Question 6 of 6: Health and Safety and Environmental Issues

Question 6: Health and Safety and Environmental Issues (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Optimum economic design versus optimum operation design

Optimum economic design is a design-stage optimization. The design variables — equipment sizes, configuration, operating targets — are still free to choose, and the objective is to minimise the total annual cost (or maximise profitability) by trading capital cost against operating cost. The classic examples are optimum pipe diameter, optimum reflux ratio, optimum insulation thickness and optimum heat-exchanger area: a larger, more expensive unit (higher fixed cost) lowers the ongoing operating cost, and the optimum is the size at which the sum of the two is least. The outcome fixes the plant that gets built.

Optimum operation design applies to an existing, already-built plant whose equipment is fixed. The capital is now sunk, so only the operating variables — throughput, temperature, pressure, reflux, recycle, feed split — can be adjusted, and the objective is to maximise profit (or minimise operating cost) within the constraints of the installed equipment. In short, economic (design) optimization chooses the best plant to build by balancing capital against operating cost; operation optimization chooses the best way to run the plant you already have, with capital cost no longer a variable.

(b) Mitigating fire, explosion and toxic release

The three hazards are attacked on the same logic — first try to remove or reduce the hazard at source (inherently safer design), then add engineered safeguards, then procedural and emergency controls:

Across all three, a formal HAZOP study, a properly designed pressure-relief and flare system, and layers of protection (the "onion" of prevention, control and mitigation) are the framework in which the specific measures sit.

(c) Preventing boilers from polluting the environment

Plant boilers pollute chiefly through their flue gas, their water streams, and their solid residues, so the actions target each:

• Air emissions. Burn a cleaner fuel (natural gas rather than heavy oil or coal) to cut sulphur and particulates; control combustion (low-excess-air firing, low-NO_x burners, flue-gas recirculation) to limit NO_x and CO; and, where a sulphur-bearing fuel is unavoidable, fit flue-gas desulphurisation (scrubbers) and particulate control (baghouse or electrostatic precipitator). Maintain good combustion efficiency to minimise CO₂ per unit of steam. Emissions are held within the limits of the Canadian Environmental Protection Act (CEPA) and the applicable provincial air-quality regulation, with continuous stack monitoring.
• Water discharges. Treat and control boiler blowdown before discharge (neutralise, cool, and manage the dissolved solids and treatment chemicals); recover condensate and recycle to reduce both make-up water and effluent; keep discharges within the Fisheries Act and provincial effluent limits.
• Solid waste and heat. Collect and properly dispose of ash and spent water-treatment chemicals; recover waste heat with economisers and air pre-heaters, which both raises efficiency and lowers the thermal and CO₂ burden.