Chemical processing tower equipment: [Weekly topic] How is pressure drop in towers generated and how can it be reduced? (2011.6.9~6.15)

Reason for pressure drop: For the gas phase to rise from the bottom of the tower to the top, in other words, gas flow occurs from high pressure to low pressure; consequently, there is inevitably pressure loss during this gas flow. How to reduce it: Through optimized design, by setting appropriate empty tower gas velocities and sieve pore gas velocities, as well as suitable internal tower components. Optimize operations to reduce the gas-phase load. There are also special considerations for pressure columns and vacuum columns.

The pressure drop in the column is mainly due to the resistance generated by the tray plates (the resistance of the packing). The resistance generated by the tray consists of three components: the dry tray resistance, the surface tension resistance, and the hydrostatic pressure of the liquid column. The resistance of the filler is mainly due to surface tension resistance, etc. The resistance of a typical packed tower is less than that of a tray tower. The general resistance is 200~300 Pa.

The resistance comes from the gas-phase flow resistance and the tray resistance. The faster the gas flow velocity, the greater the resistance ； Plate characteristics affect resistance.

The pressure drop in a distillation column is what is commonly referred to as the pressure difference between the bottom and top of the column. In the case of a floating valve column, this pressure drop consists of three components: the pressure drop across the dry plate, the pressure drop within the liquid layer, and the pressure required to overcome the surface tension of the liquid. For a fixed tower, the pressure drop across the tower is proportional to the square of the gas flow velocity.

The main resistances include gas redirection, packing, tower diameter, space, and so on

Packed tower: 1. Structure and types of packing, etc. 2. Operating load of the tower. 3. Liquid distribution efficiency. 4. Polymerization. 5. Decomposition. 6. Operational errors

The main reasons for pressure drop in towers include: the influence of the linear velocity of the gas in the empty tower, the resistance caused by the packing and trays inside the tower, the effect of the static liquid pressure within the tower, and the resistance due to surface tension. Measures to reduce pressure drop include: selecting an appropriate linear velocity for the gas in the empty tower, choosing the right type of packing as well as its height and tray opening ratio, and ensuring an appropriate distribution of the liquid phase along with the proper level of the liquid surface on the trays

The main disadvantages of plate towers are their narrow operating flexibility range, the tendency of the small-pore trays to become clogged, and the increased resistance encountered by gas and liquid streams as they pass through the floating valve trays. Effects of the linear velocity in the empty tower, the static pressure of the liquid phase inside the tower, and surface tension resistance: Solution – select an appropriate linear velocity in the empty tower, a suitable type of packing as well as its height and plate opening ratio, along with an appropriate liquid phase distribution and the static liquid level on the plates

For the generation of pressure drop in towers, refer to my reply in the link below: http://bbs.hcbbs.com/forum.php?mod=viewthread&tid=949875&page=1#pid8021467. To reduce the pressure drop: 1. For plate towers, it is possible to decrease the pressure drop by adjusting the opening ratio, the size of the downcomer, selecting the appropriate type of valve, choosing the right number of overflow paths (single overflow, double overflow, etc.), and even the diameter of the tower； 2. For packed towers, once the type of packing is chosen and the vapor-liquid load inside the tower is determined, the pressure drop is essentially fixed; only the performance of the packing can then influence it ； Of course, while ensuring the separation efficiency of the packing, it is possible to choose packing with a smaller specific surface area (packing with higher flux), which will result in a significant reduction in pressure drop. Generally, structured packing offers better flux, efficiency, and pressure drop compared to random packing and trays.

To reduce the pressure drop of a tower, for plate towers, the options available, regardless of the method used, are quite limited. Firstly, the pressure drop across the tray comes from three sources: the pressure drop due to the dry tray, the pressure drop in the liquid layer, and surface tension; the former two are the main factors. The liquid layer pressure drop is determined by the weir height and the thickness of the liquid layer above the weir (weir head); if the weir height is too low, the vapor-liquid contact is poor, which affects efficiency ； Although the weir head can be reduced through multiple overflows, an excessively low weir head leads to uneven liquid flow on the plate, facilitating backmixing which in turn affects efficiency. Therefore, for a well-designed tray, it is very limited in terms of further reducing the tray pressure drop by lowering the liquid layer pressure drop. Increasing the porosity can reduce the pressure drop across the dry plate, but considering the unevenness in the thickness of the liquid layer on the plate, to prevent leakage, the proportion of the pressure drop due to the dry plate should not be too small relative to the total pressure drop. Once this minimum ratio is determined, there is little room for adjustment in the dry plate voltage drop. Therefore, compared with high-energy packed towers, high pressure drop and low vapor phase energy are inherent drawbacks that plate towers cannot overcome, and these are also one of the main reasons why most expansion and renovation projects replace plate towers with packed towers.