Performances Of Forced Recirculating Slurry Bed Reactor Used For The Coal Liquefaction Process: An Engineering Study

Coal is the most abundant fossil fuel in China. Hence, it is strategically and economically important to develop coal liquefaction technologies as an effective way to solve the oil and energy shortage problem in our country, and in the meanwhile to avoid environmental pollution and wastes and utilize coal cleanly and effectively. Towards the end, a commercial plant of large scale production capacity has been constructed by Shenhua for producing the direct coal liquefaction oil. For the successful process design, scale up and its optimization and control, it is inevitable to derive process and engineering knowledge from the successful practice of the Shenhua direct coal liquefaction process, and implement further fundamental studies on the process. In this paper, a systematic engineering research was carried with a particular focus on the multiphase flow and reaction performances of forced recirculating slurry bed reactor used for the Shenhua coal liquefaction process.In the first place, a detailed analysis of the Shenhua direct coal liquefaction process and the reactor technology shows that a slurry bed reactor operated in the forced circulating mode of the slurry phase presents advantages such as uniform temperature distribution, good fluidization of large and heavy solid particles, and less degree of gas retention and thus effective utilization of reactor space, all of which are necessary for the highly exothermal coal liquefaction process.Secondly, on the lab scale, three phase flow hydrodynamics in a cold model slurry bed reactor of 2500 mm high and 200mm in diameter were experimentally studied, using air as the gas phase, water as the liquid phase, and glass beads or coal powders as the solid phase. The pressure distribution and its fluctuation, phase dispersion (overall and local gas and solid hold-ups) and liquid circulation velocity were measured in two systems, namely, the coal/water/air system that mimics the coal liquefaction suspensions, and the glass bead/water/air system that approximates the situation where deposition of minerals may occur during coal liquefaction; and the influence of reactor configuration, operation condition, and phase properties on the three phase flow hydrodynamics was systematically investigated. Besides, in view of the high content of calcium in Shenhua coal and tendency to solid deposition in liquefaction, simulating discharge experiments of high density glass beads via the reactor side wall were carried out and shown to be an effective method of solving the deposition problem in the liquefaction process.Thirdly, on the pilot plant scale, flow and reaction performances of a hot model forced circulation slurry bed reactor in a process supporting unit (PDU) with a capacity of 6 ton dry coal/day were investigated, and a collection and analysis of the process data was made. It is demonstrated that 1) the PDU system were successfully operated with process parameters attaining the desired ones, and properly designed for evaluating the flow and reaction performances of the PDU reactor; 2) the gas hold-up at the bottom of the PDU reactor was measured and determined by the following correlation 3) a comparison of slurry circulation intensity between three different liquefaction multiphase reactors, i.e., the PDU reactor of forced circulation of the slurry phase, a internal circulation reactor with a draft tube, and the conventional slurry bed reactor without internals, shows that the former two reactors are identical to each other and superior to the conventional one in terms of their circulation intensity; and 4) the former two reactors exhibit approximately the same coal conversions, and the PDU reactors were found to be nearly free of the calcium deposition problem owing to the intensive and forced slurry circulation.Fourthly, hydrodynamic performances of the PDU reactor were simulated by using the computational fluid dynamics (CFD) method and the FLUENT software. Spatial distribution of flow parameters, including the gas hold-up, turbulent kinetic energy, pressure and liquid velocity, was obtain and thereby used to ascertain the local distribution characteristics of flow parameter within the reactor. Predicted gas hold-up values are found to be close to those measured in the cold model reactor.Lastly, with basic assumptions, a simple reactor simulation model was developed for predicting the reaction performance of the two coal direct liquefaction reactors in the PDU process. It is shown that for the liquefaction of the Shenhua coal in the two reactors, longer mean residence time/higher reaction coefficients are favorable for the transformation or formation of corresponding product species; and compared to the NEDOL process and the Tanitoharumis used therein, the Shenhua process and the coal used exhibit characteristic of shorter residence time but higher conversions.