Process design
Process Design is the creation of processes for desired physical and/or transformation of materials.
Process design is central to chemical engineering, and like other forms of engineering design, it can be considered the summit of chemical engineering, bringing together the components of that field and placing engineering in a social, economic, and environmental context.
Processes can be designed from scratch or can be adaptations or extensions of existing facilities. The design starts at a conceptual level and ultimately takes the form of construction and fabrication plans.
Process design is distinct from equipment design, which is closer in spirit to the design of unit operations. Processes often include many unit operations, and the design is more related to systems engineering in it holistic approach.
Documentation
Engineers engaged in process design typically prepare documents as part of the design. These documents serve to define the design. They let the design engineer ensure the design fit together. They are useful in communicating ideas and plans to other process design engineers involved with the design, to external regulatory agencies, to other engineers involved in facility planning, to construction contractors, and to equipment vendors.
In order of increasing detail, these documents are:
Block flow diagrams
Process flow diagrams
Piping and instrumentation diagrams
Specifications
Block flow diagrams are composed of rectangles and lines indicating major material or energy flows.
Process flow diagrams typically show more realistic diagrams of major unit operations as well as flow line. They usually include a material balance, and sometimes an energy balance, showing typical or design flowrates, stream compositions, and pressures and temperatures.
Piping and Instrumentation Diagrams show every piping line and valve along with instrument point and control schemes.
Specifications – written requirements of equipment
Designers typically write operating plans to indicate how they expect the process to be operated.
Documents are maintained after construction of the process facility for the operating personnel to refer to. The documents also are useful when modifications to the process are planned.
Design Considerations
Designs have objectives and constraints, and even a simple process requires a trade-off among factors.
Objectives that a design may strive for include:
Throughput rate
Product purity
Constraints include:
Capital cost
Available Space
Safety Concerns
Environmental Impact and Projected Emissions
Waste production
Operating costs
Other factors that designers may include are:
Flexibility
Anticipated variability in feedstock and allowable variability in product.
Sources of Design Information
Designers usually do not start from scratch, especially for complex projects. Often the engineers have pilot plant data available or data from full-scale operating facilities. Other sources of information include published scientific data, laboratory experiments, and input from equipment vendors about the performance and prices of equipment.
Computer Help
The advent of low cost powerful computers has aided complex mathematical simulation of processes, and simulation software is often used by design engineers. Simulations can identify weaknesses in designs and allow engineers to choose better alternatives.
However, engineers still rely on heuristics, intuition, and experience when designing a process. Human creativity is an element in complex designs.