Liquefied Natural Gas (LNG for short) has the features of being convenient for long-distance transportation, low storage and transportation cost, high calorific value, cleanliness, environmental protection, etc. In LNG Receiving Station, LNG is generally used after gasification, and the cold energy released during gasification is about 830kJ/kg, which is usually abandoned with seawater or air, how to recover the cold energy of LNG has become a hot issue at home and abroad in recent years, which causes great waste of energy. This paper focuses on the discussion of the application of LNG cold energy in the air separation plant.
1. Advantages of LNG cold energy application in air separation plant
Firstly, the process of LNG cold capacity utilization is to transfer the cold capacity of LNG to the working medium which needs cooling to achieve the purpose of cold capacity recovery. The irreversible loss in the process of cold energy recovery is relatively less, which is an ideal and efficient utilization method.
Secondly, recycling the cold volume of LNG can not only improve the cyclic liquefaction rate, but also obtain a large number of liquid products and save energy consumption.
Thirdly, the cold volume of the traditional air separation process is generated by the turbine expander, and the cold volume needs to be accumulated gradually during the start-up stage. If LNG is used, a large amount of cold volume can be obtained in a very short time, the start-up time of the air separation plant can be greatly shortened.
2. Liquid air separation unit with LNG cooling capacity
The scheme of introducing LNG cooling capacity into the air separation plant can be roughly divided into two situations, one is to design a new process scheme and invest to build a new system; the other is to improve the design based on the original equipment and introduce the cold capacity of LNG.
Scheme 1 is put forward aiming at the investment and construction of the new system, as shown in Figure 1.
This scheme adopts the liquid air separation process of LNG precooled medium-pressure nitrogen circulation, among which the rectification system is basically the same as the traditional medium-pressure nitrogen circulation liquid process, and the main difference lies in the refrigeration system. In the traditional process, the circulating nitrogen is divided into internal and external circulation. The internal circulation provides cooling for the distillation column, while the external circulation provides cooling for the internal circulating nitrogen. The new process cancels the nitrogen external circulation refrigeration system, and this part of cooling capacity is replaced by the cooling capacity released from the LNG gasification process. The new process runs like this: the circulating nitrogen from the top of the lower tower absorbs heat first through the main heat exchanger 3, cools the raw material air, and then enters the circulating compressor 9 and compresses it to about 2.6MPa, after cooling down by water chilling Unit 10, directly enter LNG heat exchanger 8 to absorb the cold energy released by LNG gasification, then cool down to about 120 k, and then depressurize to about 0.55MPa through throttle valve, return to the lower tower to provide cold energy, and then enter the next cycle.
The features of the new process are as follows: 1. The nitrogen external circulation system is canceled, and the nitrogen turbine expander and booster compressor are omitted in the equipment, which makes the process organization simpler; 2. Using LNG heat exchanger to replace the Freon refrigeration unit in the traditional process can effectively recover the cold volume of LNG and save energy consumption at the same time; 3. Due to the introduction of the cold volume of LNG, it reduces the precooling temperature of circulating nitrogen, thus reducing the highest operating pressure of the system and ensuring the safety; 4. It increases the temperature of circulating nitrogen entering the compressor and avoids the difficulty of low temperature compression.
Scheme 2 is put forward aiming at the transformation of existing equipment, as shown in figure 2. This scheme is to utilize the LNG precooled medium pressure nitrogen circulation tower outside the liquefaction air separation process, and its operation mode is as follows: after the pure nitrogen from the upper tower is reheated by the main heat exchanger, A part of the extraction (the extraction quantity depends on the quantity of liquid products to be produced) is first cooled down by LNG heat exchanger 11, and then liquefied in the liquid purifier 13 to be output as liquid products. After the gas pure oxygen obtained from the top of the tower is reheated, precooled in precooler 12 first, absorb the cooling amount of circulating nitrogen, and then enter the liquid purifier 13 for liquefaction to obtain liquid oxygen, which is used as a product to lead out. At the initial stage of starting the unit, a part of nitrogen is extracted from the nitrogen products for accumulation as circulating nitrogen. When the flow rate reaches the requirement, the circulating nitrogen is isolated from the rectification system through valve switching. Circulating nitrogen is compressed to about 2.3MPa in compressor 9, then cooled by water chilling unit 10 and liquefied in LNG heat exchanger 11, after high-pressure and low-temperature cycle nitrogen throttling, provide cooling for the Heat Exchanger 13 and 12, return to the compressor inlet, and proceed the next cycle.
This scheme is improved on the basis of the traditional whole low pressure gas product flow, and its biggest advantage is that it can produce liquid products without changing any original equipment, only one circulating nitrogen compressor, three heat exchanger and one water chilling unit need to be added on the basis of the original process, which is very suitable for only producing gas products. In addition, the product liquefaction system is independent from the rectification system. Under different demands, the production of gas-liquid products can be adjusted flexibly. The highest operating pressure of this process is further reduced, which is about 0.3MPa lower than that of Scheme 1. Compared with the traditional liquid process, the two new processes have obvious improvement. The amount of circulating nitrogen required by the new process is greatly reduced, and the highest operating pressure of the system is 2.6 MPa and 2.3MPa respectively, while the traditional liquid process is about 4.2-5.0MPa. In terms of energy consumption, is it about 1.05-1.25 kW compared with the traditional process? Compared with h/kg, both of the two schemes are greatly reduced, respectively 0.317, 0.384kW? H/kg, and the root cause of energy saving lies in the reduction of circulating gas volume and the reduction of maximum operating pressure. Compared with the two new plans under the same product output, the energy consumption of liquid products in Plan 2 is higher than that in plan 1, but the quantity of circulating nitrogen and the highest pressure in Plan 1 are higher than that in plan 2, after comparison, the two new schemes are applied in different occasions and each has its own advantages. They should be selected reasonably according to different needs.
3. Conclusion
(1) the introduction of LNG cold volume in the air separation process is not only beneficial to the rational utilization of cold volume, but also beneficial to the improvement of the liquefaction rate of the air separation system and the shortening of the start-up time of the unit, it is suitable for the occasion of producing more liquid products. LNG uses liquid pump to pressurize and save energy consumption, while the increase of heat transfer pressure has little influence on the performance of air separation. It is suggested to arrange the gasification of LNG after pressurization after comprehensive consideration.
(2) liquid air separation process after the introduction of LNG cooling capacity, the high pressure nitrogen compressor, nitrogen turbine expansion machine and Freon refrigeration unit were canceled, which simplified the process organization. Gas Air separation process after the introduction of LNG cooling capacity, adding compressor, heat exchanger and other parts on the basis of not changing the existing equipment can produce liquid products easy to transport, with less investment cost and relatively low energy consumption.
(3) the liquid air separation process of introducing LNG cold volume compared with the traditional process, the amount of cyclic nitrogen needed is significantly reduced, and the highest operating pressure of the system is from 4.2 ~ 5.0MPa reduced to 2.3 ~ 2.6MPa, the unit energy consumption of liquid products ranges from 1.05-1.25 kW of the traditional process? H/kg reduced to 0.317-0.384kW? H/kg, obvious energy saving effect.
