1.1: Nature of Design
-Design is a creative activity
-Begins with specific objective or customer needs in
mind, and by developing and evaluating possible
designs, concludes the best way of achieving that
objective; be it a better chair, a new bridge, or in your
case, a new bio-chemical product or production
process.
-Designer, will be constrained by many factors, which
will help to narrow down the possible designs.
-Rarely, there will be only one solution to the problem,
just one design. Several alternative ways of meeting the
objective will normally possible,
-Constrains can appear in many forms; some will be
fixed and invariable, and some will be less rigid. Figure
1.1
-Economic consideration are a major constraint: plant
must make a profit. Process costing and economics will
be discussed
-Time also is a constraint.
-Fig 1.2 shows the design stages, which must be done
within the time limit.
-Figure 1.1
-Figure 1.2
1.1.1
Setting the Design Basis
1) Production rate
2) Purity specification
Together with the constraints that will influence the
design such as:
1) The system of units to be used
2) The national, local or company codes that must be
followed
3) Details of raw materials that are available
4) Information on potential sites where the plant
might be located, including climate data, seismic
conditions, infrastructure availability, etc.
5) Information on the conditions, availability and price of
utility services such as fuel gas, steam, cooling water,
process air, process water, electricity, etc.
1.1.2
Possible Design Concept
a) Modifications, and additions, to existing plant
b) New production capacity to meet growing sales demand.
Repetition of existing designs, with only minor design
changes, including designs of competitors to understand
whether they have compelling better cost of production.
c) New processes, develop from laboratory research, through
pilot plant, to a commercial process. Even here, most of the
unit operations and process equipment will be use
established design.
1.1.3
Fitness Testing
When design is suggested, it must be tested for fitness
for purpose
In our case (process engineer) it is usually prohibitive
and expensive to do the prototyping, which is common
in other engineering field.
Instead, the engineer build a model, from computer
simulation.
The engineer must assemble all the information needed
to model the process so as to predict its performance
against the identified objective.
1.1.3
Fitness Testing
Once the working model has been established, the
engineer can begin to determine equipment size and
cost.
1.1.4
Economic Evaluation
Usually entails analysing the capital and operating
costs of the process to determine the return of
investment.
1.1.5
Detailed Design and Equipment Selection
- Detailed specification of equipment such as vessel,
heat exchanger, pumps etc.
-This is where process engineer work with other
disciplines, i.e civil engineer for site prep, mech eng for
design of vessel and structure, piping, electrical eng for
instrumentation and control.
-Detailed design usually focus mainly on equipment
selection, rather than changes to the flow-sheet.
-Example: Use shell and tube, or plate HE. Trays or
packing for distillation column.
1.1.6
Procurement, Construction and Operation
-Last stage in design.
-Must have start-up and shutdown procedure.
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Fig 1.3
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Stage 1: Raw Material
-Some provision will have to be made to hold several
days, or weeks, storage to smooth out fluctuations and
interruptions in supply.
- The storage required depends on the nature of the
raw materials, the method of delivery. Example: If
delivered by tanker, several weeks’ stocks may be
necessary. If received by road/ rail, may be less storage
required.
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Stage 1: Feed Preparation
-Some purification/ preparation usually necessary
before they are ready for the process.
- For example: acetylene produced by carbide process
contains arsenic and sulphur compound. This must be
removed by scrubbing with concentrated sulphuric
acid before the reaction with hydrochloric acid to
produce dichloroethane.
-Feed contaminants that can poisoned catalysts,
enzymes, micro-organism must be removed.
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Stage 2: Feed Preparation
-Liquid feeds need to be vaporized before being fed to
gas-phase reactors, for example.
-Solids probably need to be crushed, ground and
screened.
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Stage 3: Reaction
-Heart of bioprocess/ chemical manufacturing process.
In the reactor, the raw materials are brought together
under conditions that promote the production of the
desired product. Some by-products will also be formed.
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Stage 4: Product Separation
-Separation of the products and by products from
unreacted material. If in sufficient quantity, the
unreacted material will be recycled to the reaction
stage/ or to the feed preparation stage.
- The by-products may also be separated at this stage.
- In some fine chemical process, it could be multiple
reaction steps, followed by one or more separation
steps.
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Stage 5: Purification
-Before sale, the main product will often need
purification to meet the product specification.
- By product also can be purified, if it produced in
economic quantities.
1.2
The Anatomy of Chemical/ Bioprocess
Manufacturing Process
Stage 5: Product storage
-Some inventory of finished product must be held to
match production with sales.
- Provision for product packaging and transport is also
needed.
- Liquids are normally dispatched in drums and in bulk
tanker. Solids in sacks, cartoon or bales.
-Example: Ibuprofen plant
1.2.1
Continuous and Batch Processes
-Continuous processes are design to operate 24
hrs/day, 7 days/ week throughout the year.
- Down time will be allowed for maintenance, catalyst/
enzyme regeneration, replacement.
- The plant operating rate usually between 90-95%.
Attainment% = hours operated/ 8760 x 100
1.2.1
Continuous and Batch Processes
-Batch processes are designed to operate intermittently,
with some, or all, of the process units being frequently
shut down and started up.
- It is quite common for batch plants to use a
combination of batch and continuous operation.
-For example, batch reactor may be used to feed a
continuous distillation column.
1.2.1
Continuous and Batch Processes
-Continuous process will usually be more economical
for large scale production.
- Batch processes are used when some flexibility is
wanted in production rate or product spec.
1.2.1
Continuous and Batch Processes
-Advantages of batch:
a) Allows production of multiple different products or
product grades in the same equipment.
b) In a batch plant, the integrity of a batch is preserved
as it moves from operation to operation. This is very
useful for quality control purpose.
c) The production rate of batch plant is very flexible.
Can be operating at low output.
d) Easier to clean and maintain sterile operation
1.2.1
Continuous and Batch Processes
-Advantages of batch:
e)Batch processes are easier to scale up from chemist’s
recipe.
f) Batch plants have low capital for small production
volumes.
1.2.1
Continuous and Batch Processes
-Disadvantages of batch:
a) Scale of production is limited.
b) Difficult to achieve economies of scale by going to
high production rate.
c) Batch-to batch quality can vary, leading to high
production of waste / off spec product.
d) Recycle and heat recovery are harder, making batch
plants less energy efficient and more likely to
produce waste by-products.
1.2.1
Continuous and Batch Processes
-Continuous Vs Batch
-Batch processes usually make sense for products that have
high value and produced in small quantities. Batch plants are
commonly used for:
a) Food products, e.g. french fries
b) Pharmaceutical products, e.g. drugs, vaccine, hormones
c) Personal care products, e.g.
d) Specialty chemicals: Rynex-for dry cleaning, Lonzaemulsifier for ice cream, Basotect, absorb sound under
Ferrari hood, Bimaz-monomers for contact lens
1.2.1
Continuous and Batch Processes
-Even in these sectors, continuous production is
favoured if the process is well understood, the
production volume is large and the market is
competitive.