Chemistry · Chemical reactions · Grade 9

Chemical industry and plants

Polyethylene from Kashkadarya gas, fertiliser from air: how does a reaction become a plant?

The situation

In a test tube, ammonia is easy to make: just heat ammonium chloride with slaked lime. But Navoiyazot turns out hundreds of thousands of tonnes of ammonia and urea a year — from nitrogen in the air and natural gas. Gas from Kashkadarya becomes polyethylene, sour gas at Mubarek becomes sulfur, and “Uzbekistan GTL” makes diesel fuel from gas. How does a laboratory reaction become a plant that runs day and night, economically and safely — and what is the future of the chemical industry?

Topic

The chemical industry: from reaction to factory. Map of the chemical industry of Uzbekistan: Navoiyazot, Maxam-Chirchiq, Ferganaazot, Ammofos-Maxam, Kungrad Soda Plant, Shurtan and Ustyurt gas-chemical complexes — from raw material…

Key ideas

Recycling and catalyst
Countercurrent flow
Chemical enterprises of Uzbekistan

Formulas

N_{2} + 3 H_{2} ⇌_{\underset{}{t, p}}^{\overset{}{Fe}} 2 NH_{3} Δ H = - 92 kJ

Ammonia synthesis is a classic example of industrial principles: catalyst, optimum temperature and pressure, recycling, heat exchange.

X_{overall} = \frac{α}{α + β - α β}

Recycling: even if the conversion per pass α is small, the overall conversion is high because the unreacted gas is returned (β is the fraction purged to stop impurities building up).

ε_{counter-current} = \frac{N T U}{1 + N T U} > ε_{co-current} = \frac{1 - e^{- 2 N T U}}{2}

Effectiveness of a heat exchanger: in counter-current flow a larger share of the heat is recovered (in co-current flow at most 50 %).

2 NH_{3} + CO_{2} \overset{}{\to} CO (NH_{2})_{2} + H_{2} O

Urea synthesis: the CO₂ “waste” of the ammonia plant becomes a raw material.

Atom economy = \frac{M (product)}{\sum M (reactants)} \cdot 100 %

A green chemistry measure: what fraction of the reactant atoms ends up in the desired product.

Theory

From chemical reaction to plant (§ 41)

Chemical production is a chemical-technological system consisting of the stages of preparing the raw materials, the chemical process in the reactor, separating and purifying the product, and neutralising the waste. When a reaction is chosen, its rate and its equilibrium are considered together. The main scientific principles are: 1) optimum conditions — temperature, pressure and catalyst are chosen as a compromise between rate and yield; 2) continuous process — the equipment works day and night in a steady regime, which makes automation easy; 3) counter-current flow — the substances (gas and liquid, hot and cold streams) move towards each other: in absorption towers, in drying, in heat exchangers — so the difference stays large along the whole path; 4) heat exchange (saving energy) — the heat of an exothermic reaction is used to warm the incoming raw material; 5) recycling — unreacted substances are separated and sent back to the reactor; 6) complex use of raw materials and waste-free technology. Measures: conversion X (what fraction of the reactant has reacted), selectivity S (what fraction of the reacted reactant became the desired product), yield Φ = X·S, productivity (product per unit time).

The chemical industry of Uzbekistan (map)

Nitrogen fertilisers: Navoiyazot (a new complex since 2020 — 660 thousand t of ammonia and 577.5 thousand t of urea per year; PVC, caustic soda and methanol since 2019), Maxam-Chirchiq (since 1940; ammonia, ammonium nitrate, urea, ammonium sulfate), Fergana Azot. Phosphorus fertilisers: Ammofos-Maxam (Almalyk, 1969) — ammophos and superphosphate from Kyzylkum phosphorite. Potassium: Dehkanabad — KCl from sylvinite. Soda: the Kungrad soda plant (2006; 200 thousand t per year). Gas chemicals: Shurtan GCC (2001; design capacity 125 thousand t of polyethylene), Ustyurt GCC (2016; 387 thousand t of polyethylene and 83 thousand t of polypropylene), Uzbekistan GTL (2021; ≈ 1.5 million t of synthetic fuel from 3.6 billion m³ of gas). Gas and oil processing: the Mubarek gas processing plant (1971; sulfur from sour gas), the Fergana (1959) and Bukhara (1997) oil refineries. The production of ammonia, sulfuric acid and soda is covered in detail in the “Reversible reactions and equilibrium”, “Sulfuric acid and the contact process” and “Alkali metals and soda” experiments.

The ammonia synthesis example: the model numbers

$N_{2} + 3 H_{2} \overset{}{⇌} 2 NH_{3}$ is exothermic and the number of moles of gas decreases: low temperature and high pressure favour equilibrium (Le Chatelier). But at low temperature the reaction is very slow, so an iron catalyst (promoted with K₂O and Al₂O₃) and a compromise of 400–500 °C and 15–30 MPa (150–300 atm) are chosen. Without a catalyst the reaction practically does not proceed even at 450 °C. In a single pass only 15–25 % of the gas is converted to ammonia (equilibrium is not reached): the ammonia is cooled, liquefied and removed, and the remaining N₂ and H₂ are returned by a recycle compressor; a small part (1.5 % in this model) is vented so that inert gases (Ar, CH₄) do not build up — this raises the overall conversion above 95 %. The hot gas leaving the reactor warms the incoming cold gas in a heat exchanger: in counter-current flow no external heating is needed (an autothermal process), but in co-current flow it is not enough.

Production of organic substances (Grade 10, 4.1)

Polyethylene: ethane is dehydrogenated at high temperature (≈ 800 °C) to give ethylene — $C_{2} H_{6} \overset{}{\to} C_{2} H_{4} + H_{2}$ , and ethylene polymerises on a catalyst: $n C H_{2} = C H_{2} \to {(- C H_{2} - C H_{2} -)}_{n}$ — Shurtan and Ustyurt GCC. PVC: vinyl chloride is made from acetylene ( $C_{2} H_{2} + HCl \overset{}{\to} C_{2} H_{3} Cl$ ) or from ethylene (chlorination and removal of HCl), and is then polymerised; the chlorine comes from the electrolysis of table salt. Urea: $2 NH_{3} + CO_{2} \overset{}{\to} CO (NH_{2})_{2} + H_{2} O$ — it uses the CO₂ formed when hydrogen is made at the ammonia plant. Methanol: from synthesis gas, $CO + 2 H_{2} \overset{}{\to} CH_{3} OH$ . Synthetic fuel (GTL): methane is converted to synthesis gas and then, in Fischer–Tropsch synthesis, to long alkanes: $n CO + (2 n + 1) H_{2} \to C_{n} H_{2 n + 2} + n H_{2} O$ . Mass calculation: the mole ratio in the reaction equation gives the consumption of raw material; for example, 1 mol (60 g) of urea needs 2 mol (34 g) of ammonia.

Prospects of chemical production (§ 38)

Green chemistry (P. Anastas and J. Warner, 1998, 12 principles): preventing waste rather than cleaning it up afterwards; reactions with high atom economy (addition and polymerisation — 100 %); catalysts; safer solvents; saving energy; renewable raw materials; biodegradable products. Waste-free and low-waste technology: the waste of one workshop is the raw material of another — at Mubarek toxic H₂S is turned into sulfur (the Claus process), sulfuric acid is made from the SO₂ in metallurgical gases, and the CO₂ of the ammonia workshop is used for urea. Deep processing of natural gas (polymers, synthetic fuel, fertilisers) creates added value compared with selling it raw. Safety and the environment: ammonia, chlorine and H₂S are toxic; constant monitoring, treatment of waste water and absorption of waste gases are compulsory at plants.

About the model

The map is schematic: the positions of plants and fields are approximate. Capacities and years come from data of the plants, the government and international industry publications (2015–2025) and may have changed since. Ammonia model: pressure is shown in MPa and converted to atmospheres inside the calculation (1 atm = 101.325 kPa); the equilibrium uses the Gillespie–Beattie correlation (ideal gas; at 20 MPa the real yield is a few per cent higher), the kinetics are schematic (approach to equilibrium in τ = 2 s with a catalyst), a single adiabatic bed (in a real column the gas is cooled between catalyst beds and the temperature is kept below ≈ 530 °C, whereas in the model it can exceed 600 °C at the outlet), constant heat capacity of the gas; heat exchanger — equal flows, NTU = 3. The plant model is not to scale. The atom economy is calculated from molar masses.

Did you know?

The Ustyurt Gas Chemical Complex started up in 2016: it is designed to process 4.5 billion m³ of gas a year and to make 387 thousand t of polyethylene and 83 thousand t of polypropylene.
The “Uzbekistan GTL” plant started up in December 2021: from 3.6 billion m³ of natural gas a year it makes ≈ 1.5 million t of synthetic fuel by the Fischer–Tropsch method — diesel, jet fuel, naphtha and liquefied petroleum gas.

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

Reaction equations — Grade 7
Combustion, flames and oxides — Grade 7
Rate of reaction and catalyst — Grade 8
Reversible reactions and equilibrium — Grade 8