KKA 3306 PROCESS AND PLANT DESIGN Lecture 4

1
KKA 3306 - PROCESS AND PLANT DESIGN Lecture 4 5

• By
• Robiah Yunus
• Faculty of engineering
• Universiti putra malaysia
• Serdang,43400 selangor.

2
PRELIMINARY PROCESS DESIGNGenerating and
Searching Among Alternatives

• Objectives
• to develop systematic methods to generate and
  search among the alternatives
• Availability of concise representations
  essential for generation of alternatives
• Number of alternatives are usually huge even for
  simple problems
• Evaluating and searching among alternatives
  requires systematic methods

3
PRELIMINARY PROCESS DESIGN Methodologies for
Evaluating Alternatives

• Total Enumeration of an Explicit Space
• Tree Search
• Evolutionary Search
• Superstructure Decision
• Establish Targets for Design
• Problem Abstraction

4
Methodologies for Evaluating Alternatives Total
Enumeration of an Explicit Space

• Method
• the most obvious
• Generate and evaluate every alternative
• locate the better alternatives by comparing
  evaluations
• feasible for small number of alternatives

5
Methodologies for Evaluating Alternatives Tree
Search

• At every node, record assessment and decisions
  prior to branching
• Backtracking to earlier node allows examination
  of other alternatives and/or alternate decision
• leads to systematic branch and bound algorithm

6
Methodologies for Evaluating Alternatives
Evolutionary Search

• Method
• suitable for good base case design
• improve design by making small incremental
  changes
• use insights obtained while evaluating for
  further improvement

7
Methodologies for Evaluating Alternatives
Superstructure of Decision

• consider all design alternatives
• selection based on evaluation of every alternative

8
Methodologies for Evaluating Alternatives
Established Targets

• Method
• Based on established design target
• Useful for heat recovery and reactor networks
• Examples of design target
• minimum utility heating or cooling
• maximum temperature or pressure

9
Methodologies for Evaluating Alternatives
Problem Abstraction

• formulate less detailed problem statement
• affects entire alternatives
• able to group parts of the problem which behave
  similarly together

10
DECOMPOSITION STRATEGIES FOR PROCESS SYNTHESIS

• Number of process alternatives are usually very
  large
• Decomposition strategies are necessary for
  evaluation of process alternatives
• Leads to decision hierarchy in generating and
  exploring alternatives
• Example
• Step 1 - Decision on reaction - raw materials/
  recycle/product recovery
• Step2 - Decision on separation task s-how to
  connect target and source streams
• Step 3 - Decision on energy network - options for
  cooling and heating

11
DECOMPOSITION STRATEGIES FOR PROCESS SYNTHESIS
Strategies

• Bounding Strategies
• Search strategy that uses bounds for evaluation
• The bounds eliminate unfavorable process
  alternatives

12
DECOMPOSITION STRATEGIES FOR PROCESS SYNTHESIS
Strategies

• Hierarchical Decomposition
• Selection of alternatives based on decision
  hierarchy classified as a s set of levels
• The levels are categorized according to process
  decisions such as
• Level 1 Batch versus continuos
• Level2 Input-output structure of the flowsheet
• level 3 Recycle structure of flowsheet
• level 4 Separation system synthesis
• level4a vapor recovery
• level4a liquid recovery
• level 6 Heat recovery network

13
DECOMPOSITION STRATEGIES Hierarchical
Decomposition

• Level 1 Batch versus continuos
• Consider batch if only the following holds
• It is necessary to set up the process as quickly
  as possible due to competition
• require short production period for a year supply
• little design information and the process is very
  sensitive
• products become obsolete in one or two years
• high- valued products
• In other cases, consider continuous process

14
DECOMPOSITION STRATEGIES Hierarchical
Decomposition

• Level2 Input-output structure of the
  flowsheet
• Consider raw material and final products streams
  in relation to overall process
• consider ways to recover components recycle,
  purge, bypass
• explores ways to handle by-products, inert and
  their effects on reaction stoichiometry

15
DECOMPOSITION STRATEGIES Hierarchical
Decomposition

• level 3 Recycle structure of flowsheet
• further explores recycle structures
• focuses more on the reactor
• consider interaction between reactor networks and
  recycle streams
• effects of reactor conditions on flowsheet
• equilibrium conditions i.e.. T and P
• adiabatic/isothermal
• excess reactants
• inert

16
DECOMPOSITION STRATEGIES Hierarchical
Decomposition

• level 4 Separation system synthesis
• products from this step are either recycled or
  processed further or removed
• concerns with selection and placement of
  separation units
• vapor recovery -
• more expensive
• effects of purging, recycle and removal
• liquid recovery
• use distillation
• sequencing and operating conditions

17
DECOMPOSITION STRATEGIES Hierarchical
Decomposition

• level 5 Heat recovery network
• deals with heat recovery networks once other
  decisions have been
• consider opportunities to exchange heat between
  process streams to reduce utility cost
• Example use condensers and reboilers of
  distillation columns top exchange heat with other
  hot and cold streams

18
DECOMPOSITION STRATEGIES A Case Study on
Synthesis of Ethanol Process

• Problem Statement
• The design team of a company is asked to
  investigate the design of a plant to convert
  approximately 75 million kg/yr. of ethylene in to
  15,000 m3/yr. of 190 proof ethanol. The ethylene
  feed is 96 mole ethylene, 3 propylene and 1
  methane. The ethylene used in the ethanol
  production is typically 99.996 pure.
• Reactions
• CH2CH2 H2O ----gt CH3CH2OH
• 2CH3CH2OH ----gt C2H5-O- C2H5 H2O

19
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process
20
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Objectives
• to use bounding strategies based on capital and
  operating costs to develop base case process
  flowsheet
• to apply decision hierarchy to generate and
  assess viable flowsheet (PFD) alternatives

21
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Bounding Strategies - Maximum Potential Profit
• develop simple economic bound based on maximum
  profit
• maximum profit revenues from ethanol - cost of
  raw materials
• data
• Physical properties, Reaction stoichiometry,
  Prices of ethanol, ethylene, water
• prices of chemicals from Chemical Marketing
  Reporter

22
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Maximum Potential Profit
• Calculations
• total sales of ethanol, EA
• US101 to US111 million per year
• total costs of raw materials, EL, PL, M
• us29 million
• maximum profit
• US72 to US82million per year
• the bound
• Annual investment cost total annual operating
  cost US72 -US82mil/ yr.
• maximum profit is sensitive to price changes

23
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Batch vs. Continuous
• choose continuous since none of the following
  factors holds for ethanol process
• It is necessary to set up the process as quickly
  as possible due to competition
• require short production period for a year supply
• little design information and the process is very
  sensitive
• products become obsolete in one or two years
• high- valued products
• ethanol is already in the market
• ethylene is available continuously throughout the
  year
• very well known chemicals
• the process must be cost effective since it will
  set the price

24
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Input- Output Structure of Flowsheet
• Feed quality - 96 mole ethylene, 3 propylene
  and 1 methane
• Information from literature
• single pass conversion only 7
• only 10 of methane are allowed into reactor to
  prevent coking.

25
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Recycle Structure of Flowsheet
• focus on reaction chemistry
• analyses of side-reactions
• undesired products
• selectivity losses
• recycle of undesired products
• effects on chemical equilibrium
• reactor build-up
• Ethanol Process
• Side reaction
• 2CH3CH2OH ----gt C2H5-O- C2H5
  H2O
• 2 mol Ethanol 1 mol Ether 1 mol
  water
• moles of desired product formed
• Selectivity ---------------------------
  ------------------
• moles of undesired product
  formed

26
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Ethanol Process
• The reaction is limited by chemical equilibrium

27
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Separation System Synthesis
• Generate and search among separation system
  alternatives using Problem Abstraction method
• The components are group into condensable and
  non-condensable gt vapor and liquid

28
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Separation System Synthesis
• Vapor Recovery
• adsorption or distillation (refrigeration)
• membrane
• purge
• Absorber is used to recover EA and DEE in
  recycle and purge streams

29
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Separation System Synthesis
• Liquid Recovery - Liquid streams from flash and
  absorber
• Distillation
• to remove water and recycle to reactor
• to remove DEE and recycle to reactor
• to recover ethanol from IPA and water

30
DECOMPOSITION STRATEGIES Synthesis of Ethanol
Process

• Hierarchical Decomposition
• Heat Recovery Network and Beyond
• Consider more constraints to refine the selection
  of flowsheet alternatives
• Consider the design of a heat exchanger network
  for energy recovery