Unit Operations in Chemical Engineering
by Chemical Engineering Guy
Introduction
S1.
Introduction
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.
Momentum Operations
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
Sizes & Schedule
Schedule 40
Schedule 80
Inner/Outside Diameter changes!
Materials
- Steel
- Copper
- Plastic (PVC)
- Ceramic
Maintenance
Fouling
Rusting
Bending
Leaking
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 + 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
Fittings&Valves
Fittings
Description:
- Used to transport fluids
- Typically made of Steel
- Several functions
- Larger drop in pressure due to friction
Math Model
- Typically based on an experimentl loss of pressure
- Related via velocity
- Based on "equivalent" length
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
- Gentle Pressure Drop
- Pressure Drop relates to Velocity
- Velocity Relates to Flow Rate
Orifice Plates
- High Pressure Drop
- High Operation Cost
- Easy to install
Agitation & Mixing
Agitation = Designed Pattern
Mixing = Random Pattern
Agitation
&
Mixing
Agitation
- Promotes mass&heat transfer by design
- Operations
- Suspensions
- Blending
- Dispersion
- Emulsifiers
Agitation Tank
- Tank/Encloser
- Impeller
- Baffling
- Shaft
- Motor
- Level
Pumps
Used to:
- Move Fluids
- Increase Pressure
Pumps
Types
Positive Displacement
Axial
Direct Application of WORK
Increae in Pressure
Axial
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
System Curve
System Head vs. Volumetric Flow Rate
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
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 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!
Fans&Blowers
Used to blow/fan air/gases
Fans
&
Blowers
Models
- Typically modelled as incompressible flow
- No compression
- Gas = Liquid
Compressor
- Increase Pressure in Gases
- Move Gases
Compressors
SBW make large process air compressors for 1.2 million ton per year PTA Unit have been running sucssefully more than 4 years.
Types Compression
Isentropic
Polytropic
Isothermal
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.
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
Heat Exchange
S3.
Heat Operations
Types of Heaters
Types
Shell & Tube
Plates
Spiral
Tubular (Double Tube)
Shell & Tube
Literally, tubes inside a shell
Multiple Tubes inside
Single Shell
Baffles
Description
- Plates Separate Flow
- Indirect Contact
Design
Number of Plates
Size of Plates
Area of Exchange
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.
Tubular Heaters
Pipe Inside Pipe
Small heat duties
Simple to construct
Main Heat Equation
Qgain=-Qlost
Q = UAT
T= Logarithmic Tempearture change
Models
AREA
More Tubes / Coils / passes
Area
Counter/Co Flow
- Temperature Profile Changes
- Efficiency might change
Logarithmic Mean Temperature Diff.
Condensers
Condenser
Function is to cool/condense
Similar as Heat Exchangers
- There is Condensate
- There must be latent heat consideraiton
Common utilities
Cooling Water
Cooling Air
Specific Refrigerants:
Ammonia
Evaporator/Reboilers
Used to evaporate&boil material
Evaporators
Evaporators
- Batch
- Natural/ Circulation (convection)
- Falling/Rising Film
- Kettle
- Thermosyphone
Batch Evaporator
Feed -> Heat -> Boil -> Remove -> Clean
Forced Circulation Evaporator
Natural Circulation Evaporator
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.
Mass Transfer Operations
S4.
Mass Operations
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
Tray Towers (1/2)
Towers used for Mass Transfer
- Main interest is to form bubbles
- Bubble-Liqui dinteraction favors transfer
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 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
What is Absorbtion?
Operation in which gas mix is contacted with a liquid in order to dissolve components of the gas into the solution.
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
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!
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
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
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
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
Solid-Fluid
Solid-Fluid Operations
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
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)
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.
Reactors
S5.
Reactor Engineering
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
Industrial Batch Reactors
Continuous Stirred Tank Reactor (CSTR)
Description
Advantages:
- Continuous operation
- Simplicity of construction
- Low operating cost & Easy to clean
Disadvantages
- Lowest conversion per unit volume
Model
Assumes Perfect Mixing
Large Volume for Conversion
Retention / Holdup Time can be very high
Volume depends on Kinetics
CSTR in Series
Increase Final Conversion
Decrease Total Volume
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
Packed Bed Reactor
High Conversions per Volume
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
Industrial Packed Bed Reactors
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
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S7.
Conclusion
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