Process simulation in process design Overview of practical design problems that can be solved with process simulation

Process simulation in process design

The goal of process design

Process design is an activity that brings the industrial process from idea to reality. Before the process design can be started, a problem must be formulated and goals of the product defined. After this step is done, further activity goes to analysis and the definition of the process flowsheet that will be constructed. The design goal might be to make a choice whether to build a new process for defined product or to reconstruct the existing one with the purpose of satisfying the market requests. This can be a change in capacity, change of the product property or similar. Process synthesis or definition of process flowsheet includes all the equipment such as reactors, distillation columns, heat exchangers, pumps, compressors, vessels, absorbers, adsorbers, evaporators, mixers etc. Interconnections between the equipment are defined. 

Laws of conservations

After the flowsheet is defined, it represents the process which must obey some fundamental laws of conservations: conservation of matter and conservation of energy. Around a given set of operations – the flowsheet section – a box can be drawn. For a given box, the amount of mass going in must equal the amount of mass going out. The same is applied for the energy and those equations represent the mass and energy balance of a given operation or set of operations and are fundamental for any process analysis. 
These tasks are done in order to define new processes or in order to reconstruct existing ones. Unrelated to which of those two activities is taking place, process design requires a lot of knowledge, data, and experience. Specific knowledge is most often protected by licences owned by the corporations.

However, at the same time, the knowledge is today more approachable than ever. Tools to utilize the knowledge are also more approachable than ever. Process simulation is one of the most important tools.

Knowledge basics

Whether the knowledge is provided by a big or small company, is the process simple or complex, whether the team is more or less experienced – all the activities related to process design require the knowledge of process simulation. 

For the development of the process model, all flowrates, compositions, temperatures and pressures of the feeds must be defined. The simulation will be used to predict the flowrates, compositions, temperatures, and pressures of the products. This is equally true for the process as a whole and for every operation of the process.

Short-cut simulation

First steps are short-cut calculations with the goal how to define working area for further detailed analysis. Short-cut methods are using less complicated algorithm and methods of calculation in order to select a number of possible solutions. Their goal is also to examine a number of design options in the terms of its feasibility with the minimum of detail to ensure the design option is worth progressing. 

Detailed calculation

After the area of interest has been defined, more detailed calculations are carried out. Those more detailed calculations are looking in the process operations in details. Therefore, they are using detailed first principles models with physical and thermodynamic properties to define material and heat balance of every single step of the operation which is part of the process. Every operation has its laws and specifics. Their general common equations are the laws of mass and energy conservation. In the details of calculations, all the specifics of equipment are analyzed. Different parameters are defined for reactors and for the distillation columns. Different types oh heat exchangers and pumps are analyzed. Great amount of knowledge has to be employed both to run the simulation and to interpret it. The knowledge that is missing might easily take the analysis and the process design in the wrong direction.
Different simulators also give different results and simulation engineer has to take all the steps of the precaution to avoid misleading results.
When coming to the point of more detailed calculations, we start facing limitations of the calculation. The more complex the process becomes, with more details included, the more it becomes a challenge for the calculation and the convergence problems are likely to occur. This is also the point where different simulation packages might show different behavior because they are using different calculation methods to overcome problems of calculation and convergence.

Equipment sizing

Those detailed calculations are also performed in order to define the characteristics for the particular pieces of equipment. Equipment must be defined to satisfy the requirements of the operation. It makes a significant difference in the operation and product quality of the column of 5 meters and 10 meters. The design has to take into account optimal operation with the requirements of product properties and costs of the equipment. 

Continuous optimization

Once the process starts its life and the basic performance of the design has been evaluated, changes can be continuously made to improve its performance. As the process starts its life, changes are also a part of it. These changes might involve the synthesis of alternative structures, changes of particular pieces of equipment etc.  

Thus, the process is simulated and evaluated again and its design is optimized continuously. Simulation is applied through every stage of the process life.