VCI Chemical Ideas

 

The Reaction

 

Some important words;

 

Raw Materials - The unprocessed substances used to produce the feedstock (usually

need to be treated or purified in some way before they can be used).

Feedstock      - The reactants that go into the chemical process.

Co-products     - A second product formed by a reaction

By-products     - Products formed by a second, unwanted reaction

Conversion     - The proportion of the feedstock that reacts in one way or another.

Selectivity     - the proportion of the feedstock that gets turned into the wanted

product.

 

The Process

 

There is a general sequence for any chemical process (see below)

 

 

 

 

 

 

 

 


                                                                                                    

Text Box: Recycle loop

Feedstock/Raw materials
 

 

 

 


However it can be organised in one of two ways ;

 

Batch               - the feedstock is put in and allowed to react….

                        - ….when the reaction is complete, it is stopped.

                        - the product is then separated and purified

                        - the process is then repeated.

           

Continuous        - feedstock is fed in at one end and the product is removed

at the other in a continuous flow.

 

 

 

 

 

 

 

ADVANTAGES

 

DISADVANTAGES

Batch :

·          Cost effective for small quantities

·          Capital cost of plant much lower

·          Slow reactions can be catered for

·          A range of products can be made in the same vessel

·          A greater % conversion is achieved compared to the same sized continuous reactor for the same time

 

·          Charging and emptying the reactor is time consuming. No product is formed in this “shut down” time

·          Larger workforce needed

·          Contamination more likely

·          Fast, exothermic reactions can be difficult

Continuous :

·          Suited to high tonnage production

·          Greater throughput. Shut down may not occur for months or years

·          More easily automated and process is better controlled as fine adjustments are possible

·          Contamination risk is lower

·          Consistent quality is ensured

·          Requires minimal labour

·          Less waste and by-products

 

·          Very much higher capital cost before any production can occur

·          Not very cost effective unless run at full capacity

·          Contamination risk is higher when used for two or more products

 

 

          Construction Materials

 

F       Must not react with the chemicals!

F       For corrosive substances, glass lined vessels, alloys or glass-reinforced plastics are often used in place of steel

 

 

Conditions for the process

 

Temperature and Pressure

- need to have optimum rate (high T and high p) but also optimum yield. If the reaction is an equilibrium then this may not be clear cut and “compromise conditions” may be necessary (e.g. the Haber process for making ammonia)

 

Catalysts

- many are made from precious metals but, since they are not used up, it is still economical to use them. However, performance can deteriorate because of impurities in the feedstock (“catalyst poisoning”) or it may get covered in some surface coating which will need to be cleaned off. Performance is judged  terms of;

§         the effect it has on the rate;

§         its life expectancy;

§         the % reclaimed when it is renewed.

 

 

 

Recycling 

- It is important to recycle unreacted feedstock but not impurities as, if this happens, their concentrations will steadily build up.

 

Heat 

- Energy loss needs to be kept to a minimum. Heat exchangers can be used to transfer heat from one part of the process to another.

 

 

                   Costs

 

Fixed costs
- those incurred regardless of how much product is made
(e.g. labour, land costs,…)

 

Variable costs
- related to how much product is made (e.g. raw materials, disposal of waste, distribution of product,…)

 

Capital costs

– relate to establishing the chemical plant, buildings etc..

 

 

Expenditure against income for a chemical process as time progresses