Coal liquefaction catalyst design

Past efforts in developing coal liquefaction catalysts have focused on alumina-supported systems and, except for exploratory studies, little attention has been given to systematic development of novel formulations. A particularly promising approach to the development of new catalysts specifically designed lor coal liquefaction processes lies in the formulation of multicomponent systems that, in comparison to work on single or bimetallic systems, are essentially unexplored. Use of multimetallic systems offers the possibility of multifunctional catalysts that are needed to perform the many different reactions encountered in coal processing. Because of its versatility for the preparation of multimetallic catalysts, the HTO system is an excellent candidate for further development. 

The difficulty in the recovery of catalysts from unreacted coal and minerals and the poor regenerability of used catalysts forces one to use disposable catalysts, especially in the primary stage. This increases the cost of coal liquefaction considerably. This section reviews the mechanism of catalyst deactivation, design of recoverable catalysts in the primary stage, and catalyst deactivation in the secondary stage. 

The recovery, regeneration, and repeated reuse of the active catalyst are of prime importance in substantially reducing the overall cost of coal liquefaction. The used catalysts usually remain in the bottoms products, which consist of nondistillable asphaltenes, preasphaltenes, unreacted coal, and minerals. The asphaltenes and preasphaltenes can be recycled with the catalyst in bottoms recycle processes. However, unreacted coal and minerals, if present in the recycle, dilute the catalyst and limit the amount of allowable bottoms recycle because they unnecessarily increase the slurry viscosity and corrosion problems. Hence, these useless components should be removed or at least reduced in concentration. If the catalyst is deactivated, reactivation becomes necessary before reuse. Thus, the design of means for catalyst regeneration and recycle is necessary for an effective coal liquefaction process. Several approaches to achieving these goals are discussed below. 

All of these problems are related to the performances of the catalysts used in coal liquefaction. Very active, durable, recoverable, and regenerable catalysts are most wanted in the primary liquefaction stage, where catalyst poisons from asphaltenes and minerals are most severe. Multifunctional catalysts should be designed by selecting supports with specific functions, such as strong but favorable interactions with catalytic species, resistance to poisons, and improved properties to allow easy recovery, while maintaining high activity. 

XIV. Design of Multistage Coal Liquefaction with Catalysts Yet to Be Developed... 

Coal liquefaction that can provide liquid fuels at the price of current petroleum (not cost but price) is one of the most important technologies that needs to be developed. The catalyst and control of its operating conditions are still key to technology for advanced coal liquefaction. The creative design of catalyst materials and reaction schemes is an important and challenging goal for the future. 

We have our work divided into process engineering, process chemistry, catalysis, and support technology. As an example, one of the indirect liquefaction projects, tube wall reactor, deals with the design and operation of high thermal efficiency catalytic reactors for syn-gas conversion. Other activities are coal liquefaction properties of coal minerals, the role of catalysts, coal liquid product stability, and environmental impact—to name a few. 

Reactions involving gas, liquid, and solid are often encountered in the chemical process industry. The most common occurrence of this type of reaction is in hydroprocessing operations, in which a variety of reactions between hydrogen, an oil phase, and a catalyst have been examined. Other common three-phase catalytic reactions are oxidation and hydration reactions. Some three-phase reactions, such as coal liquefaction, involve a solid reactant. These and numerous other similar gas-liquid solid reactions, as well as a large number of gas-liquid reactions, are carried out in a vessel or a reactor which contains all three phases simultaneously. The subject of this monograph is the design of such gas-liquid -solid reactors. 

The PIPU is a pilot plant system built in the early 1980s for studying a multitude of synthetic fuel/chemical processes. In the mid 1990s, a direct coal liquefaction reactor within the PIPU plant was reconfigured as a SBCR for FTS studies (see Figure 1.). The reactor was originally designed to operate with coarse catalyst pellets (>500 pm). Consequently, the reactor system did not contain a wax separation system sufficient for smaller catalyst particles that are typically used in FTS. Therefore, a slurry accumulator and a batch wax filtration system were installed. 

Solids play different roles in the different processes. In direct coal liquefaction, a part of the solid is dissolved in liquid (mainly in the preheater) and a part (i.e. mineral matter) may act as a catalyst for the hydrogenation reactions. In Fischer-Tropsch slurry processes, solids are catalysts. Finally, in chemical cleaning of coal, only a part of solid (i.e. sulfur) takes part in the reaction following the shrinking core diffusion/ reaction mechanism. The role of solids in the design and scaleup of the reactors for the three processes is therefore different. 

This paper presents a brief state of the art review of direct coal liquefaction. The review Includes Important pilot scale processes available for the liquefaction and a brief description of the structure of coal and the chemistry, mechanism and available lumped kinetic models for the liquefaction process. It also Includes some discussions on the role of catalysts during coal liquefaction and on the use of model compounds for the understanding of coal liquefaction kinetics. Reactor design aspects are covered In a separate paper and will not be repeated here. 

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