In chemical production, supergravity distillation equipment - tower equipment is a widely used non-standard equipment. Due to different purposes and operating principles, the structural form and operating conditions of the tower vary greatly. Here, the types, performance, and selection principles of distillation towers are mainly introduced as an example.
(1) The selection of multi-component solution distillation schemes can be divided into two categories based on the order of component separation in the distillation tower: the process of using fractions in decreasing order of volatility; The process of using fractions in increasing order of volatility; The process of staggered use of fractions according to different volatilities. The selection of the optimal separation scheme is crucial for the design of both the process flow and the distillation tower. A good separation scheme should have the characteristics of rational energy utilization, reduced energy consumption, low equipment investment, high production capacity, stable product quality, and safe operation.
(2) What is the process and form of the condenser? There are three main types of commonly used condenser layouts.
1) Integrated design. Integrating the condenser and tower has the advantage of occupying less space and saving condenser heads. The disadvantage is that the tower top structure is complex, and maintenance is inconvenient. It is often used in situations where the condenser is small or the condensate is difficult to pump, as well as in situations where pumping is dangerous.
2) Self flowing type. The condenser is installed on a platform near the top of the tower, which has similar characteristics to a monolithic structure. The condensate flows into the tower by itself, and the reflux and required level difference are obtained by changing the height of the platform.
3) Forced looping. Install the condenser at a lower location far from the top of the tower, and use a pump to provide reflux to the tower. Install a reflux tank between the condenser and the pump. This form is commonly used in large-scale production.
Fractional or complete coagulation are determined based on the following factors:
1) If the product at the top of the tower is used in a gaseous state during subsequent processing and can also meet other process requirements, then it should be discharged in a gas phase through fractional condensation. On the contrary, if liquid products are required, they should be discharged in liquid form in a fully condensed form.
2) Internal reflux control is achieved under the condition of fractional condensation. Generally, the temperature of the reflux liquid is the bubble point and the dew point of the vapor discharge. At this time, more reflux liquid circulation is required to increase reflux. If full condensation is used, the reflux liquid is sent back to the tower as subcooled liquid, and the amount of reflux flow can be controlled by the reflux liquid temperature.
3) The comparison between fractional condensation and total condensation depends on the operating pressure used, so it is necessary to consider the economic aspects of investment and operating costs when comparing fractional condensation and total condensation.
(3) What is the process and form of a reboiler? The safe operation of the reboiler is crucial in the distillation process, and the conditions that affect safe operation are also quite complex. During the distillation process of nitrobenzene, due to premature breaking of the vacuum, air entered the system under high temperature conditions, causing an explosion of nitrophenol sodium. At the same time, the use of lowering the liquid level and increasing heat flow led to an explosion of the reboiler.
Vertical reboiler (vertical thermosiphon circulation type). Good heat transfer effect; The kettle liquid is easy to clean through the tube and has a short residence time in the heating zone; If non polluting heating agents are used, fixed tube plate heat exchangers can be used to reduce the cost of heat exchangers; The piping between the reboiler and the tower kettle is short, the pressure loss in the piping is small, and the device layout is compact; Small footprint and simple foundation; The pipeline between the tower kettle and the reboiler can be equipped with a flow meter, which is easy to adjust. Due to the inability of a tower to use multiple reboiler units simultaneously during operation, it is difficult to evenly distribute the kettle liquid circulation, which limits the heat transfer area; In order to provide a circulating pressure head for the kettle liquid, the skirt of the tower needs to be raised significantly; When the evaporation efficiency of the reboiler is high, the volume expansion rate is large and the pressure loss increases, so the evaporation rate is limited to below 30%; In order to ensure the pressure balance required for thermosiphon, a weir plate should be installed at the bottom of the tower to maintain a certain liquid level at the bottom of the tower. Prevent the liquid level regulating valve from malfunctioning; When the circulation volume is large, the reboiler can be equivalent to a theoretical plate.
Horizontal reboiler (horizontal thermosiphon circulation type). The heat transfer area is larger than that of a vertical reboiler; The effective pressure head increases, and the circulation volume increases. In addition, a flow meter can be installed in the pipeline between the tower kettle and the reboiler, making it easy to adjust the flow rate. Disadvantages: Large footprint, high foundation and additional costs: It is difficult to clean the kettle liquid through the pipes. Therefore, when the kettle liquid is contaminated and has adhesive properties, U-shaped pipes are inserted into the horizontal reboiler to extract the tube bundle from the reboiler for cleaning; The evaporation rate is limited to below 30%; When the circulation volume is large, the reboiler can be equivalent to a theoretical plate.
Forced circulation reboiler. Suitable for forced circulation of high viscosity liquids that cannot circulate naturally or situations where the hydraulic head is insufficient; It can play a role in flushing and suppressing the deterioration of the heat transfer coefficient of the reboiler caused by scaling, aggregation, and coking of the kettle liquid. When multiple reboilers are used simultaneously in a tower of a large-scale device, the flow rate can be controlled by a pump to evenly distribute the kettle liquid in each reboiler; It can also operate under low evaporation rate conditions. Due to the need to add pumps, both fixed and maintenance costs are high, and forced circulation can only be carried out when natural circulation cannot be operated.
Plug in reboiler. The cylinder and piping of the circulation system that do not require a reboiler; There is no leakage problem with the kettle liquid; The small tower diameter can be restrained by snakes. Only applicable when the heat load of the reboiler is small; A tube bundle rack needs to be installed inside the tower. In order to facilitate the installation and disassembly of the tube bundle, a large-diameter manhole or handhole must be provided; When replacing the reboiler tube bundle, it must be stopped, while the external reboiler can be replaced during operation without stopping.