Large air separation plant's non freezing of the accumulator

Why is the large air separation plant's non freezing of the accumulator (or switching heat exchanger) related to the temperature difference at the cold end?

In the process of full low pressure switching air separation unit, a bypass valve is usually installed on the pipeline from the air to the switching heat exchanger and from the waste nitrogen to the switching heat exchanger, which plays a very important role in the start-up stage: part of the cooled air can return to the switching heat exchanger through the bypass valve to release the cooling capacity. Some of these substances can also be adsorbed in the adsorber of large air separation equipment, but the adsorption efficiency is only 60% ~ 65%. Because their partial pressure in liquid oxygen is very low, the amount discharged with gas oxygen is very small (except methane), and the rest will be gradually concentrated in liquid oxygen. Once it is enriched to the explosion limit, it is dangerous. The purpose of controlling low pressure during heating of fractionator is to speed up the heating speed of fractionator. If the low pressure pressure is not controlled, the heating gas will be discharged from the oxygen and nitrogen discharge pipes with small resistance, and the heating gas volume of small pipes with large resistance (such as analysis valve, liquid level gauge valve, etc.) and expander is very small, which is easy to cause the temperature of some pipes to be too high and the heating of some pipes to be incomplete.

The extraction rate of oxygen in the processing air is further reduced and the output of oxygen is reduced. Therefore, for 1000m3 / h air separation plant, it is not suitable to extract some liquid products. For 50m3 / h small medium pressure air separation unit, if 4L liquid nitrogen is extracted per hour, the air pressure needs to be increased by 0.3 ~ 0.4MPa compared with normal. When filling liquid oxygen into the tower of large-scale air separation equipment, first, pay attention to the timing. Be sure to wait until the unit cools down to the beginning of liquid generation in the main cooling, and then fill it in, so as not to damage the equipment due to thermal stress caused by excessive temperature difference; Second, pay attention to the filling pressure. Because liquid oxygen can be injected only when it is higher than the pressure at the bottom of the upper tower, but it cannot exceed the set pressure of the safety valve of the liquid oxygen storage tank to avoid the action of the safety valve.

Large air separation unit's air pressure test on site is mainly to check the air tightness of the equipment

Pay attention to the following problems during pressure test: it is strictly prohibited to use oxygen as the pressure test air source; Clean and oil-free pressure test air source shall be used for oil free equipment that will not be degreased after pressure test; The pressure gauge used for pressure test shall be calibrated and lead sealed before use. Firstly, increase the circulation to maintain the front of the machine at a high temperature (for example, about - 120 ℃), so that the carbon dioxide content in front of the machine is in an unsaturated state, and the carbon dioxide is not easy to precipitate in the machine after expansion. When the cold end temperature reaches - 150 ℃, the amount of carbon dioxide brought into the expander is very small. The bypass and circulation can be temporarily cut off, so that the temperature in front of the expander can be reduced rapidly, and the expander can not be blocked by forcibly crossing the carbon dioxide freezing area within about 10min.

According to the required heat transfer area. Multiple units can be connected in parallel and arranged in a star or multi-layer manner. Therefore, the height of the plate unit of the condensing evaporator of the air separation unit is still maintained at 1.8 ~ 2.1m. Only a few countries use a longer unit height on the thermosyphon evaporator. The acetylene content in liquid air and liquid oxygen of large air separation plant needs to be analyzed every day, so the liquid air and liquid oxygen discharge valves must be operated every day. It is often easy to open the valve, but difficult to close it. Sometimes it can't be closed at all. This is mainly because there is water in the valve stuffing box. There are often multiple automatic valves in a valve box of large air separation equipment, which rarely fail at the same time.  Therefore, its impact on pressure and flow is generally smaller than that of forced valve. It is rare that the water in the two groups of gravel is not frozen, or the automatic suction valve is not opened, resulting in a large difference in the water flow between the two groups of gravel.