ETBE Synthesis via Reactive Distillation. 1. Steady-State Simulation and Design Aspects Martin G. Sneesby,* ,† Moses O. Tade ´ , † Ravindra Datta, ‡ and Terence N. Smith † School of Chemical Engineering, Curtin University of Technology, Perth, Western Australia, and Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, Iowa 52242 Ethyl tert-butyl ether (ETBE) is an alternative gasoline oxygenate that combines the blending properties of methyl tert-butyl ether (MTBE) and the renewability of ethanol. Technologically, the best means of synthesis utilizes reactive (or catalytic) distillation to maximize hydrocarbon conversion and energy efficiency while simultaneously producing a high-purity ether product. Mathematical models of reactive distillation are based on the conventional distillation process with supplementary equations added to model the reactions present. Ether-alkene-alcohol systems are highly nonideal in the liquid phase so that careful selection of physical property routines is required to ensure satisfactory simulation results. Column simulations performed here using both Pro/II and SpeedUp show excellent agreement with previously published experimental data for a MTBE system and also agree well with each other for both MTBE and ETBE systems. A homotopy analysis was performed on the tuned simulation models to determine the effects of key design and operating variables on column performance and, subsequently, to develop a design method for reactive distillation columns. Some unusual behavior was identified in ETBE reactive distillation columns compared with either MTBE columns or conventional distillation. Introduction Changing worldwide regulations are encouraging the addition of oxygenates to gasoline sold in heavily urbanized areas to reduce emissions of carbon monoxide and unburned hydrocarbons in an attempt to combat smog and ground level ozone. The high octane rating of many oxygenates can also be utilized to eliminate leaded octane enhancers, such as tetramethyllead (TML) and tetraethyllead (TEL), from gasoline blends. To date, methyl tert-butyl ether (MTBE) and ethanol have been the most widely used oxygenates. MTBE appears to offer the best combination of oxygen content, low Reid vapor pressure (RVP), high octane, high energy content, and low cost, but ethanol has been used in gasoline for many years and has attracted particular interest as an environmentally friendly alternative to fossil fuels, as it can be produced from biomass. Many governments also offer ethanol subsidies to offset the cost differential with MTBE. Ethyl tert-butyl ether (ETBE) has emerged more recently as a potential oxygenate and offers the advantages of the blending characteristics of MTBE and the renewability of ethanol. Compared with MTBE, ETBE has a higher octane rating and a lower volatility. It is also less hydrophilic than either MTBE or ethanol and, therefore, less likely to permeate and pollute groundwater supplies. Volatile organic compound (VOC) emissions are also lessened by ETBE’s lower volatility compared with MTBE. ETBE has a slightly lower oxygen content than MTBE (and much lower than ethanol) so that larger volumes are required, but its higher cost of production remains its principal disadvantage when compared with either MTBE or ethanol. However, the high cost can generally be partially offset with renewable fuel subsidies, and ETBE production is becoming increasingly viable in the current market. Table 1 (Furzer, 1994; Brockwell et al., 1991; Lide, 1994) summarizes the key differences between ETBE and its main alternatives. A more complete comparison of the physical properties of ETBE and MTBE has been published previously (Sneesby et al., 1995). Process schemes based on reactive distillation (also called catalytic distillation where a catalyst is present) are now acknowledged to offer a technological advantage for MTBE production compared with conventional syn- thesis routes (Zhang et al., 1995) as yields are higher and operating costs are lower. The high degree of internal recycle created by the distillation operation helps overcome the thermodynamic restriction of a relatively low equilibrium constant and allows the reaction to proceed further than would otherwise be possible. The reactive distillation process is also more energy efficient than a conventional system, as heat generated by the exothermic reaction offsets the reboiler heat input and contributes to product separation. High conversion and a high ether product purity can be obtained simultaneously in a single device. The exten- sion of reactive distillation technology to ETBE synthe- sis appears to be a natural next progression. Simulation results have been published for various reactive distillation columns including several systems for MTBE synthesis (Abufares and Douglas, 1995; Sundmacher and Hoffman, 1995). However, little has been published on ETBE synthesis. As the combination of reaction and distillation in a single vessel can produce interactions between various design variables which lead to unusual responses to changes in operating conditions, it is important to fully understand these responses to avoid suboptimal performance and poor designs. Furthermore, differences in phase behavior between the MTBE and ETBE systems lead to different sets of operating conditions and considerations, and the simple extrapolation of concepts from MTBE synthesis to ETBE synthesis may prove to be misleading (Sneesby et al., 1995). Some of these issues have been addressed * Author to whom correspondence should be ad- dressed. Telephone: 619-351-3776. Fax: 619-351-3554. Email: sneesbym@che.curtin.edu.au. † Curtin University of Technology. ‡ University of Iowa. 1855 Ind. Eng. Chem. Res. 1997, 36, 1855-1869 S0888-5885(96)00283-7 CCC: $14.00 © 1997 American Chemical Society