HETP Evaluation of Structured and Randomic Packing Distillation Column

Packed columns are equipment commonly found in absorption, distillation, stripping, heat exchangers and other operations, like removal of dust, mist and odors and for other purposes. Mass transfer between phases is promoted by their intimate contact through all the extent of the packed bed. The main factors involving the design of packed columns are mechanics and equipment efficiency. Among the mechanical factors one could mention liquid distributors, supports, pressure drop and capacity of the column. The factors related to column efficiency are liquid distribution and redistribution, in order to obtain the maximum area possible for liquid and vapor contact (Caldas and Lacerda, 1988). These columns are useful devices in the mass transfer and are available in various construction materials such as metal, plastic, porcelain, ceramic and so on. They also have good efficiency and capacity, moreover, are usually cheaper than other devices of mass transfer (Eckert, 1975). The main desirable requirements for the packing of distillation columns are: to promote a uniform distribution of gas and liquid, have large surface area (for greater contact between the liquid and vapor phase) and have an open structure, providing a low resistance to the gas flow. Packed columns are manufactured so they are able to gather, leaving small gaps without covering each other. Many types and shapes of packing can satisfactorily meet these requirements (Henley and Seader, 1981). The packing are divided in random – randomly distributed in the interior of the column – and structured – distributed in a regular geometry. There are some rules which should be followed when designing a packed column (Caldas and Lacerda, 1988): a. The column should operate in the loading region (40 to 80% flooding), which will assure the best surface area for the maximum mass transfer efficiency; b. The packing size (random) should not be greater than 1/8 the column diameter; c. The packing bed is limited to 6D (Raschig rings or sells) or 12D for Pall rings. It is not recommended bed sections grater than 10m; d. Liquid initial distribution and its redistribution at the top of each section are very important to correct liquid migration to the column walls. A preliminary design of a packed column involves the following steps: 1. Choice of packing; 2. Column diameter estimation;