introduction to plant design economics
TRANSCRIPT
The purpose of engineering is to create material wealth – Douglas, Conceptual Design of Chemical Processes 1988.
How to do so ?
Raw Materials+ Other Feeds
EnergyIn
CHEMICAL PROCESS Products + Wastes
Energy Out
WHAT CAN YOU SAY ABOUT THE CHEMICAL PROCESS ?
Lecture 1. Introduction to Plant Design and Economics
In a chemical process, the transformation of raw materials into desired products usually cannot be achieved in a single step. Instead, the overall transformation is broken down into a number of steps that provide intermediate transformations. (Robin Smith, Chemical Process Design 1995)
The goal of the engineer is to design and produce artifacts and systems that are beneficial to mankind – Biegler, Grossman & Westerberg, Systematic Methods of Chemical Process Design 1997.
Raw Materials+ Other Feeds
Energy
CHEMICAL PROCESS
Products + Wastes
Energy
SO, A NUMBER OF PROCESSING STEPS MADE UP THE CHEMICAL PROCESS.
QUESTIONS?
- WHAT SORT OF PROCESSINGS ARE LOCATED IN THE CHEMICAL PROCESS? - HOW DO WE SEQUENCE THE PROCESSING STEPS?- TO WHAT EXTEND CAN WE EXPECT EACH OF THE PROCESSING STEP TO PERFORM AND WHAT FEATURES ARE REQUIRED? - HOW MUCH ENERGY IS REQUIRED AND HOW MUCH IS PRODUCED?- HOW MUCH FEEDS DO WE NEED AND HOW MUCH PRODUCTS ARE PRODUCED?- HOW MUCH WASTES ARE GENERATED?- HOW MUCH PROFIT COULD BE DERIVED?
HOW DO WE ADDRESS THIS?
Concept Design for Chemical Process
Raw Materials+ Other Feeds
Energy
CHEMICAL PROCESS
Products + Wastes
Energy
PROCESS PLANT DESIGN IS THE NAME OF THE GAME HERE !
LITTLE OR LIMITEDINFORMATIONS AREAVAILABLE
BEFORE COMING UP WITH A COMPLETE PROCESS
But so many possibilitiesor solutions !
Perhaps, the major features that distinguishes design problems from other types of engineering problems is that they are under defined; i.e., only a very small fraction of the information needed to define a design problem is available from the problem statement. (Douglas, Conceptual Design of Chemical Processes 1988)
Once the process concept has been designed which produces process flowsheet,
the equipment design then has to be performed…..
Distillation
The equipment design comprises of ;
1. Performance/Capacity Rating or Sizing
2. Vessel Mechanical Design
Equipments’ Dimensions – diameter & height
No. of stages for contact
Heat Transfer Area Needed
Wall thickness
Support DesignPipe fittings & Reinforcement
Internal Design
The process safety and control strategy has to be devised;
1. Process Safety
2. Process Control Strategy
- Material Control
- Product Control
Overall Plant and Equipment ….
- Hazard and Operability Study (HAZOP)
- Risk Assessment
HAZOP study
Risk Assessment Matrix
And Waste Treatment/Minimisation need to also be addressed…
1. Waste Treatment (conventional)
2. Waste Minimisation (sustainability)
- Suitable ‘end of pipe’ treatment on the effluent (gases & liquids)
- Adjusting processes to minimise the generation of wasteREACTOR
SEPARATION & RECYCLE SYSTEM
HEAT EXCHANGERNETWORK
UTILITIES
In finalising the process and equipment design, several stages of economic analysis could be conducted …
First step;EP 1 = Revenue – Cost of Raw Material
Second Step (after mass balance developed)EP 2 = Revenue – Cost of Raw Material - Utility
Third Step (after equipments designed)EP 3 = Revenue – Cost of Raw Material – Utility – Annualised Cost of Equipment
The economics analysis continues with other costs (manpower, insurance etc) ….
with profitability analysis conducted at the end to assess project viability ……
Pay back time,Return on InvestmentInternal Rate of Return
Finally …..
1. ADDRESS THE INDIVIDUAL PROCESS/TRANSFORMATION STEP
2. ADDRESS THE REQUIRED INTERCONNECTION BETWEEN THE STEPS
FEED REACTOR
CW
STEAM
PRODUCT 1
PRODUCT 2
RECYCLE
Lecture 2. Approaches and Stages of Process Plant Design
In the first stage, conceptual process design has to be conducted.In general conceptual process design has to address two major activities ….
……….and the target is to create the best possible feasible flowsheet for the process.
THIS IS WHAT YOU WANT AT THE END OF THE STAGE !
WHAT OTHER INFORMATIONS SHOULD BE AVAILABLE IN A FLOWSHEET ?
AND HOW DO YOU GET SUCH INFORMATION ?
FEED REACTOR
CW
STEAM
PRODUCT 1
PRODUCT 2
RECYCLE
Str1 Str2 Str3 Str4 Str5 Str6 Str7 Str8 Str9F
T
P
x
H
DEFINITION OF A FLOWSHEET.FLOWSHEET IS A DIAGRAMMATIC REPRESENTATION OF THE PROCESS STEPS AND ITS INTERCONNECTIONS.
A SYSTEMATIC APPROACH OR METHOD IS REQUIRED
However, as much as we would like to have a systematic approach or method, we must also recognised that process design is an art !
If we reflect on the nature of process synthesis and analysis, …. , we recognize that process design actually is an art, i.e., creative process. (Douglas, Conceptual Design of Chemical Processes 1988)
The approach or method should be able to help in addressing these questions …
Why is the unit operation selected ? How are the unit operations connected ?
What are the utilities required and approximate amount ? How will it be supplied to the process unit operations ?
What wastes will be generated?
1. ADDRESS THE INDIVIDUAL PROCESS/TRANSFORMATION STEP
2. ADDRESS THE REQUIRED INTERCONNECTION BETWEEN THE STEPS
FEED REACTOR
CW
STEAM
PRODUCT 1
PRODUCT 2
RECYCLE
FLOWSHEET
Now, what criteria should be adopted besides economics during the design activities?
MINIMISE COST
MINIMISE ENERGY CONSUMPTION
MINIMISE WASTES GENERATION
..... BUT HOW TO OPTIMISE ?
HIGH SAFETY & INTEGRITY
GOOD OPERATIONAL ASPECTS
QUANTIFIABLE FACTOR !
NON - QUANTIFIABLE FACTOR !
VERSUS
FEED REACTOR
CW
STEAM
PRODUCT 1
PRODUCT 2
RECYCLE
Consider back the flowsheet !
OPTIMISING THE INTERCONNECTIONSBETWEEN THE UNITS/STEPS IN THE PROCESS
?
MANY POSSIBLE CONNECTIONS WHICH LEADTO DIFFERENT STRUCTURES OF FLOWSHEETTHUS DIFFERENT ECONOMIC IMPACT EVEN TO THEEXTENT OF THE INDIVIDUAL UNIT ITSELF!
DIS-CONTINUOUS FUNCTION
STRUCTURAL OPTIMISATION
OPTIMISING A SINGLE UNIT/STEP IN THE PROCESS
EG. DISTILLATION COLUMN
TOTALCOST
RR
RR --> ENERGY
RR --> CAPITAL
CONTINUOUS FUNCTION
PARAMETER OPTIMISATION
How do we tackle them ?
Consider the approaches/methods which have been introduced to deal with such complex optimisation ?
MIXED INTEGER LINEAR/NON-LINEAR PROGRAMMING
MATHEMATICAL METHOD
USE A MATHEMATICAL PROGRAMMING METHOD TOSOLVE AND GIVE SOLUTION FOR THE PROCESS.SIMULTANEOUS SOLUTION OF ALL THE SYSTEM.
CONCEPT IS BASED ON"CREATING AND OPTIMISING A REDUCIBLE STRUCTURE"
DECISION ARE BASED ON SOLELY MATHEMATICAL OPTIMISATION CRITERIA
Grossman I. E, Comp. Chem. Eng., 9: 463, 1985
Biegler, Grossman & Westerberg, SystematicMethod of Chemical Process Design 1997
ONION MODEL
HEURISTIC METHOD
USE A SEQUENTIAL/HIERARCHICAL METHODACCORDING TO FOLLOWING SEQUENCE ;1. REACTOR2. SEPARATION AND RECYCLE SYSTEM3. HEAT EXCHANGER NETWORK4. UTILITIES
CONCEPT IS BASED ON"BUILDING AN IRREDUCIBLE STRUCTURE"
DECISION ARE BASED ON ENGINEERING GUIDELINESESTABLISHED AND MATHEMATICAL OPTIMISATION
Smith R.,Chemical Process Design 1995
HIERARCHICAL APPROACH
USE A SEQUENTIAL/HIERARCHICAL METHODACCORDING TO FOLLOWING SEQUENCE ;1. BATCH VS CONTINUOUS2. INPUT-OUTPUT STRUCTURE OF THE FLOWSHEET3. RECYCLE STRUCTURE OF THE FLOWSHEET4. GENERAL STRUCTURE OF SEPARATION SYSTEM5. HEAT EXCHANGER NETWORK Douglas.,Conceptual Design of Chemical Process 1988
HEURISTIC METHOD – The Onion Model
REACTOR
SEPARATION & RECYCLE SYSTEM
HEAT EXCHANGERNETWORK
UTILITIES
- FOLLOWS ONION LOGIC
Design starts from the centre ( heart of process ) which is the reactor.At each layer, decision has to be made to complete the design requirement for the stage. As such, many best local optimal decisions are made since the whole picture is incomplete. Unit/Equipment is added only if it is economically justified based on the current available information. This keeps the process (structure) irreducible and features which are technically/economically redundant are not included.
What do you see are the advantages and disadvantages of this method ?
Advantages Disadvantages
WHAT DO YOU NOTICE ONTHE STATEMENT CONTAIN WITHINTHE SHADED BOX ?
Compare these statementsto the onion model !
REACTOR
SEPARATION & RECYCLE SYSTEM
HEAT EXCHANGERNETWORK
UTILITIES
HEURISTIC METHOD – The Hierarchical Approach
The conceptual design is performed based on 5 different stages. The approach is somewhatdifferent by tackling the 5 different level that are classified differently. The 5 levels are ;
Level 1 Decision : Batch vs Continuous
Level 2 Decision : Fixing the Input-Output Structure
Level 3 Decision : Determining the Recycle Structure for the ProcessLevel 4 Decision : Determining the Separation System
Level 5 Decision : Determining the Heat Exchanger Network
At each level except level 1, alternatives have to be generated and assessed to see its economic and operational potential. Some level of process design has to be conducted on the units/process placed at every stages.
Semester July 2004
MATHEMATICAL METHOD – The Mixed Integer Linear/Nonlinear Programming
What are the advantages and disadvantages of this method ?
Advantages Disadvantages
- CREATION OF SUPERSTRUCTURE/HYPERSTRUCTURE
A major (super) structure is created which embedded within it all feasibleprocess (including its operations) and all feasible interconnections thatare candidates for an optimal design. The method is completely automatedand depends only on the computer programming to solve it. The design problem is formulated into sets of mathematical equations which has to besolved by the mathematical programming. Started off with many redundant features, the programming optimise and reduce the process (structure) to an optimal solution.
other processalternatives
From the conceptual process design, a flowsheet is generated for the process. The next stage would then be to address the equipment design …
FEED REACTOR
CW
STEAM
PRODUCT 1
PRODUCT 2
RECYCLE
Heat Exchanger
Type of Exchanger (Plate / Shell & Tube)Heat Transfer CoefficientHeat Transfer AreaPressure DropExchanger Configuration
Reactor
Type of ReactorReaction KineticsReaction SelectivityReactor SizingReactor Temperature & Pressure
No of stagesReflux RatioFeed LocationVapour/Liquid LoadingColumn SizingInternalsColumn Temperature & Pressure
Distillation
There are established methods for performing the design of these equipments ….
Heat Exchanger
Rating calculation that will enable the sizing of the heat exchanger to be done
Kern’s method Reactor
Reactor
From the kinetics obtained from experiment, sizing of reactor could be done based on residence time.
Distillation
No of stages calculation versus reflux ratio could be made and the feed location determined.Results are then simulated in rigorous simulation model for actual design involving capacity calculation for internals.
Fenske, Gilliland & Underwood
Heat Exchanger
Reactor
Distillation
Given that each of the equipment will normally involved vessel operated at various pressure and temperature, the design of the pressure vessel has to be conducted. The design is to be done according to standards…..
Pressure Vessel Dimension Shell ThicknessFlanges Connection & ReinforcementSupport type and DesignCorrosion AllowanceWelding specification
American Society of Mechanical Engineers (ASME)Boiler and Pressure Vessel Code.ASME Boiler and Pressure Vessel Code (BPVC) is a standard that provides rules for the design, fabrication, and inspection of boilers and pressure vessels. It is reviewed every three years.
Health, Safety and Environment aspects are increasingly gaining attention in view of their importance. Therefore the design of process plant has to take into account of the HSE particularly the safety and environment aspects where it has to be integrated with the design activities ….
Inherent SafetyRemove or attenuate conditions that could lead to the 3 incidents such as high P and T …..
Hazard Analysis
FIRE EXPLOSION TOXIC RELEASE
Auto Ignition temp.Flammability LimitsFlash PointsMinimum Oxygen concentration
Flammable liquids are more dangerous than flammable gas
Chemical Energy vs Physical EnergyDeflagration vs detonationConfined vs Unconfined Explosions (VCE)
Time weighed exposureShort term ExposureCeiling ExposureLC50 & LD50DOW Index
HAZOP
Hazard and Operability Study.
Environment
Health, Safety and Environment aspects are increasingly gaining attention in view of their importance. Therefore the design of process plant has to take into account of the HSE particularly the safety and environment aspects where it has to be integrated with the design activities ….
Environment
Waste Minimisation(Clean Process Technology)
Waste Treatment
ReactorIncrease conversion if selectivity is not an issueProduct removal for reversible reaction favoring productSet T & P to improve selectivity
DistillationRecycling waste stream to suppress by product reactionFeed purificationEliminate use of extraneous material for separation (entrainer)
Waste stream recoveryImprove heat recovery
Air EffluentParticulate, CO2, CO, SOx, NoxGravity Settlers, Inertial Collectors, Scribbers, Filters, Electrostatic precipitators, catalytic reaction etc.
Water EffluentMembrane, Adsorption, Absorption, thermal oxidation, biological treatment, membrane separation etc.