Article pubs.acs.org/EF
Improving the Interface between Fischer−Tropsch Synthesis and Refining Daniel F. Rodríguez Vallejo†,‡ and Arno de Klerk*,‡ †
Instituto de Energía Materiales y Medio Ambiente, Universidad Pontificia Bolivariana, Medellín, Circular 1 73 34, Colombia Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2 V4, Canada
‡
S Supporting Information *
ABSTRACT: In a typical industrial Fischer−Tropsch process the hot reaction product from Fischer−Tropsch synthesis is stepwise cooled, condensed, and recovered, before being separated by distillation into different cuts. A different design was proposed whereby the hot reaction products are directly introduced into a pressure distillation unit, which combines syncrude recovery and distillation. Process simulation was employed to evaluate the proposal. It was found that integration of syncrude recovery and distillation was technically viable and that it had a number of benefits compared to stepwise cooling and recovery of syncrude prior to distillation. These positive outcomes were independent of the type of Fischer−Tropsch technology used. Some notable benefits included a decrease in heating/cooling duty, improved liquid recovery, and reduced loading of tail gas separation. The proposed design also enabled other improvements, such as a strategy to improve catalyst-wax separation from slurry bubble column reactors and a strategy to reactively improve distillation performance and increase liquid yield.
1. INTRODUCTION Industrial gas-to-liquids (GTL) and coal-to-liquids (CTL) facilities are presently based on mainly indirect liquefaction technology. A generic indirect liquefaction process consists of four important elements: (1) generating syngas, either by natural gas reforming or by coal gasification, (2) cleaning and conditioning of the syngas, (3) converting the syngas into products, and (4) refining. It is a complex process. On its own, each of the four technology areas is well developed and efficient, but the same is not true of the interfaces. The inefficiency at the interfaces is best illustrated by the temperature differentials when material passes from one technology area to another (Figure 1). In this work we are focusing specifically on the interface between syngas conversion by Fischer−Tropsch (FT) synthesis and FT product refining. The syncrude from FT synthesis is a complex product, with three to four different phases at ambient conditions: gas, organic liquid, aqueous liquid, and organic solid. The composition and temperature of the syncrude product that leaves the FT reactor depend on the FT technology.1−2 The next step is to recover the syncrude from the unconverted syngas. Current industrial practice relies on a stepwise cooling and phase separation strategy.3 In some designs part of the energy is recovered by feed-product heat exchange, but for the most part the energy is removed by water or air cooling. Once the syncrude is separated from the unconverted syngas, the syncrude is refined, employing refining techniques analogous to that found in crude oil refineries. The first step is fractionation of the syncrude in an atmospheric distillation unit (ADU), with the different syncrude-cuts sent to appropriate refining units, such as oligomerization, hydrotreating, and hydrocracking. The interface between syngas conversion and syncrude refining is therefore the syncrude recovery section, which is between FT synthesis and the ADU (Figure 1). © XXXX American Chemical Society
In a crude oil refinery the interface between crude oil production and crude oil refining is physically separated, often by thousands of kilometers. The ADU is seen as the entry point into the refinery, even though in practice the ADU in a crude oil refinery is usually preceded by a desalter unit. The operation of an ADU is pertinent to this work and will be briefly described. The ADU in a crude oil refinery does not employ a standard reboiler, as is found in standard distillation columns.4 An ADU employs a fired feed preheater. The hot partially vaporized crude oil leaving the feed preheater is then allowed to dissipate the added heat to drive the distillation process in a conventional way but without a separate reboiler. In a typical crude oil refinery, the ADU accounts for about 40% of the total energy consumption during refining.5 The large amount of energy consumed by the ADU fired preheater is understandable: it is the entry point of the refinery, which represents the highest flow rate of material, and the liquid feed must be heated up and partially vaporized from near ambient conditions. In a FT facility, the division between synthetic crude oil production and refining is arbitrary, and the interface does not have to be segregated. Current design practice to perform stepwise cooling and syncrude recovery, only to be followed by reheating the feed before the ADU. This seems wasteful. Furthermore, the purpose of syncrude recovery and that of the ADU is very similar, namely, to separate syncrude from syngas and to separate syncrude into useful fractions for further refining. By introducing the hot product from FT synthesis directly into a distillation unit, it may be possible to reduce the overall energy need for syncrude separation. Considering the high consumption of energy by a refinery ADU, reducing the Received: March 29, 2013 Revised: May 10, 2013
A
dx.doi.org/10.1021/ef400560z | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Figure 1. Temperature differentials between technology areas illustrated by a coal-to-liquids design employing high temperature Fischer−Tropsch synthesis. The downstream refinery after the ADU is not shown.
size, or even eliminating the preheater before the distillation unit, will have a meaningful impact on the overall energyrelated environmental footprint of synthetic fuel refining. It is the purpose of this work to investigate the feasibility of improving the interface between FT synthesis and refining. It is suggested that the stepwise cooling, syncrude recovery, and atmospheric distillation steps can be combined into a single unit. By directly feeding the syncrude into a distillation unit, the energy can be dissipated through product fractionation, thereby avoiding successive cooling and heating by utility streams.
High temperature Fischer−Tropsch (HTFT) synthesis employs a fused Fe-based catalyst and synthesis takes place in the gas phase at around 340 °C and 2.5 MPa. At reaction conditions no liquid phase is present in the reactor. The HTFT syncrude is rich in low boiling material, with 40 wt % of the organic product boiling above 360 °C. The heavy material is a paraffinic wax. 2.1. High Temperature Fischer−Tropsch (HTFT) Design Concept. The stepwise cooling of the reactor product from HTFT synthesis and syncrude recovery that is typical of industrial operation is shown in Figure 2. Although this is a generic flow diagram, the main design elements are the same, irrespective of whether HTFT synthesis was performed in a fixed fluidized bed reactor or in a circulating fluidized bed reactor.3 The first step is to cool the hot gaseous product, typically by feed-product heat exchange, to around 150 °C. This is sufficient to condense the heaviest oil fraction. The heavy oil is used as a washing medium to remove the small amount of catalyst that was not removed by the cyclones in the HTFT reactor from the stream. The solids contaminated heavy oil is allowed to settle, producing clear oil, called decanted oil, and a small waste stream containing heavy oil and solids, called gunk. The decanted oil has a broad carbon number distribution, typically C11−C50.6
2. DESIGN CONCEPTS FT technologies can be broadly speaking classified into two categories (Table 1), which are different in important ways for the proposed integration of syncrude recovery and product fractionation.3 Table 1. Characteristics Typical of High Temperature Fischer−Tropsch (HTFT) and Low Temperature Fischer− Tropsch (LTFT) Processes description operating temperature (°C) operating pressure (MPa) reaction phase Fischer−Tropsch catalyst organic product distribution (wt%)b light gas (C1−C2) liquid petroleum gas (C3−C4) naphtha (C5-175 °C) distillate (175−360 °C) residue/wax (>360 °C) water-soluble oxygenates
HTFT
LTFT
320−360 2.0−2.5 gas Fe
170−250a 2.0−2.5 gas+liquid Fe or Co
20−25 20−25 30−35 5−10
