ASSIGNMENT CHEMICAL REACTION ENGINEERINGDecember 21, 2013

PACKED BED REACTOR (PBR):In the chemical processing, a packed bed is a hollow tube, pipe or other vessel that is filled with packing materials. The packing materials that can be randomly filled with small objects like Raschig rings or else it can be specifically designed structural packing. But for the packed bed reactors, it contains catalyst particles or adsorbents such as zeolite pellets, granular activated carbon and etc. Super Raschig ringsStructured packingThe packed bed typically used to improve contact between two phases in a chemical or similar process. Packed bed can be used in a chemical reactor, a distillation process or a scrubber. Packed bed reactor which is PBR and also known as fixed bed reactor can be used in chemical reaction. These reactors are tubular and are filled with solid catalyst particles, most often used to catalyse gas reactions. The chemical reaction takes place on the surface of the catalyst. The concept of the packed bed reactor is where the reactor is a place where the collection of fixed solid particles takes place. Then, the particles may serve as a catalyst or and adsorbent and it is continuous gas flow. This reactor used primarily in heterogeneous has phase reaction with a catalyst.

THE FIGURE OF PACKED BED REACTOR:

APPLICATION OF PACKED BED REACTOR:1. Synthesis of gas production.2. Methanol synthesis3. Ammonia synthesis4. Fischer-Tropsch synthesis5. Gas cleaning (adsorption)

ADVANTAGES PACKED BED REACTOR:1. By using a packed bed reactor is the higher conversion per weight of catalyst than other catalytic reactor.2. The reaction rate is based on the amount of the solid catalyst rather than the volume of the reactor. 3. Low operating cost and low maintenance by using this kind of reactor.4. The process using packed bed reactor operates continuously.5. Little wear on catalyst and equipment.6. Simple analysis7. Little loss or attrition.8. Only practical, economic reactor at very high pressures.9. Usually high ratio of catalyst to reactants long residence time complete reaction

CHALLENGES/LIMITATION:For this reactor which is packed bed, it consists of three aspects of limitation which are:1. Temperature control.2. Pressure drop.3. Catalyst deactivation.

For the first aspect which is TEMPERATURE CONTOL and the challenge that have to face in are:1. Endothermic reaction may die out.2. Exothermic reaction may damage the reactor.3. Selectivity control.The ways that can be used is by choose the suitable types of packed bed reactor.SINGLE BED REACTORMULTI BED REACTORMULTI TUBE REACTOR

All the particles are located in a single vessel.Several serial beds with intermediate cooling/heating stages.

Several tubes of small diameter filled with particles

Advantages/disadvantages: Easy to construct. Inexpensive Applicable when the reaction are not very exo-/endothermic.

Advantages/disadvantages: Applicable for exo-/endothermic reaction.Advantages/disadvantages Expensive High surface are for heat exchanges. (very good temperature control). Applicable for very exo-/endothermic reactions.

For the second aspect which is PRESSURE DROP:1. Friction between the gas and particle phase result in a pressure drop.2. High pressure drop will result to high compression cost.3. Some systems have low tolerance for pressure drop.4. The pressure drop is mainly dependent on reactor length, particle diameter, void fraction and gas velocity.

Large particle has to be used (dp>1mm)

POROUS CATALYST PARTICLE: The particles are porous to increase the surface area of the catalyst. Reactants are transported inside the pores by means of molecular diffusion and adsorb to the active sites where the reaction occurs. Products desorb and diffuse back to the bulk. Heat is transported by conduction.

Intra-particle diffusion/conduction may be rate determining for large particle

The third aspect of limitation is CATALYST DEACTIVATION:1. The catalyst gets deactivated if the active sites get contaminated.2. Sulphur compound deactivate Ni-catalyst Desulfurization is often necessary prior to reforming.3. Formation of carbon deposit deactivate the catalyst Large carbon deposits may clog the tubes, causing hot-spots that damage the reactor.4. Catalyst regeneration is necessary.

DISADVANTAGES OF PACEKED BED REACTOR:1. Large temperature gradient or undesired thermal gradient may occur.2. Inefficient heat exchange3. Suitable for slow-or-non-deactivating processes.4. Poor temperature control.5. Channelling may occur.6. Unit may be difficult to service and clean. 7. Swelling of the catalyst, deformation of the reactor.8. Regeneration or replacement of the catalyst is difficult- shut down is required.9. Pore diffusional problems intrude in large pelletsOVERCOMING THE DISADVANTAGES:1. Monolithic supports will overcome the problems of non-uniform flow patterns, plugging high pressure for small pellets and pore diffusional problems,2. Temperature control problems are overcome with:a. Recycle.b. Internal and external heat exchanges.c. Staged reactorsd. Cold shot coolinge. Multiple tray reactor- fluid redistributed and cooled between stages and catalyst is easily removed which varied from tray to tray.f. Use of diluents.g. Temperature self-regulation with competing reactions, one endo and one exothermic.h. Temperature control by selectivity and temporarily poisoning the catalyst.