The seven stages of chemical engineering technology from concept to industrialization (Issue 43/100) -- Columns and heat exchangers

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This post was last edited by xiouxingzhe on 2026-6-14 22:36. Seven stages of chemical technology from concept to industrialization (Issue 43/100) —— Technology finalization: Equipment selection and data sheets for towers and heat exchangers. Dear friends: Hello everyone! In the previous issue, we discussed the scale-up design of reactors. In this issue, we will continue discussing equipment selection and data sheets, focusing on towers and heat exchangers. I. The core of tower design: plate-by-plate calculation. In the design of distillation towers and absorption towers, the key task is to determine the tower diameter and the number of separation stages required through plate-by-plate calculation or calculation based on the packing sections. Process simulation software such as Aspen Plus and PRO/II can already perform plate-by-plate calculations effectively, providing sufficient accuracy for most systems. In terms of operation, a simplified method is first used to calculate the preliminary number of theoretical plates and reflux ratio, after which a more rigorous method is employed to examine each plate in detail, in order to verify whether the separation requirements are met and whether the temperature and composition distributions are appropriate. Here is an empirical value that can be shared: in the absence of any reliable data, a recommended value for the general point ratio is 0.618. This is exactly the golden ratio point, a value that is quite reliable in engineering. It means that the design point is at a sufficient safe distance from the flooding line, without being too conservative and resulting in an excessively large tower diameter. When selecting the equivalent plate height for packed towers and the tray efficiency for tray towers, it is necessary to base the choices on practical considerations. For example, the plate equalization height for 250Y fillers is usually not less than 0.5 meters; do not choose a lower value in an attempt to achieve compactness – in many cases where the separation performance does not meet the design requirements, it is because the plate equalization height was set too optimistically. The selection of tower internals cannot be ignored either. The performance of the liquid distributor and the liquid collection redistributor directly affects the separation efficiency of the packed tower. In many cases, the separation performance of towers does not meet the required standards; it is not due to an insufficient tower diameter or packing height, but rather to a poor design of the distributor – resulting in uneven initial distribution of the liquid, which prevents the packing from exerting its full separation capacity. For large-diameter towers with low liquid levels, more effort is required in the design and installation of the distributor. II. Pedestal height: Not less than 7.5 meters. The pedestal height of the tower is often overlooked by designers with insufficient experience. The purpose of the skirt design is mainly to ensure that the transfer pump at the bottom of the tower has sufficient net positive suction head. The liquid at the bottom of the tower is saturated at the operating temperature, and the pump inlet requires a certain amount of effective net positive suction head to prevent cavitation. Where does this effective net positive suction head come from? A large portion comes from the head difference between the liquid level at the bottom of the tower and the pump inlet—that is, the static head provided by the skirt. The value of 7.5 meters is a commonly used figure that takes into account the NPSH requirement of the pump at the base of the tower, the fire safety distance, and the space needed for operation and maintenance. If there are no special requirements, it is not advisable to choose a size that is too small, as this can cause the pump at the bottom of the tower to run dry or result in poor discharge from the bottom of the tower. There is another advantage to a height of over 7.5 meters: in fire scenarios, for equipment with a height exceeding 7.5 meters, the discharge capacity of its fire safety valves can be calculated according to different standards. The layout of the tower frame has also developed into a relatively mature pattern: pumps are installed on the first floor, the reflux tank is located on the second floor, and the top condenser is placed on the third floor. If four layers are required, a secondary condenser or vacuum pump is usually installed in the fourth layer. This arrangement is not arbitrary—the top condenser is placed at a high position, and the liquid outlet flows by gravity into the reflux tank ； The reflux tank is located in the middle layer, which not only facilitates the reception of condensate but also provides sufficient net positive suction head for the reflux pump. III. Heat exchanger design: Parameter selection for thermal siphon reboilers. The design of heat exchangers is carried out using specialized software such as HTRI or HTFS, with the heat load at each heat exchange point specified in the PFD heat balance sheet serving as the input parameters. Among various heat exchangers, the design parameters of thermal siphon reboilers deserve special attention. A thermosyphon reboiler utilizes the density difference between the liquid in the kettle and the vapor-liquid mixture in the reboiler to create a natural circulation, eliminating the need for pumps and ensuring reliable operation. Two key parameters need to be assigned reasonable values. One is the temperature difference between the heating medium and the material being heated; 30 to 45 degrees is a suitable range. The temperature difference is too low, resulting in insufficient driving force for heat transfer; the required heat exchange area is large, leading to high equipment costs. When the temperature difference is too large, the wall temperature of the tube may exceed the critical heat flux density of the liquid, leading to film boiling – the tube wall becomes covered with a gas film, the heat transfer coefficient drops sharply, the wall temperature rises significantly, and there is also a risk of scaling and tube damage. The other is the vaporization rate, which is usually set between 10% and 15%, with a maximum of 20%. If the vaporization rate is too high, the outlet dryness becomes excessive, which can lead to dry spots on the pipe walls, deteriorated heat transfer, and easy scaling. If the vaporization rate is too low, there is insufficient driving force for circulation, and the reboiler may be unable to establish a stable natural circulation. After the heat exchanger is designed, its dimensions are rounded to **standard values to facilitate procurement and manufacturing. The common lengths of heat exchange tubes are 2 meters, 2.5 meters, 3 meters, 4.5 meters, 6 meters, and 9 meters. These lengths are multiples or half-multiples of the basic length of 12 meters for heat exchange tubes produced in China. Using standard lengths can save on procurement costs and delivery times. IV. Issues Easily Overlooked in Heat Exchanger Design There are several issues in heat exchanger design that are easily overlooked but have a significant impact. One is the method of determining the margin. Many people simply add 20% to 30% based on the calculated area, which is not entirely reasonable. A reasonable approach is to analyze the sources of uncertainty in the heat load—fluctuations in material flow rates, deviations in material property data, and reduced heat transfer efficiency due to scaling—and combine these factors to determine a well-founded safety margin. If too much margin is taken, equipment investment increases, and control may become difficult during operation due to an excessive heat exchange area. Insufficient margin was achieved, resulting in insufficient heat exchange capacity after the device went into operation; modifying this bottleneck is even more troublesome. The other is the selection of the media for the tube and shell sides. It cannot be decided merely based on convention as to which fluid goes through the tube side and which goes through the shell side. Corrosive media are usually routed through the tube side to reduce corrosion of the shell. High-temperature and high-pressure media usually flow in the tube side, reducing the shell wall thickness. Media that tend to form scale are usually placed in the tube side, making cleaning easier. Media with high viscosity or low flow rate typically flow through the shell side, where baffles are used to increase the flow velocity and heat transfer coefficient. Attention should also be paid to the performance of the heat exchanger under low-load conditions. Many heat exchangers are designed for full load, but at low loads the flow rate decreases and the heat transfer coefficient drops, resulting in heat exchange performance that may not meet expectations. For devices with a high degree of operational flexibility, the design of heat exchangers must take into account both low-load and full-load conditions to ensure stable operation throughout the entire range of operational flexibility. V. Preparation of the data sheet: After completing the calculations for equipment selection, the results obtained for each piece of equipment are compiled into a formal equipment data sheet with a unified format. The data table for the tower includes the tag number, name, quantity, operating and design conditions, tower diameter and tangent height, material of the shell and head along with corrosion allowance, detailed specifications for the tray or packing section (tray type, spacing, overflow weir height, or packing type, height, number of sections), detailed specifications for the internal components (distributor, collection redistributor, feed distributor, etc.), skirt height and material, and a list of pipe connections. The data sheet for heat exchangers includes the tag number, name, quantity, type (fixed tube sheet, floating head, U-tube, etc.), operating and design conditions, heat transfer area, inlet and outlet temperatures and pressures of the fluids in the shell and tube sides, heat load, materials used for the shell and tube sides along with corrosion allowances, diameter, wall thickness, length, and quantity of the tubes in the tube bundle, type and spacing of baffle plates, and a list of tube connections. Every parameter on the equipment data sheet must have its source traceable back to the PFD. The operating temperature and pressure are derived from the flow data in the PFD, the heat load is calculated from the heat balance, and the material selection is based on the corrosion data in the material property tables along with the operating temperature. When this traceability chain is broken, the data becomes meaningless. Preview for the next issue: Issue 44 – Equipment Selection and Data Sheets – Containers, Pumps, Compressors, and Others. After covering towers and heat exchangers, the next issue will discuss the approaches to selecting containers, pumps, compressors, as well as filters, agitators, and other equipment, along with how to prepare their data sheets. How is the residence time of the container determined? How are the pump head and net positive suction head calculated? What parameters need to be provided in the purchase data sheet for the compressor? To be continued in the next issue.