Mar-2016

Designing a divided wall column

A refiner plans the application of divided wall column technology for greater energy efficiency in the distillation of multi-component reformate

SAŠA POLOVINA, SRE´CKO HERCEG and Ana Grani´C Šarac
INA

Article Summary

Two of the most important tasks facing refineries today are to increase conversion depth and to increase energy efficiency. To achieve deep conversion, refineries have started to invest in heavy residue processing. Increased energy efficiency is achieved by better utilisation of all energy sources and by increasing heat recovery in refinery processes.

Distillation is the most common and important physical separation method. Therefore it is not surprising that attention is dedicated to increasing energy efficiency in this process. Distillation is also a process that is responsible for a large percentage of the world’s energy consumption. Due to its significant energy inefficiencies, some call it the ‘energy dark horse’. It is known that distillation represents about 95% of all separations in the chemical industry and that, in the US alone, about 40 000 distillation columns exist in all industries.

Distillation is a thermal separation process, where reboiling part of the bottom liquid usually provides the heat required for the separation process. Condensation of overhead vapour is used to take a certain amount of heat to gain the desired product. With reboiling and condensation, a certain amount of energy is lost.

The minimum amount of energy required for a reversible distillation process (Qmin.) can be described with Equation 1:

Qmin. = ΔS Tc / (1-Tc/Th)

where ∆S = change in entropy
Tc = high temperature in the reboiler
Th = low temperature in condenser.
According to the second law of thermodynamics, change in entropy can be defined as:

ΔS = -R Σ Xiln (Xi)

where R = universal gas constant
Xi = molar fraction of component i.
The energy usage Q in conventional distillation arrangements is typically more than 50% higher than Qmin. due to inevitable entropy losses, in particular due to remixing within column sections and interconnections and temperature differences in heat exchangers.¹

Energy efficiency in conventional distillation (that is, classic distillation with one feed and two products) has been increased significantly in the last 20 years as a result of:
• Better process control systems
• Utilisation of high efficiency internals
• Using side heat exchangers enabling use of cheaper utilities
• Heat integration
• Heat pump assisted distillation.

However, the only way to minimise energy losses that are caused by a remixing effect in the separation of multi-component mixtures is to apply the principle devised by Petlyuk.² According to this, in every column in the sequence, only the easiest separation can be performed. This is the principle on which so-called divided wall columns (DWC) are based. Separation is achieved by using a vertical partition wall that divides mainly the central part of the column into a prefractionator and main column sections. In each section, two components with the greatest difference in volatility are separated, while others are allowed to distribute.³ The biggest application for divided wall columns was separation of mixtures of three products. Now, after the introduction of non-welded wall technology it is possible to use this principle for separation of multi-component mixtures consisting of four or more products.

The main obstacle to greater expansion of this technology lies in the fact that there is no reliable method for calculation of distillation and dimensioning of the column, and therefore there is a huge delay in the application of this technology. Another reason for its poor adoption is potential issues in process control. As the divided wall column has greater numbers of degrees of freedom than a conventional distillation column, for proper and safe operation of the process it is necessary to harmonise significantly larger number of variables.

Development and application of divided wall columns
In conventional distillation technology, separation of multicomponent mixtures of three products is carried out in at least two consecutive columns (the number of distillation columns required in series is equal to the number of products, minus one). Figure 1 shows an example of two possible sequences for separating three products: the so-called direct and indirect sequences.

Figure 2 shows another possibility: a sequence of three columns. The first column in the series is a prefractionator or column that has no products, but is used for sharp separation of components with the greatest difference in the relative volatility (of components A and C), while the middle-boiling component B is distributed in the top and bottom products of the prefractionator. The final separation is performed in two distillation columns. In one column, separation of components A and B is carried out, and in the other components C and B are to be separated. In this way, the middle-boiling component B is removed from the bottom of one column and from the top of the second column. This configuration, although it requires more equipment (one column and two heat exchangers) compared to the direct/indirect sequences, can lead to significant reduction in the amount of energy required to perform the same separation task, due to avoidance of energy losses related to remixing of component B.

If the reboiler of the top column and condenser of the bottom column are thermally coupled (see Figure 2), capital cost savings can be achieved. The first example of the described distillation sequence enabled reduction of energy and investment costs, by use of an additional evaporator (reboiler) and condenser located in the prefractionator.⁴ Petlyuk described the design of fully thermally coupled distillation columns.² The reboiler and condenser in this configuration are located at the main column and the reboiler and condenser of the prefractionator are replaced with thermal coupling to the main column. In this way, the heat required for separation is provided with a single reboiler and condenser. Figure 3 shows two sequences of distillation columns for separation of three products.^2,4

A study in 1972 confirmed substantial energy savings, which can be achieved by usage of a thermal connection for the separation of components with close boiling points.⁵ A later study of the separation of ternary mixtures⁶ recommended operation of Petlyuk’s pseudo-column if the concentration of component B (middle component) in a mixture is between 40% and 80%. A comparison of the optimal vapour flow in direct and indirect conventional distillation series and for Petlyuk’s pseudo and normal design showed significant savings for the latter.⁷ Underwood’s equations were used for the calculation of minimum required vapour. A further study indicated that Petlyuk’s design has five degrees of freedom, which was the basis for the further development of distillation technology.⁸

The first proposed application of the divided wall for separation of ternary mixtures consisted of the main column in a single shell with three side products, where the feed input and three side products were separated by a dividing wall.⁹ Later work enabled application of the dividing wall for separation in Petlyuk’s series.¹⁰ Using new technologies, conventional distillation separation of multicomponent mixtures of three products in two distillation columns would be replaced by separation in a single divided wall distillation column (see Figure 4).¹¹