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Chemical and Process Engineering Resources

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Category: Separation Technology
Question: Why is it sometimes necessary to distill under a vacuum and what are the special considerations involved?
Keywords: vacuum,distillation,design,pit,falls,consideration,separation,sub-atmospheric
Answer: The main reason for distilling under a vacuum is to carry out the operation at a relatively low temperature which corresponds to the lower vapor pressure imposed on the process. The design of a vacuum tower is much like a conventional one except that you are constantly under the constraint of a limited available pressure driving force within the system and this causes liquid and vapor distribution problems that affect the controls and the ultimate separation. You only have a maximum of 14.7 psi to carry out the operation and you can't afford to waste any part of it. 1. Be aware that accurate and dependable process control is essential. Refer to Greg Shinskey's great book, "Distillation Control", pages 103 and 121. Vacuum distillation is very sensitive to pressure and temperature control. This is an area that takes design experience, so don't feel bad if you make some blunders.2. Tray design is very special and usually left up to the experts who fabricate and install them. However, you must at least be knowledgeable in preselecting the type of tray that can do the job for you. The pressure drop per tray that you obtain will usually be very critical to maintain in order to keep the separation successful. In some applications, it is necessary to add an inhibitor to prevent unwanted polymerization3. Be prepared to confront a reboiler design that will require close details and scrutiny because of hot spots and the need to reduce pressure drops. You may have to employ a forced circulation type of reboiler.4. Overheads condenser design is also not your usual, conventional type of design. You will have to keep pressure drops down while maintaining vacuum. This is a very specialized area and handled by fabricator experts in real life. However, you must specify the conditions and limitations. A TEMA shell type H or J is usually used to divide the shell vacuum side flow and keep the pressure drop low. Remember, pressure drop is very expensive in this type of separations. Look for ways to reduce it.5. Because of the special conditions imposed on the reboilers and condensers you will find that you will need to concentrate on your equipment layout and vertical orientation in order to assure dependable NPSH values on your pumps and controls.All the above adds structural steel and height requirements which represent additional capital costs. Design for maximum welded construction on your process vessels, especially the towers. Avoid gasketed connection as much as you can. You must keep down the infiltration of atmospheric air into your vacuum or it will cost you capacity and temperature controls while it is operating.