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Unit Operations in Chemical Engineering

by Chemical Engineering Guy

Introduction

S1.

Introduction

Overview

Objectives

Objectives

  • Understand the importance of Unit Operations in the Chemical Industry
  • Identify relevant Unit Operations in Process Flow Diagrams
  • Get to know the basic concepts behind Unit Operations
  • Recognize typical Equipment Design & Operation
  • Pressure Changers: Pumping, piping, fittings, compressing, etc...
  • Heat Exchange: heaters, coolers, condensers, boilers
  • Separation Processes: flashing, distillation, absorbers, fractionation columns
  • Reactors: Batch, Stirred Tank, Plug Flow, etc...

What is a Unit Operation?

In chemical engineering and related fields, a unit operation is a basic step in a process.

Unit Operation

a physical change to which material is subjected especially in coordination with a unit process (as filtration, distillation, or extraction)

Main Types

Types

  • Fluid flow processes, including fluids transportation, filtration, and solids fluidization.
  • Heat transfer processes, including evaporation and heat exchange.
  • Mass transfer processes, including gas absorption, distillation, extraction, adsorption, and drying.
  • Thermodynamic processes, including gas liquefaction, and refrigeration.
  • Mechanical processes, including solids transportation, crushing and pulverization, and screening and sieving.

Examples

Diagrams

Momentum Operations

Pressure Changers...

S2.

Momentum Operations

Piping & Fittings

Piping

&

Fittings

Pipes

Description:

  • Used to transport fluids
  • Typically made of Steel
  • Several Sizing
  • Drop in pressure due to friction
  • Isothermal & Adiabatic

Piping

General

Sizes & Schedule

Schedule 40

Schedule 80

Inner/Outside Diameter changes!

Materials

  • Steel
  • Copper
  • Plastic (PVC)
  • Ceramic

Maintenance

Fouling

Rusting

Bending

Leaking

Photos

Models

  • Depend on property of fluid
  • Typically --> will yield drop in pressure
  • Pressure requirements

Model

Liquid

  • Liquid drop pressure due to friction
  • Laminar/Turbulent Flow
  • Reynolds Number (viscosity, density, velocity, diameter)
  • Energy loss --> Pressure drop
  • Roughness of material

Liquids

Friction Factor

Friction Factor + Moodys Chart

Gas/Vapor

Typically treated as "LONG" nozzles

  • isothermal
  • adiabatic
  • isentropic

Avoid flashing

Gases

Solid Transport

Either "fluid" or "semi fluid" or discrete particle

Solids

P&ID

Symbol

P&ID Symbol

Just "arrow" or "lines"

Bibliography

Fittings&Valves

Fittings

Description:

  • Used to transport fluids
  • Typically made of Steel
  • Several functions
  • Larger drop in pressure due to friction

General

Fittings

Valves

Tees/Elbows

Valves

  • Gate
  • Butterfly
  • Ball
  • Globe

Modeling

Math Model

  • Typically based on an experimentl loss of pressure
  • Related via velocity
  • Based on "equivalent" length

P&ID

Symbol

P&ID

Bibliography

Fluid Metering

Used to measure flowrates

Fluid Metering

General

  • Used to measure flow rate
  • Typically, based on velocity or pressure drops

General

Types

  • Venturi Tube
  • Orifice Plates

Model

Venturi

Tube

Venturi Tube

  • Gentle Pressure Drop
  • Pressure Drop relates to Velocity
  • Velocity Relates to Flow Rate

Orifice Plate

Orifice Plates

  • High Pressure Drop
  • High Operation Cost
  • Easy to install

P&ID

Symbol

Agitation & Mixing

Agitation = Designed Pattern

Mixing = Random Pattern

Agitation

&

Mixing

Agitation

Agitation

  • Promotes mass&heat transfer by design
  • Operations
  • Suspensions
  • Blending
  • Dispersion
  • Emulsifiers

Agitation Tank

  • Tank/Encloser
  • Impeller
  • Baffling
  • Shaft
  • Motor
  • Level

P&ID Symbol

Pumps

Used to:

  • Move Fluids
  • Increase Pressure

Pumps

Types

Positive Displacement

Gear

Screw

Lobe

Pumps

Types

Gear

Lobe

Screw

Archimides Pump/Screw

Screw

Gear

Lobe Pumps

P&ID

Symbol

Dynamic

Centrifugal

  • Axial
  • Radial

Kinetic

Axial

Direct Application of WORK

Increae in Pressure

Axial

Centrifugal/Radial

Centrifugal Force

Radial

Photos

Model

P&ID

Symbols

Cavitation

What is Cavitation?

  • Damage due to little bubbles
  • Recall that bubbles are gas, they are compressible
  • The impeller will experiment different Pressures/Forces

  • This is due to the pressure changes in the
  • Inlet (Suction)
  • Eye (impeller center)
  • Outlet (Discharge)
  • Recall that for a substance
  • If the Pressure decreases
  • The boiling temperature decreases

NSPH-R

NSPHr (Net Specific Pressure at Head Required)

  • Supplier will typically set it for the design
  • You may calculate/experiment it if not given
  • This is the limit pressure.
  • The min required pressure so it won’t cavitate

Pump

vs.

System Curve

System Curve

System HEAD

System HEAD

System Curve

System Head vs. Volumetric Flow Rate

  • Different Pipe Sizes

Valves

Effect of Valves

Pump Curve

Pump HEAD

Solving for "pump"

  • Suction
  • Discharge

Pump Curve Design

Different Pump Curves

  • Pump Design
  • Head vs. Flows

Pump Efficiency

  • Diameter
  • Head
  • Volumetric Flow
  • Efficiency
  • NPSH-R
  • Power

Pump Systems

Pump arrangement is crucial

Pumping Systems

Series

Pump in Series

  • Ideal when Pressure increase is needed
  • Adding a pump:
  • Will increase the pressure
  • Flow rate must remain the same
  • Series Pressure!
  • Pressure is NOT constant
  • Volumetric Flow IS constant

Parallel

Parallel Pumps

  • Ideal when Flow varies
  • When Adding a Pump:
  • Pressure is maintained
  • The system’s capacity is increased
  • Parallel Quantity! Flow Rate increases
  • Pressure will remain the same!

Compare

Fans&Blowers

Used to blow/fan air/gases

Fans

&

Blowers

Models

Models

  • Typically modelled as incompressible flow
  • No compression
  • Gas = Liquid

Photos

Fan & Blowers

P&ID

P&ID

Compressor

  • Increase Pressure in Gases
  • Move Gases

Compressors

General

Axial

Centrifugal

SBW make large process air compressors for 1.2 million ton per year PTA Unit have been running sucssefully more than 4 years.

Models

Types Compression

Isentropic

Polytropic

Isothermal

Min. Work

Isentropic Compression

  • A compressor can be idealized as internally reversible and adiabatic, thus an isentropic steady state device, meaning the change in entropy is 0.

  • By defining the compression cycle as isentropic, an ideal efficiency for the process can be attained, and the ideal compressor performance can be compared to the actual performance of the machine.

Polytorpic Compression

  • Polytropic - This model takes into account both a rise in temperature in the gas as well as some loss of energy (heat) to the compressor's components.

  • This assumes that heat may enter or leave the system, and that input shaft work can appear as both increased pressure (usually useful work) and increased temperature above adiabatic (usually losses due to cycle efficiency).

  • Compression efficiency is then the ratio of temperature rise at theoretical 100 percent (adiabatic) vs. actual (polytropic).

  • Polytropic compression will use a value of n between 0 (a constant-pressure process) and infinity (a constant volume process).

  • For the typical case where an effort is made to cool the gas compressed by an approximately adiabatic process, the value of n will be between 1 and k.

Isothermal Compression

Isothermal - This model assumes that the compressed gas remains at a constant temperature throughout the compression or expansion process.

Isothermal compression or expansion more closely models real life when the compressor has a large heat exchanging surface, a small gas volume, or a long time scale (i.e., a small power level).

Compressors that utilize inter-stage cooling between compression stages come closest to achieving perfect isothermal compression.

P&ID

Symbol

Fluidization

is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a fluid (liquid or gas) is passed up through the granular material.

Fluidized Beds

Main Applications

Applications

Fluidized

Adsorbtion

Fluidized Reactor

Fluidized Combustion

Fluidized Drying

P&ID

Symbol

Heat Exchange

S3.

Heat Operations

Heaters

Heaters

&

Coolers

Types of Heaters

Types

Shell & Tube

Plates

Spiral

Tubular (Double Tube)

Shell & Tube

Shell & Tube

Literally, tubes inside a shell

  • Hot Fluid
  • Cold Fluid

Multiple Tubes inside

Single Shell

Baffles

Parts

Arrangement

Configuration

Maintenance (1/2)

Maintenance (2/2)

Plates

Description

  • Plates Separate Flow
  • Indirect Contact

Design

Number of Plates

Size of Plates

Area of Exchange

Industrial Exchanger

Maintenance

Spiral

Spiral

The main advantage of the SHE is its highly efficient use of space.

This attribute is often leveraged and partially reallocated to gain other improvements in performance, according to well known tradeoffs in heat exchanger design.

A notable tradeoff is capital cost vs operating cost.

A compact SHE may be used to have a smaller footprint and thus lower all-around capital costs, or an oversized SHE may be used to have less pressure drop, less pumping energy, higher thermal efficiency, and lower energy costs.

Design

Maintenance

Tubular

Tubular Heaters

Pipe Inside Pipe

Small heat duties

Simple to construct

Industrial Heaters

Maintenance

Main Heat Equation

Qgain=-Qlost

Q = UAT

T= Logarithmic Tempearture change

Models

AREA

More Tubes / Coils / passes

Area

Co/Counter

Flow

Counter/Co Flow

  • Temperature Profile Changes
  • Efficiency might change

Temperature

The "REAL" Change

Logarithmic Mean Temperature Diff.

P&ID

Symbol

Condensers

Condenser

Function is to cool/condense

Similar as Heat Exchangers

- There is Condensate

- There must be latent heat consideraiton

Types

Types

Surface

Shell&Tube

Utilities

Common utilities

Cooling Water

Cooling Air

Specific Refrigerants:

Ammonia

P&ID

Symbol

Evaporator/Reboilers

Used to evaporate&boil material

  • High Energy requirements

Evaporators

Types

Evaporators

  • Batch
  • Natural/ Circulation (convection)
  • Falling/Rising Film
  • Kettle
  • Thermosyphone

Batch Evaporator

Feed -> Heat -> Boil -> Remove -> Clean

Forced Circulation Evaporator

Natural Circulation Evaporator

Falling Film Evaporators

Rising Film Evaporators

Kettle Reboiler

Thermosyphone

P&ID

Diagram

Furnace

A furnace is a device in which heat is generated and transferred to materials with the object of bringing about physical and chemical changes. The source of heat is usually combustion of solid, liquid or gaseous fuel, or electrical energy applied through resistance heating (Joule heating) or inductive heating.

Types

Types

Blast Furnace

Industrial Furnaces

P&ID

Symbol

Reference

Books

Mass Transfer Operations

S4.

Mass Operations

Liquid-Gas

Liq-Gas

Gas Dispersion

Gas Dispersion

  • Gas is dispersed in liquid as Foams
  • Interaction between Gas - Liq
  • Bubbles --> favors greater Area of exchange
  • Cocurrents vs. Countercurrent

Calculations & Design

  • Gas Bubble Diameter
  • Height of Column
  • Number of Stages / Amount of Packaging
  • Diameter of Tower
  • Holdup / Residence Time
  • Mass Transfer Coefficients
  • Bubble Size/Formation
  • Velocity & Flow Rates

Sparged Vessel

AKA Bubble Column

Tray Towers (1/2)

Towers used for Mass Transfer

  • Main interest is to form bubbles
  • Bubble-Liqui dinteraction favors transfer

Tray Towers (2/2)

Bubble Cap Trays

  • Pressure Drop
  • Flooding Control
  • Shell
  • Tray Type & Size and Spacing
  • Number of Trays / Stages

Sieve Trays

Pressure Drop

Flooding Control

Diameter

Tray Type & Size and Spacing

Number of Trays / Stages

Liquid Dispersion

Liquid Dispersion Operations

Overall operation is to disperse the liquid in gas.

Venturi Scrubber

Suspended solids presence

Considered as Single Stage Process

Spray Towers

Spray Nozzle --> Disperses Liquid

High Pumping Costs

Low dP

Wetted Wall Towers

  • Also called falling-film column
  • Transfer is between the liquid and Gas
  • Mostly countercurrent opeartion
  • Multiple Tube application
  • dP is very lowe

Packed Towers

  • Flooding
  • Packaging Sizing & Arrangement
  • Random vs. Ordered/Regular Packaging

Absorbtion

What is Absorbtion?

Operation in which gas mix is contacted with a liquid in order to dissolve components of the gas into the solution.

  • Cleaning Gases

Solubility of components in gases/liquids are analyzed

  • Selection (Volatility, Solubility, Cost, Corrosivness, Viscosity, etc.)

Topics: Henry Law, Ideal systems

Single Component (Ideal & Dilute) systems

Single Component (nonideal)

Multicomponent Component

Single Stage

1 Equilibrium Stage: L-G

Nontypical

Multiple Stage

  • Multiple Equilibrium Stages
  • More mass transfer

Sour Gas - Ammine System

Sour = high H2S content

Sweet = low H2S Content

Flashing

Flashing

Vapor-Liquid Equilibrium = 1-Stage

  • At least 2 components
  • Feed composition is fixed, T,P is given
  • Liquid / Vapor will set due to T/P
  • Different Compositions
  • Saturation!

T-XY diagrams or P-XY

Flash Drum

Typically:

  • P is assigned via Pressure (or atmospheric)
  • T is given by the Heater

Distillation

Distillation is the process of separating the components or substances from a liquid mixture by selective boiling and condensation.

Binary Distillation

Binary Distillation

Ponchon and Savarit Method

McCabe and Thiele Methods

  • Enriching vs. Stripping Sections
  • Feed Tray Poisition
  • Recycle Ratio
  • Min. Reflux Ratio
  • Optimum Reflux Ratio
  • Reboiler & Condenser Duties
  • Total/Partial Condenser
  • Number of Stages/Trays
  • Tray Efficiency

Tray towers

Models & Diagrams

HTU and NTU

Design (1/2)

Column heigh&diameter

Design (2/2)

Multi-Comp.

Distillation

Multicomponent

likely Software Modeled

Examples:

  • Naptha/Light Alkane Mix
  • Petroleum
  • Solvent Mix

Specific Applitactions and Rigurous Models

Light vs. High Key Components

Fractionation Column

Petroleum/Crude Oil Fractionation

P&ID

Symbols

Liq-Liq

Liq-Liq Extration

  • Pretty similar to Gas-Liquid Interaction
  • It exploits solubility of Component X into phases 1 and 2
  • Single & Multistages
  • aka Solvent Extraction
  • Distribution Coefficient!

Solvent Selection

  • General = Insolubility, Selectivity, Distribution Coefficient
  • Phys/Chem Props = ensity, Interfacial Tension, Chemical Reactivity, Viscosity, Vapor Pressure, Freezing point
  • Safety&Health = Toxicity, Flammability
  • Economic = Cost, Recoverability

Ternary Diagrams

DOF = C-P+2 = 3-1+2= 4

P/T Constnat = 4-2 = 2; i.e. 2 compositions

Composition Diagrams

Used to predict equilibirum

No. Stages Required

Column Equipment (1/2)

Column Equipment (2/2)

Mixer-Settlers

P&ID Symbol

Solid-Fluid

Solid-Fluid Operations

Adsorption

What is Adsorption

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface

  • ability of certain solids to concentrate in specific subtances from solution into a surface

Industrial Adsorption Columns

P&ID Symbol

Drying

What is Drying?

Decrease in moisture/humidity content

Typicalinsoluble solids

  • Minerals (Zinc oxide, Ores, ) Paper, Wood, Tabacco Leaf

Typical soluble solids

  • Hydrated Crystals (CuSO4*5H2O)

Psychrometric Chart

Humidity Calculation (P=1atm)

Batch Dryers

Time Cycle

Batch Drying

Tunnel Dryers

Turbo Type Dryer

Rotatory Dryer (1/2)

Rotatory Dryer (2/2)

Crystallization

Cristallization

Crystallization is the (natural or artificial) process by which a solid forms, where the atoms or molecules are highly organized into a structure known as a crystal.

Crystallizers

P&ID Symbol

Reference

Books

Reactors

S5.

Reactor Engineering

Batch

Batch Reactor

Batch Cycles

High Conversions

Fine Chemistry

Description

Advantages

  • High Conversion Rate
  • Easy to Build/Operate/Maintain,
  • Good for Rapid Reactions

Disadvantages

  • Poor Temp. Control
  • High Labor Cost
  • Hard to Scale
  • Long Idle Times

Model

Batch Cycle $$$

Industrial Batch Reactors

CSTR

Continuous Stirred Tank Reactor (CSTR)

Continuous

Cheap Opeartion

Poor Temp. Control

Description

Advantages:

  • Continuous operation
  • Simplicity of construction
  • Low operating cost & Easy to clean

Disadvantages

  • Lowest conversion per unit volume

Model

Assumes Perfect Mixing

  • Heat/Mass gradients

Large Volume for Conversion

Retention / Holdup Time can be very high

Volume depends on Kinetics

Industrial Tank Reactors

CSTR in Series

Increase Final Conversion

Decrease Total Volume

PFR

Plug Flow Reactor (PFR)

Continuous, inline

Cheap build/operation

Poor Temp./Prs Control

PFR

Advantages

  • High Conversion per Unit Volume
  • Low operating (labor) cost)
  • Continuous Operation
  • Good heat transfer

Disadvantages

  • Undesired thermal gradients may exist
  • Poor temperature control
  • Shutdown and cleaning may be expensive

Model

Assumes:

  • one long reactor or many short reactors in a tube bank
  • no radial variation in reaction rate (concentration)
  • concentration changes with length down the reactor

PBR

Packed Bed Reactor

Packaging/Catalyst

High Conversions per Volume

High $ Maintenance

Description

Advantages

  • Achieves reactions
  • High conversion per unit mass of catalyst
  • Continuous operation

Disadvantages

  • Low operating cost
  • Undesired thermal gradients may exist
  • Poor temperature control
  • Channeling may occur
  • Unit may be difficult to service and clean

Description & Model

Common operation (phase)

  • Gas phase/ solid catalyzed
  • Gas-solid rxns

Tubular reactor that is paced with solid catalyst particles

Used primarily in heterogeneous has phase reactions with a catalyst

Catalyst / Packaging

Industrial Packed Bed Reactors

P&ID

Symbols

Reference Books

S6.

PFD

&

P&ID

PFD

What are PFD?

A process flow diagram (PFD) is a diagram commonly used in chemical and process engineering to indicate the general flow of plant processes and equipment.

The PFD displays the relationship between major equipment of a plant facility and does not show minor details such as piping details and designations.

More on PFD

Typically, process flow diagrams of a single unit process will include the following:

  • Process piping
  • Major equipment items
  • Control valves and other major valves
  • Connections with other systems
  • Major bypass and recirculation (recycle) streams
  • Operational data (temperature, pressure, mass flow rate, density, etc.), often by stream references to a mass balance.
  • Process stream names

P&ID

Thanks for Joining us!

Keep it up!

S7.

Conclusion

Contact Info

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