Question

1- Assume that you are going to build up a model for a system where unit operation and unit processes are carried out. Explain the various steps involved in the process of model building for the system you have chosen.

Answer 1

For any process, a set of unit operations handling some unit processes are employed in such a way that they interact with the feed and the intermediate products, and with each other in a meaningful manner to give rise to products in the most economic way. From a point of view of the chemical industry, the following factors mostly affect its performance:

1. Chemistry employed
2. Thermodynamics
3. Reaction Kinetics
4. Process design and Mechanical design
5. Unit operations and Unit processes
6. Economics

Process design and building of a sustainable model is an important factor as it acts as the spine of the whole industry. It basically involves the determination of the correct or the most appropriate position of a unit operation or process. A process model is basically defined as a graphic representation representing a correct workflow that gives rise to the most economic product.

The two main parameters involved in process modelling are:

1.) Process Steps:

• Prepare the reactants: Employ different unit operations to prepare the raw materials that have to be reacted. For example, In a sulphuric acid plant, SO2 is generated by burning molten Sulphur in excess air. The air is made free from moisture either by passing it through a bed of silica gel or through the sulphuric acid shower itself. The Sulphur (solid) is melted in pits and the molten sulphur is filtered using leaf filters before feeding it to the combustion chamber.
• Carry out the reaction: This involves the selection of the most suitable reactor and determination of the rector size along with any additional aid for heating or cooling the process and /or use of any catalyst (along with provisions to reuse the catalyst).
• Separation of the products formed: Usually, the product stream out of the reactor contains a mixture of many chemicals and only one of them is the desired entity. Hence, a series of unit operations are involved to separate the desired product to get nearly 100% of the desired product. For example, a unit operation like distillation is used to separate products which have a significant difference in boiling points.
• Purification of products: This is the final step of the process which mainly involves the separation of any foreign species that may degrade the quality of the final product.

2.) Design Steps:

• Collection of all laboratory and process development information.
• Prepare Flowsheets: This is the most important step as it produces guidelines to employ the process steps. The different types of flowsheets are discussed later after this section.
• Selection of conventional design procedures for equipment designated on flowsheets.
• Reactor Design: The most critical factor that affects the overall product formation and hence the economy of the system. The reactor design mainly involves the determination of the size of the reactor (volume) considering the mass and the energy balance across the reactor.
• Selection of control instrumentation for controlling, monitoring and analysis of different sections the system. In the chemical industry, most of the process occurs in confined vessels and it is physically impossible to monitor the conditions of the chamber, which gives rise to a demand of appropriate control system to analyse, monitor and adjust the process variables.
• Selection and sizing of material handling equipment like conveyors, pumps, piping networks etc.
• Design of all process auxiliaries
• Preparation of complete layout of the system by inserting the important parameters like flow-rate, composition, temperature, pressure and toxicity(if any) in the process flow diagram constructed earlier.
• The itemisation of each and every element for cost estimation

Mainly, the above mention steps are represented altogether as a process flow diagram which contains some common standard figures( viz. figure of pumps, valves etc) that is readable to a Process engineer. There are mainly three types of Process flow diagrams:

• Block diagrams: Rectangular boxes interlinked with single-headed arrows defining the flow of the process. This type of diagram is recommended for preparing preliminary flowsheets as mentioned in point 2 of the design steps above.
• Simplified Engineering flowsheets: Mostly used to make the final flow sheet. It contains all the necessary parameters that have to be known to a process engineer in charge of the system. Such flowsheets exist in all levels of complexities and are considered as the final model of the system.
• Other Design flow sheets: These type of flowsheets mainly emphasize on the design parameters instead of process variables. It contains information like the size of the tank/reactor, specifications of the pumps, stirrers, pipings etc, specification of electrical fittings and other such components.

These are the main steps to be considered while modelling any complex system involving various unit operations and processes