uo unit 1.pdf unit operations food engineering

Unit-1
Introduction to Unit Operations: Fundamental Concepts
Unit operation:
The study of process engineering is an attempt to combine all forms of physical processing into a
small number of basic operations, which are called unit operations. for example, the baking of
bread, the freezing of meat, the tempering of oils.
Or
Unit operations in food processing refer to the fundamental physical processes that involve a
change in the properties of food materials. These are standardized steps used to transform raw
materials into finished products, each focused on achieving a specific change, such as mixing,
heating, or separating.
Unit operations are categorized based on the type of process, such as:
Mechanical (e.g., size reduction, mixing, separation)
Thermal (e.g., heating, cooling, drying)
Mass transfer (e.g., evaporation, filtration, distillation)
Examples of Unit Operations in Food Processing:
Mixing: Combining ingredients to form a uniform mixture.
Pasteurization: Heat treatment to kill harmful microorganisms.
Drying: Removal of water to extend shelf life.
Freezing: Lowering temperature to preserve food.
Separation: Removal of unwanted components (e.g., solid-liquid separation).
Conservation of Mass and Energy
The law of conservation of mass states that mass can neither be created nor destroyed. Thus
in a processing plant, the total mass of material entering the plant must equal the total mass
of material leaving the plant, less any accumulation left in the plant. If there is no
accumulation, then the simple rule holds that "what goes in must come out".
For example, when milk is being fed into a centrifuge to separate it into skim milk and
cream, under the law of conservation of mass the total number of kilograms of material
(milk) entering the centrifuge per minute must equal the total number of kilograms of
material (skim milk and cream) that leave the centrifuge per minute.
The law of conservation of energy states that energy can neither be created nor destroyed.
The total energy in the materials entering the processing plant plus the energy added in the
plant must equal the total energy leaving the plant.

This is a more complex concept than the conservation of mass, as energy can take various
forms such as kinetic energy, potential energy, heat energy, chemical energy, electrical
energy and so on.
During processing, some of these forms of energy can be converted from one to another.
Mechanical energy in a fluid can be converted through friction into heat energy. Chemical
energy in food is converted by the human body into mechanical energy.
For example, consider the pasteurizing process for milk, in which milk is pumped through
a heat exchanger and is first heated and then cooled. The energy can be considered either
over the whole plant or only as it affects the milk. For total plant energy, the balance must
include: the conversion in the pump of electrical energy to kinetic and heat energy, the
kinetic and potential energies of the milk entering and leaving the plant and the various
kinds of energy in the heating and cooling sections, as well as the exiting heat, kinetic and
potential energies.
In milk pasteurization, milk flows through a machine called a heat exchanger where it is
heated to kill bacteria and then cooled to a safe temperature. Here's how energy works in
this process:
1. Energy In:
o Electrical Energy: The pump uses electricity to move the milk. This electrical energy is
partly converted into:
 Kinetic Energy: Helps the milk flow.
 Heat Energy: Slightly warms the milk due to friction.
2. Heating Section:
o The milk absorbs heat energy to raise its temperature for pasteurization.
3. Cooling Section:
o Heat is removed from the milk to cool it down. This energy is transferred to cooling
water or another medium.
4. Energy Out:
o The milk leaves the system with some remaining kinetic energy (still moving) and
potential energy (depends on its height in the system).
o Any heat left in the milk and the energy in the cooling system also exit.
1. Conservation of Mass:
The law of conservation of mass states that mass cannot be created or destroyed in a closed system;
it can only change form or be transferred. This principle applies to all food processing operations,
where the total mass of input must equal the total mass of output, including products, by-products,
and waste.
General Equation:
Input Mass = Output Mass + Accumulation

Example in Food Processing:
In mixing operations, the total mass of ingredients (flour, water, sugar, etc.) should equal
the mass of the final dough.
In evaporation, the initial mass of a liquid is equal to the mass of the concentrated product
plus the mass of the evaporated water.
2. Conservation of Energy:
The first law of thermodynamics, or the law of conservation of energy, states that energy
cannot be created or destroyed; it can only be converted from one form to another. In food
processing, energy is often transferred as heat or work.
Overall View of an Engineering Process
In any engineering process, raw materials are transformed into desired products through a series
of controlled steps called unit operations. The process involves inputs such as raw materials,
energy, labour, and equipment. Outputs include products, by-products, waste, and unused energy.
Control mechanisms ensure the process runs efficiently and effectively.
Key Components of an Engineering Process:
Inputs:
o Raw materials
o Energy (heat, electricity)
o Labour and equipment
Unit Operations:
o Individual steps where changes take place (mixing, heating, separating, etc.)
Outputs:
o Final products
o By-products
o Wastes
o Wasted energy
Control:
o Monitors and regulates energy and material flows to optimize the process.
Using a material balance and an energy balance, a food engineering process can be viewed
overall or as a series of units. Each unit is a unit operation. The unit operation can be
represented by a box as shown in Fig

In engineering, everything is based on measurements, which must be accurate and clear. When we
make a measurement, we are comparing something unknown with something known. For example,
weighing an object compares its weight with a standard weight, like one kilogram. The result is
expressed as a multiple of the known quantity, such as how many kilograms the object weighs.
A measurement consists of three parts:
1. Dimension: What is being measured (e.g., length, weight).
2. Unit: A standard quantity used for comparison (e.g., meter, kilogram).
3. Number: The ratio of the measured quantity to the standard unit (e.g., 1.18, which means
the length is 1.18 times the length of one meter).
For instance, if a rod is 1.18 meters long, we can break this down as:
Dimension: Length
Unit: Meter
Number: 1.18 (the length of the rod is 1.18 times the length of one meter)
Though it seems simple to say that the rod is 1.18 meters long, this statement is very important
because all engineering is built on measurements, which need to be understood and used correctly.
In engineering, we can express most everyday quantities using a small set of basic dimensions:
length, mass, time, and temperature. To make calculations easier, force is also considered a
dimension, even though force can be expressed using the other dimensions (since weight is a force,
which is mass times gravity).
These dimensions are represented by symbols:
Length = [L]
Mass = [M]
Time = [t]
Temperature = [T]
Force = [F]

These symbols are always written inside square brackets, which is the standard way to show
dimensions.
Here’s a simpler explanation with examples of dimensions:
Length = [L] Area = [L]² (because area is length × length) Volume = [L]³
(because volume is length × length × length)
Velocity = [L] / [t] (because velocity is distance traveled per unit of time)
Acceleration = [L] / [t]² (because acceleration is the change in velocity per time)
Pressure = [F] / [L]² (because pressure is force per unit area)
Density = [M] / [L]³ (because density is mass per unit volume)
Energy = [F] × [L] (because energy is force times distance)
Power = [F] × [L] / [t] (because power is energy per unit time)
In these examples:
[L] represents length,
[M] represents mass,
[t] represents time,
[F] represents force.
This shows how different physical quantities are related to these basic dimensions.
* Dimensional constants: Dimension less ratios:
I) Reynold's no. =
𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑜𝑟𝑐𝑒𝑠
𝑉𝑖𝑠𝑐𝑜𝑢𝑠 𝐹𝑜𝑟𝑐𝑒𝑠
=
ρ 𝐿2 𝑉2
µ 𝐿 𝑉
=
ρ L V
µ
=
𝐿 𝑉
𝜈
where, 𝛎 = µ /𝜌= kinematic viscosity
II) Froud no;
𝐺𝑟𝑎𝑣𝑖𝑡𝑦 𝐹𝑜𝑟𝑐𝑒𝑠
=
ρ 𝐿2 𝑉2
µ 𝐿3 𝑔
=
𝑉2
𝐿 𝑔
=
𝑉
√𝐿𝑔
III) Euler's no;
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
=
ρ 𝐿2 𝑉2
𝑃 𝐿2
=
ρ 𝑉2
𝑃
=
1
𝑁𝑒𝑤𝑡𝑜𝑛 𝑛𝑜.

(Euler no =
1
𝑁𝑒𝑤𝑡𝑜𝑛 𝑛𝑜.
)
IV) Mach no;
𝐸𝑙𝑎𝑠𝑡𝑖𝑐 𝐹𝑜𝑟𝑐𝑒𝑠
=
ρ 𝐿2 𝑉2
𝐸 𝐿2
=
ρ 𝑉2
𝐸
=
𝑉2
𝑣𝑠
2
V) Weber no;
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑇𝑒𝑛𝑠𝑖𝑜𝑛
=
𝐿2 𝑉2
𝜏 𝐿
=
ρ L 𝑉2
𝜏
Unit Consistency and Unit Conversion
Unit consistency means that the units used for measurements should come from the same
system. In this book, we use the SI system (International System of Units), which is the
standard for physical measurements worldwide.
While most countries use the SI system, which includes units like meters and kilograms, the
USA often uses feet and pounds instead. Using a consistent system helps avoid confusion and
makes calculations easier to understand and compare.
Let’s say you are given this data:
Flow rate: 1.3 liters per minute (L/min)
Time: 18.5 hours
To find the total amount of liquid that flowed, you need to multiply the flow rate by the time.
However, the flow rate is in minutes, and the time is in hours, so they don’t match. You need to
convert everything to the same unit.
Steps for Conversion and Calculation:
1. Convert time to minutes:
o There are 60 minutes in 1 hour.
o Multiply 18.5 hours by 60 to get the total time in minutes:
18.5 hours×60=1110 minutes18.5 , text{hours} times 60 = 1110 ,
text{minutes}18.5hours×60=1110minutes
2. Calculate the total flow:
o Now that both units are in minutes, multiply the flow rate by the total time:
Total flow=1.3 L/min×1110 min=1443 liters

EXAMPLE 1.1 Velocity of flow of milk in a pipe.
Milk is flowing through a full pipe whose diameter is known to be 1.8 cm The only measure
available is a tank calibrated in cubic feet, and it is found that it takes 1 h to fill 12.4 ft3. What is
the velocity of flow of the liquid in the pipe in SI units?

EXAMPLE 1.2. Conversion of grams to pounds
Convert 10 grams into pounds.
EXAMPLE 1.3
Thermal conductivity of aluminium: conversion from fps to SI units. The thermal conductivity of
aluminium is given as 120 Btu ft-l
h-l
°F-l
. Calculate this thermal conductivity in J m-l
S-l
°C

What Are Dimensionless Ratios?
A dimensionless ratio is just a comparison between two quantities where the units cancel out,
leaving only a number without any units. This makes it easy to compare things because you don’t
need to worry about the units.
Example 1: Speed of a Car
When someone says a car is going "twice the speed limit", we are dealing with a dimensionless
ratio. This is because we are comparing the car's speed to the speed limit, but the units (like
kilometers per hour or miles per hour) cancel out in the ratio. You’re just saying the car is 2 times
the speed limit, which is a pure number.
Why Use Dimensionless Ratios?
1. Simplify Comparisons:
o It’s easier to understand and compare quantities when expressed as ratios. For
example, saying a car is going at "twice the speed limit" is simple and easy to
grasp, without needing to think about the units.
2. Used in Engineering:
o In process engineering, dimensionless ratios are very helpful. They allow
engineers to compare an unknown material or factor to a well-known one, making
it easier to understand and analyze without worrying about units.

Example 2: Specific Gravity
Another common example is specific gravity, which is used to compare the density of different
substances. It is defined as the ratio of the weight of a substance to the weight of an equal volume
of water.
For example:
If a substance has a specific gravity of 2, it means the substance is twice as heavy as an
equal volume of water.
If the specific gravity is less than 1, the substance is lighter than water and will float.
If the specific gravity is greater than 1, the substance is heavier than water and will sink.
Since the units of weight cancel out in the ratio (weight is measured in the same units for both
water and the substance), specific gravity is a dimensionless ratio.
In Simple Words:
A dimensionless ratio is just a comparison between two things that cancels out the units,
leaving a number.
They help us compare things quickly and easily.
Engineers use them to compare different materials, factors, or systems without worrying
about units like meters, seconds, or kilograms.
What is Evaporation?
Evaporation is the process where liquid changes into a gas or vapor at the surface of the liquid,
typically when it’s heated. In evaporation, the liquid molecules at the surface gain enough energy
to break free and enter the gas phase.
Example: Water boiling on a stove.
Liquid Characteristics Important in Evaporation:
When evaporating a liquid, several characteristics influence how efficiently it happens:
1. Boiling Point: The temperature at which a liquid turns into a gas.
2. Heat of Vaporization: The amount of heat needed to convert a unit mass of liquid to vapor
at constant temperature.
3. Viscosity: How thick or sticky the liquid is. Higher viscosity liquids are harder to
evaporate.
4. Surface Area: A larger surface area allows more liquid to be exposed to the environment
for evaporation.

What is a Single-Effect Evaporator?
A Single-Effect Evaporator is a device used to heat a liquid and turn part of it into vapor (gas).
The process happens in one stage or "effect," hence the name single-effect.
How Does It Work?
1. Feed Liquid:
The liquid (for example, water or juice) enters the evaporator from the top. This is called
the feed.
2. Heat the Liquid:
Steam (hot vapor) is pumped into the evaporator to heat the liquid. The heat makes part
of the liquid turn into vapor (gas).
3. Vapor and Liquid Separation:
o The vapor rises to the top and leaves the evaporator.
o The remaining liquid (now more concentrated because some of it turned into vapor)
exits the evaporator from the bottom.
4. Output:
o The vapor that comes out is collected and can be used for other purposes (like heating or
cooling).
o The concentrated liquid is removed and is much thicker than when it entered.
A Single-Effect Evaporator heats a liquid using steam, turns part of it into vapor, and
leaves the rest as a concentrated liquid. This happens in one step.

Multiple Effect Evaporation (MEE):
MEE is a process that improves energy efficiency by using the vapor from one evaporator (effect)
to heat the next one. This reduces steam consumption and operational costs. It is widely used in
industries like food processing, sugar, and chemicals.

uo unit 1.pdf unit operations food engineering

Performance of Evaporators:
When using an evaporator, several key factors affect its efficiency and performance. These
factors are
Boiling Point Elevation (BPE):
 What it is: When a solution contains dissolved substances (like salt), its boiling point
rises.
 Why it matters:
o A higher boiling point means more heat is needed to evaporate the liquid.
o This affects the efficiency of heat transfer.

o Design Impact: Engineers must consider BPE to ensure enough heat is supplied for
evaporation.
2. Capacity:
What it is: The amount of vapor produced per hour (measured in kg/hr).
What affects it:
o Size of the heat transfer area.
o The temperature difference between the heating steam and the liquid.
o Characteristics of the liquid (e.g., viscosity).
Why it matters: A higher capacity means the evaporator can process more liquid in less
time
3. Economy:
What it is: The ratio of water evaporated to steam used.
Single Effect Evaporator: Economy is close to 1 (1 kg of water evaporated per 1 kg of
steam).
Multiple Effect Evaporator (MEE): Economy improves significantly because steam is
reused.
o Example: A three-effect evaporator can evaporate 3 kg of water with the same 1 kg of
steam, giving an economy of 3.
Why it matters: Higher economy means lower energy costs and better efficiency.
4. Heat Balance:
What it is: Ensures that all heat entering the system (through steam) is accounted for.
Components of heat balance:
o Heat used to evaporate the liquid.
o Heat losses (e.g., to the surroundings).
Why it matters: A proper heat balance ensures energy is not wasted and helps optimize
the process.
Types of Evaporators:
Evaporators come in different designs depending on how the liquid flows through them. Here are
two common types:
 How it works:
The liquid flows through the evaporator only once and exits.
 When to use:
Ideal for heat-sensitive liquids (like milk) that can get damaged by too much heat.
Works well with non-scaling liquids (liquids that don’t leave deposits).
 Example:
Used in processes where gentle heating is required.

Non-Scaling Liquids
Non-scaling liquids are liquids that do not leave solid deposits (scale) on the surfaces of
equipment, like evaporators or heat exchangers, during heating or evaporation.
Examples of Non-Scaling Liquids:
Distilled water
Clean solvents
Liquids with low mineral content
Circulation Evaporators:
How it works:
o The liquid circulates multiple times through the evaporator before exiting.
o This allows for better heat transfer and more efficient evaporation.
When to use:
o Suitable when higher evaporation rates are needed.
o Helps achieve more uniform heating of the liquid.
Example: Commonly used in sugar and chemical industries where thorough evaporation
is needed.
Specific Evaporator Designs:
Simple Types of Evaporators:
1. Short Tube Evaporators:
o What they are: Have short vertical tubes.
o Where used: Common in industries like sugar production.
o Best for: Liquids that are thin or flow easily (low viscosity).
2. Long Tube Evaporators:
o What they are: Have long, vertical tubes.
o Why used: Good for liquids that need high heat transfer.
o Operation: Can handle liquids that move upward (rising film) or downward (falling
film).
3. Agitated Film Evaporators:
o What they are: Use a mechanical stirrer to spread the liquid into a thin layer on the
surface.
o Best for: Thick (viscous) or heat-sensitive liquids.
o Benefits: Prevents deposits (fouling) and allows faster evaporation.
Short Tube: Good for easy-flowing liquids.
Long Tube: Ideal for efficient heat transfer.
Agitated Film: Perfect for thick or delicate liquids.

uo unit 1.pdf unit operations food engineering

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