**3. Slurry bed reactors for carbon one chemistry**

#### **3.1 Fischer-Tropsch synthesis**

FTS has been practiced on a large scale by Sasol in South Africa since the mid-1950s in tubular fixed-bed reactors and circulating fluidized-bed reactors using iron catalysts. The commercial scale conventional bubbling fluidized-bed reactor and slurry bubble column reactor (SBCR) were placed on stream by Sasol in 1989 and 1993, respectively. At present, the multi-tubular fixed bed reactor, fluidized bed reactor, gas-liquid-solid slurry reactor, and microchannel reactor are used for FTS. The reactor type and the operating conditions are the governing factors in controlling the product distribution with chain growth probability, catalyst activity, and product selectivity during FTS [9, 10]. The multi-tubular fixed bed reactor and slurry reactor are used for the low-temperature process, while the circulating fluidized bed reactor, fixed fluidized bed rector, and microchannel reactor are used for the high-temperature process. The high-temperature process yields large amounts of olefins, a lower boiling range, and very good gasoline while producing the substantial amounts of oxygenates. The low temperature process yields much more paraffin and linear products and can be adjusted to very high wax selectivity. The primary diesel cut and wax cracking products can give excellent diesel fuels. The very linear primary gasoline fraction needs further treatment to attain a good octane number. Olefin and oxygenate levels for the low-temperature process are lower than those for the high-temperature process.

#### *3.1.1 Fixed bed reactor*

The fixed bed reactor for FTS, such as the Arge reactor at Sasol with 12 m height and 0.5 m diameter, consists of a shell containing 2050 tubes packed with Fe-based catalyst that produced the 600–900 bbl/day/reactor [11]. Where, the heat generated in highly exothermic synthesis reaction of 220°C reaction temperature, and 25–45 bar pressure was used for the generation of steam on the shell side of the reactor. Similarly, some other reactors using Co-based catalyst were also introduced at Bintulu Malaysia, Las Raffan, Qatar, and the Pearl GtL facility which resulted the rate of 3000–140,000 bbl/day with 85–95% C5 + selectivity [12]. Despite the robust nature and high productivity, the fixed-bed multi tubular reactors contain the disadvantages of design complexity, high cost, low catalyst utilization, high pressure drops, and insufficient heat removal due to poor heat conductivity [13].

#### *3.1.2 Fluidized bed reactor*

Fluidized-bed reactors being deployed for the high temperature FT process of entire gas phase, have been designed to comparatively improve the efficiency of fixedbed reactors in terms of superior heat transfer and temperature control during highly exothermic FT reactions; avoiding intraparticle diffusion, pressure drop and the

reaction rate and the better mixing of catalyst particles while giving high production capacity [14]. However, it finds some limitations of difficulty in scale-up for causing the agglomeration and blockage of the fluidization and containing a high risk of attrition and heavy product deposition on catalyst, while requiring the special equipment (cyclones) for catalyst separation. Since, the technical and economic problems of the industrial scale reactor, discouraged their application in 1956 by Hydrocol and later by SASOL in 1980s [15].

#### *3.1.3 Microchannel reactor*

The recent advancement in terms of a new type of reactor—the microchannel reactor, which consist of a large number of parallel channels with diameters <1 mm and with the catalyst on a thin layer inside the channel walls. It has been demonstrated in different studies that the large temperature gradients in the furnace-heated conventional fixed-bed reactor can be avoided in the microchannel reactor under the same operating conditions, achieving the best catalyst utilization and thus a high productivity for the large transfer surface area with high heat transfer coefficient between the bed and wall [9]. However, the main challenges of the difficulty in changing the catalyst and high cost of scaling up several microchannel reactors to be operated in parallel, still avoided its commercial FT plant.

#### *3.1.4 Slurry reactor*

To replace the fixed bed reactor for the low-temperature FT process, the Sasol slurry reactor was developed. In 1993, a single slurry reactor with 5 m in diameter and 22 m in height realized the industrial production for FTS. Compared with the other reactors, the slurry reactors for FTS are less expensive to construct, maintain and operate [16]. It was reported that the capital cost required for a large-scale slurry reactor was less than 40% of that needed for an equivalent multi-tubular fixed bed reactor: The catalyst usage of the slurry reactor is about a third of that of the fixed bed reactor with a promise of even better performance, due to the catalyst's effectiveness and the higher average temperature used in the slurry reactor. Krishna and Sie compared the several reactor types for the FTS process and concluded that the slurry reactor was the best reactor type for large-scale plants [17].

The adopted catalysts for FTS in slurry bed reactors are commonly the iron-based or cobalt-based catalysts [18]. Parts of FTS using slurry bed reactors are listed in **Table 1**. The most common FTS catalysts have been usually regarded as Fe-based and Co-based catalysts, in which Fe-based catalyst have been synthesized by precipitation method [19]. While, the FTS process being carried out by cobalt-based catalysts were usually suspended in inert liquid [13], by impregnation method using Al2O3, SiO2 or TiO2 as support, with the Co loading of 10–30% (wt.). Sasol has developed cobaltbased catalyst SAC 2-100SB and applied in slurry bed reactor for FTS, providing this Co catalyst to two sets of natural gas-based synthetic oil production plants in Nigeria and Qatar. The Co catalyst show stable catalytic activity; however, it is only used for low-temperature FTS due to the high price of cobalt.

The pure iron catalysts are easily worn and deactivated in FTS, which needs to enhance the catalytic activity, selectivity, and stability by introduction of additives (electronic additives and structural additives). Fe/Cu/K catalyst is one of the most successful industrial catalysts for low-temperature FTS (200–250°C) [20]. The precipitated iron-based catalyst prepared by Sasol in South Africa exhibits


#### **Table 1.**

*Installations for FTS with slurry reactors [18].*

high activity, selectivity, and stability, and it has been industrialized. Subsequently, Synfuels China Technology Co., Ltd. proposed a high-temperature FTS in a slurry bed reactor (260–290°C), which greatly improve the steam pressure generated by the FTS reaction and significantly enhance the overall energy utilization efficiency of the FTS process. And based on the development of industrial iron-based catalyst for high-temperature FTS reaction, a complete set for high-temperature FTS process and product processing technologies were developed.

## **3.2 Methanol synthesis**

The syngas conversion to methanol is a strongly exothermic reaction (CO + 2H2 = CH3OH, <sup>Θ</sup> ∆H298K = −90.37 kJ/mol). Inspired by the FTS in a slurry bed reactor, a liquid-phase methanol synthesis was first proposed in 1975. Compared with the gas-solid phase fixed-bed reactor, the gas-liquid-solid three-phase methanol synthesis selected long-chain hydrocarbons with high heat capacity and thermal conductivity liquid inert medium to remove the reaction heat, which makes the catalyst bed operated at a uniform temperature and easy to control. The slurry methanol synthesis can use the feed gas with high CO concentration to improve the single-pass conversion of CO, and the outlet methanol concentration is as high as

15–20%; moreover, there will be no local overheating and excessive temperature rise in catalyst bed [21].

In early studies, the catalysts applied in slurry methanol synthesis were mainly commercial fixed-bed CuZnAl catalysts [22]. In recent years, the slurry methanol catalysts have been intensively studied [23]. It is found that that the precipitation sequence of CuZnAl catalyst affected the catalytic performance. The catalyst prepared by precipitation of Al first and then co-precipitation of Cu and Al exhibit the highest activity and stability; the precipitation and aging temperature can not only affect the phase composition of CuZnAl catalyst precursor, but also affect its crystallinity. In addition, the introduction of microwave radiation heating can make the copper-based catalyst particles size smaller and more uniform, and the strengthened synergistic effect between copper and zinc is beneficial to improve the catalytic activity and stability.
