With the increase of cooling capacity and the same amount of backflow air, the cooling capacity will be insufficient, so the temperature in the middle will rise. For the group with high temperature in the middle, turn down the air inlet stop valve (or diaphragm butterfly valve) to reduce the air throughput. Due to the sufficient cooling capacity, the temperature in the middle can be reduced. Attention should be paid to keep the total amount of air unchanged.DEAR Air separation plant.In the start-up operation, attention must be paid to gradually reduce the temperature difference at the hot end of the cold accumulator or the switching heat exchanger; before accumulating enough liquid, do not rush to start the liquid oxygen pump or deliver oxygen. If the cooling loss is limited to a minimum, more cooling capacity can be used to liquefy the gas and the start-up time can be shortened.The cross-sectional area of plate fin heat exchanger is smaller than that of switching heat exchanger due to the increase of flow velocity. Therefore, for the same equipment, the non switching plate fin heat exchanger has the advantages of light weight, simple piping, convenient operation and quick start-up, so it is widely used.
DEAR Air separation plant.When the temperature of the cold end of the switching heat exchanger reaches - 60 ～ - 150 ℃, the circulation can be used. The function of the circulation is to reduce the temperature difference at the cold end. When passing through the water freezing stage before - 60 ℃, it is easy to ensure the temperature difference of water self-cleaning (between 12 ℃ and 28 ℃), so the circulating current can not be used. In particular, the central cooling method can accelerate the cooling of the cold end of the switched heat exchanger.DEAR Air separation plant.In some processes, the crude neon and helium must go through two steps of hydrogen and nitrogen removal. Hydrogen is removed by catalytic oxidation to generate water, which is then removed by desiccator. The remaining nitrogen is removed by condensation or low temperature adsorption with activated carbon. Because the difference of boiling point between neon and helium is about 23K, condensation method can be used to separate pure neon and helium mixture. Because the freezing temperature of neon is - 248.7 ℃, the gas helium and solid neon can be separated by freezing method.
For switching heat exchanger, one of the reasons for the expansion of temperature difference at the hot end is that there is too much cooling capacity of the backflow gas. For example, if the circulating gas volume or intermediate pumping volume is too large, the cooling capacity can not be fully recovered in the heat exchanger, the temperature of the backflow gas out of the heat exchanger will decrease, and the temperature difference at the hot end will expand. At this time, the circulation or pumping volume should be adjusted properly.The gas-liquid two phase moves vigorously on the sieve plate to form a foam layer, which increases the contact area and disturbance degree of the gas and liquid, and makes the gas and liquid energy transfer well in heat and mass. The effect is better than that of the packed tower. There is neither packing nor sieve plate in the hollow tower. The cooling water is sprayed downward by layers through the spray device, the air flows from bottom to top, and the gas and liquid are mixed directly.
The main way to adjust the temperature of the heat exchanger is to adjust the middle temperature and keep the deviation as small as possible. It is the most common and effective method to adjust the middle temperature by changing the ring flow (or middle pumping).DEAR Air separation plant.Heat exchange equipment is an essential equipment to achieve air liquefaction, separation and maintain the normal operation of the device. Moreover, whether the heat exchanger works normally or not directly affects the economy of the air separation unit. Although there are many types of heat exchangers in air separation plant, they can be divided into three types according to their heat transfer principles: 1) inter wall type. 2) Regenerative. 3) Hybrid.DEAR Air separation plant.According to the quantity of products, it can be divided into small equipment below 800m3 / h, medium equipment between 1000-6000m3 / h and large equipment above 10000m3 / h. According to different separation methods, it can be divided into low temperature distillation, molecular sieve adsorption and membrane permeation.