A Brief Discussion on Coal Tar Suspended Bed Hydrogenation:

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A brief discussion on coal tar slurry bed hydrogenation: Recently, interest in slurry bed hydrogenation has grown significantly. In a broad sense, slurry beds include fluidized beds and slurry beds; the former uses molybdenum-nickel particle catalysts, while the latter employs iron-based powder catalysts. To date, we have classified slurry beds into three generations: The first generation consists of slurry beds that use micron-sized powdered iron-based catalysts; a representative example is the German company Wiba (VCC in China). Although such systems achieve a conversion rate of over 90% for vacuum residue, their gas yield of over 15% and yield of low-value hydrogenated oil slurry of over 10% mean they do not offer significant advantages over the delayed coking process, which yields around 78% liquid product. The second generation consists of slurry-bed systems that use nanoscale iron sulfate-based catalysts; a representative example is CanMet’s technology from Canada. China’s Shenhua direct liquefaction coal-to-oil process is related to this technology, but the issue of low activity of iron catalysts still persists. The third generation is the molecularly homogeneous suspended bed MCSH for molybdenum-based liquid catalysts at the molecular level, which represents a qualitative improvement in terms of catalyst technology and reaction engineering. In particular, the reactor is simplified to an empty tower; the absence of slurry and resistance to coking result in lower costs for capital investment as well as for subsequent operation and maintenance. In the promotion of technologies within the industry on a global scale, the conversion rates for standard promotional methods are as follows: conversion rate for molybdenum-based fluidized-bed asphalt heavy oil (slag reduction) ~70%, and conversion rate for iron-based suspended-bed asphalt heavy oil (slag reduction) ~90%. In fact, no industrial setup can achieve it (but the marketing isn’t malicious; after all, we poor researchers can indeed achieve it in the laboratory, and there are also industrial setups that go all out to deliver an impressive result and keep working at full capacity for a few days). The fluidized bed is the device used worldwide to process the most bitumen-heavy oils, but the conversion rate is generally kept between 35% and 55%. Apart from the Shenhua and Yanchang facilities that are in operation or in the process of being brought online, almost all industrial and pilot-scale units using iron-based suspended beds abroad have been dismantled or abandoned. However, the author estimates that it would be fortunate if these two facilities could achieve a conversion rate of over 50% (which would already be enough to please the management greatly). After all, the domestic industrial versions are essentially reduced versions of foreign original technologies; this is evident in the reduction of the pressure from 300 kilograms to less than 220 kilograms. Theoretically, iron-based catalysts are not sufficient to maintain hydrogenation equilibrium under such conditions, unless thermal cracking is carried out in an ultra-high temperature hydrogen atmosphere. Someone asked me what I thought about Sanju Environmental Protection, known as the “queen of domestic enterprises” in this field, which has developed a suspended-bed technology with independent intellectual property rights and a conversion rate of over 90%. Firstly, the author has no comment to make on this 90% figure; it’s not possible to determine what products are included in that 90% of the liquid yield. If that is indeed the case, then the reaction temperature must be very high (typically 470°C), unless lighter raw materials are used or part of the raw material that produces residues is removed (which is an option). The author believes that the powder-iron slurry bed has always been a problem; it has a reputation for high asphalt conversion rates, and this blame has to be taken on by itself. (R&D engineers have more than once locked themselves in a room to spank sales engineers, under the pretense of cleaning up the mess those engineers have created.) To be honest with you all, this is also feasible, but without ultra-high pressure to back it up, it’s a bit problematic. Of course, using hydrogen pressure to lower the temperature of the slurry bed reactor also helps to reduce gas yield, which in turn improves the technical and economic viability. Speaking of reaction temperature, the author would like to elaborate a bit more on this topic. Theoretically, the reason why slurry bed hydrogenation is classified under hydrocracking, and its reaction mechanism being pyrolysis rather than hydrogenation catalysis, is that the reaction temperature is a very important indicator and parameter. Whether it is a fluidized bed or a suspended bed, the reaction temperature should be above 410°C (up to 470°C in the VCC process, and 410°C–450°C in fluidized beds); otherwise, not only will it be impossible to crack and lighten asphaltenic heavy residue oils, but even vacuum gas oils with a dry point of less than 524°C will not achieve a high conversion rate, and can only be used for pre-hydrogenation. Sinopec’s 50,000-ton fluidized bed demonstration unit in Jinling was forced to shut down indefinitely due to coking and wear at high temperatures. In China, Xin Qiyuan, as a private enterprise, has found an alternative approach: it reduces the operating temperature of its 50,000-ton boiling unit (with circulating oil, this can be considered as 100,000 tons) to below 400°C. It also processes and removes the reduced-residue fraction from the mixed oil, and can process anthracene oil as well, using it as a pre-hydrogenation or pretreatment unit for subsequent fixed-bed reactors. While this helps prevent the reactors from remaining idle, the author estimates that at such a temperature, the conversion rate of mixed oils with a temperature above 360°C will likely not exceed 20%. This approach deviates from the original purpose of boiling-bed technology as well as from the principles of hydrocracking. If it is used merely as an optional pre-hydrogenation pretreatment reactor, the author believes it would be better to use a smaller fixed-bed pre-hydrogenation reactor. Considering potential technical improvements that could enable this boiling bed to handle heavier oils, it might be possible to bring this device back in line with its intended purpose; therefore, we recommend that the design temperature for the new boiling-bed reactor be 490°C, so as to achieve the optimal operating temperature of 450°C for molybdenum-based catalysts. Undoubtedly, compared to suspension beds that use iron-based catalysts, the advantages of bubbling beds lie in their high activity of molybdenum-based catalysts, lower reaction temperatures, the absence of bottom slurry, and relatively lower gas yields; however, due to the need for high-pressure on-line catalyst replacement and the requirement for suspended particles, bubbling bed reactors are complex and require a large proportion of internal circulation, the reaction is controlled by liquid-solid diffusion, and the system is prone to coking. This is also the driving force behind the use of molecular homogeneous technology to meet the production needs of enterprises aiming to upgrade existing fluidized bed systems. Chen Song (PhD)