Fundamentals of Applied Chemistry

Everything in the first fifteen units happened in a test tube. This unit asks a different question: how do you take a reaction that works on 5 g in a laboratory and make it work on 500 tonnes a day, profitably, safely, and without poisoning the river next door? Applied chemistry — also called chemical technology or chemical engineering science — is the study of that scale-up. It is the bridge between the balanced equation and the bag of urea a farmer in Chitwan actually buys.

16.1 Chemical industry and its importance

Chemical manufacture is divided by scale and value:

Class	Tonnage	Value	Examples
Heavy (bulk) chemicals	millions of tonnes/year	low per kg	H₂SO₄, NH₃, NaOH, Na₂CO₃, cement, urea
Fine chemicals	hundreds to thousands of tonnes/year	high per kg	dyes, flavours, pesticide actives, reagents
Speciality chemicals	small, made to a specification	sold on performance	paints, adhesives, catalysts, detergents
Pharmaceuticals	kg to tonnes	very high per kg	paracetamol, antibiotics, vaccines

Why the industry matters:

1. Food. Nitrogen fertilizers made from atmospheric nitrogen feed roughly half the world's population. Nepal has no fertilizer factory of its own and imports 100% of its chemical fertilizer, at a cost of more than Rs 40 billion a year.
2. Health. Medicines, disinfectants, chlorine for drinking water, oxygen for hospitals, and the plastics that syringes and IV sets are made from.
3. Infrastructure. Cement, steel, glass, paints and bitumen. Nepal has 13 integrated and 16 grinding cement plants with a combined capacity of about 12.3 million tonnes a year, including the state-owned Hetauda Cement (using limestone from Bhaise, Majhuwa and Jogmara in Makawanpur) and Udayapur Cement.
4. Energy. Refining, battery chemicals, and the electrolysis of water for hydrogen.
5. Economy. The output of one chemical plant is the raw material of the next, so a single plant creates a chain of downstream industry and employment.

The industry's signature is that a single cheap raw material tree feeds everything. Air gives N₂ and O₂; brine gives Cl₂, NaOH and Na₂CO₃; sulphur or pyrites give H₂SO₄; hydrocarbons and water give H₂ and synthesis gas. Almost every product in Unit 17 grows from one of these four roots.

16.2 Stages in producing a new product

A new chemical product is not invented and then built. It passes through a fixed sequence of stages, each of which can kill the project. The later a project is abandoned, the more money is lost — so each stage is designed to answer one question as cheaply as possible.

Two stages deserve extra attention.

• The pilot plant (stage 3) exists because chemistry does not scale linearly. Doubling the size of a reactor multiplies its volume by 8 but its surface area only by 4, so a reaction that was easy to cool in a 250 cm³ flask can run away in a 25 m³ vessel. Mixing, heat transfer, corrosion, catalyst life and the handling of solids can only be measured, not predicted.
• Process design (stage 4) fixes the flow sheet: which units, in what order, at what temperature and pressure, with what recycle streams. Every later cost is locked in here.

16.3 Economics of production; cash flow in the production cycle

A plant is built only if it will make money. The money involved falls into two capital sums and two running costs.

Term	Meaning	Examples
Fixed capital	one-off spending to build the plant	land, reactors, pipework, instruments
Working capital	money tied up to keep it running	stock of raw material, product in store, unpaid bills
Fixed costs	paid whether or not anything is produced	depreciation, interest on loans, salaries, insurance, rates
Variable costs	proportional to output	raw materials, catalyst, electricity, steam, packaging

For a selling price S per kilogram, a variable cost V per kilogram and fixed costs F per year, the profit on n kilograms a year is

The quantity (S-V) is the contribution per kilogram. Setting P=0 gives the break-even point — the output at which the plant stops losing money:

Because F is spread over every kilogram, the cost per kilogram falls as output rises. This is the economy of scale, and it is the single strongest reason that bulk chemical plants are enormous.

The curve above is the cash-flow diagram of the production cycle. Read it left to right: money flows out during research, design and construction; the outflow is worst at commissioning, when the plant is complete but selling nothing; then sales begin and the curve climbs. Where it crosses zero, the project has returned exactly what was spent — that time is the payback period. Nothing after that point is profit until the plant is shut down and its working capital recovered.

16.4 Running a chemical plant; designing a chemical plant

Designing a plant

Design begins with the flow sheet — a block diagram naming every unit and every stream. Once the flow sheet is fixed, the designer works through:

1. Choice of route. Of the reactions that give the product, pick the one with the cheapest raw materials, the highest atom economy and the fewest steps.
2. Reaction conditions. Temperature, pressure and catalyst are chosen by balancing equilibrium yield against rate and cost — the compromise that dominates Unit 17.
3. Plant location. Near the raw material if the raw material is bulky (cement near limestone); near the market if the product is bulky or dangerous (sulphuric acid); always near water, power, transport and labour, and away from dense settlement.
4. Materials of construction. Stainless steel, lead lining, glass lining or titanium, chosen against corrosion at the operating temperature.
5. Energy integration. Exothermic reactions raise steam that drives the next stage. In a contact-process plant almost all the process heat comes from the reaction itself.
6. Safety and effluent. Pressure-relief systems, interlocks, and treatment for every waste stream, decided at the design stage — not afterwards.

Running a plant

Task	What it involves
Raw-material supply	continuous, of guaranteed purity; catalyst poisons excluded
Process control	automatic control of temperature, pressure, flow, pH, level
Quality control	sampling and analysis of product against a written specification
Maintenance	planned shutdowns; corrosion and catalyst-activity monitoring
Safety	training, protective equipment, leak detection, emergency plans
Waste management	scrubbing of gases, effluent treatment, solid-waste disposal

Why catalysts get "poisoned" in exam answers

A catalyst is expensive and must last for years. Impurities that adsorb on its surface and block the active sites are called poisons — arsenic oxide for the V₂O₅ of the contact process, sulphur compounds and CO for the iron of the Haber process. This is why every flow sheet in Unit 17 has a purification stage before the catalyst chamber. Naming the poison earns the mark.

16.5 Continuous and batch processing

Feature	Batch	Continuous
Scale suited to	small, up to a few thousand t/yr	very large, bulk chemicals
Equipment	one general-purpose vessel, many products	dedicated plant, one product
Capital cost	low	high
Labour per tonne	high	low
Product uniformity	varies from batch to batch	very uniform at steady state
Traceability	excellent — each batch numbered	harder; needs continuous sampling
Down-time	large (charge, clean, discharge)	small; runs months between shutdowns
Control	manual or semi-automatic	fully automatic
Typical products	medicines, dyes, paints, cosmetics	H₂SO₄, NH₃, urea, NaOH, cement, petrol

16.6 Environmental impact of the chemical industry

Every kilogram of product leaves the plant with some mass of waste attached to it. The standard measure is the E-factor: kilograms of waste per kilogram of product. It rises steeply as tonnage falls and chemistry gets more complicated.

The main impacts, with the chemistry behind each

Impact	Chemistry	Control measure
Acid rain	SO₂ and NOₓ escaping to air: SO₂ + H₂O → H₂SO₃; 2SO₂ + O₂ → 2SO₃ → H₂SO₄	scrubbing flue gas with lime slurry; double absorption in the contact process
Global warming	CO₂ from fuel, from reforming and from CaCO₃ → CaO + CO₂ in cement kilns	energy integration, alternative fuels, carbon capture
Ozone depletion	CFCs release Cl atoms: Cl + O₃ → ClO + O₂	Montreal Protocol; replacement by HFCs and hydrocarbons
Water pollution	acidic or alkaline effluent, heavy-metal ions (Hg²⁺, Cr⁶⁺, Pb²⁺), organic load	neutralisation, precipitation as hydroxide or sulphide, biological treatment
Solid waste	CaCl₂ from the Solvay process, phosphogypsum, spent catalyst, fly ash	re-use in road building and cement; metal recovery from spent catalyst
Air-borne dust	cement, lime and fertilizer dust causing respiratory disease	bag filters, electrostatic precipitators, wet spraying
Thermal pollution	warm cooling water lowers the dissolved oxygen of a river	cooling towers before discharge
Major accidents	the Bhopal disaster of December 1984, in which methyl isocyanate leaked from a pesticide plant and killed thousands	inventory reduction, containment, interlocks, emergency planning

The modern answer to all of this is green chemistry: design the waste out rather than treat it afterwards. The most useful single number is the atom economy, which measures how much of the mass of the reactants ends up in the wanted product:

Unlike percentage yield, atom economy is fixed by the equation — a reaction with a poor atom economy is wasteful even at 100% yield.

Chapter summary

• The chemical industry converts cheap raw materials — air, brine, water, sulphur, limestone, hydrocarbons — into heavy, fine, speciality and pharmaceutical chemicals. Nepal imports 100% of its chemical fertilizer.
• New products pass through seven stages: market research → laboratory synthesis → pilot plant → process and plant design → construction and commissioning → full production → marketing and review, with a go/no-go decision at each.
• Costs are fixed capital and working capital, fixed costs and variable costs. Profit P = n(S-V) - F and the break-even output is n_{\text{BE}} = F/(S-V).
• The cash-flow curve is negative through research and construction, reaches its minimum at start-up, and crosses zero at the end of the payback period.
• Plant design fixes the route, the conditions, the location, the materials of construction, energy integration and effluent treatment. Purification comes before every catalyst bed because catalysts are poisoned.
• Batch = one charge at a time, cheap, flexible, traceable, small scale. Continuous = steady state, high capital, low labour, uniform product, bulk scale.
• Environmental damage includes acid rain (SO₂, NOₓ), CO₂ emission, ozone depletion, effluent and heavy metals, dust and solid waste.
• Atom economy = (M_{\text{product}}/\sum M_{\text{reactants}})\times 100\% is a property of the equation, not of the operator; percentage yield is not.

Practice questions

Group A — Multiple choice (1 mark each)

1. Which of the following is a fixed cost of a chemical plant? [1]araw materialsbpackagingcdepreciationdelectricity
2. The stage at which a reaction is first tested on the kilogram scale is the [1]abench synthesisbpilot plantccommissioningdmarket survey
3. Which process would normally be operated batchwise? [1]amanufacture of sulphuric acidbmanufacture of ammoniacmanufacture of a tablet coatingdmanufacture of cement
4. The atom economy of the addition reaction C₂H₄ + Br₂ → C₂H₄Br₂ is [1]a50%b75%c100%dimpossible to calculate
5. The lowest point of the cumulative cash-flow curve of a project occurs at [1]athe end of researchbstart-up of the plantcthe break-even pointdthe end of the plant's life

Answers to Group A

1. (c) — depreciation is paid whether or not the plant produces anything.

2. (b) — the pilot plant is the kg-scale trial between bench and full plant.

3. (c) — small tonnage, many different specifications, batch traceability needed.

4. (c) — in an addition reaction all reactant atoms appear in the product.

5. (b) — the plant is fully paid for but has not yet sold anything.

Group B — Short answer (5 marks each)

1. Define the chemical industry. Classify chemical products into four groups with one example of each. [5]
2. List, in order, the stages in producing a new chemical product, and state why a pilot plant is needed even when the bench reaction works perfectly. [5]
3. Distinguish between batch and continuous processing under any five headings. [5]
4. A plant has fixed costs of Rs 60 million per year. The variable cost is Rs 28 per kg and the selling price is Rs 40 per kg. Find(a) the break-even output in tonnes and(b) the profit at an output of 8,000 tonnes per year. [5]
5. State four environmental impacts of the chemical industry, giving one chemical equation or species responsible for each and one method of control. [5]

Answers to Group B

2. Market research → bench synthesis → pilot plant → process and plant design → construction and commissioning → full production → marketing and review. The pilot plant is needed because heat transfer, mixing, corrosion, catalyst life and solids handling do not scale with volume: the surface-to-volume ratio falls as size rises, so a reaction that is easy to cool in a flask can run away in a large reactor.

4. Contribution = 40 - 28 = Rs 12 per kg. (a) n_{\text{BE}} = 60\times10^{6}/12 = 5.0\times10^{6} kg = 5000 tonnes per year. (b) P = 8.0\times10^{6}(12) - 60\times10^{6} = 96\times10^{6} - 60\times10^{6} = Rs 36 million per year.

5. Any four from the impact table — e.g. acid rain (SO₂ + H₂O → H₂SO₃; controlled by lime scrubbing), global warming (CaCO₃ → CaO + CO₂ in cement kilns; controlled by energy integration), water pollution (Cr⁶⁺, Pb²⁺ in effluent; controlled by precipitation and neutralisation), dust (cement dust; controlled by electrostatic precipitators).

Group C — Long answer (8 marks each)

1. (a) Draw and explain the cash-flow diagram for the production cycle of a new chemical product, marking the research, construction, start-up, break-even and profit regions. [4](b) Explain the factors that must be considered when choosing the site and designing a new chemical plant. [4]
2. (a) Define atom economy and percentage yield and distinguish between them. [3](b) Ethanoic acid can be made by CH₃OH + CO → CH₃COOH or by CH₃CH₂OH + O₂ → CH₃COOH + H₂O. Calculate the atom economy of each route. (M: CH₃OH = 32, CO = 28, CH₃COOH = 60, CH₃CH₂OH = 46, O₂ = 32, H₂O = 18.) [3](c) State which route a manufacturer would prefer on environmental grounds and why. [2]

Answer to Group C question 2

(a) Atom economy is the fraction of the total mass of the reactants that appears in the desired product, (M_{\text{product}}/\sum M_{\text{reactants}})\times100\%; it is fixed by the balanced equation. Percentage yield is (actual mass obtained / theoretical mass from the equation) \times 100\%; it depends on how well the reaction and the separation are carried out. A reaction can have a high yield and a poor atom economy, or the reverse.

(b) Route 1: reactants 32 + 28 = 60; product 60; atom economy = (60/60)\times100\% = 100\%. Route 2: reactants 46 + 32 = 78; product 60; atom economy = (60/78)\times100\% = 76.9\%.

(c) Route 1 — the carbonylation of methanol — is preferred: every atom of the reactants ends up in the product, so there is no by-product to separate, treat or dispose of, and no raw material is bought only to be thrown away. Route 2 wastes 18 g of water per 60 g of acid, and the dilute water must be removed by distillation, which costs energy.