# Unit operations: the building blocks of any process

> Any transformation process, however complex, breaks down into a small number of standard steps: heat, separate, react, move. Recognising them lets you read any plant.

- Source : https://industryhub.site/en/learn/process/operations-unitaires
- Track : Process
- Level : Discovery
- Reading time : 6 min
- Language : English

## In brief

A unit operation is an elementary physical or chemical step — heat exchange, distillation, filtration, reaction, pumping. Any process reduces to a chain of these blocks, each with its own physics and sizing rules.

## Break it down to understand it

A process plant can look impenetrable: hundreds of pipes, vessels and pumps. But behind the apparent disorder lies a simple idea, set out by chemical engineering a century ago: **any process reduces to a chain of a small number of standard steps**, called unit operations. Whether you make gasoline, milk powder or a drug, you find the same blocks, assembled differently.

## Heat transfer: heating and cooling

Many steps are about adding or removing heat. You heat to react, to evaporate, to sterilise; you cool to condense, to stabilise, to store. The typical equipment is the **heat exchanger** (two fluids separated by a wall), the **furnace**, the **condenser** and the **evaporator**. The thermal power to supply comes from the sensible-heat balance:

$$\dot{Q} = \dot{m}\,c_p\,\Delta T$$

For example, heating **10 t/h** of water ($\dot{m} = 2.78$ kg/s, $c_p = 4.18$ kJ/kg·K) from 20 to 80 °C requires $\dot{Q} = 2.78 \times 4.18 \times 60 \approx \mathbf{700\ kW}$. Energy efficiency is the key game here: recovering heat from a hot outgoing stream to preheat an incoming one is one of the first sources of savings in a plant.

## Separation: sorting the constituents

Splitting a mixture into its components is probably the richest family. **Distillation** separates by boiling point — it is the heart of a refinery. **Filtration** and **settling** separate a solid from a liquid. **Absorption** captures a gas into a liquid, **drying** removes water, **extraction** isolates a compound using a solvent. These operations consume most of the energy of a chemical process — distillation alone can account for **40% and more** of a refinery's energy spend.

## Reaction: transforming matter

The **reactor** is the chemical heart: this is where molecules recombine into the desired product. Temperature, pressure, residence time and mixing are controlled there to reach the right yield without runaway. It is also the most safety-sensitive operation: an exothermic reaction that runs away is a classic scenario for a [HAZOP](/en/standards/iec-61511) risk study.

## Moving fluids

Nothing happens without moving the material. **Pumps** push liquids, **compressors** push gases, **valves** dose and route the flows. Sizing a control valve goes through its flow coefficient — that is what the [Cv/Kv calculation](/en/tools/process/valve-cv-kv) is for. This "plumbing" connects all the other operations together.

## Why this decomposition is powerful

Each unit operation has its own physics, equations and sizing rules, independent of the product being made. An engineer who masters distillation can size it just as well for oil as for alcohol. That is what makes process engineering universal: you do not relearn the whole plant each time, you assemble known blocks. Reading a [P&ID](/en/learn/process/lire-un-pid) is precisely about recognising these blocks and how they chain together.