Isothermal reactors flow through packed beds

How wide a range of linear velocity must be studied The following table shows the results of a calculation of the effect of linear velocity on the fractional conversion from an ideal PFR. The reaction considered is A R, which was assumed to be irreversible and first order in A, with A r = 0. The reactor was assumed to be isothermal. The mass-transfer coefficient was assumed to be proportional to the square root of linear velocity, a relationship that is reasonably typical for flow-through packed beds. Finally, to provide a starting point for the calculation, rjkylc/kc was taken to be 1.0 when the outlet fractional conversion of A was 0.50. When rjkylc/kc = 1, the resistance to extmial transport is equal to the resistance to reaction inside the catalyst particle. At this condition, /ly arbitrarily was assigned a value of 1. 

Gas-solid reactions are carried out on a commercial basis using fixed-bed, moving-bed, and fluidized-bed reactors. The fixed-bed reactor is an unsteady-state system as reactive gas is fed on a continuous basis through the reactor that is packed with a finite quantity of solid reactant. The solid is depleted and breakthrough of the gas reactant occurs after a certain reaction time. In the moving-bed reactor, both solid and gas are fed on a continuous basis and overall operation is steady state. The fluidized-bed reactor, where small solid particles are fluidized by upward flow of gas, also operates in a steady-state manner. Diffusional reaction resistances are reduced because of the small solid particles while solid backmixing reduces solid concentration gradients and promotes isothermal operation. 

We conduct the reaction in a packed bed reactor, assuming that the heat transfer is sufficiently fast for the reactor to be isothermal. The mass W of catalyst in a region of volume V in the reactor is W = ps — solid catalyst and void fraction of the bed. Let T a(IT) be the flow rate (moles per unit time) of A passing through the particular surface in the reactor for which the mass of catalyst in the region between this surface and the inlet is W. The mole balance on A for the region between Wand W + SW is... 

The differentia reactor is relatively easy to construct at a low cost. Owing to the low conversion achieved in this reactor, the heat release per unit volume will be small (or can be made small by diluting the bed with inert. solids) so that the reactor operates essentially in an isothermal manner. Vtlien operating this reactor, precautions must be taken so that the reactant gas or liquid does not bypass or channel through the packed catalyst, but instead flows uniformly across the catalyst. If the catalyst under investigation decays rapidly, the differential reactor is not a good choice because the reaction rate parameters at the start of a run will be different from those at the end of the run. In some cases sampling and analysis of the product stream may be difficult for small conversions in multicomponent systems. 

The effectiveness factor is required to estimate the average rate of consumption of reactants in the catalytic pores based on temperatnre and molar density at the external surface or in the adjacent bulk gas stream moving through the fixed-bed reactor. Hence, plng-flow design equations presented in Section 27-6 for packed catalytic tubular reactors must include the effectiveness factor in the rate law, even when isothermal operation is a reasonable assnmption. 

Apparatus. The system was constructed by modifying an existing trickle-flow once through PDU reactor. A 2.5 cm diameter reactor was packed with a mixture of 100 cm of 1/16-inch extrudate catalyst and 100 cm of 1.0 mm silicon carbide non-porous Inerts, producing a 60 cm long catalyst bed. There were 25 cm long sections of Inerts above and below the catalyst. Isothermal conditions were maintained throughout the bed within about 5 C. 

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