Question

answer these two questions (each question should have 200 words or more)  

1.Importance of reactors in various chemical process industries

2.Recent developments made in packed and fluidized bed columns

Answer 1

Ans 1:

A reactor used in industrial processes is defined as a vessel designed in such a way to carry out a Chemical reaction effectively and safely. The design of a chemical reaction depends on many factors, out of which the thermodynamics and the kinetics of reaction are the most important factors.

Based on the application they have to be used, the size of reactors varies from a few cubic centimeters to very vast structures having capacities of even 400 cubic meters or more. The process is carried out in either batch mode or continuous mode, so we have two broad categories of reactors.

1). Batch reactors

2). Continuous reactors

In batch reactors, a batch comprising of the feed components in an appropriate proportion is charged into a reactor. Suitable reaction conditions are maintained and the required time is provided for the reaction to occur. After the time completion, the product is withdrawn from the reactor and a fresh feed batch is charged again. Batch reactors have flexibility of operation and are mostly used in industries where a wide range of products have to be manufactured having different reactants and the reactant conditions. They are generally used in small applications, in laboratory works and pharmaceutical industries.

The continuous reactors operate at steady state, i.e . the feed continuously enter the column and the same amount is withdrawn as product stream. They are employed when large quantities of chemical have to be produced, these reactors are operated continuously for several months without a shutdown. The product from these reactors are of more consistent quality as the reaction parameters ( temperature, pressure, residence time) are much better controlled in comparison to batch process.

Various types of continuous reactors are-

1. Continuous stirred tank reactor (CSTR) or Mixed flow reactor (MFR)

2. Tubular or plug flow reactor (PFR)

3. Fluidized bed reactor

4. Fixed bed reactor

In a CSTR, 2 or more reactants in a solution or as a slurry are introduced in reactor which is continuously agitated with an impeller. There is a homogeneous concentration through out the reactor and the same concentration is present at reactor outlet.

In a PFR, one or more fluid reagents flow at a very high velocity through pipe or tube. As the reagents pass, chemical reaction takes place and transformation into product starts taking place. At high velocities, the products are unable to dilute back and there is little or no backmixing. PFR is advantageous in higher order reactions because they maintain a higher concentration of reactant in the reactor in comparison to CSTR.

In fixed bed reactors (heterogeneous phase), a catalyst bed of solid particles is formed. The fluid (gas) passes through this bed and the reaction takes place to form desired product. For example vanadium pentoxide bed is used in sulphur industry for the oxidation of sulphur into sulphur dioxide. Multiple beds are also used in reactors with the installation of cooling/ heating system in between to control the reaction conditions .

In a fluidized bed reactor, the catalyst very fine in size is made to fluidize in a reactor with the help of some fluid. A continuous contact is provided to the reactant fluids with the catalyst for reaction to take place. For example - the hydrogenation reaction in Hydrogen peroxide plant takes place in fluidized palladium catalyst.

All natural processes are spontaneous i.e. they tak place at their own and a finite gradient exists befween the states of the two systems. As all spontaneous processes are irreversible, so all natural processes are irreversible. For eg- rivers flow from higher altitude to lower and this process cannot be reversed. The rain falls from a sky is a natural process and irreversible. The transformation of a plant into tree is a natural process and so many more.

Ans 2:

The recent development in packed and fluidised bed columns includes incorporation of computational fluid dynamic analysis to analyse flow patterns and interphase contact, which could improve underlying mass transfer phenomena and heat transfer rate for better control and stability.

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