Proper design and operation of product draws from distillation towers should not be taken for granted. Improper draw operation is a routine source of problems with tower operation. The major aspects of tower draw design related to (1) liquid and vapor draws; (2) partial and total draws; and (3) draws from both packed and trayed towers will be covered. Case histories highlight the important points for correct draw design. Our objective here is to present general issues on drawing streams and then follow this with specific practical aspects applicable to both trayed and packed towers. These same rules apply equally to other vapor-liquid service towers (absorption and stripping) as well as to fractionation services.
Normally, new towers are not as susceptible to draw design problems as revamped towers. In revamping, a game of inches is usually required to fit the required internals into the vessel. Often, a tempting place to economize on space is on spacing and design of auxiliaries. Avoid this temptation. Additionally, standard design practices for new equipment are often inappropriately applied to complex revamp situations. Failures in auxiliary equipment, such as tower draws and feeds cause more tower failures than improper design of the trays of packing themselves.
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