Distillation column high-purity

The product from the hydrolysis reaction is distilled to remove residual water (4). In subsequent distillation columns high-purity MEG is recovered (5) and small amounts of co-produced di-ethylene glycol are removed (6). The homogeneous catalyst used in the process concentrates in the bottom of column 5 and is recycled back to the reaction section. 

In the laboratory and on a semi-technical scale, plate columns are used mainly for sp>ecial purposes, for instance in constantly-occurring separations where a distillate of high purity has to be prepared. DistiUations carried out for comparison with industrial plate colmnns may similarly require the use of laboratory plate columns. In analytical work at atmospheric pressure the sieve-plate column has proved reliable. 

In this chapter, we study distillation columns that have more than the normal two product streams. These more complex configurations provide savings in energy costs and capital investment in some systems. Sidestream columns are used in many ternary separations, and the examples in this chapter illustrate this application. However, a sidestream colmnn can also be used in a binary separation if different purity levels are desired. For example, two grades of propylene products are sometimes produced from a single column. The bottoms stream is propane, the sidestream is medimn-purity propylene, and the distillate is high-purity polymer-grade propylene. 

Figure 5.8. Residue curve map and separation sequence for zone b in the synthesis of MTBE by reactive distillation. Remark high purity MTBE (pseudo azeotrope) is recovered at the bottom of the column in an indirect separation operating pressure is set at 11-10 ...

Azeotropic distillation A third component called a hght-entrainer is added, which carries one of the components overhead in a distillation column and forms two liquid phases in a decanter. A two-column system is used with one Uquid phase from the decanter fed to one column and the other Uquid phase fed to the second column. High-purity products are produced from the bottom of the two columns. 

As shown in Fig. 10.6, the vapor from the reactor flows into the bottom of a distillation column, and high-purity dichloroethane is withdrawn as a sidestream several trays from the column top. The design shown in Fig. 10.6 is elegant in that the heat of reaction is conserved to run the separation and no washing of the reactor... 

The wet ester is distilled in the dehydration column using high reflux to remove a water phase overhead. The dried bottoms are distilled in the product column to provide high purity acrylate. The bottoms from the product column are stripped to recover values and the final residue incinerated. Alternatively, the bottoms maybe recycled to the ester reactor or to the bleed stripper. 

Avoid attempts to recover simultaneously both high and low boiling nodes in high purity from mixtures of >3 components, particularly in columns that reflux compositions different from the distillate composition, ie, reflux of one phase from a decanter, as such operations may be difficult to control. 

A number of processes have been devised for purifying thionyl chloride. A recommended laboratory method involves distillation from quinoline and boiled linseed oil. Commercial processes involve adding various high boiling olefins such as styrene (qv) to react with the sulfur chlorides to form adducts that remain in the distillation residue when the thionyl chloride is redistilled (179). Alternatively, sulfur can be fed into the top of the distillation column to react with the sulfur dichloride (180). Commercial thionyl chloride has a purity of 98—99.6% minimum, having sulfur dioxide, sulfur chlorides, and sulfuryl chloride as possible impurities. These can be determined by gas chromatography (181). 

Whereas there is extensive Hterature on design methods for azeotropic and extractive distillation, much less has been pubUshed on operabiUty and control. It is, however, widely recognized that azeotropic distillation columns are difficult to operate and control because these columns exhibit complex dynamic behavior and parametric sensitivity (2—11). In contrast, extractive distillations do not exhibit such complex behavior and even highly optimized columns are no more difficult to control than ordinary distillation columns producing high purity products (12). 

General. With simple instrumentation discussed here, it is not possible to satisfactorily control the temperature at both ends of a fractionation column. Therefore, the temperature is controlled either in the top or bottom section, depending upon which product specification is the most important. For refinery or gas plant distillation where extremely sharp cut points are probably not required, the temperature on the top of the column or the bottom is often controlled. For high purity operation, the temperature will possibly be controlled at an intermediate point in the column. The point where AT/AC is maximum is generally the best place to control temperature. Here, AT/AC is the rate of change of temperature with concentration of a key component. Control of temperature or vapor pressure is essentially the same. Manual set point adjustments are then made to hold the product at the other end of the column within a desired purity range. The technology does exist, however, to automatically control the purity of both products. 

Superfractionation is an extension of distillation using smaller diameter columns and 100 or more trays to achieve reflux ratios exceeding 5 1. This equipment separates a narrow range aunponents such as of high-purity solvents, e.g., isoparaffins or individual aromatic compounds foi use. IS petrochemicals. 

The conventional process consists of a reactor followed by eight distillation columns, one liquid-liquid extractor and a decantor. The reactive distillation process consists of one column that produces high-purity methyl acetate that does not require additional purification and there is no need to recover unconverted reactant. The reactive distillation process costs one fifth of the conventional process and consumes only one fifth of the energy. 

A single-column distillation configuration called Flash Compact System has been proposed which is capable of delivering an equivalent high purity product. The key advantage lies in the lower capital and operating costs. The feed is heated and pre-flashed and then sent to a distillation column as two. separate vapour and liquid feeds. 

To produce a high purity product two distillation columns are operated in series. The overhead stream from the first column is the feed to the second column. The overhead from the second column is the purified product. Both columns are conventional distillation columns fitted with reboilers and total condensers. The bottom products are passed to other processing units, which do not form part of this problem. The feed to the first column passes through a preheater. The condensate from the second column is passed through a product cooler. The duty for each stream is summarised below ... 

Another type of nonlinear control can be achieved by using nonlinear transfonnations of the controlled variables. For example, in chemical reactor control the rate of reaction can be controller instead of the temperature. The two are, of course, related through the exponential temperature relationship. In high-purity distillation columns, a transformation of the type shown below can sometimes be useful to "linearize the composition signal and produce improved control while still using a conventional linear controller. 

The aromatics-laden or fat solvent is fractionated in a distillation column. The widely different boiling points of the solvent and aromatics make the separation relatively easy and clean. The solvent is recycled back to the beginning of the process. The arorhatic extract, called crude benzene, is usually passed through a clay treater to remove any olefins that sometimes get created in the process and then distilled once again to produce high purity benzene. 

---