Dividing Wall Column

The dividing wall column is a concept frequently used in direct column coupling. In all processes where multicomponent mixtures have to be separated into pure fractions, dividing wall columns are applicable. They are particularly suited to obtain pure medium boiling fractions.

A sequential system with at least two columns as shown in Figure 7.14 is required for the separation of a three-component mixture into its pure fractions in conventional systems.

A three-component mixture can be separated by using a dividing wall column in only one apparatus as illustrated in Figure 7.15. In the middle part of the column a vertical wall is introduced, creating a feed and draw- off section in the column. The dividing wall permits the low-energy separation of the low and high boiling fractions in the feed section. The medium boiling fraction is concentrated in the draw-off part of the dividing wall column (www.montz.de).

Dividing wall columns reduce investment and operating costs and therefore are an alternative to multicolumn systems. Investment costs can be reduced by 20 to 30%, and operating costs reduced by approx. 25% [10].

Policy That Has Two Columns
Figure 7.14 Conventional separation scheme of a ternary mixture in two columns.
Separation Column Images
High Boiling Component C Figure 7.15 Separation of a ternary mixture in a dividing wall column.

However, the drawback of a dividing wall column is that all the heat is consumed at the highest temperature level and removed at the lowest temperature level. In addition, the different separation tasks on the right and left sides of the partition must be carried out at the same packing height. Thus, the amounts of liquid in both parts of the column must be controlled carefully.

7.4.4.2 Indirect Coupling of Columns

In recent years, multistage distillation for the separation of multicomponent mixtures has received increasing attention. The basic idea is to use the overhead vapor of one column as the heat source in the reboiler of the next column. The columns can be heat integrated in the direction of the mass flow (forward integration) or in the opposite direction (backward integration). A common practice is to operate the columns at different pressures to obtain the necessary temperature gradient. Relatively large pressure differences are required to obtain a significant temperature change because the vapor pressure of liquids increases exponentially with temperature.

The separation in two thermally coupled columns is often more economical than separation in a single column [2]. A serial and a parallel connection of two thermally coupled columns are shown in Figure 7.16. The pressure of column 2 is lower than the pressure of column 1. Therefore, the overhead temperature of column 1 is higher than the bottom temperature of column 2, enabling waste heat use from column 1 to heat column 2. As a result, the heat requirements can be almost halved.

A pinch analysis can be used to systematically establish ways of integrating energy streams into the entire process and to show concrete interconnection options. A pinch analysis requires merely a list of the hot and cold streams present in the process (inlet and outlet temperature, mass flow and heat capacity). This method is discussed in detail in Chapter 6.

Column 1 Column 2

Pi Pl>P2 P2

Column 1 Column 2

Pi Pl>P2 P2

Feed w

Reboiler \

Feed

' Condenser

Distillate

Reboiler

Reboiler i

Bottom Product

Condenser

Distillate

Reboiler

Bottom Product

Figure 7.16 Indirect thermal column coupling; (a) serial connection, (b) parallel connection.

Ada Bathroom Requirements
Figure 7.17 Typical distillation and purification sequence for bioethanol.

An example: Increasing oil prices and growing environmental concerns in recent years have been the major driver in the development of renewable biofuels. The use of ethanol as a fuel has been growing exponentially around the world. Until now, most of the bioethanol production concepts are based on sugar and starch crops as feedstock. Bioethanol is produced by fermentation technology leading to a product concentration of approx. 10% ethanol in water. The raw alcohol is separated by distillation and purified to fuel ethanol by dehydration, usually by means of a molecular sieve in a pressure swing adsorber. Distillation and dehydration represents the largest fraction of the energy used in the production of ethanol. The design is therefore approached with energy conservation in mind, using combinations of multipressure cascades, heat recovery and thermal coupling. Figure 7.17 shows a typical distillation and purification sequence for bioethanol. Here, the overhead vapors from columns 2 and 3 are reused to heat columns 1 and 2. The energy used to regenerate a molecular sieve dehydration unit is also recovered to preheat the feed.

The composite curves of this typical distillation and purification sequence for bioethanol are shown in Figure 7.18. Due to the heat integration, a reduction of approx. two-thirds of the energy consumption without heat integration can be achieved.

Enthalpy

Figure 7.18 Composite curves of a typical distillation and purification sequence for bioethanol.

Enthalpy

Figure 7.18 Composite curves of a typical distillation and purification sequence for bioethanol.

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment