Heat transfers during catalytic

It is well known that during liquefaction there is always some amount of material which appears as insoluble, residual solids (65,71). These materials are composed of mixtures of coal-related minerals, unreacted (or partially reacted) macerals and a diverse range of solids that are formed during processing. Practical experience obtained in liquefaction pilot plant operations has frequently shown that these materials are not completely eluted out of reaction vessels. Thus, there is a net accumulation of solids within vessels and fluid transfer lines in the form of agglomerated masses and wall deposits. These materials are often referred to as reactor solids. It is important to understand the phenomena involved in reactor solids retention for several reasons. Firstly, they can be detrimental to the successful operation of a plant because extensive accumulation can lead to reduced conversion, enhanced abrasion rates, poor heat transfer and, in severe cases, reactor plugging. Secondly, some retention of minerals, especially pyrrhotites, may be desirable because of their potential catalytic activity. 

The concepts discussed so far indicate that the major challenge in asymmetric operation is correct adjustment of the loci of heat release and heat consumption. A reactor concept aiming at an optimum distribution of the process heat has been proposed [25, 26] for coupling methane steam reforming and methane combustion. The primary task in this context is to define a favorable initial state and to assess the distribution of heat extraction from the fixed bed during the endothermic semicycle. An optimal initial state features cold ends and an extended temperature plateau in the catalytic part of the fixed bed. The downstream heat transfer zone is inert, in order to avoid any back-reaction (Fig. 1.13). 

Flue-gas from boilers fired with liquid or solid fuels contains fly-ash and gaseous contaminants such as CO, NOx, SO2, or volatile organic compounds (VOCs). Emission regulations require their removal, which is achieved by a sequence of after-treatment processes. The after-treatment usually comprises a filter to remove solid particulates operated at approximately 150 °C, a wet scrubber for the removal of SO2 with an alkaline solution operated at approximately 50 °C, and finally a selective catalytic reduction (SCR) unit, which converts NOx to N2 with the help of NH3 at approximately 370 °C (Fig. 15.1) [4]. During this process, the flue gas is cooled down and then heated up again, which requires additional heat transfer equipment, with its inherent energy losses. 

The discussion in the previous sections has evidenced that the use of biphasic systems has solved, at least in various cases, the problem of homogeneous catalyst recovery and recycle, but there still exists the problem of the cost of recycle and especially of reaction rate per volume of reactor, which derives in large part from mass- and heat-transfer limitations, but also from the low amount of catalytic centers per volume of reactor necessary to avoid side reactions and maintain a high selectivity, and/or limit catalyst deactivation or loss. These aspects often emerge only during the scaling-up and industrialization of the reaction and this is one of the reasons why many interesting reactions at the laboratory scale fail in commercialization. 

Liu et al. (2004, 2005) examined a three-dimensional non-linear coupled auto-catalytic cure kinetic model and transient-heat-transfer model solved by finite-element methods to simulate the microwave cure process for underfill materials. Temperature and conversion inside the underfill during a microwave cure process were evaluated by solving the nonlinear anisotropic heat-conduction equation including internal heat generation produced by exothermic chemical reactions. 

In 1943 Houdry Process Corp. announced the Houdry adiabatic process for catalytic cracking which eliminated the molten salt heat transfer system. This process did not go commercial for catalytic cracking, but it was successfully used for butane dehydrogenation to produce butenes for synthetic rubber production during World War II. 

The kinetics of the many commercially important reactions is derived from experimental investigations that are further simplified with substantive assumption on reaction mechanism. During the kinetics study of heterogenous catalytic reactions, mass and heat transfer may affect the observed kinetics and must be avoided. 

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