Savings using divided wall columns
Divided wall columns can provide substantial energy and capital cost savings compared to conventional distillation methods.
PRAFULL PATIDAR and AJAY GUPTA
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Distillation is the most widely applied separation technology in the petroleum and chemical process industries. However, inefficiencies in the distillation process lead to considerable energy consumption. With energy costs rising, finding ways to reduce energy consumption has become an important requirement in the industry. Often simple measures such as selecting proper operating pressure, optimum feed tray location, optimum reflux ratio, avoiding over-purification and minimising recycle can help towards reduction in energy consumption.1 The minimum energy required for separation of components using distillation for a reversible distillation process is given by the second law of thermodynamics:
∫(Qrev/T) = ΔS (1)
However, to realise a reversible distillation column, difficult practical requirements, like an infinitesimal amount of heat supply and removal from an infinite number of column sections in the bottom and top of a column respectively, are needed (see Figure 1). In real distillation columns, the change in entropy is always greater than the minimum due to irreversibilities brought in by mixing of streams which are not in equilibrium with each other, such as where feed, reflux and reboil streams mix with other streams. Other sources of irreversibility are pressure loss in the flow through a column and supply and removal of heat to the column at a non-zero temperature difference in the reboiler and condenser respectively. Typical distillation columns used in real life consume more than 50% of the minimum required energy.
In order to make distillation more energy efficient, heat integration and novel distillation configurations can make a significant difference. A divided wall column (DWC) is one such promising technology. Let us first look at the evolution of a DWC from conventional distillation sequences.
Conventional column sequence
Single sidedraw column
In order to separate a multi-component mixture, a sequence of distillation columns is often used. Let us consider separation of three components, A, B and C, where A is the lightest and C is the heaviest. In some cases, it is possible to get pure B from a sidedraw stream in a single column if the sidedraw is a liquid stream from a stage above the feed tray where the concentration of B maximises, provided component A is present in much smaller quantity in the feed. Otherwise, when component C is present in much smaller quantity in the feed, a vapour sidestream rich in component B can be drawn from a stage below the feed location with maximum B concentration. However in both cases, it is required that the relative volatilities should be high between the separating components, and intermediate boiling component B should be present in large concentration in the feed. In addition, taking out a vapour sidestream is practically difficult.
Direct and indirect sequences
More commonly employed distillation configurations are the direct sequence and the indirect sequence. As Figure 2 shows, in a direct sequence, A is removed first, followed by B and C in the next column, whereas in the indirect sequence C is removed in the first column, followed by separation of A and B in the second column.
In these conventional configurations, in the first column the maximum concentration of middle boiling component occurs at some intermediate tray and decreases towards the column exit. If this component is not removed at its maximum concentration, thermal inefficiency increases due to its remixing with other components in the stages that follow. This remixing problem can be avoided by thermal coupling of the two columns.
Thermally coupled columns
Partially thermally coupled columns
In the direct sequence, if we remove the reboiler from the first column and supply the required boil-up from the reboiler of the second column, we get the so-called side rectifier arrangement shown in Figure 3a. Alternatively, we can also get a side rectifier configuration by removing the reboiler from the second column and supplying the necessary boil-up from the reboiler of the first column. Similarly, in the indirect sequence, we can get rid of the condenser from one of the two columns, with a single condenser serving both the columns to provide the necessary liquid reflux required. This results in the side stripper arrangement shown in Figure 3b.
Fully thermally coupled columns
To further improve on the conventional sequences discussed above, a better arrangement would be to use the so-called Petlyuk column arrangement shown in Figure 4.3 In this arrangement, we perform a sharp split between A and C in the prefractionator while B is distributed between the A and C rich streams. The top stream from the prefractionator containing A and B is fed to the upper part of the second column while the bottom stream containing B and C is fed to the lower part of the second column. The upper part of the second column performs separation of A and B whereas the bottom part performs the B and C separation. Like the side rectifier or the side stripper configurations, these two columns are further thermocoupled using only one reboiler and condenser installed with the second column. Vapour-liquid traffic in the prefractionator is maintained by providing part of the vapour and liquid streams from the second column to the bottom and top sections of the prefractionator, respectively. A side draw from the second column provides pure B product whereas the top and bottom streams give pure A and B.
Compared to the conventional direct or indirect configurations, the advantages of the Petlyuk arrangement are a reduced number of equipment as well as increased thermodynamic efficiency, with energy savings to the tune of 20-50% depending upon the components being separated and the design adopted. This energy saving is primarily obtained by avoiding the remixing problem discussed earlier.
Divided wall column
The number of columns required can be further reduced to a single column if the prefractionator in the Petlyuk arrangement is integrated in the same shell as the second column. This is done by dividing the column into two sections using a partition wall (see Figure 5a), each section taking the role of one of the two columns discussed above. Thermodynamically, this divided wall column is equivalent to the Petlyuk configuration, with the added advantage of a lesser column. In addition to energy savings, capital cost is reduced by typically 30%. Equivalent DWC configurations with side rectifier and side stripper distillation arrangements are shown in Figures 5b and 5c, respectively. Table 1 shows the savings realised using DWC, reported by several companies.
Important aspects which make Petlyuk columns thermodynamically more efficient are:3
• The difference between the composition of the feed and the composition present at the feed stage of the column is reduced. This decreases losses due to mixing of streams of different compositions that are not in equilibrium with each other.
• At the end of the first column, mixing losses due to reflux and reboiled vapour streams are negligible; these streams are from the other column and are near to equilibrium composition with the leaving streams.
• Remixing of the intermediate component at the end of the first column and separation in the second column, resulting in thermodynamic losses, are avoided.
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