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Abstract
An industrial separation system consisting of four pressure staged distillation columns has been studied to see if multi-effect integration can be applied to any two columns in the sequence. Shortcut equations and Vmin-diagrams have been used for screening purposes to find the columns with the highest potential for energy savings. The most promising case has then been further studied using rigorous simulation tools to verify the results from the shortcut approach. Three cases have been simulated: a non-integrated base case (existing), a multi-effect indirect split arrangement (ISF) and a multi-effect prefractionator arrangement (PF). The results showed that when considering the existing number of stages available the ISF arrangement was the best, however when considering infinite number of stages the PF arrangement was the best (as expected).
1- Introduction
Multi-effect (also called pressure-staged) distillation means that the column pressures are adjusted such that the cooling (energy removal) in one column can be used as heating (energy input) in another column.
The separation of a hydrocarbon feed into four products using four sequential distillation columns have been studied in this paper to see if any of the four columns are suitable for heat integration by using a multi-effect prefractionator arrangement.
Multi-effect integration of prefractionators has been considered in the literature by authors like Cheng and Luyben (1985) and Emtir et al. (2001), who demonstrated that this arrangement can have high energy savings. In terms of industrial examples there is no knowledge of the multi-effect prefractionator arrangement being used. There are, nevertheless, examples of other multi-effect arrangements in use. Examples in literature includes a binary multi-effect distillation described by O’Brien (1976), the feed-split arrangement presented by Gross et al. (1998) and the forward-integrated indirect split arrangement (ISF) for the methanol-water separation as described by Engelien, Larsson and Skogestad (2003).
In this revamp case study we investigate if the multi-effect prefractionator arrangement can be implemented in an industrial context. Three separation tasks from a gas processing facility are investigated, in order to see if an integrated prefractionator arrangement can be suitable for an industrial application.
The methods presented in Engelien and Skogestad (2004) are applied in order to screen the three cases based on minimum vapour flowrate criteria. Also the required pressure levels for multi-effect integration was calculated for each case. From these preliminary calculations a candidate for integration was identified for which further rigorous simulations were carried out to compare energy consumption, pressure and temperature levels for the new multi-effect system with that of the existing distillation arrangement. Finally an exergy analysis was made in order to determine the efficiencies of the different arrangements.
2. Systems Studied
We consider the separation of a light hydrocarbon mixture into five products: ethane, propane, i-butane, n-butane and gasoline (pentane). The four two-product columns presently used for this task are denoted I, II, III and IV in Figure 1. The present pressure and temperature levels are indicated in the figure. An example of a multi-effect integration of columns III and IV is shown in Figure 2. This is only one possibility as there are three adjacent pairs of columns that are candidates for being replaced by multi-effect prefractionator arrangements in a possible revamp of the plant:
Case 1. Columns I and II for the separation of ethane, propane and butane (+ higher).
Case 2. Columns II and III for the separation of propane, butane and gasoline.
Case 3. Columns III and IV for the separation of i- butane, n-butane and gasoline.
3. Minimum Vapour Flowrate – Shortcut Calculations
The first task is to determine if any of the three cases are suitable for integration using multi-effect distillation. Shortcut methods have been used to calculate the minimum vapour flow requirement for each of the separations. Simple flash calculations have also been made to determine the required pressure levels.
For simplicity the mixtures have been taken as ternary mixtures for the shortcut calculations. Hydrocarbons of C5 or higher have therefore been assumed to be n-pentane and the small presence of CO2 in the feed to Column (I) has been neglected. Further, in the shortcut simulations for Case 1 the small amounts of i-butane and n-pentane have been lumped together as n-butane. For Case 2 the i-butane and n-butane have been considered to be n-butane. The ternary feeds to each case are marked in Table 1 as A, B and C. The specifications of the five products are given in the right hand column. Also given is the relative volatility of each component, relative to the heaviest component considered; n-pentane. These relative volatilities are found from literature (Smith, 1995 and Kister, 1992). For the shortcut analysis the relative volatilities have been assumed to be independent of pressure, but this assumption is relaxed later when studying the most promising alternative in more detail. In addition the analysis assumes sharp splits, liquid feeds, constant molar flows.
The Vmin-diagram gives the minimum energy requirements (in terms of vapour flow V) as a function of the distillate fraction  = D/F for the first column in a two-column sequence. Engelien and Skogestad (2004) show how to draw the Vmin-diagram and how to use it to compare the multi-effect prefractionator arrangement with other multi-effect systems and the existing non-integrated direct split (Case 1 and 2) and indirect split (Case 3) arrangements. We can also compare the Vmin to that of the Petlyuk arrangement, which is the best of the adiabatic systems (Halvorsen, 2003).
Using the relative volatility data and the simplified feed compositions in Table 1 minimum vapour flow (Vmin) diagrams for each of the three cases were plotted in Figure 3, Figure 4, and Figure 5. For clarity the feed composition and relative volatility used are given in each of the diagrams. The results for some other sequences are summarised in Table 2.