Fixed bed system

While the geometry of the systems depicted in Fig. 7.10 differs quite appreciably, the actual movement of the reaction front will be identical, provided 

Gas-solid contacting in a fixed-bed arrangement, (a) Packed bed, (b) packed bed with cross flow contacting. 

The principal physical phenomena with which we are concerned in the representation of fixed bed reactors are fluid flow, heat transfer, and mass transfer. 

The assembly of particles in packed beds represents a rather complex geometry and, while a number of interesting studies have been made of the flow field in packed beds [5-7] for most practical problems we have to resort to empirical relationships between the pressure drop and the flow rate of the fluid. Of the numerous correlations proposed for relating pressure drop to flow rate, the Ergun equation [8] is perhaps the most widely accepted  

Surface area of sphere of equal volume to the particle 

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Two new processes usiag 2eohte-based catalyst systems were developed ia the late 1980s. Unocal s technology is based on a conventional fixed-bed system. CR L has developed a catalytic distillation system based on an extension of the CR L MTBE technology (48—51). 

A fluidi2ed-bed catalytic reactor system developed by C. E. Lummus (323) offers several advantages over fixed-bed systems ia temperature control, heat and mass transfer, and continuity of operation. Higher catalyst activity levels and higher ethylene yields (99% compared to 94—96% with fixed-bed systems) are accompHshed by continuous circulation of catalyst between reactor and regenerator for carbon bum-off and continuous replacement of catalyst through attrition. 

In the late 1980s, however, the discovery of a noble metal catalyst that could tolerate and destroy halogenated hydrocarbons such as methyl bromide in a fixed-bed system was reported (52,53). The products of the reaction were water, carbon dioxide, hydrogen bromide, and bromine. Generally, a scmbber would be needed to prevent downstream equipment corrosion. However, if the focus of the control is the VOCs and the CO rather than the methyl bromide, a modified catalyst formulation can be used that is able to tolerate the methyl bromide, but not destroy it. In this case the methyl bromide passes through the bed unaffected, and designing the system to avoid downstream effects is not necessary. Destmction efficiencies of hydrocarbons and CO of better than 95% have been reported, and methyl bromide destmctions between 0 and 85% (52). 

The MTZ is that length of the adsorbent bed where the adsorbate concentration in the liquid varies from the influent concentration to zero or the specification value. At the same contact time, the single fixed-bed system at the higher LV stays onstream about 15 percent longer. 

Fixed-bed systems are the most common, but some countercurrent fluidized beds are in use. Flow diagrams are given in reference 47. The superficial velocities of gases in fixed beds should be about 1 ft/sec (0.3 m/sec) and those for liquids about 1 ft/min (0.3 m/min).48 See references 48 and 49 for more design information. 

Still another advantage of fluidized bed operation is that it leads to more efficient contacting of gas and solid than many competitive reactor designs. Because the catalyst particles employed in fluidized beds have very small dimensions, one is much less likely to encounter mass transfer limitations on reaction rates in these systems than in fixed bed systems. 

The term three-phase fluidization, in this chapter, is taken as a system consisting of a gas, liquid, and solid phase, wherein the solid phase is in a non-stationary state, and includes three-phase slurry bubble columns, three-phase fluidized beds, and three-phase flotation columns, but excludes three-phase fixed bed systems. The individual phases in three-phase fluidization systems can be reactants, products, catalysts, or inert. For example, in the hydrotreating of light gas oils, the solid phase is catalyst, and the liquid and gas phases are either reactants or products in the bleaching of paper pulp, the solid phase is both reactant and product, and the gas phase is a reactant while the liquid phase is inert in anaerobic fermentation, the gas phase results from the biological activity, the liquid phase is product, and the solid is either a biological carrier or the microorganism itself. 

Rosen, J. B. J. Chem. Phys. 20 (1965) 387. Kinetics of fixed bed systems for solid diffusion into spherical particles. 

The ion-exchange process has been used effectively in the field of waste disposal. The use of continuous ion exchange and resin regeneration systems has further improved the economic feasibility of the applications over the fixed-bed systems. One of the reported [1] special... 

The fixed bed system, which offers the same advantage of low raw material cost, also is licensed widely despite the possible disadvantages of reactor corrosion problems, low single-pass conversion, and poor tem-... 

The Ergun equation relates the pressure drop in a packed bed to the flow rate and the properties of particle and gas. However, the application of this equation has been extended beyond the limits of fixed bed systems since it was first formulated in 1951. Thus, a detailed account of the origin of this equation is necessary. 

Much of the benefit of countercurrent operation, without the problems associated with circulation of the adsorbent, can be achieved by using a multiple-column fixed-bed system with an appropriate sequence of column switching, designed to simulate a counterflow system. The general scheme is illustrated in Fig. 13. Such systems are widely used in wastewater treatment,... 

While the evaluations of cost and plant size discussed in the preceding paragraphs have been devoted to fixed bed,systems, the conclusions are valid for all coal gasification techniques. Estimates of fluid bed gasifiers have also been prepared. (2) Unfortunately, insufficient data are available to substantiate the operability and actual productivity which must form the basis for any cost estimate. Using these tentative costs, we find that costs for LBG or MBG might be below 3.00/million Btu. For entrained flow systems, still under development, costs in the below 3.00/million Btu range are estimated. However, actual implementation of these advanced systems or even state-of-the-art... 

The major advantage of these fixed-bed systems is that the fixed-bed technology can be simple, and will require minimum scale-up studies. As indicated earlier, this type of design has been successfully scaled-up to commercial size directly from bench-scale pilot plant data many times. 

Still another multi-reactor approach is to divide the MTG reaction into two steps as shown in Figure 7. In the first step, methanol is partially dehydrated to form an equilibrium mixture of methanol, dimethyl ether and water over a dehydration catalyst. About 15% of the reaction heat is released in this first step. In the second step, this equilibrium mixture is converted to hydrocarbons and water over ZSM-5 catalyst with the concomitant release of about 85% of the reaction heat. Though this two step approach does not have any of the inherent complications of the previously mentioned multibed reaction systems, it leaves one with a substantial amount of the reaction heat (85%) still to be taken over one catalyst bed. This requires a fairly high recycle stream to moderate the temperature rise over the second reactor. Such a high recycle design would require careful engineering in order to transfer heat efficiently from the reactor effluent to the recycle gas and reactor feed. However, this two stage reactor system is the simplest of the fixed-bed systems to develop. 

Another reactor system which has several attractive features for heat removal is the tubular, heat-exchange reactor. Good temperature control can be achieved in the tubular reactor if the coolant approximates an isothermal heat sink. Light gas recycle can be reduced significantly compared to fixed-bed systems. Tubular reactors have been used for Fischer-Tropsch reactions and for synthesis of methanol and phthalic anhydride, for example. 

The advantage of fixed bed systems is that the relatively high activity per unit weight allows manufacturers to process large quantities of product through relatively small reactors in short times. The short residence time in these reactors also reduces development of undesirable color and flavor compounds. 

In the mixed-phase CD reaction system, propylene concentration in the liquid phase is kept extremely low (<0.1 wt%) due to the higher volatility of propylene to benzene. This minimizes propylene oligomerization, the primary cause of catalyst deactivation and results in catalyst run lengths of 3 to 6 years. The vapor-liquid equilibrium effect provides propylene dilution unachievable in fixed-bed systems, even with expensive reactor pumparound and/or benzene recycle arrangements. 

The simplest solid desiccant fixed bed system consists of a minimum of two dryer vessels or towers. One vessel is drying the process... 

In the vast majority of experimental studies, the backmixing characteristics of a flowing phase are examined using a -pulse tracer input. For the fixed-bed systems shown in Fig. 3-2, if a perfect pulse input is used, then, as shown by Levenspiel,5 6 the axial dispersion coefficient or the Peclet number can be obtained from the variance of the RTD curve. For example, for a closed system and large extent of dispersion, the variance, it, is related to the Peclet number by the equation... 

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