Techno-Economic Analysis of the Ethanol Dehydration Processes: Isothermal Vs. Adiabatic Operation

The present work addresses the dehydration of ethanol over zeolite catalyst, where main products are ethylene and diethyl ether (DEE) while some of hydrocarbons of C3+ are produced by oligomerization, cracking and hydrogenation of ethylene [1,2]. In the suggestion of kinetic mechanism for the dehydration of ethanol, two different pathways are considered; either ethanol is directly converted to ethylene or diethyl ether (DEE) is formed and then, cracked to ethanol and ethylene. For quantitative analysis, reaction rates are developed on the basis of Langmuir-Hinshelwood equation and kinetic parameters are estimated by fitting experimental data under a variety of conditions. Temperature shows positive effects on the conversion, while too high temperature decreases the selectivity of ethylene. The effects of water are also shown that the conversion is decreased with increasing water amount due to the competitive adsorption. In the feed, DEE is also included with varying amount to evaluate the role of each pathway in such a way that the developed model is used to determine the condition for maximizing the yield of ethylene.

The kinetic model was extended to develop pilot-scale reactors under the assumption of both adiabatic and isothermal operation, and then, the separation systems were suggested to produce pure ethylene (purity >99 wt.%). In the case of an isothermal reactor, the effects of the number and size of tubes on the capital expenses of the reactor were evaluated and the most appropriate one was used in the design of the process. Meanwhile, an adiabatic reactor was considered to be composed of several reaction tubes with inter-heating system incorporated between them. Despite the series of reaction tubes, the conversion was below 100%, indicating that additional separation system such as a cryogenic distillation should be employed for the recovery of ethylene from by-products, compared to the isothermal operation.

For preliminary quantitative analysis for the economics of both processes, the optimal size of each reactor was determined, and the purchase and installation costs were estimated using several correlations reported in the literature. The isothermal operation showed that the operation expenses for the separation were increased with increasing operation temperature, while the capital cost for the reactor was decreased due to the increased reaction rates, resulting in the decrease of total cost in the process. Meanwhile, although the adiabatic case had merits in the design of the reactor tubes, significant increase in the expenses for the separation was observed because of the increased operation cost by additional use of the steam in the reboiler and the cooling media in the condenser. In conclusion, the comparison of two processes based on different operation mode would provide the useful data on the design of the optimal pilot-scale process.

References

[1] Gayubo et al., Ind. Eng. Chem. Res. 2001, 40, 3467–3474.

[2] Galadima and Muraza, Ind. Eng. Chem., 2015, 31, 1–14.

[3] Chiang and Bhan, J. Catal., 2010, 271, 251–261.

[4] Phung and Busca, Chem. Eng. J., 2015, 272, 92–101.

[5] Cameron et al., Process Design for the Production of Ethylene from Ethanol, 2012.

[6] Jernberg et al., Ethanol Dehydration to Green Ethylene, 2015.

Acknowledgements

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the ‘‘Energy Efficiency & Resources Programs’’ (Project No. 20153010092090) of the Ministry of Trade, Industry & Energy (MOTIE), Republic of Korea, and the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry & Energy (No. 20154010200820).