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Working with an extractive distillation process

In the mid-60s, Krupp Uhde developed the Morphylane process (extractive distillation process) originally for the recovery of high-

purity benzene from hydro-refined coke oven benzole, the reason being that the existing liquid-liquid methods of extraction were unable to process coke oven benzole due to its high aromat-ics content. The liquid-liquid extraction techniques exploit the different solubilities of aromatics and non-aromatics in a polar solvent (Figure 1).

Two liquid phases are formed: the aromatics, which tend towards the solvent phase, usually the heavy phase, due to their higher polarity and

Gerd Emmrich, Helmut Gehrke and Uwe Ranke Krupp Uhde GmbH

thus higher solubility, and the light phase, which consists mainly of non-aromatics.

The extraction process essentially consists of four process units:• Extractor, where the feedstock is brought into contact with the solvent• Extractive stripper, where the non-aromatics which have also been dissolved are stripped off• Solvent stripper, where the aromatics are sepa-rated from the solvent• Raffinate washer for the recovery of the solvent residue which has been dissolved in the raffinate.

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A cost comparison between progressive extractive distillation of aromatics versus a more conventional technology

Figure 1 Morphylex liquid-liquid extraction

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It is obviously a very long-winded process. However, if the aromatics content of the feed material is too high, formation of two phases does not take place as the aromatics act as a solubility agent for non-aromatics. As a result, extractive separation of the aromatics and non-aromatics is not possible because the entire feed material dissolves in the solvent.

Polar solvents, as commonly used for this purpose, also demonstrate another exceptional physical property in addition to the solubility differences of aromatics and non-aromatics. Their polarity can influence hydrocarbon vapour pressures. They lower vapour pressures of all hydrocarbons contained in the solution, albeit to

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different degrees. What does this mean, exactly?The molecular structure of N-formylmorpho-

line is shown in Figure 2, the solvent used in Krupp Uhde’s proprietary Morphylex and Morphylane processes. The different molecular groups on both sides of the molecule have an electrical effect, making the molecule act like a small dipole.

The electrical effect acts on the double bond of the hydrocarbons. A loose bond is formed imped-ing the mobility of the molecules concerned. The more double bonds a molecule has, the greater its movement is impeded and the more difficult for it to be converted from a liquid to a vapour state in the presence of such solvents.

How significant are these vapour pressure changes?

Example: the first temperature column in Table 1 shows the boiling points of the pure components such as benzene (80°C), methyl-cyclohexane (MCH) (101°C) and n-heptane (98°C). If these components are now mixed with NFM to a ratio of 15 mol% hydro-carbons:85 mol% NFM, the resulting mixtures have boiling points of 135°C for benzene, approximately 110°C for MCH and 103°C for n-heptane. The reduction of the vapour pressure is

Figure 2 Characteristics of N-formylmorpholine (NFM)

Components NFM Boiling points15 mol% 85 mol% Pure comp °C Mix °C2,2-dimethylpentane 79.19 83.432,4-dimethylpentane 80.49 84.742,2,3-trimethylbutane 80.88 85.21N-heptane 98.43 103.15Cyclohexane 80.72 87.39Methylcyclohexane 100.93 109.55Benzene 80.09 134/70

Initial boiling points of NFMl HCs

Table 1



dramatic in the case of benzene, while in the case of n-heptane, it is only minimal.

The most important point, however, is that in the pure component mixture benzene is the low boiler, while in the mixture containing the solvent, it is the high boiler. This is the principle of extractive distillation where, at first, a close-boiling distillation cut is produced; this is then mixed with a polar solvent to increase boiling differences between components, thus increasing the relative volatility of the non-aromatics. Azeotropes are also destroyed. The principle arrangement of extractive distillation is very simple (Figure 3).

The extractive distillation column shown in Figure 3 consists of three sections: Stripping section, rectifying section and solvent recovery section.

The feed material, in this case the benzene fraction, is added between the stripping and the rectifying section; the solvent NFM is introduced above the rectifying section. The vapourised benzene is removed in the enrichment zone by the influence of NFM. Some light non-aromatics are also dissolved and stripped off in the strip-ping section.

Solvent vapours also flow to the top of the column together with non-aromatics, according to their partial pressure. These solvent traces are


washed back into the column in the solvent recovery section by non-aromatics reflux. The solvent-benzene mixture which accumulates in the bottom of the ED column is transported to the stripper column, which operates under a slight vacuum. Here the benzene is separated from the solvent.

The NFM in the benzene vapours is washed back by the reflux to less than 1ppm. The condensed benzene meets all product specifica-tions. The solvent, which contains a minimal benzene content, is fed back to the extractive distillation column from the bottom of the strip-ping column after intensive heat exchange.

In principle, this is a conventional two-column distillation system in which the low-boiling component is taken overhead in the first column, the medium boiling component leaves as over-head in the second column and the high-boiling component accumulates in the bottom of the second column. The only difference is that the high-boiling component is recycled back to the first column as extraction solvent. Separation of a three-component mixture (ABC) into individ-ual components A, B and C, by distillation in direct sequence is shown in Figure 4a. The three components can also be separated using a main column and a side column. Depending on the boiling conditions of component B, either a side

Figure 3 Morphylane extractive distillation



stripper or a side rectifier can be used. These are both thermally coupled columns as the two columns are connected with each other at the reboiler or condenser side.

These side columns can also be incorporated in one column by dividing the column into sections. If the two aforementioned methods are combined, a fully thermally coupled column is obtained. If the side column is then incorporated in the main column, the result is the divided-wall column (Figure 4b).

What is the thermodynamic advantage of the thermally coupled column over the conventional two-column system?

If one considers the concentration profile of an

extractive distillation process in a two-column system, where mol fractions of benzene are plot-ted against the number of theoretical stages, it can be shown that the benzene concentration drops off again before entering the second column. This is called back-mixing. Therefore, additional energy must be expended in column 2 in order to reverse this back-mixing.

However, if one considers the concentration profile of a thermally coupled single-column extractive distillation process, it can be seen that the benzene with the highest concentration occurs in the separated section. How, then, can this principle of thermally coupled columns be applied to extractive distillation (Figure 5)?

A main column with a side rectifier can be used as an example of extractive distillation. If the composition in the bottom of the extractive distillation column is taken into consideration, it can be seen that it must be similar to that on the stripper column feed tray. Therefore, the draw to the side rectifier must be positioned at such a location that the packing below the draw in the main column corresponds to that of the stripper column, such that this bottom packing, combined with the side rectifier, corresponds to the strip-per column (packings V and IV), while the column section above the side draw constitutes the extractive distillation column (packings I, II and III).

A single-column extractive distillation configu-ration of this type is shown in Figure 6. Feed material, the benzene or benzene/toluene frac-tion, is added above packing III on the feed side.

Packing II, above which the solvent is added, is located above the feed entry point. Packing I, which plays the role of removing solvent traces from the non-aromatic vapours by means of reflux, is arranged at the column head. Solvent traces are also removed on the product side of the baffle plate through aromatics reflux onto packing IV, in this


Figure 4a Direct sequence

Figure 4b Thermally coupled columns


case, however, from the pure aromatics.Below the baffle plate there is a chimney tray,

which directs the entire liquid through a reboiler heated with hot, stripped solvent. Packing V, in which the aromatics are stripped from the solvent to a residual content of 0.1–0.2 per cent, by means of a bottoms reboiler, is located below the chimney tray.

Column operationAromatics are removed from the vaporised feed material by the solvent in packing II until a residual content of 0.5–1 per cent is reached. However, some of the non-aromatics are also dissolved. These are stripped off by aromatic vapours in packing III. The solvent traces, which flow to the column head with the non-aromatics (proportionate to the vapour pressure of the solvent) are washed back with the non-aromatics reflux.

The solvent traces are removed from the aromatics vapours in packing IV, again by reflux. It is, however, packing V, where the aromatics are stripped off from the solvent, that is of crucial importance. Extractive distillation can only be effective if the aromatics content is dras-tically reduced to ~0.1 per cent. Intensive

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Figure 5 Thermally coupled columns

Figure 6 Thermally coupled extractive distillation



aromatics-stripping is crucial for the aromatics yield.

The wall forms two separate chambers. The vapours which enter both sides have the same composition and their non-aromatics content must conform with the product quality for pure aromatics. The purity of the aromatics is regu-lated in packing III, the yield is regulated in packings II and V and the solvent is retained in packings I and IV.

Overall column control is the most critical factor in the process.

In order to ensure that the aromatics stripping in the bottom of the column is intensive enough, the column operates under a vacuum. Special significance is attached to the pressure regula-tion since it also regulates purity and yield. If, for example, the temperature in the upper section of packing IV rises, this is a sign that the percentage of solvent vapours is increasing. If the pressure in the aromatics reflux drum is now raised, fewer vapours flow to the condenser while the reflux rate remains constant.

As a result, solvent is washed back to the lower packing section by the increased reflux ratio. If, for example, the temperature in packing II rises, this is a sign that the benzene front in the pack-ing is rising, which in turn results in an increased

benzene loss. Such a column configuration was first demonstrated in an Aspen Plus steady state simulation.

It was found that the system was surprisingly quick to converge, it demonstrated the required values for product purity and yield and attained the expected reduction in energy consumption. The only thing still to do was to test the process and, in particular, the control concept in a pilot plant.

For this, a pilot plant equipped with a DCS system at the laboratory/pilot plant test centre in Ennigerloh, Germany, was assembled to demonstrate comprehensively the mode of oper-ation. The aim of the experiments was both to confirm the simulation calculations and also to demonstrate the controllability of the column as needed in commercial scale applications.

Simulation calculations for the recovery of pure benzene from reformate or fully hydrogen-ated pyrolysis gasoline reveal energy savings of 16 and 20 per cent, respectively, in the new tech-nology application (Figure 7).

The catalytic reformate gives a lower energy saving as the percentage of non-aromatics in the feed to the extractive distillation process is higher than in the pyrolysis gasoline cut. This produces a higher percentage of overhead prod-


Figure 7 Comparison of concepts



uct (raffinate). As the quantity of heat required for vaporising the raffinate is the same for conventional ED and progressive ED, the energy saving is correspondingly lower if there is a high percentage of non-aromatics in the feed fraction.

To confirm the simulation results, a pilot-scale column with an internal diameter of 72mm was installed. Its dimensions were based on those of the columns and other equipment (eg reboiler, feed plates) which already have been used at Krupp Uhde facilities for the development of the Morphylex and Morphylane technologies. A typi-cal arrangement of the column details is illustrated in Figure 8.

Number 022 in detail drawing A shows the cover plate welded to the aromatics discharge side above the wall and the vapour draw for the aromatics. Detail drawing B shows an intermedi-ate flange with a feeder on the left-hand side of the wall and an aromatics reflux feeder on the right-hand side of the wall. The design is based largely on the conventional divided-wall columns installed at Krupp Uhde facilities some time ago.

Special semicircular structured packings were manufactured for the vertical wall section of the column. The structural geometry corresponds to that of commercial packings. The semicircular elements were installed at a 90 degree tray angle

in order to achieve good liquid distribution across the column section. Resistance thermom-eters were installed directly in the structured packings at close intervals to each other to allow the temperature profiles to be measured as accu-rately as possible.

To compensate for heat losses of the column, each individual column element is insulated and equipped with electric tracers. The tracers are set in accordance with the temperature level for each experimental run. As a rule, heat balances of approximately 105 and 110 per cent of the theoretically required heat quantity are achieved at the test facility. In order to achieve steady-state conditions and thus guarantee stable operation with sustainable results, the column was operated continuously in a number of runs, each lasting five days at least. The total mass and heat balances were recorded at hourly intervals and balances for each component were also listed separately.

Figure 9 shows the temperature profile recorded on the ED side and stripper side sections of the column for one test phase compared with the simulated temperature profile. The correlation between the two profiles is extremely good, especially in the divided wall section of the column. This clearly demonstrates


Figure 8 Realisation



that Krupp Uhde’s existing simulation model can also be applied to the new process equipment.

Figure 10 illustrates the results of two evalu-ated test phases with a C6 cut of pyrolysis gasoline and reformate respectively. The respec-tive values obtained in the pilot plant for the benzene content in the raffinate, the content of non-aromatics in the benzene and the heat duty are compared with the corresponding simulation values.

The heat duties for the pilot plant column were only around 5 per cent higher than calculated for the same product purities. This confirms the

substantial energy savings of progres-sive extractive distillation (with only one column) over conventional extractive distilla-tion (with two columns), arrived at in the simulation calculations.

Two additional facts which favour the use of the progressive extrac-tive distillation were observed in the pilot

plant operations. Surprisingly a very simple star-tup behaviour and a very robust and stable mode of operation could be noticed.

Economic viabilityIn order to ascertain how the concept of progres-sive ED compares with the conventional technology the required investment costs were calculated. This was done on the basis of a completed project for conventional benzene ED to allow a direct comparison between the two methods. The progressive ED was designed for the same general parameters. The findings of the


Figure 9 Temperature profiles

Figure 10 Comparison of simulation and pilot plant test runs


pilot tests were also taken into account. The cost of the designed process vessels and

machinery was then calculated using up-to-date databases for material prices. A substantial part of the equipment is no different from the conven-tional technology and could therefore be adopted directly from the completed project.

Bulk materials and construction costs were calculated using a common factor calculation method and verified and adjusted on the basis of the completed project. Engineering costs were calculated in the usual way for such projects and then also verified on the basis of completed projects.

The final cost estimate made in this way revealed a cost advantage of approximately 20 per cent for progressive ED, which was lower than expected. This is partly due to the lower condensation temperature at the ED head which is caused by the vacuum operation and which thus requires more condensing surface. However, the economic advantage of progressive ED becomes clearer when the energy consumption and utilities consumption are also considered.

If the different temperature levels for the inte-gration of external energy are taken into account, a cost saving in steam consumption of approxi-mately 20 per cent can be assumed in the case of progressive ED. Cooling water costs are also reduced by approximately 10 per cent. The elec-tric energy requirement remains unchanged.

For an aromatics plant which recovers approx-imately 40t/h of high purity benzene from 50t/h of a pyrolysis gasoline cut, an operating cost saving of at least DM1 million per annum can be expected (approximately DM3 per tonne of benzene produced). If the investment cost savings for a plant of this size are then spread over six years through allocation to the benzene

quantity produced, this reflects an additional saving of approximately DM2 per tonne.

This may be an insignificant amount with current benzene prices of approximately DM800. However, if the already highly developed state-of-the-art in aromatics extraction is taken into account, this may prove to be the decisive factor in favour of the more modern alternative. Also, the high oil prices at the time of writing are an indication of the fact that energy is not expected to become cheaper in the future.

This article is based on a paper presented at the European Refining Technology Conference, Rome, 13-15 November 2000.

Gerd Emmrich is research and development manager in Krupp Uhde’s Oil and Gas Division. He graduated as a chemical engineer from Rheinisch Westfaelische Technische Hochschule, Aachen, and is involved in the development of various refining processes in the field of refineries and petrochemicals.Helmut Gehrke is head of Krupp Uhde´s laboratory/pilot plant at Ennigerloh, Germany. He holds an MSc and a PhD in chemistry from the University of Essen and his main fields of activities are thermal separation processes and GC analytics.Uwe Ranke is head of the process department at Oil & Gas of Krupp Uhde. During his professional career he has been working in the fields of hydrocarbon processing and gasification for 15 years. He holds a PhD from the University of Bochum.

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