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The prospects for producing low-carbon olefins directly from syngas are slim. Author/Source: China Chemical Industry News. Date: 10-12-2018. Clicks: 42. Interview with Zhao Tiejun, Director of the Joint R&D Center at Jiangsu Jicui Topso. In recent years, the direct production of low-carbon olefins from syngas has attracted considerable attention within the industry. What are the obstacles to the industrial application of direct synthesis of olefins from syngas? How far is this technology from being put into industrial use? With these questions in mind, a reporter recently interviewed Zhao Tiejun, director of the Joint R&D Center at Jiangsu Jicui Topso. “Based on the currently published research progress, the industrialization of direct olefin production from syngas still faces many challenges, with certain aspects even approaching a ‘valley of death’. ”In Zhao Tiejun’s view, the direct production of low-carbon olefins from syngas has a bleak outlook. Low space-time yield of the catalyst From a reaction perspective, to ensure a certain conversion rate of syngas as well as selectivity for the desired products, it is necessary to increase the space-time yield of the catalyst. In terms of the separation process, energy consumption is a key factor, especially when it comes to separating low-carbon olefins and alkanes, as this process requires significant amounts of energy. The space-time yield is low; a large amount of unconverted material needs to be recycled, which in turn increases the energy consumption for separation. In this sense, catalyst activity is a fundamental requirement; that is, to reduce the amount of syngas that needs to be recycled, it is necessary to maximize the conversion rate of the syngas, which is generally around 90%. Under such conditions, the selectivity for low-carbon olefins in the reaction process should not decrease significantly. However, based on the publicly reported progress to date, there is still a significant gap in this area. The catalyst based on the Na-Fe/ZnO system reported by Peking University exhibits high activity, with a CO conversion rate of up to 83%; the selectivity for CO2 is also good. Moreover, a large proportion of the products are olefins, although the selectivity for light olefins is 26.5%. Generally speaking, to maintain high catalyst activity, it is necessary to increase the proportion of hydrogen; however, increasing this proportion may lead to the hydrogenation of the low-carbon olefins in the product, thereby reducing the catalyst’s selectivity. As a result, there is a trade-off between high activity and high selectivity for such catalysts, and it is not possible to have both simultaneously. It is well known that the space-time yield of the catalyst decreases at high conversion rates. For another catalyst approach to the direct production of low-carbon olefins from syngas, namely the oxide and molecular sieve composite catalyst route, the conversion rates reported so far are only around 20%, and that is even under conditions of hydrogen **excess. When the conversion rate reaches 90%, the space-time yield of the Zn-CrOx-SAPO composite catalyst decreases by more than 80%. For a low-carbon olefin production plant with a capacity of 300,000 tons, the amount of catalyst required will exceed 1,500 tons, whereas for MTO plants of the same scale, the catalyst requirement is less than 150 tons. Therefore, from this perspective, maintaining the selectivity for light olefins while further increasing the space-time yield of syngas is an issue that must be resolved before the industrialization of direct synthesis of low-carbon olefins from syngas. A more realistic goal is to use a catalyst amount similar to that in the MTO process. In other words, for Zn-CrOx+MSAPO system catalysts, to be competitive with the current MTO process while maintaining the same selectivity, their activity needs to be increased by a factor of 10. The separation process requires high energy consumption. If the CO conversion rate is not high, then there will be a large amount of unconverted CO and H2 in the products, which significantly increases the energy demand of the downstream separation processes. At the same time, the process of directly producing olefins from syngas often also contains a certain amount of methane; to prevent this methane from accumulating in the reaction system, it must be vented or separated from CO and hydrogen. If separating methane from CO is considered, the lowest temperature of the cooling system may need to be below minus 140 degrees Celsius. Therefore, the separation process of light components in the direct production of low-carbon olefins from syngas will be a highly energy-intensive process. Furthermore, as CO and methane are separated from the system, some ethylene is often carried away as well. For a typical 300,000-ton ethylene plant, an ethylene loss of 0.1% means 300 tons of ethylene are lost, resulting in a loss of nearly 3 million yuan; this does not even include the syngas present in the off-gases. The lower the CO conversion rate in the syngas unit, the higher the proportion of residual syngas, and thus the more ethylene is carried away. Additionally, a larger syngas circulation volume is required to increase the overall conversion rate, but this entails higher compression energy consumption. Therefore, although the direct conversion of syngas into olefins eliminates the methanol synthesis step compared to coal-based olefin production, the scale of the downstream separation systems and their operating costs increase significantly. As a result, the equipment investment may even exceed that of the coal-based olefin process, and the operating costs due to high energy consumption are also very substantial. Difficulty in matching catalysts to reactors: The compatibility between Fertor reactors and catalysts plays a decisive role in the process of producing olefins directly from syngas. Different types of reactors require catalysts with varying mechanical strength, shapes, and even manufacturing processes; in other words, it is necessary to tailor the catalysts to the specific requirements of each reactor. Based on current reports, there are catalysts at two different temperature levels for the direct conversion of syngas into olefins: 250 degrees Celsius (Co2C) and 350–400 degrees Celsius (Zn-CrOx-SAPO). For 250 degrees Celsius, the type of reactor that can be used is either a fixed-bed reactor with efficient heat exchange or a slurry bed using a high-boiling-point solvent. Compared to Fe-based catalysts, the Co-based catalysts used at this temperature level produce less carbon deposition; therefore, the risk of catalyst deactivation due to carbon deposition is relatively low. However, Co is sensitive to the reaction atmosphere, and its particles tend to sinter easily, which are issues that industrial catalysts must take into account and address. Furthermore, during the reaction process, there is a change in the active phase from Co to Co2C, and the formation of carbides may cause structural changes in the catalyst. If a fixed-bed reactor with strong heat transfer is used, the issues that need to be addressed are catalyst shaping and pretreatment. Since fixed-bed reactors use catalysts on a millimeter scale, the ratio of hydrogen to CO within the catalyst particles varies greatly; macroscopically it is 1 or 0.5, but the actual ratio inside the catalyst particles can even reach 100. Therefore, even though the problem of low space-time yield of catalysts has been resolved, systematic research is still needed to implement the direct conversion of syngas into olefins in fixed-bed reactors, covering not only catalyst formulation and pretreatment but also the coordination between granular catalysts and the reactor. The advantage of a slurry bed is its strong heat exchange capacity; it also allows the use of powder catalysts, which offer relatively high efficiency. However, the disadvantage of the slurry bed is severe backmixing; in other words, the olefins produced are prone to being further saturated by unconverted hydrogen, which results in a **decrease in the selectivity of the olefins obtained**. Furthermore, for slurry-bed catalysts, a challenge lies in how to produce spherical catalysts of appropriate size through special processes while ensuring good mechanical strength. For catalysts designed to operate at temperatures of 350 degrees Celsius or 400 degrees Celsius, a fluidized bed reactor may be a suitable choice; fixed-bed or circulating-fluidized-bed reactors can be used. These reactors require higher mechanical strength from the catalysts, which poses a significant challenge for composite oxide-molecular sieve catalytic systems – it is not easy to mix the two types of catalysts while maintaining the desired spatial distribution within the reactor. The above are just a few of the basic technical challenges associated with the process of directly producing low-carbon olefins from syngas. From a macroscopic perspective, its economic viability is also affected by challenges related to MTO itself, as well as shale gas and conventional oil production methods. But even from a technical perspective, it is clear that this seemingly revolutionary approach still has numerous fundamental issues that have not been addressed; without resolving these basic problems, it is impossible to transform these laboratory results into practical applications.