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Catalysts change the reaction mechanism and therefore the rate of the reaction. Packed bed catalytic reactors are specially designed reactors that accelerate the rate of reaction by the use of latest technological equipment. If the reaction rate increases, the reaction volume will decrease, reducing the cost of the reactor. Many chemical syntheses are impractical without using a catalyst. Catalytic Pellet Selection If a pure catalyst is structurally weak and cannot be formed into a pellet or is too expensive to use as a pellet, then the catalyst is deposited as a thin film on an inert support. Because the reaction rate is proportional to the catalyst surface area, the pellet must be porous to achieve a large surface area. |
Besides chemical properties of the catalyst, the mechanical properties of the support material must also be considered when selecting a catalyst. Support materials are mostly alumina, silica, activated carbon or diatomaceous earth, but alumina is more widely used than the other materials . Pellets are usually molded or extruded into spheres, cylinders, or rings.
PACKED BED CATALYTIC REACTOR
Extrusion is a lower cost operation than molding. The most common pellet diameters are 1/32, 1/16, and 1/8 in (0.794, 1.59, and 3.18 mm). Pellets should have a high compressive strength to resist crushing and abrasion and a low pressure drop to minimize compressor and power costs. Because pellets are packed in a bed, the bulk crushing strength of the pellets limits the bed height. Trambouze et al. define bulk crushing strength as the stress that produces 0.5 % fines as determined by compressing the pellets in a press. Pellet strengths vary from 1.0 to 1.3 MPa (145 to 189 psi) for several pellets tabulated by Trambouze et al.
Selecting a pellet size, shape, and porosity (void fraction in the pellet) is a trade-off between achieving high reactivity, high crushing strength, and low pressure drop. Promoting high reactivity requires a porous pellet with a large internal surface area, which requires small pores. Small pores, however, lower the diffusion rate, reducing the pellet activity. The rate of diffusion increases with increasing pore size, but the increased pore size reduces surface area and therefore reactivity. Consequently, there is an optimum pore size that maximizes pellet reactivity. Reactor reactivity increases if the pellet diameter is reduced, allowing more pellets to be packed into a reactor, but then the pressure drop is increased. Low pressure drop is achieved using large pellets, but' then this reduces the catalyst surface area for a unit volume of reactor. Also, crushing strength decreases with increasing porosity particularly when the porosity is above 50% .
Approximate Packed Bed Catalytic Reactor Sizing
After selecting a reactor type and catalyst configuration, the next step is to calculate the reactor volume. Before undertaking a detailed calculation, we need to estimate the reactor volume. A quick estimate is sometimes needed to check an exact calculation or to prepare a budget for a proposal. For packed bed or homogenous reactors, the space velocity is a way of rapidly sizing reactors. Space velocity is defined as the ratio of the volumetric feed flow rate to the reaction volume or the ratio of mass feed flow rate to the catalyst mass. The volume or the ratio of mass feed flow rate to the catalyst mass. The volumetric feed-gas flow rate is calculated at a standard temperature and pressure. Thus, the space velocity is defined by:
GHSV = hourly volumetric feed-gas flow rate/reaction volume
LHSV = hourly volumetric liquid-feed flow rate/reaction volume
WHSV = hourly mass feed flow rate/catalyst mass
The units of space velocity are the reciprocal of time. Usually, the hourly volumetric feed-gas flow rate is calculated at 60 °F (15.6 °C) and 1.0 arm (1.01 bar). The volumetric liquid-feed flow rate is calculated at 60 °F (15.6 °C). Space velocity depends on the design of the reactor, reactor inlet conditions, catalyst type and diameter, and fractional conversion. Walas has tabulated space velocities for 102 reactions. For example, for the homogeneous conversion of benzene to toluene in the gas phase, the hourly-volumetric space velocity is 815 h"1. This means that 815 reactor volumes of benzene at standard conditions will be converted in one hour.
Although space velocity has limited usefulness, it allows estimating the reaction volume rapidly at specified conditions. Other conditions require additional space velocities. A kinetic model is more useful than space velocities, allowing the calculation of the reaction volume' at different operating conditions, but a model requires more time to develop, and frequently time is not available.
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