Vacuum distillation column design

Figure 7-4. The Scientific Design Co. process for producing ethylene glycols from ethylene oxide (1) feed tank, (2) reactor, (3,4,5) multiple stage evaporators, 4 operates at lower pressure than 3, while 5 operates under vacuum, evaporated water is recycled to feed tank, (6) light ends stripper, (7,8) vacuum distillation columns.

The extract is vacuum-distilled ia the solvent recovery column, which is operated at low bottom temperatures to minimise the formation of polymer and dimer and is designed to provide acryUc acid-free overheads for recycle as the extraction solvent. A small aqueous phase in the overheads is mixed with the raffinate from the extraction step. This aqueous material is stripped before disposal both to recover extraction solvent values and minimise waste organic disposal loads. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

The dehydrogenation reaction produces crude styrene which consists of approximately 37.0% styrene, 61% ethylbenzene and about 2% of aromatic hydrocarbon such as benzene and toluene with some tarry matter. The purification of the styrene is made rather difficult by the fact that the boiling point of styrene (145.2°C) is only 9°C higher than that of ethylbenzene and because of the strong tendency of styrene to polymerise at elevated temperatures. To achieve a successful distillation it is therefore necessary to provide suitable inhibitors for the styrene, to distil under a partial vacuum and to make use of specially designed distillation columns. 

Precondensers are recommended for any ejector system when the pressure conditions and coolant temperature will allow condensation of vapors, thus reducing the required design and operating load on the ejectors. This is usually the situation when operating a distillation column under vacuum. The overhead vapors are condensed in a unit designed to operate at top column pressure, with only the non-condensables and vapors remaining after condensation passing to the ejector system. 

Select design pressure drop for operations. Suggested values of below 1.0 in. water/ft. Low-pressure, atmospheric, and pressure columns usually require 0.5 to 0.7 in. water/ft, with absorbers and strippers around 0.2-0.6 in. water/ft. For vacuum distillation low values of 0.05-0.6 in. water/ft are often necessary, usually depending on the required boiling point of the bottoms. 

Process vapours from the esterification reactors and EG from the EG-vapour jet, as well as from the vacuum stages of the spray condensers, are purified in the distillation unit. The distillation unit commonly consists of two or three columns and is designed for continuous operation. The purified EG is condensed at the top of the third vacuum rectification column and returned to the process via a buffer tank. Gaseous acetaldehyde and other non-condensables are vented or burned and high-boiling residues from the bottom of the third column are discharged or also burned. 

A properly designed deodorizer or stripper vessel, located after the distillation column and operating at high vacuum and with open or sparging steam, will remove close boiling impurities and most odor bodies. Remaining color bodies are removed by adsorption with activated carbon. 

Entrainment Corrections. Above about 80% of flood, the recirculation of liquid as entrainment between trays undermines the countercurrent action of the tray column, and efficiency therefore suffers. This is a particular problem in vacuum distillation where it may be optimum to allow a certain amount of liquid entrainment in initial design. Figure 13.41 shows an approximate method for entrainment correction to column efficiency or Murphree efficiency. The abscissa scale is the same parameter used for flooding prediction (Figure 13.32(b)). The ordinate value is used to correct from a dry to a wet efficiency (with entrainment) ... 

The Fina/Badger distillation section consists of three distillation columns. All the columns are designed to operate under vacuum to minimize temperature and polymer formation. The first column in the sequence splits the benzene and toluene byproducts from the unconverted EB and styrene product. The benzene and toluene mixture is typically sent to an integrated EB plant where it is further fractionated. In this case, the benzene by-product is ultimately consumed in the EB unit and the toluene becomes a by-product stream from the EB plant. 

In the following discussion of design-parameters, it should be evident that not all parameters are of equal importance in all operations. Pressure drop, for example, is of central importance in vacuum crude stills-but of little import in the liquid-liquid extraction of penicillin from fermentation mashes stage efficiency is not important in the design of a whiskey distilling column, which requires few stages, but it can be critical for a deisobutanizer, which may require over 100 stages. 

Case Study 1 is on retfofitting the vacuum system of a crude distillation column in a typical petroleum refinery. Design details for the present operation (Case 1 A) are shown in Figure 11.5. Pressure at the top of the distillation colunm is 48 torr, vacuum system suction flow rate is 8500 kg/h and discharge pressure is 865 torr. The suction stream contains 8000 kg/h of water vapour and 500 kg/h of non-condensable gases (molecular... 

We have a serious problem with styrene. If any stream contains more than 50 wt% styrene, the temperature of the stream must not exceed 145°C. Otherwise, the styrene will polymerize. This must be carefiilly considered when establishing the operation conditions for the two distillation operations. You may have to operate one or both colunms under vacuum. This will require you to estimate the amount of air that leaks into the vacuum columns and select and cost vacuum systems. In designing the distillation system, you are to consider the direct sequence and the indirect sequence. 

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