Ionic Liquids: From Knowledge to Application - ACS Publications

Chapter 18

Downloaded by OHIO STATE UNIV LIBRARIES on June 29, 2012 | http://pubs.acs.org Publication Date (Web): December 18, 2009 | doi: 10.1021/bk-2009-1030.ch018

Separation of Aromatic and Aliphatic Hydrocarbons with Ionic Liquids: A Conceptual Process Design G.Wytze Meindersma and A.B. de Haan Department of Chemical Engineering and Chemistry/SPS, Eindhoven University of Technology, Eindhoven, The Netherlands

Presently, there are no processes available to separate low concentration (< 20%) aromatic hydrocarbons from mixed aromatic aliphatic hydrocarbon streams, such as a feed stream to naphtha crackers, which may contain 10 to 25% of aromatic components, depending on the source of the feed (naphtha or gas condensate). Present practice is removal of the aromatic hydrocarbons from the C5+-stream in the naphtha cracker by extractive or azeotropic distillation. If a major part of the aromatic compounds present in the feed to the crackers could be separated upstream of the furnaces, it would offer several advantages: higher capacity, higher thermal efficiency and less fouling. The improved margin will be around €20/ton of feed or €48 million per year for a naphtha cracker with a feed capacity of 300 ton h-1, due to lower operational costs. Extraction with sulfolane will result in a negative margin of M€10 per year. Therefore, a conceptual process for the extraction of aromatic hydrocarbons with the ionic liquid 1butyl-4-methylpyridinium tetrafluoroborate was developed using ASPEN. The investment costs are estimated at M€56 and the annual costs about M€28 per year, resulting in a positive margin of about M€20 per year.

© 2009 American Chemical Society In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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Introduction Most ethylene cracker feeds contain 10 to 25% of aromatic components, depending on the source of the feed (naphtha or gas condensate). In Figure 1, a simplified flow scheme of a naphtha cracker is depicted and a typical naphtha feed composition is given in Table 1. The aromatic compounds present are not converted into olefins and even small amounts are formed during the cracking process in the cracker furnaces (1). Therefore, they occupy a large part of the capacity of the furnaces and they put an extra load on the separation section of the C5+-aliphatic compounds. This is illustrated by the bold line in the process scheme in Figure 1, which the aromatic hydrocarbons follow. Moreover, the presence of aromatic compounds in the feed to the cracker also has a negative influence on the thermal efficiency. Aromatic compounds present in the feed tend to foul the radiation sections (coking of the coils) and the Transfer Line Exchangers. Lights (CH4/H2/CO)

Hydrogen Furnace Steam

C1/C2

Quench

Acetylene Ethylene

Cold Oil

Naphtha Steam

DMF

Water

C2/C3

Natural Gas

C2=/C2

Ethane

Sour Gases (CO2, H2S)

Propylene

C3/C4 C3=/C3 Compression & Sour Gas Removal

Propane C4-fraction C4/C5

Quench Oil Recovery

C5+-fraction Pyrolysis Gasoline

Cold Oil / C9+-fraction

Figure 1. Simplified flow scheme of a naphtha cracker If a major part of the aromatic compounds present in the feed to the crackers could be separated upstream of the furnaces, it would offer several advantages: higher capacity, higher thermal efficiency and less fouling. The improved margin for the removal of 10% aromatic hydrocarbons from the feed to the naphtha cracker will be around €20/ton of feed or €48 million per year for a cracker with a feed capacity of 300 ton h-1, due to the lower operational costs. However, although separation of aromatic and aliphatic hydrocarbons after the furnace section is industrial practice, no suitable technology is currently available for the separation of aromatic compounds from the feed to cracker plants. Given this tremendous economic potential, the objective of this project is

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

257 the development of a separation technology for the selective upstream recovery and purification of aromatic compounds benzene, toluene, ethylbenzene and xylenes (BTEX) from liquid ethylene cracker feeds.


Table 1. Typical composition of a naphtha feed containing 10 wt% aromatics [Sabic]

The separation of aromatic hydrocarbons (benzene, toluene, ethylbenzene and xylenes) from C4 - C10 aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes. The conventional processes for the separation of these aromatic and aliphatic hydrocarbon mixtures are liquid extraction, suitable for the range of 20-65 wt% aromatic content, extractive distillation for the range of 65-90 wt% aromatics and azeotropic distillation for high aromatic content, >90 wt% (2). Typical solvents used are polar components such as sulfolane (37), N-methyl- pyrrolidone (NMP) (6) N-formylmorpholine (NFM), ethylene glycols (7-9) or propylene carbonate (10). Overviews of the use of extraction and extractive distillation for the separation of aromatic hydrocarbons from aliphatic hydrocarbons can be found elsewhere (11-14). Preliminary calculations, with confidential information from UOP, showed that extraction with conventional solvents, such as sulfolane, is not an option since additional separation steps are required to purify the raffinate, extract and solvent streams, which would induce high investment and energy costs. The costs of regeneration of sulfolane are high, since the sulfolane, which has a boiling point of 287.3 °C, is in the current process taken overhead from the regenerator and returned to the bottom of the aromatics stripper as a vapour (15). The application of ionic liquids for extraction processes is promising because of their non-volatile nature (16). This facilitates solvent recovery using


258 techniques as simple as flash distillation or stripping. The extraction of toluene from mixtures of toluene and heptane is used as a model for the aromatic/aliphatic separation. A conceptual process using the ionic liquid 1butyl-4-methylpyridinium tetrafluoroborate ([C4m4py][BF4]) as extractant was developed for this separation and with this process design, the optimal requirements for ionic liquids for extraction of aromatic hydrocarbons from mixed aromatic-aliphatic process streams can then be determined.


Methods and Materials Extraction with ionic liquids Ionic liquids are organic salts that are liquid at low temperature (1014), it is impossible to synthesise all ionic liquids and measure their properties. Thus, to determine suitable ionic liquids for a certain problem, simulation tools will be very useful. A dielectric continuum model (COSMO-RS), which is a quantum chemical approach, is chosen by a large number of authors for the a priori prediction of activity coefficients and other thermophysical data using only structural information of the molecules. This method enables to screen ionic liquids based on the surface charge, the polarity. In addition, it is possible to calculate activity coefficients at infinite dilution. Activity coefficients at infinite dilution of ionic liquids are useful for screening purposes, but for separations with ionic liquids, real distribution coefficients and selectivities at finite dilutions will have to be obtained, as these are concentration dependent, as can be seen in Figure 3. Ionic liquids are not always better than conventional solvents, but in a large number of publications, no benchmarks with conventional solvents are mentioned. Sulfolane is possibly a better extraction solvent for the separation of aromatic hydrocarbons in the concentration range of 0.4 to 0.6 aromatic mole fraction, since the toluene distribution coefficient is equal or higher than that of the ionic liquid [C4mpy][BF4]. However, at higher concentrations than 0.65 mole fraction, extraction with sulfolane is less feasible, because the aromatic/aliphatic selectivity is near 1 and the selectivity with the ionic liquid is still around 20, as is shown in Figure 3b. Too much emphasis still exists on ionic liquids with [PF6]- as anion, because these are versatile ionic liquids, but in industry, these ionic liquids will never be used, due to their instability in water and HF formation (27).


271 Task-specific ionic liquids are generally more selective than standard ionic liquids and, therefore, more focus must be on the development of these ionic liquids.

Acknowledgements UOP is acknowledged for the supply of cost estimates. SenterNovem and DSM (now Sabic) are acknowledged for their financial support.


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Ionic Liquids: From Knowledge to Application - ACS Publications

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