# How a chemical process works: reactions, separations, catalysts and flowsheets: Reading a flowsheet, and safety as a design input

Source page: https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/p5/

[Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 2: How a chemical process works: reactions, separations, catalysts and flowsheets](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/) Part 5 of 5

## Reading a flowsheet, and safety as a design input

A flowsheet compresses a whole plant into one diagram: what goes in, what happens at each step, and what comes out. With a material and energy balance attached, it shows where money and risk sit. Working through the contact process for sulphuric acid ties together everything so far. Then we look at hazards, because Kent's safety chapter treats them as a design input from the first sketch.

### 5.1 Walking through the contact process step by step

Shreve calls a skilfully drawn flowchart the most concise description of a process that exists, and recommends that it show utilities and labour as well as materials. Read a flowsheet from left to right, asking at each box what changes, at what temperature and pressure, and where heat goes in or out.

In a sulphur-burning Sulphuric acid plant, molten sulphur is kept liquid in steam-heated lines and sprayed into a furnace. Shreve notes that sulphur-burning plants are the simplest and cheapest because the gas needs no special cleaning. Combustion air is first dried in a tower of 93 to 98% acid, because water vapour would later condense as corrosive acid. Burning gives a gas of about 10 to 12% sulphur dioxide, and the heat raises steam in a waste-heat boiler.

The gas then enters a converter of four or five fixed beds of vanadium pentoxide catalyst on silica. Kent gives the operating range for this catalyst as about 410 to 440 C, with caesium-promoted versions working at 360 to 400 C. Each bed heats up as it reacts, and boilers, superheaters or gas-gas exchangers between beds bring the temperature back down. After two or three beds the gas passes to an intermediate absorber, where sulphur trioxide dissolves in circulating 98% acid at 60 to 80 C. Kent explains that 98.3% acid is used because the vapour pressures of SO3, H2SO4 and water are lowest there. The gas is reheated, passes over the last beds and goes to a final absorber.

Water is added to the absorber acid to hold its strength, and product acid is drawn off. Acid coolers remove the heat of absorption into cooling water. Shreve reports that vanadium catalysts can last up to 20 years and resist most poisons, apart from fluorine and arsenic.

Process flowchart Double absorption contact process for sulphuric acid (sulphur burning) Inputs **Sulphur**0.327 t per t of 100% acid (theoretical) **Dry air**supplies O2: 0.490 t O2 per t acid (theoretical) **Water**0.184 t per t acid (theoretical) **Electricity**about 3% of energy input, mainly the blower Process steps 1 **Sulphur melting and air drying**

steam-heated sulphur; air dried in 93-98% acid at 50-60 C
 2 **Sulphur furnace and waste-heat boiler**

S + O2 → SO2, -298 kJ/mol; gas about 10-12% SO2
 3 **Converter beds 1-3 (V2O5 catalyst)**

SO2 + 1/2 O2 → SO3, -98 kJ/mol; enter about 425-440 C, rise towards 600 C, cooled between beds
 4 **Intermediate absorber**

SO3 absorbed in 98% acid at 60-80 C; equilibrium shifted
 5 **Reheat and final converter bed(s)**

remaining SO2 converted; overall 99.7% or more
 6 **Final absorber and acid coolers**

SO3 + H2O → H2SO4, -130 kJ/mol; heat rejected to cooling water
 Outputs **Sulphuric acid**1 t (100% basis) **High-pressure steam**more than 1.3 t per t acid at 40-60 bar **Warm cooling water**carries about 40% of the heat **Tail gas**about 0.3% of SO2 unconverted in a five-bed plant

Theoretical quantities calculated here from the equations; conditions from Kent Ch 25 and Shreve Ch 19.
 Source: Kent (2012), Ch 25; Shreve (1984), Ch 19, Kent pdf pp. 1016-1017; Shreve pdf pp. 347-354

- Shreve (1984), Ch 2 Flowcharts · pdf p. 26
- Shreve (1984), Ch 19 Contact process · pdf pp. 347-354
- Kent (2012), Ch 25 Sulfur and Sulfuric Acid · pdf pp. 1016-1017

### 5.2 The material and energy balance tells you what the plant earns

A material balance tracks every kilogram in and out. Shreve calls it the essential first step in any process study, because cost is driven by how materials are used. An energy balance does the same for heat and power. For the contact process both can be worked from source figures.

Materials first. One tonne of pure acid contains 0.327 tonnes of sulphur, 0.490 tonnes of oxygen taken from air, and 0.184 tonnes of water added in the absorbers; these add up to 1.001 tonnes after rounding. Losses are small. A single absorption plant converting 98% of its sulphur dioxide would release about 13 kg of SO2 per tonne of acid (2% of 0.327 tonnes of sulphur, doubled because SO2 weighs twice as much as the sulphur in it). At 99.7% conversion this falls to about 2 kg. Shreve reports that US rules of the time required new sulphur-burning plants to match 99.7% conversion, which is why double absorption became standard.

Energy next. The three reaction heats add to about 5.4 GJ per tonne, matching Kent's 63 MW for a 1,000 tonne-per-day plant. Kent says 55 to 60% is typically recovered as high-pressure steam, about 3.0 to 3.2 GJ per tonne, and Shreve puts the steam at more than 1.3 tonnes per tonne of acid in large plants. Most of the rest leaves in cooling water.

The balance turns into money. At Brennan's 2018 price of $10 to $16 per tonne of high-pressure steam, 1.3 tonnes is worth $13 to $21 per tonne of acid, a real credit against the cost of sulphur. Shreve gives the example of a Swedish 2,200 tonne-per-day plant supplying 52 MW to district heating. The catalyst inventory is also set by the balance: Kent gives 150 to 200 litres per tonne-per-day of capacity, or 150 to 200 cubic metres for a 1,000 tonne-per-day plant.

Data Material and energy balance per tonne of sulphuric acid (100% basis)

| Item | Quantity | How obtained |
|---|---|---|
| Sulphur in | 0.327 t | 32/98 from the equation |
| Oxygen in (from air) | 0.490 t | 48/98 |
| Water in | 0.184 t | 18/98 |
| SO2 to stack, single absorption (98%) | about 13 kg | calculated |
| SO2 to stack, double absorption (99.7%) | about 2 kg | calculated |
| Heat released | 5.4 GJ | sum of reaction heats; matches 63 MW at 1,000 t/day |
| Recovered as HP steam (55-60%) | about 3.0-3.2 GJ; over 1.3 t steam | Kent; Shreve |
| Lost to cooling water | about 40% of heat | Kent |
| Electricity | about 3% of energy input | Kent |
| Catalyst inventory | 150-200 L per t/day capacity | Kent, double absorption |

 Source: Kent (2012), Ch 25; Shreve (1984), Ch 19; Brennan (2020), Ch 4, Kent pdf pp. 1016-1017; Shreve pdf pp. 347-349; Brennan pdf p. 110

- Shreve (1984), Ch 2 Material balances · pdf pp. 25, 29
- Shreve (1984), Ch 19 · pdf pp. 347-349
- Kent (2012), Ch 25 · pdf pp. 1016-1017
- Brennan (2020), Ch 4 Table 4.4 · pdf p. 110

### 5.3 Inherent safety: remove the hazard before adding protection

Kent separates occupational safety (slips, falls, cuts) from process safety, which is about preventing fires, explosions and toxic releases. The common thread in process accidents is loss of containment: material escaping from the pipes and vessels meant to hold it, through rupture, overpressure, corrosion or human error.

Protection is layered. Kent describes the Swiss cheese picture, where each barrier has holes and an accident happens only when the holes line up. The layers run from the vessel and piping itself, through the basic control system and alarms, to an independent safety instrumented system that trips the plant, then relief valves, and finally dikes, fire protection and emergency response.

Kent ranks strategies by reliability. Least reliable are procedures, which depend on people. Better are active engineering controls, then passive features such as dikes that work without any device operating. Most reliable is inherent safety, a term coined by Trevor Kletz of ICI: designing the hazard out. His principles are to use less hazardous materials, keep inventories small, run at less severe conditions and simplify equipment. They are easiest to apply at the laboratory or concept stage.

Inherent choices involve trade-offs. Kent's example: supplying chlorine from many small cylinders instead of one big tank means more connections and more chances of a leak, but each leak is much smaller. Another: a reactor designed to 50 psig to make release less likely stored so much energy that, when a runaway did burst it, the damage was far greater than a low-pressure vessel would have caused.

Relief systems are the last engineered layer. Designers pick the worst credible scenario, such as fire, runaway reaction or loss of cooling, calculate the flow to be vented, and size the device. Kent reports a key finding of the DIERS industry programme: runaway reactions often vent a two-phase froth that needs two or more times the relief area of vapour alone.

Data Inherently safer design principles with examples

| Principle | Meaning | Example |
|---|---|---|
| Intensification | reduce the quantity of hazardous material | smaller inventories; smaller, more responsive equipment |
| Substitution | use a safer material | less hazardous solvent or reagent |
| Attenuation | run at less severe conditions | lower temperature or pressure |
| Limitation of effects | lay out equipment to limit consequences | spacing, I, L or H layouts, less congestion |
| Simplification | avoid needless complexity | avoid multiproduct operation in one unit |
| Error tolerance | equipment that tolerates upsets | design vessels for full vacuum |
| Avoid knock-on effects | stop one failure spreading | open construction |

 Source: Kent (2012), Ch 2 Inherently safer concepts, pdf pp. 71-72, 96  In the data

#### Reactive chemistry is a known risk

Kent cites a US Chemical Safety Board review of 167 chemical reactivity incidents from 1980 to 2001, which caused 108 deaths. In over 90% of them, the information needed to prevent the incident was already documented and publicly available. Many happened where no reaction was intended, during storage, blending or distillation. At Napp Technologies in New Jersey in 1995, water mixed by accident with aluminium powder and sodium hydrosulphite during blending; five people died. Kent's conclusion is that the problem is not reactive chemicals but how they are managed.

- Kent (2012), Ch 2 Safety Considerations in the Chemical Process Industries · pdf pp. 61-67, 71-72, 96-97

### 5.4 Flixborough and Bhopal turned inventory into a design variable

Kent's case histories show how management failures turned process hazards into disasters, and how the lessons fed back into design.

Flixborough, England, 1974. The Nypro plant oxidised cyclohexane to make cyclohexanone, a step towards Caprolactam and so Nylon (PA6, PA66). Six reactors in series ran at 155 C and 8.8 bar and held 120 tonnes of cyclohexane, a flammable liquid far above its boiling point. When one reactor cracked, the plant was rushed back into service with a 20-inch pipe bypassing it, supported on scaffolding, with bellows at each end and a dog-leg to cover the level difference. The only drawings were in chalk on the workshop floor, and there was no qualified mechanical engineer on site. The pipe failed. About 30 tonnes of cyclohexane formed a vapour cloud that exploded, killing 26 people, injuring 36 and destroying 1,821 houses. A key conclusion, Kent notes, was that limiting inventory of flammable material should be a specific design objective in major hazard plants.

Bhopal, India. A Union Carbide India plant used methyl isocyanate (MIC), an extremely toxic intermediate, to make the pesticide Sevin. About 120 to 240 gallons of water got into an MIC storage tank. The MIC reacted, heating and pressurising the tank until the rupture disc burst. Roughly 27,000 lb escaped over two hours. The scrubber and flare were sized for 10,000 lb per hour of routine vent flow, not a runaway in storage; the refrigeration was off, the flare was down for repair, and the water curtain could not reach the gas. A shanty town of about 100 people per acre had grown next to the plant. Kent reports about 1,700 to 2,700 deaths, possibly more, and 50,000 people seriously affected. Contributing causes included a larger inventory of MIC than was needed.

Data Three landmark accidents in Kent's case histories

| Case | Process | Immediate cause | Toll | Design lesson |
|---|---|---|---|---|
| Flixborough, 1974 | cyclohexane oxidation for caprolactam | temporary bypass pipe failed; vapour cloud exploded | 26 dead, 36 injured, 1,821 houses destroyed | limit inventory; control plant modifications |
| Bhopal | MIC for the pesticide Sevin | water entered MIC tank; runaway hydrolysis | about 1,700-2,700 dead (possibly more); 50,000 seriously affected | minimise toxic inventory; size relief and scrubbers for runaway; control land use near plant |
| Phillips, Pasadena, 1989 | high-density polyethylene loop reactor | reversed air hoses opened a valve during maintenance; over 85,000 lb of gas released in 90-120 s | 23 dead, over 130 injured, nearly $750 million damage | back-up isolation (double valve or blind) when opening lines; space control rooms away from reactors |

 Source: Kent (2012), Ch 2 Case histories, pdf pp. 113-115  Case

#### What the cases teach an investor

Neither disaster began with exotic chemistry. Both began with large inventories of a hazardous material, safety systems that were switched off or undersized, and missing expertise on site. Kent's chapter states the industry view that process incidents are, at root, management system failures. For a fund looking at a chemical plant, the practical questions follow: how much hazardous material is stored and why, whether relief and scrubbing are sized for the worst case rather than normal venting, how changes to the plant are reviewed, and who on site has the authority to stop work. These are design and governance choices, not luck.

- Kent (2012), Ch 2 Case histories · pdf pp. 113-115

### See it in the table
[H₂O Water](https://www.materialinsights.org/commodities/water/)[Al Aluminium](https://www.materialinsights.org/commodities/aluminum/)[V Vanadium](https://www.materialinsights.org/commodities/vanadium/)[As Arsenic](https://www.materialinsights.org/commodities/arsenic/)[Cs Caesium](https://www.materialinsights.org/commodities/cesium/)[H2SO4 Sulphuric acid](https://www.materialinsights.org/chemicals/sulphuric_acid/)[Cl2 Chlorine](https://www.materialinsights.org/chemicals/chlorine/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/)[CPL Caprolactam](https://www.materialinsights.org/chemicals/caprolactam/)[PA Nylon (PA6, PA66)](https://www.materialinsights.org/chemicals/nylon/)

### Check your understanding

Which strategy does Kent rank as the MOST reliable way to control a hazard?
 Kent orders the hierarchy procedural, active, passive, inherent, from least to most reliable.

Why do relief systems for runaway reactions often need much larger vents than simple vapour relief?
 The DIERS programme found two-phase flow is easy to trigger in a runaway, and under-sizing for it leaves the vessel unprotected.

In the contact process energy balance, roughly what share of the heat leaves in cooling water from the acid coolers?
 Kent says 55 to 60% is recovered as high-pressure steam and about 40% is lost as warm water. Some plants have pushed recovery to about 70%.

[← Part 4: Unit operations, their energy bills, and batch versus continuous](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/p4/)[Mark read · Wrap-up →](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/review/)

## Other sections

- [Module overview](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/index.md)
- [The anatomy of a chemical plant](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/p1/index.md)
- [Reaction basics that decide the economics](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/p2/index.md)
- [Catalysts: what they change and what they cannot](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/p3/index.md)
- [Unit operations, their energy bills, and batch versus continuous](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/p4/index.md)
- [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/how-chemical-processes-work/review/index.md)