What is reactors, catalyst and batch type reactors, advantages and disadvantages of reactors.

 WHAT IS TREACTORS AND WHY WE USE REACTORS ?

REACTORS :

Introduction:-

     The basic objective in designing a reactor is to produce a specified product at a given rate from known reactants. The principal features of the reactor are the following:

i)                            The overall size of the reactor, its general configuration and the more important dimensions of any internal structures.

ii)                        The exact composition and physical condition of the products emerging from the reactor. The composition of the products must of course lie within any limits set in the original specification of the process.

iii)                    The temperatures prevailing within the reactor and any provision which must be made for heat transfer.

iv)                     The operating pressure within the reactor and any pressure drop associated with the flow of the reaction mixture.

Reactors used for biomass pyrolysis is most commonly classified depending on the way the solids move through the reactor.

TYPES OF REACTORS   

       There are four types of reactor mainly use in pyrolysis process:-

Type A:   No solid movement through the reactor during      pyrolysis (batch reactors)

Type B:   Moving bed (shaft furnaces)

TypeC: Movement caused by mechanical forces (e.g.,     rotary kiln, rotating screw etc.)

Type D:  Movement caused by fluid flow (e.g., fluidized bed, spouted bed, entrained bed etc.)

     Charcoal making today is mostly based on Type A and Type B reactors, Type A reactors are more common in developing countries.

   

    Pyrolytic reactors can also be classified depending on   the way heat is supplied to biomass as follows:

Type 1: Part of the raw material burns inside the reactor   to provide heat needed to carbonize the remainder.

 Type 2: Direct heat transfer from hot gases produced by combustion of one or more of the pyrolysis products or any other fuel outside the reactor.

Type 3: Direct heat transfer from inert hot material (hot gases or sand introduced into the reactor).

Type 4: Indirect heat transfer through the reactor walls (i.e. external heat source due to combustion of one of or more pyrolysis products or any other fuel).

                                   CATALYST REACTORS

    Basically  two types of catalyst reactor are use in this process:-

1         1.   Homogeneous

2        2.  Heterogeneous

 

Homogeneous and Heterogeneous Reactors :-

Chemical reactors may be divided into two main categories, homogeneous and heterogeneous. In homogeneous reactors only one phase, usually a gas or a liquid, is present. In heterogeneous reactors two, or possibly three, phases are present, common examples being gas- liquid, gas solid, liquid-solid and liquid-liquid systems.

                                       BATCH REACTORS

It is the simplest type of reactor. The immobilized enzyme is placed in a container with the reactants, and the reaction is allowed to proceed until the desired level of conversion is reached. Some stirring or agitation of reaction mixture is also required. Many modifications of these reactors have been designed to simplify recovery and reuse of the enzyme composite.

ADVANTAGES OF REACTORS

The increase in fluidized bed reactor used in today’s industrial world is largely due to the inherent advantages of the technology.

i)   Uniform Particle Mixing: Due to the intrinsic fluid-like behavior of the solid material, fluidized beds do not experience poor mixing as in packed beds. This complete mixing allows for a uniform product that can often be hard to achieve in other reactor designs. The elimination of radial and axial concentration gradients also allows for better fluid-solid contact, which is essential for reaction efficiency and quality.

 

ii)              Uniform Temperature Gradients: Many chemical reactions require the addition or removal of heat. Local hot or cold spots within the reaction bed, often a problem in packed beds, are avoided in a fluidized situation such as a fluidized bed reactor. In other reactor types, these local temperature differences, especially hotspots, can result in product degradation. Thus fluidized bed reactors are well suited to exothermic reactions. Researchers have also learned that the bed-to-surface heat transfer coefficients for fluidized bed reactors are high.

 

Ability to Operate Reactor in Continuous State: The fluidized bed nature of these reactors allows for the ability to continuously withdraw product and introduce new reactants into the reaction vessel. Operating at a continuous process state allows manufacturers to produce their various products more efficiently due to the removal of startup conditions in batch processes.

DISADVANTAGES OF REACTORS 

   As in any design, the fluidized bed reactor does have it draw-backs, which any reactor designer must take into consideration.

i)                Increased Reactor Vessel Size: Because of the expansion of the bed materials in the reactor, a larger vessel is often required than that for a packed bed reactor. This larger vessel means that more must be spent on initial capital costs.

ii)             ii) Pumping Requirements and Pressure Drop: The requirement for the fluid to suspend the solid material necessitates that a higher fluid velocity is attained in the reactor. In 47 order to achieve this, more pumping power and thus higher energy costs are needed. In addition, the pressure drop associated with deep beds also requires additional pumping power

iii)         Particle Entrainment: The high gas velocities present in this style of reactor often result in fine particles becoming entrained in the fluid. These captured particles are then carried out of the reactor with the fluid, where they must be separated. This can be a very difficult and expensive problem to address depending on the design and function of the reactor. This may often continue to be a problem even with other entrainment reducing technologies.

iv)          Lack of Current Understanding: Current understanding of the actual behavior of the materials in a fluidized bed is rather limited. It is very difficult to predict and calculate the complex mass and heat flows within the bed. Due to this lack of understanding, a pilot plant for new processes is required. Even with pilot plants, the scale-up can be very difficult and may not reflect what was experienced in the pilot trial.

v)             Erosion of Internal Components: The fluid-like behavior of the fine solid particles within the bed eventually results in the wear of the reactor vessel. This can require expensive maintenance and upkeep for the reaction vessel and pipes

Pressure Loss Scenarios: If fluidization pressure is suddenly lost, the surface area of the bed may be suddenly reduced, this can either be an inconvenience (e.g. making bed restart difficult), or may have more serious implications, such as runaway reactions (e.g. for exothermic reactions in which heat transfer is suddenly restricted).