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Second key point of process design: Distillation tower and shell-and-tube heat exchanger, Distillation tower 1. Packed tower: (a) It is determined whether flooding will occur based on the pressure drop per meter of packed layer height. Typically, the flooding pressure drop per meter of packing is 0.017–0.025 Kg/cm2; operating below this value represents normal and stable operating conditions. Typically, the pressure drop per meter of packing is between 0.0043 and 0.009 Kg/cm2. Under these operating conditions, the height equivalent to a packed bed, HETP, is at its lowest, which means the separation efficiency is highest. 2. Due to wind loads and foundations, among other factors, the height of the tower generally does not exceed 53 meters. 3. For small towers with a diameter of less than 900 mm, packed towers are commonly used. This is based on the high manufacturing costs of small-diameter plate towers. 4. The typical overall tower efficiency is usually between 60% and 90%. 5. Typically, the spacing between sieve tray plates is 300–400 mm ； The spacing between vacuum trays is 500–750 mm. If ease of maintenance is taken into account, the board spacing should be larger; the minimum requirement from a mechanical design perspective is 460 mm. Shell-and-tube heat exchanger 1. Flow direction arrangement of the heat exchange medium: (a) Place highly corrosive fluids inside the tubes, so that only a small amount of expensive alloy tubing is required. If corrosive fluids flow between the shells, not only expensive shell materials are required, but the pipes inside the shells also need to be made of corrosion-resistant materials. (b) Place the scale-prone fluid inside the pipe, and dirt can be properly removed through flow rate control. During maintenance, the straight pipe sections can be mechanically cleaned without removing the tube bundle. (c) By arranging fluids for high-temperature/high-pressure operation within tubes, the need for special and expensive manufacturing materials can be eliminated. (d) Placing the medium with a lower flow rate on the shell side can enhance its economic performance. This is because low-flow-rate fluids are more likely to generate turbulence favorable for heat transfer on the shell side than inside the tubes. 2. Under various operating pressure conditions, the reasonable pressure drop in heat exchangers is as follows: For operating pressures within a reasonable range, the pressure drop is one-tenth of the absolute pressure at vacuum to atmospheric pressure; for pressures of 1–1.7 Kg/cm2, it is half of the gauge pressure; for pressures above 1.7 Kg/cm2, it is 0.35 Kg/cm2 or more. When cooling fluids with high viscosity, operating in the co-current direction is better than counter-current heat exchange. Because cold fluids can achieve a higher heat transfer coefficient. 4. The empirical relationship between the shell diameter and the number of tubes per row is: D=1.75×d×(n×Np)0.47, where D is the inner diameter of the shell, d is the outer diameter of the tube, n is the number of tubes per row, and Np is the number of tube passes per shell pass.