Process Design on the FE Chemical Exam
Process design and economics accounts for 5 to 8% of the FE Chemical exam. This section tests your ability to read and interpret process diagrams, size basic equipment, and evaluate process economics. These skills are fundamental to chemical engineering practice and appear in various forms throughout your career.
Process Flow Diagrams (PFDs)
A process flow diagram shows the major equipment, flow paths, and operating conditions for a chemical process. PFDs include:
- Major equipment items (reactors, columns, heat exchangers, pumps, compressors) shown as standard symbols
- Process streams with flow rates, compositions, temperatures, and pressures
- Energy streams (heating, cooling, power)
- Stream numbers that reference a separate stream table with detailed information
On the FE exam, you may be asked to identify equipment from PFD symbols, trace flow paths through a process, or use information from a PFD to calculate a missing stream property using material or energy balances.
Piping and Instrumentation Diagrams (P&IDs)
P&IDs provide more detail than PFDs, showing all piping, valves, instruments, and control systems. Key elements include:
- Instrument symbols: Circles with letters indicating the measured variable (T for temperature, P for pressure, F for flow, L for level) and function (I for indicator, C for controller, T for transmitter)
- Valve symbols: Gate, globe, ball, butterfly, check, and control valves each have distinct symbols
- Control loops: Show the connection between sensors, controllers, and final control elements
Know how to read the ISA (International Society of Automation) instrument identification system used on P&IDs. A tag like TIC-101 means Temperature (T), Indicating (I), Controller (C), instrument number 101.
Equipment Sizing Basics
Heat Exchangers
Size heat exchangers using the heat duty equation: Q = U times A times LMTD, where Q is the heat duty, U is the overall heat transfer coefficient, A is the heat transfer area, and LMTD is the log mean temperature difference. Given three of these four quantities, solve for the fourth. Overall heat transfer coefficient values for common services are available in reference tables.
Pumps
Pump sizing requires calculating the total dynamic head (TDH), which includes static head (elevation change), friction losses in piping, and pressure difference between source and destination. Pump power equals the flow rate times TDH times the specific weight of the fluid, divided by the pump efficiency.
Distillation Columns
Basic column sizing involves determining the number of theoretical stages using the McCabe-Thiele method, then applying a tray efficiency to determine the actual number of trays. The column diameter is sized based on vapor velocity to prevent flooding or weeping.
Reactors
Reactor sizing uses reaction kinetics and the design equation appropriate for the reactor type. For a CSTR (continuous stirred-tank reactor), the volume equals the flow rate times the extent of conversion divided by the reaction rate at the exit conditions. For a PFR (plug flow reactor), integrate the design equation over the conversion range.
Process Safety
Process safety topics on the FE Chemical exam include hazard identification methods (HAZOP, What-If analysis), relief device sizing basics, and fire and explosion hazards. Understand the concepts of flash point, autoignition temperature, flammability limits (lower explosive limit and upper explosive limit), and the importance of proper ventilation and inerting.
Know the basic principles of pressure relief systems: every pressure vessel needs overpressure protection, and relief valves must be sized to handle the worst-case scenario (fire case, blocked outlet, runaway reaction, etc.).
Process Economics
Beyond the general engineering economics tested on all FE exams, the FE Chemical exam may include questions on:
- Capital cost estimation: Order-of-magnitude, factored, and detailed estimates
- Operating cost components: Raw materials, utilities, labor, maintenance, overhead
- Profitability measures: Return on investment, payback period, net present value
- Cost scaling: The six-tenths rule for estimating equipment costs at different capacities
Study Recommendations
Process design questions often involve reading diagrams and applying basic engineering judgment. Practice interpreting PFDs and P&IDs from textbooks or online resources. For equipment sizing, focus on understanding the fundamental design equations and when to apply each one. The FE Reference Handbook provides many of the necessary formulas and coefficients.