23-Chem-A5 Chemical Plant Design and Economics · December 2019
Question 2 of 6: The Ten Essential Elements of a Process Flow Diagram
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams / EGBC — December 2019 — 16-Chem-A5 Chemical Plant Design and Economics. Three-hour closed-book examination; one aid sheet (both sides) and an approved Sharp/Casio calculator are permitted. Six questions are printed and any five constitute a complete paper (each worth 20 marks); all six are solved below for completeness. The two calculation questions (Q3, Q4) are worked with explicit engineering-economy factors; the four discussion questions (Q1, Q2, Q5, Q6) are answered as structured lists with supporting description, as the paper directs.
Reference texts: M. S. Peters, K. D. Timmerhaus & R. E. West, Plant Design and Economics for Chemical Engineers (5th ed., McGraw-Hill) — profitability measures (rate of return, incremental analysis), straight-line depreciation, after-tax cash flow, and the anatomy of a process/economic study; R. Turton, R. C. Bailie, W. B. Whiting & J. A. Shaeiwitz, Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — the process flow diagram and its information content, equipment/economics; G. Towler & R. Sinnott, Chemical Engineering Design (Coulson & Richardson Vol. 6, 2nd ed.) — utilities, offsites and storage; O. Levenspiel, Chemical Reaction Engineering (3rd ed.) and H. S. Fogler, Elements of Chemical Reaction Engineering — reactor scale-up. Engineering-economy factors follow the standard notation $(A/P,i,n)$ and $(P/A,i,n)$; as the question specifies straight-line depreciation, that method is used throughout (rather than the Canadian CCA declining-balance system).
Question 2: The Ten Essential Elements of a Process Flow Diagram (20 marks)
Overview. The Process Flow Diagram (PFD) is the master process document: it presents the process topology, the stream information and the equipment information on a single sheet, and it is the basis from which piping-and-instrumentation diagrams (P&IDs), equipment specifications and cost estimates are later developed. A complete, professional PFD contains the following ten essential elements.
Figure 1 — Skeleton PFD illustrating the essential elements: labelled and tag-numbered major equipment, directional process streams, external feed and product arrows, a utility (steam) connection, a recycle loop through a heat exchanger, and the points at which operating conditions and stream data are reported.
All major process equipment. Every significant vessel, reactor, column, pump, compressor, heat exchanger, drum and tank is drawn to a recognised symbol and given a descriptive name.
Equipment identification (tag) numbers. Each item carries a unique tag (e.g. P-101, E-102, R-201) keyed to an equipment legend, so equipment can be cross-referenced on other documents.
Process flow streams. All process lines connecting the equipment are shown, each assigned a unique stream number for reference to the stream table.
Direction of flow / process topology. Arrows show the direction of every stream and the interconnection of the equipment, establishing the sequence of operations from feed to product.
Operating conditions. The temperature and pressure at key locations (equipment inlets/outlets and important points along the process) are stated, usually with flag or balloon symbols.
Stream flow rates. The mass and/or molar flow rate of each numbered stream is given, defining the plant throughput and the basis of the material balance.
The stream (flow-summary) table. A tabulated material balance lists, for every stream number, the temperature, pressure, vapour/liquid fraction, total flow and component flows or compositions — the quantitative core of the PFD.
Major utility streams. The principal utilities supplied to or removed from equipment (steam, cooling water, refrigerant, fuel, air, etc.) are shown at the equipment they serve, with their conditions.
Basic control loops. The few key control loops that govern the operation of the process (e.g. reactor temperature, column reflux, level control) are indicated to show how the process is regulated.
Title block, legend and notes. A title block (process name, drawing and revision number, company, date), a legend of symbols and tag conventions, and any explanatory notes complete the drawing and make it a controlled, self-describing document.
Together these elements let a reader reconstruct the entire material and energy balance of the process and understand how it is operated, without reference to any other drawing.
