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Ch.E 4049/18/2010

Department of Polymer & Process Engineering U.E.T.Lahore 1

University of Engineering & Technology, Lahore

PLANT DESIGN

Dr.G.M.Mamoor


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LECTURE 1: THE DESIGN PROCESS


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Objectives

On completing this part of the course, you should:

Be knowledgeable about the kinds of design decisions

that challenge process design teams.

Have an appreciation of the key steps in carrying out a

process design. This course, as the course text, isorganized to teach how to implement these steps.

Be aware of the many kinds of environmental issues and

safety considerations that are prevalent in the design of a

new chemical process.

Understand that chemical engineers use a blend of hand

calculations, spreadsheets, computer packages, and

process simulators to design a process.


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The Design Process

Primitive Design Problems

Example

Steps in Designing and Retrofitting Chemical Processes

Assess Primitive Problem

Process Creation

Development of Base Case

Detailed Process Synthesis - Algorithmic Methods

Process Controllability Assessment

Detailed Design, Sizing, Cost Estimation, Optimization

Construction, Start-up and Operation

Environmental Protection

Safety Considerations


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Primitive Design Problems

The design or retrofit of chemical processes begins with the desireto produce profitably chemicals that satisfy societal needs that

arise in the broad spectrum of industries that employ chemical

engineers:

petrochemicals,

petroleum products

industrial gases

foods

pharmaceuticals

polymers

coatings

electronic materials

bio-chemicals

Partly due to the growing awareness of the public,many design projects involve the redesign, or

retrofitting, of existing chemical processes to solve

environmental problems and to adhere to stricter

standards of safety


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Origins of Design Problems

Often, design problems result from the explorations of

chemists, biochemists, and engineers in research labs to satisfy

the desires of customers to obtain chemicals with improved

properties for many applications

However, several well-known products, like Teflon (poly-tetrafluoroethylene), were discovered by accident.

In other cases, an inexpensive source of a raw material(s)

becomes available

Other design problems originate when new markets are

discovered, especially in developing countries

Yet another source of design projects is the engineer himself,

who often has a strong inclination that a new chemical or route

to produce an existing chemical can be very profitable.


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Typical Primitive Design Problem

Consider, the need to manufacture vinyl chloride (VC),

C CH Cl

H H

A typical primitive problem statement is as follows:

An opportunity has arisen to satisfy a new demand for VC monomer(VCM), on the order of 800 million pounds per year, in a petrochemical

complex on the Gulf Coast, given that an existing plant owned by thecompany produces one-billion pounds per year of this commodity chemical.Since VCM is an extremely toxic substance, it is recommended that all newfacilities be designed carefully to satisfy governmental health and safety

regulations.


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Steps in Process Design

Assess PrimitiveProblem

Detailed Process

Synthesis -Algorithmic

Methods

Developmentof Base-case

Plant-wideControllabilityAssessment

Detailed Design,Equipment sizing, Cap.Cost Estimation,

Profitability Analysis,Optimization


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Assess PrimitiveProblem

Developmentof Base-case

Detailed Process

Synthesis -Algorithmic

Methods

Plant-wideControllabilityAssessment

Detailed Design,Equipment sizing, Cap.Cost Estimation,

Profitability Analysis,Optimization


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Assess Primitive Problem

Process design begins with a primitive design problem that expresses thecurrent situation and provides an opportunity to satisfy a societal need.

Normally, the primitive problem is examined by a small design team, who

begins to assess its possibilities, to refine the problem statement, and to

generate more specific problems: Raw materials - available in-house, can be purchased or need to be

manufactured?

Scale of the process (based upon a preliminary assessment of the

current production, projected market demand, and current and

projected selling prices)

Location for the plant

Refined through meetings with engineering technical management,business and marketing.

Brainstormingto generate alternatives


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Example: VC Manufacture

To satisfy the need for an additional 800 MMlb/yr of VCM, thefollowing plausible alternatives might be generated:Alternative 1. A competitors plant, which produces 2 MMM lb/yr of VCM

and is located about 100 miles away, might be expanded to produce the

required amount, which would be shipped. In this case, the design team

projects the purchase price and designs storage facilities.

Alternative 2. Purchase and ship, by pipeline from a nearby plant, chlorine

from the electrolysis of NaCl solution. React the chlorine with ethylene to

produce the monomer and HCl as a byproduct.

Alternative 3. Since the existing company produces HCl as a byproduct in

large quantities are produced, HCl is normally available at low prices.

Reactions of HCl with acetylene, or ethylene and oxygen, could produce 1,2-dichloroethane, an intermediate that can be cracked to produce vinyl chloride.


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Survey Literature Sources ?

You suggest the resources


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Environmental Issues in Design

Handling of toxic wastes 97% of hazardous waste generation by the chemicals and nuclear industry is wastewater(1988 data).

In process design, it is essential that facilities be included to remove pollutants fromwaste-water streams.

Reaction pathways to reduce by-product toxicity As the reaction operations are determined, the toxicity of all of the chemicals, especiallythose recovered as byproducts, needs to be evaluated.

Pathways involving large quantities of toxic chemicals should be replaced byalternatives, except under unusual circumstances.

Reducing and reusing wastes Environmental concerns place even greater emphasis on recycling, not only for unreacted

chemicals, but for product and by-product chemicals, as well. (i.e., production ofsegregated wastes - e.g., production of composite materials and polymers).


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Environmental Issues in Design (Contd)

Avoiding non-routine events Reduce the likelihood of accidents and spills through thereduction of transient phenomena, relying on operation at the

nominal steady-state, with reliable controllers and fault-

detection systems.

Design objectives, constraints and optimization Environmental goals often not well defined because economic objective

functions involve profitability measures, whereas the value of reduced

pollution is often not easily quntified economically.

Solutions: mixed objective function (price of reduced pollution), orexpress environmental goal as soft or hard constraints.

Environmental regulations = constraints


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Safety Considerations

Example Disaster 1 Flixborough: 1st June 1974http://www.hse.gov.uk/hid/land/comah/level3/5a591f6.htm

50 tons of cyclohexane were released from Nypros KA plant(oxidation of cyclohexane) leading to release of vapor cloud andits detonation. Total loss of plant and death of 28 plant personnel.

Highly reactive system - conversions low, with large inventory inplant. Process involved six, 20 ton stirred-tank reactors.

Discharge caused by failure of

temporary pipe installed to

replace cracked reactor.

The so-called dog-leg was

not able to contain theoperating conditions of the

process (10 bar, 150 oC)


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Safety Considerations (Contd)

Example Disaster 2 Bhopal: 3rd December 1984

http://www.bhopal.com/chrono.htm Water leakage into MIC (Methyl isocyanate) storage tank leading to

boiling and release of 25 tons of toxic MIC vapor, killing more than3,800 civilians, and injuring tens of thousands more.

MIC vapor released because the refrigeration system intended to coolthe storage tank holding 100 tons of MIC had been shut down, the

scrubber was not immediately available, and the flare was not inoperation.

Bhopal - What can we learn? Avoid use of hazardous materials. Minimize stocks of hazardous

materials (what you dont have, cant leak). Carry out HAZOP analysis.

Train operators not to ignore unusual readings.

Keep protective equipment in working order.

Control building near major hazards.


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Safety Considerations (Contd)

Example Disaster 3 Challenger: 28th January 1986http://www.onlineethics.com/moral/boisjoly/RB-intro.html

An O-ring seal in one of the solid booster rockets failed. A high-pressure flameplume was deflected onto the external fuel tank, leading to a massive explosionat 73 sec from lift-off, claiming the Challenger with its crew.

The O-ring problem was known several months before the disaster, but down-

played by management, who over-rode concerns by engineers.

Challenger - What can we learn? Design for safety.

Prevent management over-ride of

engineering safety concerns.

Carry out HAZOP analysis.


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Safety Issues: Fires and Explosions

Compound LFL (%) UFL (%)

A t l ne 2.5 1

Cyclohexane 1.3

Ethylene 2.7 36asoline 1.4 7.6

ydrogen 4. 75

Flammability Limits of Liquids and asesLFL and UFL (vol %) in Air at 25 oCand 1 Atm

These limits can be extended for mixtures, and for elevated

temperatures and pressuresWith this kind of information, the process designer makes sure

that flammable mixtures do not exist in the process during

startup, steady-state operation, or shut-down.


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Design Approaches for Safety

Techniques to Prevent Fires and Explosions Inerting- addition of inert dilutant to reduce the fuel concentration below the LFL

Installation of grounding devices and anti-static devices to avoid the buildup of static

electricity

Use of explosion proof equipment

Ensure ventilation - install sprinkler systems Relief Devices

Hazard Identification and Risk Assessment the plant is carefully scrutinized to identify all sources of accidents or hazards.

Hazard and Operability (HAZOP) study is carried out, in which all of the

possible paths to an accident are identified. when sufficient probability data are available, a fault tree is created and the

probability of the occurrence for each potential accident computed.


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The Design Process - Summary

Steps in Designing and Retrofitting Chemical Processes Assess Primitive Problem Process Creation

Development of Base Case

Detailed Process Synthesis - Algorithmic Methods

Process Controllability Assessment

Detailed Design, Sizing, Cost Estimation, Optimization

Construction, Start-up and Operation

Environmental Protection

Environmental regulations | design constraints

Safety Considerations Should strive to design for inherently safe plants


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