Chemistry · Chemistry of the elements · Year 4

Chemical plant: reactors and workshops

How much current does a tonne of chlorine need, and how is 99.7 % of SO₂ turned into SO₃?

The situation

The Almalyk Mining and Metallurgical Combine makes sulfuric acid from the SO₂ in the gas of its copper-smelting furnaces; Navoiyazot and Maxam-Chirchiq make ammonia, nitric acid and nitrogen fertilisers from natural gas; and Navoiyazot also produces chlorine and caustic soda from table salt. The laboratory test tube becomes reactors, heat exchangers and columns that run day and night in a plant. Which reactor will be smaller, how is 99.7 % of the SO₂ turned into SO₃, and how much electrical energy does a tonne of chlorine need?

Topic

Chemical plant: reactors, sulfuric acid, ammonia, chlor-alkali. A tour of three plants: the Almalyk Mining and Metallurgical Combine sulfuric acid shop (from SO₂ in copper smelter gas), Navoiyazot (ammonia, nitric acid, caustic soda and chlorine) and…

Key ideas

Conversion and selectivity
Contact converter
Electrolysis energy

Formulas

X_{CSTR} = \frac{k τ}{1 + k τ}, X_{PFR} = 1 - e^{- k τ}

First-order reaction: continuous stirred-tank (CSTR) and plug-flow (PFR) ideal reactors; $τ = V / v_{0}$ — mean residence time.

SO_{2} + \frac{1}{2} O_{2} \overset{\overset{}{V_{2} O_{5}}}{⇌} SO_{3} Δ H = - 99 kJ/mol

Catalytic (contact) oxidation is exothermic and reversible — the gas is cooled after each layer.

X = X_{3} + (1 - X_{3}) X_{4}

Double contact: the SO₂ left after intermediate absorption is oxidised again in the fourth layer.

W = \frac{U \cdot z F}{M η}

Electrolysis energy per 1 kg of product ( $z$ — number of electrons, $η$ — current efficiency).

m = \frac{M I t η}{z F}, δ = \frac{m}{ρ A}

Faraday's law and the thickness of an electroplated coating.

Theory

Chemical technology and the chemical industry of Uzbekistan

Chemical technology is the science of economical and safe ways of turning raw materials into useful products: when a laboratory reaction becomes a plant process, scale, continuity, heat and energy, and waste become decisive. History: in the 18th century, the lead-chamber process for sulfuric acid and Leblanc soda; at the end of the 19th century, the contact process; in 1913, the first Haber–Bosch ammonia synthesis plant; in the 20th century, petrochemicals and polymers; today, green chemistry, energy saving and waste-free technologies. In Uzbekistan the chemical industry took shape mainly in the 1940s–1970s: nitrogen fertiliser production in Chirchik (today Maxam-Chirchiq), the Almalyk Mining and Metallurgical Combine (copper, zinc, molybdenum, gold, silver, and sulfuric acid from metallurgical gases), Ammofos-Maxam in Almalyk (phosphorus fertilisers), Navoiyazot and Ferganaazot; in the years of independence the Kungrad soda plant and the Shurtan and Ustyurt gas-chemical complexes (polyethylene) were built, and capacity for PVC, caustic soda and methanol was commissioned at Navoiyazot. Main products: mineral acids (sulfuric, nitric, phosphoric), salts and soda, nitrogen and phosphorus fertilisers, chlorine and alkali, polymers. Uzbek chemist-scientists include the academicians O. S. Sadykov (bioorganic chemistry, President of the Academy of Sciences) and S. Yu. Yunusov (alkaloid chemistry), among others; for more, see the “From alchemy to the periodic law” experiment, and for school level the “Chemical industry and plants” experiments.

Basic concepts: chemical process systems, processes and reactors

A chemical process system consists of apparatus and the streams linking it: series, parallel, recycle (return) and bypass connections (the arrows on the stage show the flow direction). Types of process: hydromechanical (settling, filtration, mixing), heat transfer (heating, cooling, evaporation; the basic equation is $Q = K F Δ t_{avg}$ , where $K$ is the heat-transfer coefficient and $F$ the surface area), mass transfer (absorption, distillation, extraction, adsorption, drying; $M = K_{y} F Δ y_{avg}$ ) and chemical. Classification of reactors: by degree of mixing — ideal mixing (CSTR: the concentration throughout the volume is as low as at the outlet) and ideal plug flow (PFR: the flow moves layer by layer), with cascades and real reactors between them; by mode of operation — batch, continuous, semi-continuous; by heat regime — isothermal, adiabatic, polytropic; by phases — homogeneous and heterogeneous (fixed and fluidised catalyst bed). For the same conversion a stirred-tank reactor is larger than a plug-flow reactor, and a cascade approaches a PFR. Performance indicators: conversion $X$ , yield (the fraction turned into product), selectivity $S = n_{R} / (n_{A, 0} - n_{A})$ , productivity and intensity, and consumption coefficients (t of raw material / t of product). Material balance: input = output (+ accumulation); heat balance: the heat released $Q = (- Δ H) v_{0} c_{A 0} X$ is removed by a cooler, otherwise adiabatic heating occurs, $Δ T = (- Δ H) c_{A 0} X / (ρ c_{p})$ . Industrial catalysis: catalyst + promoters (K₂O, Al₂O₃ on iron; K₂SO₄ on V₂O₅), contact apparatus (layered, tubular, fluidised-bed), poisons (compounds of S, As, CO, Cl) — the raw material is purified; for more, see “Catalysis and enzymes”. Organic and polymer materials: methanol from synthesis gas $CO + 2 H_{2} \overset{}{\to} CH_{3} OH$ (Cu/ZnO/Al₂O₃, 5–10 MPa, 220–280 °C); ethanol by hydration of ethylene $C_{2} H_{4} + H_{2} O \overset{}{\to} C_{2} H_{5} OH$ (H₃PO₄ catalyst, ≈ 300 °C, 7 MPa) or by fermentation; polymer synthesis in bulk, solution, suspension and emulsion — see “Polymer synthesis and kinetics”; viscose and acetate fibres, moulding (pressing), injection moulding, extrusion and calendering — see “Chemistry of polymer materials”.

Raw materials, beneficiation and energy

Raw materials are mineral (ores, salts, phosphorites), plant and animal, oil, gas, coal, air and water. An ore is often poor (in Almalyk copper ore the copper content is a few tenths of a per cent), so it is beneficiated: mechanically (crushing, sieving, gravity separation by density), thermally (roasting, thermal decomposition), electromagnetically (magnetic separation — magnetite) and physico-chemically by flotation: particles of a non-wettable (hydrophobic) mineral stick to air bubbles and rise with the froth, while the waste rock sinks (flotation reagents — collectors and frothers). Complex use of raw materials: from copper ore, molybdenum, gold, silver and selenium are obtained along with the copper, and the SO₂ in the smelting gases is turned into sulfuric acid, which in turn turns phosphorite into fertiliser: the waste of one unit is the raw material of another. Energy: fuel (natural gas is the main one in Uzbekistan), electrical energy (electrolysis) and steam; heat recovery — sulfuric acid plants raise steam from the heat of sulfur combustion and SO₂ oxidation and generate electricity; in an ammonia plant the heat of reaction gives high-pressure steam.

Sulfuric acid: the contact process

Sources of SO₂: burning sulfur $S + O_{2} \overset{}{\to} SO_{2}$ (≈ 10–12 % SO₂), roasting pyrite, and non-ferrous metallurgical gases (Almalyk MMC: the gas of the copper-smelting furnaces — a waste product is put to use and the air is cleaned). Cleaning: dust is removed in cyclones and electrostatic precipitators, compounds of arsenic, selenium and fluorine (poisons for V₂O₅) and the acid mist are removed in washing towers, and the gas is dried with 93–96 % H₂SO₄ — on the stage, switch the cleaning off and see how the catalyst activity falls. Catalytic oxidation: $SO_{2} + \frac{1}{2} O_{2} \overset{}{⇌} SO_{3}$ , ΔH ≈ −99 kJ/mol, catalyst V₂O₅ + K₂SO₄ (promoter) on a silica support, 420–600 °C (below ≈ 390 °C it “does not light off”, above ≈ 630 °C it is destroyed). The reaction is exothermic and the number of moles of gas decreases — a low temperature favours the equilibrium, a high temperature favours the rate. This is why the multi-layer contact apparatus is used: in each layer the gas heats up adiabatically (with 10 % SO₂, each 1 % of conversion ≈ 3 °C), the straight line on the X–T diagram approaches the equilibrium curve; between layers the gas is cooled in heat exchangers and the next layer gives further conversion. Single contact gives ≈ 96–98 %; with double contact (after layer 3 the SO₃ is absorbed in an intermediate absorber, and the remaining SO₂ is oxidised in layer 4 in a gas rich in oxygen) the conversion is ≥ 99.7 % — the waste gas contains a few hundred ppm of SO₂. Absorption: SO₃ is absorbed not in water but in 98.3 % H₂SO₄: at this (azeotropic) concentration the total pressure of water and SO₃ vapours above the acid is lowest; with water, SO₃ forms a fine H₂SO₄ mist in the gas phase, which is not absorbed and escapes. Oleum is a solution of SO₃ in H₂SO₄ (for example 20 %); it is made in the first (oleum) absorber. Production cost (Kimtex L24): raw material, energy, wages and depreciation, minus the revenue from steam sales; when capacity grows, the fixed cost per 1 t falls. School level: the “Sulfuric acid and the contact process” experiment.

Ammonia and nitric acid

Ways of fixing nitrogen: the arc process (N₂ + O₂ → 2NO, a great deal of energy), the cyanamide process (CaC₂ + N₂), ammonia synthesis (Haber–Bosch) — the main method today; in nature, nodule bacteria. Nitrogen–hydrogen mixture: natural gas is freed of sulfur, then steam reforming $CH_{4} + H_{2} O \overset{}{\to} CO + 3 H_{2}$ (Ni, 800–900 °C); in secondary reforming air is added (this is the source of the nitrogen); then the CO shift $CO + H_{2} O \overset{}{\to} CO_{2} + H_{2}$ , the CO₂ is absorbed and the remaining CO is methanated (it poisons the iron catalyst). The result is H₂ : N₂ = 3 : 1 with ≈ 1 % inerts (CH₄, Ar). Synthesis: $N_{2} + 3 H_{2} \overset{}{⇌} 2 NH_{3}$ , ΔH = −92 kJ; Fe with K₂O and Al₂O₃ promoters, 400–500 °C, 15–30 MPa; medium-pressure schemes (≈ 25–32 MPa) are widespread. Synthesis tower: inside a thick-walled high-pressure shell are the catalyst beds and an internal heat exchanger — the incoming cold gas is heated by the outgoing hot gas. Compromise: at low temperature the equilibrium is favourable but the reaction is slow; at high temperature it is fast but the equilibrium NH₃ is small — the NH₃ at the outlet has an optimum temperature (the curve on the board). In a single pass only ≈ 15–30 % of the synthesis gas reacts, so a recycle scheme is used: cooling, separating NH₃ by liquefaction, and returning the remaining gas — overall utilisation ≈ 97 %. So that inerts do not build up, part of the gas is released (purge) — and H₂ is lost with it: if the purge is small, inerts accumulate and the conversion per pass falls; if it is large, H₂ is wasted (H₂ from the purge gas is recovered on a membrane). Nitric acid (Ostwald): $4 NH_{3} + 5 O_{2} \overset{}{\to} 4 NO + 6 H_{2} O$ on a Pt–Rh gauze at ≈ 900 °C, with a contact time of about a thousandth of a second and an NO yield of 95–98 % (at lower temperature more N₂ and N₂O form); $2 NO + O_{2} \overset{}{\to} 2 NO_{2}$ — faster in the cold and at high pressure; $3 NO_{2} + H_{2} O \overset{}{\to} 2 HNO_{3} + NO$ — in the absorption column, giving 55–60 % acid (azeotrope ≈ 68 %); concentration with H₂SO₄ or Mg(NO₃)₂ up to 98 %. At Navoiyazot and Maxam-Chirchiq this acid is used to make ammonium nitrate $NH_{3} + HNO_{3} \overset{}{\to} NH_{4} NO_{3}$ . Equilibrium theory — “Reversible reactions and equilibrium”; school level — “Nitrogen, ammonia and nitric acid” and “Chemical industry and plants”.

Electrochemical production; safety and the model

Electrolysis of table salt solution: at the anode $2 Cl^{-} - 2 e^{-} \overset{}{\to} Cl_{2}$ , at the cathode $2 H_{2} O + 2 e^{-} \overset{}{\to} H_{2} + 2 OH^{-}$ ; overall $2 NaCl + 2 H_{2} O \overset{}{\to} Cl_{2} + H_{2} + 2 NaOH$ : with 1 t of Cl₂ come 1.13 t of NaOH and 28 kg of H₂. Diaphragm process: the anode and cathode compartments are separated by a porous diaphragm (formerly asbestos, now asbestos-free), ≈ 3.2–3.6 V, the alkali is 10–12 % and mixed with NaCl — it is concentrated by evaporation. Mercury-cathode process: the cathode is flowing mercury, which forms sodium amalgam (decomposition voltage ≈ 3.1 V, working voltage ≈ 4–4.5 V); in a separate decomposer the amalgam gives pure 50 % NaOH with water; mercury escapes into the environment — under the Minamata Convention (2013) such production must stop by 2025. Membrane process: a cation-exchange perfluorinated membrane lets only Na⁺ through, ≈ 2.9–3.5 V, pure 32–35 % NaOH — the most economical (≈ 2.2–2.5 MWh/t Cl₂); new capacity is built with this process. The energy is $W = U \cdot 2 F / (M_{Cl_{2}} η)$ , where $η$ is the current efficiency. Aluminium: Al₂O₃ is electrolysed in molten cryolite at 950–970 °C, the carbon anode burns away, 4–4.5 V, ≈ 13–15 kWh per 1 kg of Al. Electroplating: the surface is degreased and etched, then coated; Faraday's law $m = M I t η / (z F)$ , from which the current efficiency is $η = Δ m z F / (M I t)$ — the measured mass gain divided by the theoretical mass (Kimtex L12, L13); nickel (Watts bath, 50 °C, 1–5 A/dm², η ≈ 95–97 % in the working range) and acid copper (0.5–4 A/dm², η ≈ 98–100 %); if the current density exceeds the limit, hydrogen is also released at the cathode — the efficiency falls and the coating “burns”. Faraday's laws — “Electrolysis, Faraday's laws”; the chemistry of electrolysis — “Electrolysis and corrosion”. Safety: chlorine is a green-yellow, very toxic gas, ≈ 2.5 times heavier than air, and forms an explosive mixture with hydrogen; mercury vapour and its compounds are toxic, and in nature it turns into methylmercury, which accumulates in the food chain; SO₂, SO₃, NH₃ and NO₂ are toxic; oleum fumes in air and burns — do not pour it into water (it is diluted by adding it to concentrated acid), and pour acid into water, not the other way round; high pressure of 15–30 MPa; if alkali gets in the eye, rinse with plenty of water; nickel salts are allergens; in the laboratory, safety goggles, gloves and a fume cupboard. Model: the reactors are ideal (first order, isothermal); the contact apparatus uses adiabatic layers, a Kp correlation and semi-empirical kinetics, and the catalyst activity in uncleaned gas is taken as 30 %; ammonia — an equilibrium fitted to Larson–Dodge data, approach to equilibrium by the Arrhenius law, 3 % NH₃ after the separator; the NO yield, electrolyser voltages, plating efficiency and cost prices are model values for teaching; the layouts are not to scale, and the plating time is speeded up (1 s ≈ 1 min).

Did you know?

Wilhelm Ostwald patented the method of making nitric acid by oxidising ammonia on platinum in 1902, and in 1909 he received the Nobel Prize in Chemistry for his work on catalysis.
Fritz Haber received the 1918 Nobel Prize for ammonia synthesis, and Carl Bosch, who brought it to industrial scale, received the 1931 Nobel Prize for high-pressure chemical methods.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Chromium and manganese: coloured ions — Grade 9
Iron and its compounds — Grade 9
Patterns in s- and p-block elements — Year 1
d- and f-block elements: colour, redox — Year 1
Complex compounds and ligands — Year 1