperature of the expander is, the lower the expansion refrigeration efficiency is!
4Under the extreme engineering conditions (no positive return flow resistance loss, no heat exchange temperature difference, no cooling dissipation loss), in the open heat pump expansion refrigeration liquefaction process scheme, the feed gas pressure for liquefaction and the pressure of the working medium for expansion refrigeration cycle have no significant impact on the open heat pump expansion refrigeration efficiency, but under the actual engineering conditions, The pressure of the feed gas used for liquefaction and the pressure of the working medium of the expansion refrigeration cycle will have a significant impact on the liquefaction efficiency of the open heat pump expansion refrigeration. If it is not properly selected, it will significantly reduce the liquefaction efficiency of the open heat pump expansion refrigeration under the same equipment performance parameters!

Kaifeng Kaixing Air Separation Equipment Co., Ltd. is an energy-saving and environmental protection emerging technology service enterprise integrating air separation equipment, cryogenic technology application, energy-saving technology promotion, and contract energy management. The main products are complete sets of air separation plants, dynamic air separation plants, oxygen compressors, nitrogen compressors, turboexpanders, and air compressors.