In chemical engineering, process design is
the design of processes for desired physical and/or chemical transformation of
materials. Process design is central to chemical engineering, and it can be
considered to be the summit of that field, bringing together all of the field's
components.
Process design can be the design of new facilities or it can
be the modification or expansion of existing facilities. The design starts at a
conceptual level and ultimately ends in the form of fabrication and
construction 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.
Documentation
Process design documents serve to define the design and they
ensure that the design components fit together. They are useful in
communicating ideas and plans to other engineers involved with the design, to
external regulatory agencies, to equipment vendors and to construction
contractors.
In order of increasing detail, process design documents
include:
- Block Flow Diagrams (BFD): Very simple diagrams composed of rectangles and lines indicating major material or energy flows.
- Process Flow Diagrams (PFD's): Typically more complex diagrams of major unit operations as well as flow lines. They usually include a material balance, and sometimes an energy balance, showing typical or design flowrates, stream compositions, and stream and equipment pressures and temperatures.
- Piping and Instrumentation Diagrams (P&ID's): Diagrams showing each and every pipeline with piping class (carbon steel or stainless steel) and pipe size (diameter). They also show valving along with instrument locations and process control schemes.
- Specifications: Written design requirements of all major equipment items.
Process designers also typically write operating manuals on
how to start-up, operate and shut-down the process.
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 facility are planned.
A primary method of developing the process documents is process flowsheeting.
Design Considerations
Designs have objectives and constraints, and even a simple
process requires a trade-off among such factors.
Objectives that a design may strive to include:
- Throughput rate
- Process yield
- Product purity
Constraints include:
- Capital cost
- Available space
- Safety concerns
- Environmental impact and projected effluents and emissions
- Waste production
- Operating and maintenance costs
Other factors that designers may include are:
- Reliability
- Redundancy
- 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 proprietary design criteria provided by process licensors,
published scientific data, laboratory experiments, and input.
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.
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