Unit Operation is not just a word but almost a world in process engineering & a wizard of process engineers. In this blog, you would know unit operation definition, its need & significance, industrial examples, classification & frequently asked questions. Every chemical engineer should at least know information shared in this post for good career in process engineering. After reading this post, you would be having detail information about unit operation.
Table of Contents
What Is Unit Operation
Every unique substance in this universe is combination of different chemical constituents. As each material has different chemical constituent mixture &/or proportion, it shows different physical & chemical properties. As per the human need, we use natural material or make new material from natural ones. Hence, we need to perform physical, chemical or both changes on the materials so as to get desired product. The physical changes done to materials keeping it chemically consistent comes under Unit Operation.
In simpler terms, Unit Operation in process industry are the processes which involves physical changes in materials to achieve transportation of materials, separation of material into its constituent, change dimensions / state or prepare newer materials without altering any chemical properties of the material.
Classification of Unit Operation
As shown in figure, there are four types of unit operations in chemical engineering.
1) Heat Transfer Operation
2) Mass Transfer Operation
3) Fluid Flow Operation
4) Mechanical Operation
Let's understand each one by one-
1) Heat Transfer Operation-
It deals with the processes wherein thermal energy transfer takes place. The thermal energy transfer is due to temperature difference predominantly.
Conduction, convection, evaporation, condensation processes comes under it.
2) Mass Transfer Operation-
It is the operation which deals with the separation of the species on the basis of chemical potential difference. It is very important because it is applied to remove unwanted species present & get the desired product.
Distillation, Absorption, Humidification, Leaching, Extraction, Adsorption, Drying, Crystallization are processes of Mass Transfer.
3) Fluid Flow Operation-
As name suggests, Fluid flow operation is related to fluid flow. The fluid name is collectively given to liquid, gaseous state of matter. Here, the driving force to move the fluid is pressure difference.
Pumping, Compression, Fluidization are Fluid flow operation processes.
4) Mechanical Operation-
Mechanical Operation deals with all about solid size reduction, transportation, handling & storage. It is the operation that focuses only on solid state of matter. Many a times, it is grouped with Fluid flow operation.
Processes include- Size reduction, Mixing, Filtration, Screening, Sedimentation.
| Type of operation | Primary Mechanism | Examples | Industrial Applications |
|---|---|---|---|
| Fluid Flow Operations | pressure difference | Pumping, Compression, Fluidization | Oil refining, Gas processing, Pneumatic conveying |
| Mechanical Operations | Mechanical forces | Size reduction, Mixing, Filtration, Screening, Sedimentation | Mining, Food processing, Waste treatment |
| Heat Transfer Operations | Thermal Energy transfer primarily due to temperature difference |
Conduction, Convection, Evaporation, Condensation | Distillation plants, Dairy processing, Chemical manufacturing |
| Mass Transfer Operations | Species transfer due to chemical potential difference |
Distillation, Absorption, Extraction, Adsorption, Drying, Crystallization | Pharmaceutical purification, Water treatment, Petrochemicals |
Unit Operation Everyday Example
You might be thinking that all these processes explained would be happening industrially only. It's is not true!
Chemical Engineering is an amazing field where you can see all the things that you read in your daily life. Let's take an example-
Lemonade
I am sure that you might have taken lemonade in summers to be hydrated but do you know that multiple unit operations are involved in making of the lemon juice? Let's see the complete lemonade making journey-
Step 1: Lemon Selection & Washing 🍋
Picking fresh lemons and washing them in water.
- Unit Operation Involved: Washing/Rinsing (Mechanical Operation)
- Purpose: Remove dirt, pesticides, and contaminants from the lemon surface
- Engineering Principle: Mechanical separation and cleaning using liquid flow
Step 2: Cutting Lemons into Pieces ✂️
Slicing lemons into halves or quarters.
- Unit Operation Involved: Size Reduction (Mechanical Operation)
- Purpose: Increase surface area to enhance juice extraction
- Engineering Principle: Mechanical breakdown to increase contact area for better mass transfer
Step 3: Squeezing Lemons
Applying mechanical pressure to extract juice.
- Unit Operation Involved: Mechanical Extraction/Pressing (Mechanical Operation)
- Purpose: Separate liquid juice from solid pulp and skin
- Engineering Principle: Applying pressure to extract liquid
Step 4: Filtering Pulp & Seeds 🍲
Pouring juice through a strainer to remove pulp and seeds.
- Unit Operation Involved: Filtration (Mechanical Operation)
- Purpose: Separate solid particles (pulp, seeds) from liquid juice
- Engineering Principle: Using a porous medium to separate solids from liquids
Step 5: Mixing Sugar & Water 🥄
Adding sugar to water and stirring well until dissolved.
- Unit Operation Involved: Mixing (Mechanical Operation) + Mass Transfer
- Purpose: Achieve uniform distribution of sugar throughout water; sugar dissolves into solution
- Engineering Principle: Mechanical mixing creates homogeneous mixture; sugar molecules disperse uniformly
Step 6: Cooling the Mixture ❄️
Letting the lemonade cool below room temperature, then refrigerate with ice.
- Unit Operation Involved: Heat Transfer (Heat Transfer Operation) - Cooling/Refrigeration
- Purpose: Reduce temperature for better palatability
- Engineering Principle: Heat removal through convection and conduction
Step 7: Adding Carbonation (Optional) 💨
Adding sparkling water or use a carbonator to introduce CO₂ bubbles.
- Unit Operation Involved: Mass Transfer (Gas-Liquid Absorption)
- Purpose: Dissolve CO₂ gas into liquid to create fizzy sensation
- Engineering Principle: Gas dissolves in liquid under pressure (Henry's Law)
And TA-DA!...You're lemonade is ready 🥛
Summary Table for Lemonade
| Step | Action | Category | Purpose | Engineering Principle |
|---|---|---|---|---|
| 1 | Washing | Mechanical | Remove contaminants | Liquid-solid separation |
| 2 | Size reduction | Mechanical | Increase surface area | Particle size reduction |
| 3 | Pressing/Extraction | Mechanical | Extract juice | Liquid-solid separation |
| 4 | Filtration | Mechanical | Remove pulp & seeds | Solid-liquid separation |
| 5 | Mixing | Mechanical + Mass Transfer | Distribute sugar uniformly | Homogenization |
| 6 | Cooling | Heat Transfer | Reduce temperature | Heat removal |
| 7 | Carbonation | Mass Transfer | Dissolve CO₂ | Gas absorption |
Unit Operation Industrial Example
After understanding day-to-day example, let's undarstand an Industrial example. This will give you industrial view point of how things work.
Sugar Manufacturing from Sugarcane
We use sugar on daily basis but hardly think about how it is creation of process engineers.
Step 1: Cane Cleaning & Preparation
Harvested sugarcane is washed and cut into smaller pieces.
- Unit Operation Involved: Washing + Size Reduction (Mechanical Operation)
- Purpose: Remove dirt and prepare cane for juice extraction
- Engineering Principle: Mechanical breakdown increases surface area for better extraction
Step 2: Juice Extraction (Pressing)
Sugarcane is squeezed through heavy rollers to extract the juice.
- Unit Operation Involved: Mechanical Extraction/Pressing (Mechanical Operation)
- Purpose: Separate liquid sugar juice from solid fiber (bagasse)
- Engineering Principle: Applying pressure separates liquid from solid
Step 3: Juice Clarification (Impurities removal)
Raw juice is heated and treated to remove particles and impurities.
- Unit Operation Involved: Heat Transfer + Settling + Filtration (Mechanical Operation)
- Purpose: Produce clear juice for crystallization
- Action:
- Heating: Juice is heated to denature unwanted proteins & helps to coagulate impurities
- Liming: Lime (calcium hydroxide) added to make juice alkaline & precipitate impurities.
- Settling: Juice flows into large tanks to settle heavy particles
- Filtration: Final polishing through filters to remove odor & color
- Engineering Principle: Multiple stages progressively remove different types of impurities
- Result: Brown raw juice is now light golden clear juice
Step 4: Juice Concentration
Water is evaporated from juice in multiple stages to concentrate the sugar.
- Unit Operation Involved: Evaporation (Heat Transfer Operation)
- Purpose: Remove most of water; concentrate sugar
Step 5: Cooling & Crystallization 🧊
Hot concentrated syrup is gradually cooled, and sugar crystals begin to form and grow.
- Unit Operation Involved: Crystallization ( Mass Transfer Operation)
- Purpose: Convert dissolved sugar into solid sugar crystals.
- Engineering Principle: Supersaturation + slow cooling + agitation = perfect crystals
Step 6: Centrifugal Separation
Sugar crystals and molasses are separated using centrifugal force.
- Unit Operation Involved: Centrifugation (Mechanical Operation)
- Purpose: Separate solid sugar crystals from liquid molasses
- Engineering Principle: Centrifugal force separates components by density
Step 7: Drying
Remaining moisture is evaporated from sugar crystals.
- Unit Operation Involved: Drying (Mass Transfer Operation)
- Purpose: Remove final remaining moisture; prevent clumping in storage
- Engineering Principle: Heat and air flow remove moisture through evaporation
Step 8: Cooling to Room Temperature
What happens: Dried hot sugar is cooled before packaging.
- Unit Operation Involved: Heat Transfer (Heat Transfer Operation) - Cooling
- Purpose: Prevent moisture absorption from air during storage
- Engineering Principle: Heat removal via conduction and convection
- Why This Matters: Hot sugar absorbs moisture from air; cool sugar is stable
Technically application of chemical engineering is over here, the product goes for quality testing & upon positive results, sugar is packed, stored, distributed & then comes to our house.
Summary Table for sugar crystals from sugarcane
| Step | Action | Category | Purpose | Equipment |
|---|---|---|---|---|
| 1 | Cleaning & Size reduction | Mechanical | Prepare for extraction | Washers, shredders |
| 2 | Juice Extraction | Pressing (Mechanical) | Extract liquid from solid | Roller mills |
| 3 | Clarification | Heat + Settling + Filtration | Remove impurities | Heaters, settling tanks, filters |
| 4 | Evaporation | Heat Transfer | Concentrate sugar | Multi-effect evaporators |
| 5 | Crystallization | Heat Transfer + Mass Transfer | Form sugar crystals | Crystallizers with cooling coils |
| 6 | Centrifugation | Mechanical Separation | Separate crystals from molasses | Industrial centrifuges |
| 7 | Drying | Mass Transfer | Remove final moisture | Rotary dryer |
| 8 | Cooling | Heat Transfer | Attaining room temperature | Air coolers |
Now I am sure that you would have understood the subject in detail. But you might have some doubts, let me try to answer them in FAQ (Frequently Asked Questions). If you have any questions other than FAQs then ask me in comments section.
FAQ (Frequently Asked Questions)
1. Is the distillation unit process or unit operation?
Answer- Distillation is a part of unit operation. Distillation does not involve chemical changes & it is meant to separate the fluids based upon a physical property i.e relative volatility. Hence, distillation is a unit operation, not unit process.
2. Is heating a unit operation?
Answer- Yes, heating is a unit operation. Heating involves transfer of thermal energy. Hence, it becomes part of heat transfer operation.
3. Is filtration a physical or chemical change?
Answer- Filtration is a physical change. Filtration is a separation process which involves physical separation based upon the particle size i.e heavy size particles retain on filter & less size particles permeates through the filter. The heavy & light particle is seen relative to filter pore size.
4. What is unit operation according to pharmaceutical engineering?
Answer- Pharmaceutical includes size reduction , sieving, mixing, drying, filtration, centrifugation, crystallization, distillation, evaporation and various types of heat and mass transfer processes as unit operation.
5. What is a unit operation in the food industry?
Answer- Food industry include a wide range of unit operations like cleaning, sorting, size reduction (cutting, milling), mixing, heat treatment like pasteurization, sterilization, cooking , separation like filtration, centrifugation, sieving, for concentration-evaporation is performed, drying, cooling, freezing & many more.