Batch distillation

Details Batch distillation Batch distillation

2001-12-05

2003-04-22

Doerrler, William C. (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Separation of gas mixture

Reexamination Certificate

active

06550274

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to batch distillation, and in particular to batch distillation processes and apparati to separate multi-component mixtures containing three or more components.
New, high-value specialty chemicals and pharmaceutical drugs are continually introduced in the market. Generally, these chemicals are separated through batch distillation. Batch distillation, as against continuous distillation, is used because it provides operational flexibility. It is not uncommon to use the same batch distillation equipment for a large number of products. In batch distillation, one distillation column is used to separate a multi-component mixture into several product streams. On the other hand, a continuous distillation system uses a number of distillation columns. The operating flexibility and equipment cost make a batch distillation column quite attractive for numerous distillation applications. It is well known, however, that batch distillation requires much more heat duty than continuous distillation (comparative energy consumption in batch and continuous distillation, O. Oppenheimer and E. SØrensen, Computers Chem. Engng, Vol 21, Suppl., pp S529-S534, 1997). An objective of the present invention is to reduce the energy consumption of batch distillation, reduce the time taken to distill a batch, and provide more choices for operating modes.
For the distillation of a multi-component mixture, both continuous and batch distillations are used. Generally when large quantities are to be distilled, continuous feed distillation is used, otherwise batch distillation is preferred.
The details of continuous multi-component feed distillation column arrangements can be found in U.S. Pat. Nos. 5,970,742 and 6,106,674, both assigned to Air Products and Chemicals, Inc. Consider the separation of a ternary mixture ABC having components A, B and C into three product streams each enriched in one of the components. A is the volatile component, C is the heavy component and B is of intermediate volatility. The continuous feed distillation schemes use two distillation columns. U.S. Pat. No. 5,970,742 discusses five well-known schemes: direct sequence, indirect sequence, side rectifier, side stripper and thermally coupled columns. These are described in FIGS. 1 through 5 of the '742 Patent. This patent also describes some new continuous feed distillation schemes for multi-component feed distillation. In the direct and indirect sequences, feed is continually fed to a distillation column, a product stream is produced from one end of this column and a mixture from the other end is sent to the other column for further distillation. Each distillation column has a reboiler and a condenser. In thermally coupled column systems, fewer reboilers and condensers are used. This is achieved by having a two-way communication between the two distillation columns. In a two-way communication mode, when a vapor stream is sent from one column to another column, a return liquid stream is implemented between the two distillation columns. The side rectifier, side stripper and thermally coupled columns use thermal coupling to not only reduce the total number of reboilers and condensers in a continuous multi-component feed distillation but also to reduce the total heat demand for distillation.
Recently, cost reduction for continuous multi-component feed distillation columns with thermal coupling was suggested by building two or more distillation column's functions in a single shell column. Some of these configurations are also known as divided wall columns. U.S. Pat. No. 2,471,134 was the first to describe a fully coupled divided wall column arrangement. U.S. Pat. Nos. 3,844,898 and 3,959,085 describe concentric cylinders in lieu of side stripper type configurations to distill a continually fed multi-component feed stream. Some recent examples of divided wall or partitioned distillation columns are given in two U.S. Patents assigned to Air Products and Chemicals, Inc.; namely U.S. Pat. Nos. 6,240,744 and 6,250,106. Two recent publications describe continuous multi-component feed distillation columns with partitions and multiple reboilers and condensers in detail (“More Operable Fully Thermally Coupled Distillation Column Configurations for Multi-component Distillation” R. Agrawal, Trans. IChemE, Vol 77, Part A, pp 543-553, 1999; and “Multi-component Distillation Columns with Partitions and Multiple Reboilers and Condensers”, R. Agrawal, Ind. Eng. Chem. Res., Vol. 40, pp. 4258-4266, 2001).
Besides the continuous feed distillations, batch distillations are also used to separate multi-component mixtures. A recent review article provides a good survey of the state of the art (“New Era in Batch Distillation: Computer Aided Analysis, Optimal Design and Control”, K. J. Kim and U. M. Diwekar, Reviews in Chemical Engineering, Vol 17, pp 111-164, 2001). A conventional batch distillation column
110
is shown in FIG.
1
. It consists of a reboiler
130
at the bottom and a condenser
160
at the top. The distillation column
110
includes devices that promote vapor-liquid contact for mass transfer and provide separation stages. Structured packing, dumped packing and several types of trays are used for this purpose. In a typical process, the multi-component mixture is charged either in the bottom sump of the column
110
or in the reboiler
130
. Then heat is added to the reboiler and vapor ascends the column. The vapor is condensed in the condenser
160
through heat removal and collects in the reflux drum
162
. Initially the condensed liquid is sent through line
166
to the distillation column and no product is collected from line
170
. This operation is continued until the liquid holdup in both the column and in the reflux drum is met. After that the apparatus is run at total reflux until the acceptable purity of the volatile component A is achieved in the reflux drum. At that point, volatile product is withdrawn through line
170
according to a prescribed mode. The three modes often used are (i) constant reflux ratio, (ii) variable reflux ratio, and (iii) optimal reflux ratio. Reflux ratio is defined as the ratio of liquid flow rate in line
166
to the product flow rate in line
170
. In variable reflux ratio mode, the reflux ratio is varied to keep the product composition constant. In optimal reflux ratio mode, reflux ratio is constantly changed to meet an objective function; e.g., to minimize the batch time for distillation. Generally, in constant reflux ratio and optimal reflux ratio modes, the composition of product in line
170
varies with time. When the product is collected in a storage vessel, however, the average composition meets the required specification. After the volatile component has been collected, a slop cut containing other components is collected through line
170
. This is done until the desired purity of the intermediate volatility component is achieved. Then this product is collected through line
170
according to a prescribed mode. The slop cut is generally recycled to the distillation system in the next batch. The distillation is continued until all the desired product streams have been collected from the top reflux drum. In the end, the heavy component is recovered from the sump or the reboiler through line
140
. When the batch distillation is run in this mode, the distillation column arrangement is known as “traditional batch column,” “conventional batch distillation,” “ordinary column,” or “batch rectifier.”
In recent times, other batch distillation configurations have been suggested. These are also discussed in Chapter 9, pages 417-420, of the textbook by Doherty and Malone (Conceptual Design of Distillation Systems, M. F. Doherty and M. F. Malone, McGraw Hill, 2001). In a batch stripper case, most of the multi-component mixture is initially charged at the top of the column, such as in the reflux drum
162
. The column is started by having some of the multi-component mixture charged to the bottom reboiler
130
and heat is provided to start

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