SYNTHESIS AND PROCESS DESIGN ERT 416 AKMAL HADI BIN MA’ RADZI SCHOOL OF BIOPROCESS ENGINEERING

SYNTHESIS AND PROCESS DESIGNERT 416

AKMAL HADI BIN MA’ RADZI

SCHOOL OF BIOPROCESS ENGINEERING

Objectives

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, is organized 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.

On completing this part of the course, you should:

Schedule - 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

Primitive Design Problems The design or retrofit of chemical processes begins with the desire

to 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

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

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.

Typical Primitive Design Problem

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 the company produces one-billion pounds per year of this commodity chemical. Since VCM is an extremely toxic substance, it is recommended that all new facilities be designed carefully to satisfy governmental health and safety regulations.”

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

C CH Cl

H H

Assess Primitive Problem

Steps in Process Design and Retrofit

Detailed Process

Synthesis -Algorithmic

Methods

Development of Base-case

Plant-wide Controllability Assessment

Detailed Design, Equipment sizing,

Cap. Cost Estimation, Profitability Analysis,

Optimization




Detailed Process


Methods




Optimization

SECTION A


Assess Primitive Problem Process design begins with a primitive design problem that

expresses the current situation and provides an opportunity to satisfy a societal need.

Normally, the primitive problem is examined by a small design team, to refine the problem statement and 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 Brainstorming to generate alternatives

Example: VC Manufacture To satisfy the need for an additional 800 MMlb/yr of VCM, the following

plausible alternatives might be generated: Alternative 1. A competitor’s 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. Alternative 3. Design an electrolysis plant. One possibilty is to electrolyze

the HCl, available from within the petrochemical complex, to obtain H2 and Cl2. React chlorine, according to alternative 2. Elsewhere in the petrochemical complex, react hydrogen with nitrogen to form ammonia or with CO to produce methanol

Survey Literature Sources

SRI Design Reports Encyclopedias

Kirk-Othmer Encyclopedia of Chemical Technology (1991)

Ullman’s Encyclopedia of Industrial Chemistry (1988) Handbooks and Reference Books

Perry’s Chemical Engineers Handbook (1997)CRC Handbook of Chemistry and Physics

IndexesSee Technion Library

Patents and internet







Optimization

Detailed Process


Methods

SECTION B





Detailed Process


Methods



Optimization

SECTION C



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 from waste-water streams. Reaction pathways to reduce by-product toxicity

As the reaction operations are determined, the toxicity of all of the chemicals, especially those recovered as byproducts, needs to be evaluated.

Pathways involving large quantities of toxic chemicals should be replaced by alternatives, 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 of segregated wastes - e.g., production of composite materials and polymers).

Environmental Issues in Design (Cont’d)

Avoiding non-routine events– Reduce the likelihood of accidents and spills through the

reduction 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”), or express environmental goal as “soft” or “hard” constraints.

– Environmental regulations = constraints

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

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

Highly reactive system - conversions low, with large inventory in plant. 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 the operating conditions of the process (10 bar, 150 oC)

Safety Considerations Flixborough - What can we learn?

Develop processes with low inventory, especially of flashing fluids (“what you don’t have, can’t leak”)

Before modifying process, carry out a systematic search for possible cause of problem.

Carry out HAZOP analysis Construct modifications to same standard as original plant. Use blast-resistant control rooms and buildings

T. Kletz, “Learning from Accidents”, 2nd Ed. (1994)

Safety Considerations (Cont’d)

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 than 3,800 civilians, and injuring tens of thousands more.

MIC vapor released because the refrigeration system intended to cool the storage tank holding 100 tons of MIC had been shut down, the scrubber was not immediately available, and the flare was not in operation.

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

hazardous materials (“what you don’t have, can’t leak”). Carry out HAZOP analysis. Train operators not to ignore unusual readings. Keep protective equipment in working order. Control building near major hazards.

Safety Considerations (Cont’d)

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

An O-ring seal in one of the solid booster rockets failed. A high-pressure flame plume was deflected onto the external fuel tank, leading to a massive explosion at 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.

Safety Issues: Fires and Explosions

Compound LFL (%) UFL (%)

Acetylene 2.5 100

Cyclohexane 1.3 8

Ethylene 2.7 36

Gasoline 1.4 7.6

Hydrogen 4.0 75

Flammability Limits of Liquids and Gases LFL and UFL (vol %) in Air at 25 oC and 1 Atm

These limits can be extended for mixtures, and for elevated temperatures and pressures (see Seider et al, 2003).

With 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.

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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SYNTHESIS AND PROCESS DESIGN ERT 416 AKMAL HADI BIN MA’ RADZI SCHOOL OF BIOPROCESS ENGINEERING