Because the low temperature liquid is easy to store and transport, and easy to operate, large equipment will release the excess oxygen and nitrogen to liquid oxygen and liquid nitrogen, and supply them to nearby users or for peak regulation, it can greatly increase the rate of using oxygen and nitrogen, and the economic benefit is very considerable. For the users who buy liquid products, using the liquid after vaporization is more economical than using the air separation plant with high energy consumption and small scale to produce gas. This makes the market demand of liquefaction equipment increase day by day.
Liquefaction equipment is divided into two processes: medium pressure and low pressure. The low-pressure circulating liquefaction process is generally applicable to small-scale liquefaction units with a small output of 1000 m 3/h (LN2); while the medium-pressure circulating liquefaction process is applicable to those with a output of more than 1000 m 3/h (LN2) liquefaction Unit of medium or above scale. This is because although medium pressure process investment is relatively large, with the increase of liquid production, its advantages of low energy consumption and less land occupation are more and more obvious.
I. Brief introduction of technological process
There are mainly two forms of medium pressure cyclic liquefaction process, which are divided according to the refrigeration equipment adopted by high-temperature cold source. There are medium pressure cyclic liquefaction process with refrigeration machine and medium pressure cyclic liquefaction process with high-temperature expansion machine.
1. Medium pressure circulation liquefaction process with refrigerator
After the combination of the low-pressure nitrogen from the low-pressure pipe network and the low-pressure return nitrogen from the main heat exchanger E1 (see figure 1), it is compressed to 0.58MPa (A) by the air supply compressor tc1 ), it enters into the main heat exchanger E1 in a state higher than critical pressure after it is compressed by the circulating compressor TC2 and pressurized by the booster of the turbine expansion machine, be extracted at appropriate temperature, and enter into the main heat exchanger after cooling by the low temperature cooler.
Figure 1
When this positive air is cooled to the intake temperature of the low-temperature expansion machine, most of the medium-pressure nitrogen will be extracted from the middle of the heat exchanger, and the expansion and refrigeration will be removed from the expansion machine. After the expansion, the nitrogen will reheat and be inhaled by the circulating compressor; while the other part of nitrogen continues to cool and liquefaction in the main heat exchanger, and the latter part is used as the output cold box of product liquid with pressure after the cold end of the main heat exchanger ., The other part of liquid enters into the low-pressure channel of the main heat exchanger after throttling vaporization, and exchanges heat with oxygen from the low-pressure pipe network. After reheating, it is inhaled by the feed gas compressor tc1, the main heat exchanger of oxygen is cooled and liquefied by liquid nitrogen and sent out of the cold box.
2. Medium pressure circulation liquefaction process with high temperature expander
Nitrogen from the pipe network is pressurized in the circulating nitrogen compressor, and the backward device is divided into two ways. One of them enters into the main heat exchanger for cold pumping out of the expansion machine (heating machine) for expansion, and then goes back to the main heat exchanger for heat recirculation, and then goes back to the entrance of nitrogen compressor for continuous circulation. The second route enters into the (cold, warm) Supercharger, and the two levels of pressurization are respectively cooled to normal temperature through the cooler, then enter into the main heat exchanger for cooling, and extract most of the expansion machine (cooler) from the middle of the main heat exchanger for expansion and refrigeration, after expansion, nitrogen is reheated through the main heat exchanger to return to circulation. Nitrogen press entrance continues to circulate; in addition, a small part of pressurized nitrogen is extracted from the cold end of the main heat exchanger, and liquid is sent out of the cold box as a product, the other part goes back to the main heat exchanger after throttling through the throttle valve to return to the circulation nitrogen compressor entrance to continue the circulation.
Figure 2
When producing liquid oxygen, the oxygen from the pipe network directly enters the Oxygen channel of the main heat exchanger and exchanges heat with the return gas, which is cooled and liquefied, and the liquid nitrogen is no longer output into the cold box, it is sent out of the cold box as a cold source and other return gas after being reheated by the main heat exchanger. If the liquid oxygen pressure is not high enough, the liquid oxygen pump can be used to pressurize and then output the cold box.
II. Process features
In the process of medium pressure cyclic liquefaction, since the liquefied nitrogen is cooled down under the pressure of super critical point, the condensation process with constant temperature will not appear in the process of liquefaction. In this way, the cooling capacity of the return air in the low temperature section of the heat exchanger can be fully utilized. On the other hand, it adopts secondary cooling (the first grade adopts refrigeration machine or expansion machine, and the second grade adopts expansion machine) and the first grade throttling refrigeration cycle to transfer the high-temperature cooling capacity to the associated low-temperature stage, greatly reduce the circulating gas, that is, reduce the energy consumption ,. Moreover, because the medium-pressure liquefaction equipment is larger than the low-pressure liquefaction equipment, with the increase of equipment scale, the efficiency of machine parts such as nitrogen compressor and turbine expander will be improved, and the loss of relative cooling is also small.
Generally speaking, the energy consumption of the process of medium pressure circulation liquefaction is 20% lower than that of the low pressure circulation ~ 30%. For example, for the 100 t/d liquefaction unit, the nitrogen pressure of the raw material is 5KPa, the oxygen pressure of the raw material is 10KPa, and its energy consumption is 0.69kw/m3 of liquid nitrogen and 0.83kw/m3.
Under the premise that the power of the circulating nitrogen compressor is certain, increasing the discharge pressure and increasing the discharge capacity is the same. The main feature of medium pressure cycle is that nitrogen is liquefied under the supercritical pressure, which is an important factor for its energy saving. Under the condition of ensuring the nitrogen cooling and liquefaction under the supercritical pressure, the effect of increasing the exhaust volume of the circulating nitrogen compressor is more obvious than that of increasing the exhaust pressure. For the process of adopting the refrigeration machine, the discharge pressure of the circulating nitrogen compressor should generally be 3.0 ~ 3.3 MPa (A); for the process of adopting high-temperature expander, the discharge pressure of circulating nitrogen compressor should be 2.4 ~ 2.8MPa(A ),
In the process of medium pressure, increasing the pre-machine temperature of low temperature expansion machine appropriately can increase enthalpy drop and reduce circulating air volume, which will certainly cause the increase of temperature before throttling of liquid products and increase the throttling vaporization rate, but the comprehensive effect is good.
