PROCESS DESIGN AND CONTROL A Process Synthesis Approach To Investigate the Effect of the Probability of Chain Growth on the Efficiency of Fischer-Tropsch Synthesis Peter Mukoma, Diane Hildebrandt,* and David Glasser COMPS, School of Chemical and Metallurgical Engineering, UniVersity of the Witwatersrand, P/Bag 3, WITS 2050, South Africa To evaluate a process at the early stage of its development, one must be able to perform simple calculations to compare all the alternatives. This must be done on a reasonably realistic basis, so that one can make credible decisions; however, the process should not be so detailed and laborious that it becomes too “expensive” to perform for alternatives that can ultimately be discarded. A methodology for performing these calculations has been developed, and this will be illustrated with a case study on Fischer-Tropsch synthesis (FTS). The effect of designing a Fischer-Tropsch (FT) process targeting a particular value of the probability of chain growth (R) on the overall carbon efficiency has been studied using simplified FTS flowsheet models. Two process configurationssnamely, the once-through and recycle processesshave been compared, and it is observed that, for a fixed production rate of liquid fuels at 100% CO conversion, the carbon efficiency for the process with a recycle stream is higher than that of the once-through process for all values of R. However, if the aim is to maximize diesel production by hydrocracking the waxes, it has been determined that an optimal R value should be sought to reduce the cost of hydrocracking very heavy waxes. The incorporation of wax hydrocracking in the two processes reduces the carbon efficiency at all R values beyond ∼0.7, thereby making it uneconomical to produce very-long-chain hydrocarbons. 1. Introduction Fischer-Tropsch synthesis (FTS) is a process that produces liquid hydrocarbons from synthesis gas (CO, H 2 ), and it is a promising option for the production of environmentally friendly chemicals and fuels from coal and natural gas. Presently, Fischer-Tropsch (FT) catalyst/process technology suffers from the following limitations: (i) limited selectivity for premium products (e.g., light olefins, gasoline, or diesel); (ii) catalyst deactivation; (iii) high capital cost; 1 (iv) heat removal, because the reaction is highly exothermic; and (v) less- than-optimum thermal efficiency. 1 Several factors have led to renewed interest in the use of FT technology for the conversion of natural gas and coal to liquid fuels. Some of the major factors that are influencing this renewed interest include (i) an increase in the known reserves of natural gas, (ii) the need to monetize remote or stranded natural gas; (iii) environmental pressure to minimize the flaring of associated gas; and (iv) the need to reduce dependence on crude oil. Because of this renewed interest, more FT plants are likely to be built. Existing FT plants are very capital-intensive processes, so it is anticipated that future plants will be designed based on the available raw materials (coal or natural gas) and the specific needs of a particular economy. For this reason, it is appropriate to make a process evaluation by examining alternative process configurations at the early stage of design. Jess et al., 2 in their paper, advocated the use of low-cost technology for countries in remote areas where the cost of natural gas is low as the only economical solution for the conversion of natural gas to higher hydrocarbons using FTS. This technology may not be highly efficient but it will bring benefits to the economy. This concept of a low-cost FT process is based on the use of a nitrogen-rich syngas, which does not utilize a recycle loop (once-through process) to avoid any nitrogen buildup in the system. To achieve a reasonable efficiency in a once-through FT process of the type proposed by Jess et al., 2 a reasonably high per-pass CO conversion should be achieved. One possibility for achieving this is the use of multiple reactors in series. 3 In contrast, the objective of the recycle process is to achieve higher reactor productivity using higher syngas flow rates, because of the recycle and low single-pass CO conversion. It is possible that, for the same reactor volume and catalyst loading, the recycle process could have a higher production rate of hydrocarbons than a single-pass operation. The drawback to the recycle process is the level of investment, which is likely to be higher due to the separation of hydrocarbon products, CO 2 , and H 2 O from the exit stream before syngas and the lighter gases can be recycled. The second choice of using reactors in series (especially if the same volume reactors are to be used) is complicated by the fact that additional fresh syngas might have to be added to the syngas that is leaving the previous reactor to obtain the required feed rate. However, in the case where catalyst activity is such that effectively 100% conversion can be achieved in a single per-pass conversion, the once-through process should be the configuration of choice, because it will not require any compression and reforming of the recycle stream and no air separation. This is especially true if the cost of methane and or coal is low. Even if 100% conversion is not achieved, if the * To whom the correspondence should be addressed. Tel.: +2711 717 7527. Fax: +2711 717 7557. E-mail address: diane.hildebrandt@ comps.wits.ac.za. 5928 Ind. Eng. Chem. Res. 2006, 45, 5928-5935 10.1021/ie0505256 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/25/2006