Reactor types and catalysts test

Reactor types

Reactors can be classified based on different criterion such as size, reactor material, methods of charging and discharging or type of fluid flow. For fluid – solid heterogeneous system reaction rate is based on mass of solid catalyst rather than on reactor volume. In laboratory scale, the reactions are carried out in micro reactors with diameters ranging from 1 – 5 mm for testing up to 0.1 – 1gm of catalysts. In industry, diameter and height of reactors can vary from 1 to 10 m and 2 to30 m respectively. The reactor materials also vary depending on usage. Laboratory reactors are usually made of glass, quartz or stainless steel with few mm wall thicknesses. The large industrial reactors depending on usage are usually made of mild steel or stainless steel or other alloys. The wall thickness can range from of 6-15 mm depending on process pressure. Depending on the operation, reactor can be batch, mixed flow or packed bed type. The choice of reactor depends on the type of reaction. For examples, many hydrogenation reactions are carried out in CSTR while many of the solid catalysts are tested in packed bed reactor. Reactor selection also depends on the type of catalyst and its activity, selectivity and deactivation behavior. The schematic diagram of different type of reactors is shown in Fig 1.

For study of catalytic reactions in laboratory different reactors are used. For example fluid-solid catalytic reactions are studied in (1) packed bed reactors heated in furnace, (2) Carberry reactors equipped with rotating catalyst basket and cooling or heating jacket or (3) Berty reactor with internal circulation and cooling or heating jacket. For reactions involving gas-liquid-solid, slurry CSTR with cooling or heating jacket or packed bed reactor with downflow, upflow or countercurrent flow of gas and liquids are used. Laboratory reactors are mainly used for measuring reaction kinetics and catalyst activity at different conditions of temperature and pressure.

Fixed bed reactor

The fixed bed or packed bed reactors are most commonly used for study of solid catalyst. A fixed bed reactor usually consists of a cylindrical vessel packed with catalyst pellets and easy to design and operate. The metal support grid and screen is placed near the bottom to support the catalyst. Inert ceramic balls are placed above the catalyst bed to distribute the feed evenly.

Advantages of packed bed or fixed bed reactor include ideal plug flow behavior, lower maintenance cost and reduced loss due to attrition and wear. Heat management is very important aspect for design of fixed bed reactor. Poor heat distribution may result in non uniform reaction rates and consequently low reactant conversion. Poor heat transfer may also result in generation of hot spots and thermal degradation of catalyst. However, the situations are observed more in large fixed bed and for highly exothermic or endothermic reactions when temperature control is difficult. The regeneration or replacement of catalyst is also difficult in fixed bed reactors and process needs to be shutdown. Another major disadvantage of packed bed reactor is plugging of bed due to coke deposition which results in high pressure drop. High pressure drop is also observed for small beads or pellets of catalysts. However, increase in pellet size increases the pore diffusion limitation.

Catalyst pellet sizes are usually in the range of 1 to 10 mm. Non-uniform packing of catalysts can cause channeling of fluids leading to poor heat and mass transfer. The column to particle diameter is maintained in between 10 to 20 to minimize channeling. The bed voidage is usually 70 to 90 %. Plug flow behavior is ensured by maintaining ratio of reactor length to catalyst particle diameter greater than 50. The allowed pressure drop is less than 0.5 inch water per foot of bed depth. Usually the ratio of bed height to diameter is maintained greater than 0.5.

For better heat management for very highly exothermic (or endothermic) reaction the multitubular reactor is used with catalyst packed inside the tubes. The cooling (or heating) fluid flows through the shell side. The length is limited by allowable pressure drop. The multitubular reactor has high surface area for heat transfer per unit volume. For determination of heat transfer and mass transfer properties several correlations are available in literature.

Fluidized bed reactors

In fluidized bed reactor catalyst pellets of average size less than 0.1 mm are fluidized by the reactant fluid. The linear velocity is maintained above the minimum fluidization velocity required to obtain the fluidized bed. As the superficial velocity increases, the bed expands and become increasingly dilute. At high enough linear velocity, the smallest catalyst particles escape from the bed and have to be separated from exhaust gases and recycled.

In fluidized bed, heat transfer is much better resulting in more uniform temperature compared to packed bed reactor. Frequent regeneration of catalyst can be done without any shutdown of the process. However, fluidized bed is a complicated system to operate and requires extensive investments and high operating and maintenance cost. Other major disadvantages are attrition and loss of catalysts due to fluidized condition. Modeling of fluidized bed flow is complex. The fluidized bed is assumed to consist of bubble and emulsion phases which can be modeled respectively by plug flow and CSTR, as the emulsion phase is assumed to be well mixed. Correlation for heat and mass transports are available in literature. The reactor is extensively used for catalytic cracking process.

Carberry reactor or Berty reactor

For catalytic investigations, reactors equipped with rotating basket or fixed basket with internal circulation can be used. These CSTR type reactors are used to minimize the inherent mass and heat transfer limitations observed in fixed-bed reactors. These reactors are frequently used in industry to evaluate reaction mechanism and reaction kinetics. The most common type of reactors used are Carberry and the Berty reactors.

The main feature of the Carberry reactor is that the catalyst particles are contained in a spinning basket or embedded in the blades of a spinning agitator. The mounted catalyst is rapidly rotated resulting in good mixing between reactants in fluid phase and the solid catalyst. This minimizes the mass transfer and heat transfer resistances. The basket or impellers can spin up to 2,500 rpm.

The Berty reactor uses an internal recycling to achieve perfectly mixed behavior. The catalyst is contained in a fixed bed basket through which the reacting gases circulate. The catalyst basket is equipped with large diameter impellers rotated in order to circulate gases & liquids past solid catalysts. An internal recirculation rate of 10 to 15 times of the feed rate effectively eliminates external diffusion resistance and temperature gradient. Retaining the solid catalyst in a spinning woven wire mesh basket allows gas / liquid circulation with low pressure drop. Circulating the reactants past the catalyst minimize wearing & breakage.