Distillation Operation Difficulties Vapour Flow Conditions Adverse vapour flow conditions can cause foaming entrainment weeping/dumping flooding
Foaming Foaming refers to the expansion of liquid due to passage of vapour or gas. Although it provides high interfacial liquid-vapour contact, excessive foaming often leads to liquid buildup on trays. In some cases, foaming may ma y be so bad that the foam mixes with liquid on the tray above. Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but is sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always reduced. Entrainment Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by high vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate. Excessive entrainment can lead to flooding. Weeping/Dumping This phenomenon is caused by low vapour flow. The pressure exerted by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency.
water can vaporize so rapidly that it causes an over pressure event. This can and will damage trays. This effects column efficiency and p roduct quality.
Column Diameter
Most of the above factors that affect column operation is due to vapour flow conditions: either excessive or too low. Vapour flow velocity is dependent on column diameter. Weeping determines the minimum vapour flow required while flooding determines the maximum vapour flow allowed, hence column capacity. Thus, if the column diameter is not sized properly, the column will not perform well. Not only will operational problems occur, the desired separation duties may not be achieved. State of Trays and Packings
Remember that the actual number of trays required for a particular separation duty is determined by the efficiency of the plate, and the packings if packings are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the column. Tray efficiencies are affected by fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus appropriate materials should be specified for tray construction. Weather Conditions
Most distillation columns are open to the atmosphere. Although many of the columns are insulated, changing weather conditions can still affect column operation. Thus the reboiler must be appropriately sized to ensure that enough vapour can be generated during cold and windy spells and that it can be turned down sufficiently during hot seasons. The same applies to condensers. These are some of the more important factors that can cause poor distillation column
http://facstaff.cbu.edu/rprice/lectures/distill7.html
Distillation VII: Equipment and Column Sizing In order to have stable operation in a distillation column, the vapor and liquid flows must be managed. Requirements are: vapor should flow only through the open regions of the tray between the downcomers do wncomers liquid should flow only through the downcomers liquid should not weep through weep through tray perforations liquid should not be carried up the column entrained in the vapor vapor should not be carried down the column in the liquid vapor should not bubble up through the downcomers These requirements can be met if the column is properly sized and the tray layouts correctly determined. Tray layout and column internal design is quite specialized, so final designs are usually done by specialists; however, it is common for preliminary designs to be done by ordinarily superhuman process engineers. These notes are intended to give you an overview of how this can be done, so that it won't be a complete mystery when you have to do it for your design project. Basically in order to get a preliminary sizing for you column, you need to obtain values for the tray efficiency the column diameter the pressure drop the column height
Primary References: 1. Douglas, James M., Conceptual Design of Chemical Processes , McGraw-Hill, 1988, pp. 453-457. 2. Kister, Henry Z., Distillation Z., Distillation Design Design, McGraw-Hill, 1992, pp. 275-282. 3. Luyben, William L., "Introduction" in Practical Distillation Control (W.L. (W.L. Luyben, ed.), Van Nostrand Reinhold, 1992, pp. 10-11. 4. McCabe, W.L., J.C. Smith, P. Harriott, Unit Operations of Chemical Engineering , Engineering , 5th Edition, McGraw-Hill, 1993, pp. 560568. 5. Seader, J.D. and Ernest J. Henley, Separation Process Principles, Principles , John Wiley, 1998, pp. 305-312.
