The term trickle bed is used here to mean a reactor in which a liquid phase and a gas phase flow concurrently downward through a fixed bed of catalyst particles while reaction takes place. The earliest antecedent probably was the so-called “trickling filter” which has long been used for removal of organic matter from wastewater streams by aerobic bacterial action. Biological growths are allowed to attach themselves to a bed of stone or other support over which the wastewater is allowed to trickle in contact with air. Another antecedent appeared in the 1930s when Duftschmid and co-workers at the I.G. Farbenindustrie used evaporative cooling of an inert oil which flowed upwards through a catalyst bed to remove the heat of reaction in a version of the FischerTropsch process for synthesis of liquid fuels from H2 and CO and a later version at the U.S. Bureau of Mines operated with concurrent upflow but without evaporative cooling. This was superceded by an ebulliating bed reactor because of difficulties with cementation of the catalyst in the fixed bed, but commercial scale operations of this process in Germany and elsewhere have all utilized completely vapor-phase reaction (Storch et al., 1951; Crowell et al., 1950; Benson et al., 1954; Kastens et al., 1952). Trickle-bed reactors have been used to a moderate extent in chemical processing, but most of the published information about their industrial applications concerns the processing with hydrogen of various petroleum fractions, in particular the hydrodesulfurization or hydrocracking of heavy or residual oil stocks and the hydrofinishing or hydrotreating of lubricating oils. Their commercial development during the 1950s by the Shell Companies and by the British Petroleum Company has been described by van Deemter (1964), Le Nobel and Choufoer ( 1959), and Lister (1964). The development of hydrocracking and hydrodesulfurization processes by Chevron, Esso (Exxon), Gulf, Union Oil, and others has been described in the proceedings of the various World Petroleum Congresses. Information is fragmentary on uses in chemical processing. A process for synthesis of butynediol (HOCH2C = CCH20H) from aqueous formaldehyde and acetylene uses trickle-bed flow over a copper acetylide catalyst and recycle of the product stream for heat removal (Brusie et al., 1963). Other trickle-bed studies of this reaction are given in a thesis by W. Bill, as reported by Bondi (1971), who also cites trickle-bed studies by H. Hofmann on the hydrogenation of glucose to sorbitol. Kronig (1963) describes a trickle-bed process used in one or more commercial plants for selective hydrogenation of acetylene to remove it in the presence of butadiene in Cq hydrocarbon streams. Operation at 10” to 20°C and 1.9 to 5.8 atm pressure allows liquid-phase processing which reportedly gives long catalyst life, unlike gas-phase processing in which polymers rapidly build up on the catalyst. The standard commercial process for manufacture of hydrogen peroxide utilizes a working solution of an alkyl

Trickle-bed reactors

Phenomenological kinetics of complex systems: the development of a generalized severity parameter and its application to lignocellulosics fractionation

Publisher Summary Chemical mechanisms have been employed in hydrocarbon combustion as a means of understanding the underlying phenomenology of the combustion process in terms of the elementary reactions of individual species. This chapter provides an introduction to most of the mathematical methods that have been used for the construction, investigation, and reduction of combustion mechanisms. The use of algebraic manipulation in techniques, such as the quasi-steady-state approximation (QSSA) and lumping, make the production of a reduced mechanism essential and make subsequent calculations as simple as possible. Computational singular perturbation (CSP) is an alternative to the rate-of-production and sensitivity methods for mechanism reduction and provides an automatic selection of the important reactions as well as time-scale analysis. The simplest and most widely used technique involving the separation of time scales is the QSSA; however, a possible limitation is that it may not provide the minimum low-order system. Chemical lumping can prove very useful in areas, such as the combustion of hydrocarbon mixtures or soot formation. Several programs are available for the investigation and reduction of combustion mechanisms, including MECHMOD, a code for the automatic modification of CHEMKIN format combustion mechanisms, and KINALC, which is an almost automatic program for the investigation and reduction of gas-phase reaction mechanisms. KINALC is a postprocessor to CHEMKIN-based simulation packages SENKIN, PREMIX, OPPDIF, RUN1DL, PSR, SHOCK and EQLIB. Because models in combustion are expected to cover a wide range of conditions, it is natural to expect that a different approach might be used for different cases.

Chapter 4 Mathematical tools for the construction, investigation and reduction of combustion mechanisms

A general analysis of exact lumping in chemical kinetics

The field of catalytic cracking is reviewed. Cracking uses the largest tonnage of commercial catalysts. It produces the largest volume of products. It has been instrumental in creating the most varied, richest, and most influential of our industries. It is therefore not surprising that a great deal is known about cracking catalysts and catalytic cracking. What is surprising is that a coherent picture of the chemistry of this process is only now beginning to emerge. In brief, the picture is as follows. Catalytic cracking occurs on acid catalysts containing strong Bronsted and Lewis sites. It is accompanied by a vast variety of other processes such as skeletal isomerization, cyclization, aromatization, disproportionation, and dehydrogenation. Each of these processes occurs on sites of appropriate strength and nature and not on other sites which may be present. The weak sites catalyze reactions such as cis-trans isomerization, stronger sites catalyze double-bond isomerization, followed in turn by skeletal isomerization, cracking, and coke formation on sites of increasing strength. Further complications are introduced by the size, tortuosity, and surface configuration of the pores present in crystalline cracking catalysts. Appreciation of the significance of these effects seems to be new to the field of heterogeneous catalysis. Itmore » now seems certain that they have enormous potential to affect selectivity and to introduce specificity to the list of important properties of heterogeneous commercial catalysts. 365 references, 25 figures, 22 tables.« less

Philip Hutchinson

Papers

Lumping of mixtures with many parallel N-th order reactions