Operation of Azeotropic Distillation Columns

Design and Operation of Azeotropic Distillation Columns Simulation and design of azeotropic distillation columns is a difficult computational problem, but one fhat is readily handled, in most cases, by widely available commercial computer process simulation packages [Glasscock and Hale, Chem. Eng., 101(11), 82 (1994)]. Most simma-... 

The main distillation types include atmospheric, vacuum, steam, azeotropic, extractive, and pressure distillation [45]. AU of these distillation methods can be carried out in a batch or continuous marmer with the exception of extractive distillation, which is solely continuous by nature. Gomplex solvent systems often require the use of multiple distillation columns in series to purify certain solvents that are not easily separated. The energy consumption in distillation columns can therefore be quite large because of the continuous operation of condensers and reboilers over extended periods of time. In order to cut down on these costs, both vacuum and steam distillation can be employed ]45]. 

The overhead stream of the distillation column may be a low-boiling binary azeotrope of one of the keys with the entrainer or more often a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation results in two liquid phases, one of which contains the bulk of one of the key components and the other contains virtually all of the entrainer which can be returned to the column. Figure 13.29(a) is of such a flow scheme. When the separation resulting from the phase split is... 

The possibility of combining two different separation units into one, hybrid, process has not been considered in this chapter. Hybrid processes are quite novel and have only very recently been considered by industry and have, therefore, so far not made it into the standard textbooks. A hybrid process has the combined benefits of both of the component units and the benefits should theoretically outweigh the disadvantages. An example is a hybrid of a distillation column and a pervaporation unit for azeotropic separation, where the distillation unit alone is limited by the azeotropic point. Again, a lot of research is currently devoted to this type of operation and it is generally believed that it will become more widely used in the future. 

Step 1. For this process we must be able to set the production rate of vinyl acetate while minimizing yield losses to carbon dioxide. During the lifetime of the catalyst charge, catalyst activity decreases and the control system must operate under these different conditions. To maintain safe operating conditions, the oxygen concentration in the gas loop must remain outside the explosivity region for ethylene. The azeotropic distillation column must produce an overhead product with essentially no acetic acid and a bottoms product with no vinyl acetate. The absorber must recover essentially all of the vinyl acetate, water, and acetic acid from the gas recycle loop to prevent yield losses in the CCf removal system and purge,... 

The first three of these are solely VLE-based approaches, involving a series of simple distillation column operations and recycles. The final approach relies on distillation (VLE), but also exploits another physical phenomenon, hquid-liquid phase formation (phase splitting), to assist in entrainer recovery. This approach is the most powerful and versatile. Examples of industrial uses of azeotropic distillation grouped by method are given in Table 13-20. 

In the second step, a distillation-reaction system is appHed to prevent hydrolysis of epichlorohydrin, by removing epichlorohydrin and water as an azeotropic mixture from the top of the distillation column. This operation is known as steam-stripping. In addition to being used in the synthesis of epichlorohydrin, AC is also used as a raw material for synthesizing other aHyl compounds such as aHyl esters, aHyl ethers, and aUylamines by nucleophilic substitution, utilizing the easily substituting property of its chloride group. 

When a multicomponent fluid mixture is nonideal, its separation by a sequence of ordinaiy distillation columns will not be technically and/or economically feasible if relative volatiK-ties between key components drop below 1.05 and, particularly, if azeotropes are formed. For such mixtures, separation is most commonly achieved by sequences comprised of ordinary distillation columns, enhanced distillation columns, and/or liquid-liquid extraction equipment. Membrane and adsorption separations can also be incorporated into separation sequences, but their use is much less common. Enhanced distillation operations include extractive distillation, homogeneous azeotropic distillation, heterogeneous azeotropic distillation, pressure-swing distillation, and reactive distillation. These operations are considered in detail in Perry s Chemical Engineers Handbook (Perry and Green, 1997) and by Seader... 

When operating homogeneous azeotropic distillation towers, a convenient vehicle for permitting the compositions to cross a distillation boundary is to introduce a membrane separator, adsorber, or other auxiliary separator. Tbese are inserted either before or after the condenser of the distillation column and serve a similar role to the decanter in a heterogeneous azeotropic distillation tower, with the products having their compositions in adjacent distillation regions. 

For on-site separation/purification of recovered solvent it is necessary to consider the number and complexity of distillations needed to obtain materials which are suitably pure for reuse. Where mixtures must be separated into individual solvents this can require several distillations, particularly where the solvents form azeotropes - this can significantly add to costs. The major costs associated with solvent purification are normally the capital required for distillation columns, energy and the additional staffing needs to oversee the operation. Where azeotropic distillations are required the cost of distillation columns can be greater than the capital cost of the recovery unit itself and staffing costs can be a significant variable cost (particularly if batch distillation is required). 

The impact of inaccurate -model parameters can be very serious. The parameters have a major influence on the investment and operating costs (number of stages, reflux ratio), The influence of the -model parameters on the results is especially large if the separation factor is close to unity. Poor parameters can either lead to the calculation of nonexisting azeotrojjes in zeotropic systems (see Section 11.1) or the calculation of zeotropic behavior in azeotropic systems. Poor parameters can also lead to a miscibility gap which does not exist." In the case of positive deviation from Raoult s law a separation problem often occurs at the top of the column, where the high boiler has to be removed, since at the top of a distillation column the most unfortunate separation factors are obtained. 

This theoretical study is focused on the process combination of a distillation column and a pervaporation unit located in the side stream of the column. This hybrid membrane process can be applied for the separation of azeotropic mixtures such as acetone, isopropanol and water. Water is removed from the side stream of the column by pervaporation, while pure acetone and isopropanol are obtained at the top and bottom of the column. Detailed simulation studies show the influence of decisive structural parameters like side stream rate and recycle position as well as operational parameters like reflux ratio and mass flow on concentration profiles, membrane area and product compositions. 

Distillation is still the most common unit operation to separate liquid mixtures in chemical and petroleum industry because the treatment of large product streams and high purities with a simple process design is possible. Despite of this the separation of azeotropic mixtures into pure components requires complex distillation steps and/or the use of an entrainer. Industrial applied processes are azeotropic, extractive or pressure swing distillation (Stichlmair and Fair, 1998). Another sophisticated method for the separation of binary or multicomponent azeotropic mixtures is the hybrid membrane process, consisting of a distillation column and a membrane unit. 

This heterogeneous azeotropic distillation column system is ready to mn. There are two degrees of freedom needed to be determined in this system. One is at the operating specification in the configuration page tab of the column set-up. Another external degree of freedom is the flowrate of the makeup stream, which is set in the Streams page tab of the column... 

Acetic acid (HAc) dehydration is an important operation in the production of aromatic acid, such as terephthalic acid or in the manufacture of cellulose acetate. Although acetic acid and water do not form an azeotrope, using simple distillation to separate these two components requires many equilibrium stages and thus is impractical. The reason is because the system has a tangent pinch on the pure water end (see Fig. 9.1) where the jc and y curves are very close together. Therefore, it is more customary to use an entrainer via a heterogeneous azeotropic distillation column system for this separation if high-purity water must be produced. 

In this section, design and operation of an industrial column for acetic acid dehydration via heterogeneous azeotropic distillation is investigated. The entrainer used for this industrial column to aid the acetic acid and water separation is also isobutyl acetate. This entrainer is circulating inside the column through OR stream from a decanter. Multiple column feed streams from various parts of the upstream process are fed into this column. The feed components besides acetic acid and water also include small amount of methyl acetate and m-xylene. These components are intermediate boilers and tend to accumulate inside the column. They cannot leave the column system through either the top decanter AO stream or... 

Extractive distillation usually is preferred over azeotropic distillation, if both methods can be used to separate the feed components. The bulk of the solvent in extractive distillation is not vaporized in each cycle, as compared to azeotropic distillation where the entrainer is recovered from the overhead vapor stream. The energy input necessary to effect separation usually is lower for extractive distillation than for azeotropic distillation. Also, an extractive distillation column can operate over a wider range of pressures than an azeotropic distillation column, because the azeotropic composition is a function of pressure. Finally, there usually is a wider choice of solvents than entrainers, thus enabling the designer to minimize added component cost. 

A detailed discussion of azeotropes is beycwid the scope of this brief introduction. The control engineer should be aware that the existence of azeotropes imposes restrictions on die operation and performance of a distillation column. 

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