How a chemical process works: reactions, separations, catalysts and flowsheets: Key terms, quiz and sources
Understanding Chemicals / Module 2: How a chemical process works: reactions, separations, catalysts and flowsheets Wrap-upHow a chemical process works: reactions, separations, catalysts and flowsheets: review
Key terms
- Unit operation
- A physical processing step, such as distillation, filtration or heat transfer, that recurs across many industries.
- Stoichiometry
- The fixed ratios in which substances react, set by the balanced chemical equation.
- Conversion
- The fraction of a feed that reacts to anything at all, often quoted per pass through the reactor.
- Selectivity
- The amount of desired product formed divided by the amount of reactant converted.
- Yield
- The fraction of raw material that ends up as the desired product.
- Recycle and purge
- Returning unreacted feed to the reactor, while bleeding off a small stream so that inert gases do not build up.
- Equilibrium constant
- A number describing the balance point of a reversible reaction at a given temperature; large means mostly products.
- Le Chatelier's principle
- A system at equilibrium shifts to oppose a change in temperature, pressure or concentration.
- Activation energy
- The energy barrier molecules must cross to react; a catalyst provides a route with a lower barrier.
- Heat of reaction
- The heat released (exothermic) or absorbed (endothermic) when a reaction occurs, per mole.
- Homogeneous catalyst
- A catalyst dissolved in the same phase as the reactants, typically a metal complex in solution.
- Heterogeneous catalyst
- A solid catalyst, usually metal or oxide particles on a porous support, in contact with gas or liquid reactants.
- Catalyst deactivation
- Loss of activity by sintering (heat), poisoning (chemical blocking) or fouling (physical deposits).
- Reflux
- Condensed liquid returned to the top of a distillation column to improve separation, at the cost of more boiling.
- Multiple-effect evaporation
- Using vapour from one evaporator to heat the next at lower pressure, so each tonne of steam boils off several tonnes of water.
- Material and energy balance
- An account of every input and output of mass and energy across a process or unit.
- Inherent safety
- Designing a process so the hazard is removed or reduced, rather than controlled by added protection.
- Relief system
- Valves, discs, headers and flares that vent a vessel safely when pressure exceeds design limits.
Try it in the table
- Open the Sulphuric acid tile and find the plant sizes and sulphur source mentioned. Using 0.327 tonnes of sulphur per tonne of acid, estimate how much sulphur a 2,800 tonne-per-day plant burns in a year.
- Open the Ethylene oxide tile and look at its main uses. Using the selectivity table in this module, work out how much ethylene a plant making 300,000 tonnes a year would save by moving from 82% to 90% selectivity.
- Open the Ammonia tile and compare its energy or feedstock figures with Shreve's 40 to 45 GJ per tonne for early 1980s plants. Note what has changed and why recycle and compression matter so much.
- Open the Nitric acid tile and check which downstream products, such as Ammonium nitrate, take most of the output. Then relate its cost to the 0.279 to 0.291 tonnes of ammonia per tonne in Kent's utility table.
- Open the Chemical process catalysts tile and the Autocatalysts tile. Compare which catalysts are recovered and recycled and which are thrown away, using the Ziegler-Natta and platinum examples in this module.
- Open the Small-molecule APIs or Custom synthesis (CDMO) tile and look for plant or batch sizes. Compare them with Pollak's rule of 15 to 30 tonnes a year per cubic metre of reactor.
Final quiz
Shreve says methanol and ammonia plants are four to five times larger than they would be if conversion equalled yield. Why?
Low conversion per pass means each tonne of product requires many passes, so compressors, converters and separators are sized for a far larger gas flow than the net output.Kent's table shows dual-pressure nitric acid plants using 0.279 t of ammonia per tonne of acid. The theoretical minimum is 0.270 t. What is the overall ammonia efficiency?
0.270 divided by 0.279 is about 0.97. The loss is mostly ammonia burned to nitrogen rather than nitric oxide, plus a little used to abate NOx.Which statement about a catalyst is correct?
Catalysts change rates and selectivity but not equilibrium. They do change over time through sintering, poisoning and fouling.The contact process converter cools the gas between catalyst beds because:
The reaction is exothermic. Shreve's table shows Kp falling from 897 at 400 C to 9.5 at 600 C, so the gas is cooled to about 430 C before later beds.Which is the main reason heterogeneous catalysts are more widely used in industry than homogeneous ones?
Wittcoff notes heterogeneous catalysts are easily recovered and work in fixed or fluidised beds at high temperature. Homogeneous catalysts are often more selective but must be separated from the product.A four-effect evaporator concentrates juice from 15% to 65% solids. Roughly how much steam does it need per 100 t of juice?
About 77 t of water must be removed. At 4 t of water per tonne of steam that is about 19 t of steam.Pollak's multipurpose fine chemical plant has 24 m3 of reactors and cost $21 million. Compared with a continuous commodity plant, it is:
At 15 to 30 t per m3 a year it makes 360 to 720 t, so capital is roughly $29,000 to $58,000 per annual tonne, far above a commodity plant. Its value lies in flexibility for small-volume, high-value products.What design lesson did Kent draw from Flixborough?
120 tonnes of hot cyclohexane under pressure made the explosion possible. Limiting inventory, a core inherent safety principle, became a design objective for major hazard plants.Sources for this module
- Kent, J.A. (ed.), Handbook of Industrial Chemistry and Biotechnology, 12th ed., Springer, 2012. Ch 2 Safety Considerations (Murphy); Ch 5 Green Engineering; Ch 6 Industrial Catalysis (Farrauto); Ch 10 Synthetic Organic Chemicals (Barnicki); Ch 22 Synthetic Nitrogen Products (Maxwell); Ch 25 Sulfur and Sulfuric Acid; Ch 26 Salt, Chlor-Alkali; Ch 35 Sugar · pdf pp. 61-72, 95-97, 113-115 (Ch 2); 176-177 (Ch 5); 213-226, 237-240 (Ch 6); 327 (Ch 10); 902-904, 913-915 (Ch 22); 1016-1017 (Ch 25); 1022 (Ch 26); 1409 (Ch 35)
- Austin, G.T., Shreve's Chemical Process Industries, 5th ed., McGraw-Hill, 1984. Ch 1 Chemical Processing; Ch 2 Chemical Processing and the Work of the Chemical Engineer; Ch 18 Nitrogen Industries; Ch 19 Sulfur and Sulfuric Acid; Ch 37 Petroleum Processing; Ch 38 Petrochemicals · pdf pp. 19-30, 322-328, 347-354, 751-752, 773-774
- Wittcoff, H.A., Reuben, B.G. and Plotkin, J.S., Industrial Organic Chemicals, 2nd ed., Wiley, 2004. Ch 3 Ethylene; Ch 9 Xylenes; Ch 10 Methane; Ch 16 Industrial Catalysis · pdf pp. 151-153, 346-348, 391-392, 553-562
- Brennan, D.J., Process Industry Economics, 2nd ed., Elsevier, 2020. Ch 4 Operating cost estimation; Ch 7 Process technology evolution and adoption · pdf pp. 106-110, 126-127, 187-188
- Pollak, P., Fine Chemicals: The Industry and the Business, 2nd ed., Wiley, 2011. Ch 5 Facilities and Plants; Ch 7 Cost Calculation · pdf pp. 74-75, 97-99