Moving from Batch to Continuous Operation for the Liquid Phase Dehydrogenation of Tetrahydrocarbazole Yan Shen, †,∥ Azman Maamor, †,⊥ Jehad Abu-Dharieh, † Jillian M. Thompson,* ,† Bal Kalirai, ‡ E. Hugh Stitt, § and David W. Rooney* ,† † CenTACat, School of Chemistry and Chemical Engineering, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, U.K. ‡ Robinson Brothers, Ltd., Phoenix Street, West Bromwich B70 0AH, U.K. § Johnson Matthey Catalysts, PO Box 1, Belasis Avenue, Billingham, TS23 1LB, U.K. * S Supporting Information ABSTRACT: Despite the numerous advantages of continuous processing, high-value chemical production is still dominated by batch techniques. In this paper, we investigate options for the continuous dehydrogenation of 1,2,3,4-tetrahydrocarbazole using a trickle bed reactor operating under realistic liquid velocities with and without the addition of a hydrogen acceptor. Here, a commercial 5 wt % Pd/Al 2 O 3 catalyst was observed to slowly deactivate, hence proving unsuitable for continuous use. This deactivation was attributed to the strong adsorption of a byproduct on the surface of the support. Application of a base washing technique resolved this issue and a stable continuous reaction has been demonstrated. As was previously shown for the batch reaction, the addition of a hydrogen acceptor gas (propene) can increase the overall catalytic activity of the system. 1. INTRODUCTION Motivated by a desire for flexibility, liquid phase processing of fine chemicals occurs largely under batch operation, allowing use of installed equipment for a wide variety of reactions. It is generally considered that the initial capital costs are also relatively low when compared to continuous operation and installation is relatively simple, meaning that, if necessary, additional capacity can be easily accommodated by numbering- up or scaling out. However, the time required for starting up each batch reaction and the subsequent downtime including cooling as well as post reaction cleanup can be longer than the reaction time itself, resulting in significant loss of manufacturing time and increased labour. Conversely, continuous operation occurs under steady state for extended times, thereby requiring smaller equipment and facilities as well as less labour and downtime. With decreased manual input, there is also a reduced chance for exposure to risk, leading to improved process safety particularly in cases where toxic, flammable, or explosive materials are used. Decreasing start-up and cool-down cycles also reduces the time required to produce a given campaign, resulting in savings in both energy and materials when compared to a batch process. Furthermore, the more stable and consistent reaction conditions generally result in a higher quality product when compared to batch production. Rode et al. demonstrated the advantages of continuous processing for the selective hydrogenolysis of glycerol to 1,2- propanediol, where the conversion and selectivity to the desired product increased from 34% to 65% and 84% to over 90%, respectively, 1 on changing from batch to continuous packed bed operation. These improvements in rate and selectivity were attributed to a number of factors including in situ catalyst activation and suppression of side reactions from a lower contact time. A factor of 3 decrease in the contact time afforded by continuous operation was also observed to improve the selectivity for the dehydration of glycerol to acetol from 55% to 70%. 2 While not all liquid phase processes are suitable for conversion from batch to continuous operation, there are cases where a larger scale fine chemical process could benefit from being run continuously. To determine when continuous operation would be advantageous, Calabrese and Pissavini compiled a table of guidelines for initial assessment of flow reactor applicability, in which severe reaction conditions such as pressures greater than 120 bar, temperatures less than −10 °C, formation of toxic byproducts, highly exothermic reactions, and sequential reactions reducing product selectivity amongst others are given as reasons to consider use of a flow reactor. 3 Anderson discussed similar advantages amongst others for continuous operation for the production of pharmaceutical materials, citing improved control of mixing and heat transfer, more facile operation for both cryogenic and high-temperature processes, removal of reactive intermediates resulting in improved selectivity, and the removal of toxic compounds. 4 Further discussion on the topic of batch to continuous processing is given in the article by Stitt and Rooney. 5 Herein we expand on our work on the dehydrogenation of 1,2,3,4-tetrahydrocarbazole (THCZ) by investigating the liquid phase dehydrogenation reaction under continuous operation using a trickle bed reactor (TBR). Carbazoles can be produced from coal tar and crude oil or through synthetic routes such as the Graebe−Ullmann reaction 6 and the liquid-phase dehydrogenation of 1,2,3,4- Received: August 12, 2013 Published: March 11, 2014 Article pubs.acs.org/OPRD © 2014 American Chemical Society 392 dx.doi.org/10.1021/op400217d | Org. Process Res. Dev. 2014, 18, 392−401