Design

The Design ProcessDESIGN AND ANALYSIS II - (c) Daniel R. Lewin1

054402 Design and Analysis II

LECTURE 1: THE DESIGN PROCESS

Daniel R. Lewin

Department of Chemical Engineering

Technion, Haifa, Israel


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

Ref: Seider, Seader and Lewin (1999), Chapter 1


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 ofchemists, biochemists, and engineers in research labs tosatisfy the desires of customers to obtain chemicals withimproved 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.


Typical Primitive Design Problem

A typical primitive problem statement is as follows:

“An opportunity has arisen to satisfy a new demand for VCmonomer (VCM), on the order of 800 million pounds peryear, in a petrochemical complex on the Gulf Coast, giventhat an existing plant owned by the company produces one-billion pounds per year of this commodity chemical. SinceVCM is an extremely toxic substance, it is recommendedthat 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






Methods




Optimization

SECTION A





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, 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, andcurrent and projected selling prices)

– Location for the plant Refined through meetings with engineering technical

management, business and marketing. 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.


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– ...

Indexes– See Technion Library

Patents Internet








Optimization


Methods

SECTION B








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 productand by-product chemicals, as well. (i.e., production ofsegregated wastes - e.g., production of composite materialsand 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



Example Disaster 1 – Flixborough: 1st June 1974http://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)



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 1984http://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 1986http://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 GasesLFL 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.


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”

Assess Primitive Problem - covered today

Process Creation - next week

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