Economic Considerations in Process Design and Engineering

This is an extract of a lecture I delivered at the workshop on 'Chemical Engineering and Economics' organised on 13th October by the Lote Regional Centre of Indian Institute of Chemical Engineers.

Of all the phases that a chemical plant project goes through from concept to commissioning, it is the design and engineering phase that offers the maximum scope and opportunities for economy.

Chemical plant design and engineering has three elements to it:

• Safety
• Reliability
• Economy

Safety and reliability are not as easily quantifiable as economy. Economy in design should not be achieved at the cost of acceptable limits of safety and reliability.

Chemical plant design still continues to be governed by a large set of heuristics (rules of thumb). Unquestioning adherence to such heuristics can lead to uneconomic design. Depending on the situation heuristics and assumptions must be challenged by process engineers. Now that software have speeded up calculations, it would be worthwhile to examine several alternatives rather than rely on heuristics.

Chemical Engineering is largely an empirical science. Empirical correlations form the backbone of even the most sophisticated software for equipment design. Various physical and chemical properties that are required to be plugged into these correlations are not easily and accurately available, especially for mixtures. These are often predicted, extrapolated and even guess estimated. To account for these intrinsic uncertainties, a safety factor is often added.

Safety factors are also usually added to account for future expansion and as a cushion for fulfilling performance guarantees. Often factors get multiplied at the hands of several agencies, resulting in extreme over design.

There are also some hidden safety factors, for example

• Roughness factor in pressure drop calculation
• Fouling factor in heat exchanger design
• Corrosion allowance

There is a mistaken notion that providing design margins improves the reliability of the equipment. Design margins are more often than not generously specified for heat exchangers and pumps and this not only results in higher cost but also leads to sub-optimal performance of the equipment.

Though firmly rooted in chemical engineering principles, the design of a chemical plant requires a multi-disciplinary approach. Through relocating, rearranging and reconfiguration of equipment, it is possible to reduce plant piping and save both capital and operating costs. Value Engineering is a systematic process designed to focus and improve upon the major elements of complex or high cost projects. It is a process that employs a multi-disciplinary team of experts to develop recommendations aimed at improving the value of a project during its design phase.