Introduction to Design: Lesson 1 A

Reasons why companies value ChEs:

chemical engineers can develop an understanding of the important underlying physical science relevant to the problem and use this understanding to:

create a plan of action, set detailed specifications, leading to a predicted financial outcome

Design is a creative activity that does not exist at the start of the project

begins with a specific objective or customer need in mind and by developing and evaluating possible designs to the best way of achieving that objective

Fixed and invariable constraints

from laws of physics, government regulations, and standards

Less rigid constraints

can be relaxed by the designer as part of the general strategy for seeking best design

External constraints

constraints outside the designer’s influence

set the outer boundary of possible designs

Internal constraints

constraints within the designer’s control

choice of process, process conditions, materials, and equipment

Economic considerations

plants must make profit

Time

limits the number of alternative designs that can be considered

Design is an iterative procedure

as the design develops, the designer will be aware of more possibilities and more constraints, and will be constantly seeking new data and ideas

Determine customer needs

Set design specifications

Set design specifications

Generate design concepts

Generate design concepts

Predict fitness for service

• R&D if needed

• Build performance models

Predict fitness for service

• R&D if needed

• Build performance models

Evaluate economics, optimize and select design

Evaluate economics, optimize and select design

Detailed design and equipment selection

Detailed design and equipment selection

Procurement and construction

Procurement and construction

Begin operation

Designs start with a perceived need

The need is the public need for the product, creating a commercial opportunity, as foreseen by the sales and marketing organization

this overall objective the designer will recognize sub-objectives (i.e., the requirements of the various units that make up the overall process)

Before starting work, the designer should obtain a

complete and unambiguous statement of the requirements

“Should haves”

are parts of the initial specification that may be thought desirable, but that can be relaxed if required as the design develops

the designer should always question the design requirements (the project and equipment specifications)

and keep them under review as the design progresses.

The most important step in starting a process design

translating the customer need into a design basis

Design basis

Include the production rate and purity specifications of the main product, together with information on constraints that will influence the design

System of units

National, local or company design codes

Available raw materials

Information on potential plant sites

Information on utility services

the creative part of the design process

generation of possible solutions to the problem for analysis, evaluation and selection

No design is entirely novel

“me third” syndrome

Commercialization of new technology is difficult and expensive and few companies are willing to make multi-million dollar investments in technology that is not well proven

“not invented here” syndrome

when innovation is wanted, previous experience, through prejudice, can inhibit the generation and acceptance of new ideas

Modifications and Additions to existing plants

Usually carried out by the plant design group

New production capacity to meet growing sales demand, and the sale of established processes by contractors

Repetition of existing designs, with only minor design changes, including designs of vendor’s or competitor’s processes carried out to understand whether they have a compellingly better cost of production

New processes, developed from laboratory research, through pilot plant, to a commercial process

Even here, most of the unit operations and process equipment will use established designs

“process synthesis”

The design engineer very seldom sits down with a blank sheet of paper to create a new design from scratch

The first step in devising a new process design

sketch out a rough block diagram showing the main stages in the process

to list the primary function (objective) and the major constraints for each stage

When design alternatives are suggested

must be tested for fitness for purpose, i. e., the design engineer determines how well each design concept meets the identified need.

the design engineer builds a mathematical model of the process, usually in the form of computer simulations of the process, reactors and other key equipment.

may include a pilot plant or other facility for predicting plant performance and collecting the necessary design data;

design data is collected from an existing full-scale facility or can be found in ChE literature

The process of design selection can begin

Once the designer has identified a few candidate designs that meet the customer objective

The primary criterion for design selection is usually economic performance, although factors such as safety and environmental impact may also play a strong role.

The economic evaluation usually entails analyzing the capital and operating costs of the process to determine the return on investment.

Every design will have several possible variants that make economic sense under certain conditions.

The extent of process heat recovery is a trade-off between the cost of energy and the cost of heat exchangers (usually expressed as a cost of heat exchange area).

regions where energy costs are high, designs that use a lot of heat exchange surface to maximize recovery of waste heat for re-use in the process will be attractive.

regions where energy costs are low, it may be more economical to burn more fuel and reduce the capital cost of the plant

The design engineer will find that several designs have very close economic performance,

the safest design or that which has the best commercial track record will be chosen.

After the process or product concept has been selected, the project moves on to detailed design.

The detailed specifications of equipment such as vessels, exchangers, pumps and instruments are determined.

The design engineer may work with other engineering disciplines, such as civil engineers for site preparation, mechanical engineers for

design of vessels and structures and electrical engineers for instrumentation and control.

Contractors

companies engage specialist Engineering, Procurement and Construction (EPC)

detailed design decisions tend to focus mainly on equipment selection though,

rather than on changes to the flowsheet.

The EPC companies also have a great deal of experience in field construction, inspection, testing and equipment installation

EPC firms are able to place bulk orders for items such as piping, wire, valves, etc., and can use their purchasing power to get discounts on most equipment

can therefore normally contract to build a plant for a client cheaper (and usually also quicker) than the client could build it on their own.

Raw material storage

Feed preparation

Feed Preparation

Reaction

Reaction

Product Separation

Product Separation

Product Purification

Product Purification

Product Storage

The storage required depends on the nature of the raw materials, the method of delivery, and what assurance can be placed on the continuity of supply

If materials are delivered by ship (tanker or bulk carrier) several weeks’ stocks may be necessary; whereas if they are received by road or rail, in smaller lots, less storage will be needed

Some purification and preparation of the raw materials will usually be necessary before they are sufficiently pure, or in the right form, to be fed to the reaction stage.

Feed contaminants that can poison process catalysts, enzymes or micro-organisms must be removed.

Liquid feeds need to be vaporized before being fed to gas- phase reactors and solids may need crushing, grinding and screening.

reaction stage

the heart of a chemical manufacturing process.

the raw materials are brought together under conditions that promote the production of the desired product; almost invariably, some by-products will also be formed, either through the reaction stoichiometry, by side-reactions, or from reactions of impurities present in the feed

After the reactor(s) the products and by-products are separated from any unreacted material.

If in sufficient quantity, the unreacted material will be recycled to the reaction stage or to the feed purification and preparation stage.

The by-products may also be separated from the products at this stage.

In fine chemical processes there are often multiple reaction steps, each followed by one or more separation steps.

Product Purification

Before sale, the main product will often need purification to meet the product specifications.

If produced in economic quantities, the by-products may also be purified for sale.

Product Storage

Some inventory of finished product must be held to match production with sales.

Provision for product packaging and transport is also needed, depending on the nature of the product.

Liquids are normally dispatched in drums and in bulk tankers (road, rail and sea), solids in sacks, cartons or bales.

The amount of stock that is held will depend on the nature of the product and the market

Ancillary Processes

provision must be made for the supply of the services (utilities) needed; such as, process water, cooling water, compressed air and steam

Facilities are also needed for maintenance, fire fighting, offices and other accommodation, and laboratories.

Continuous processes are designed to operate 24 hours a day, 7 days a week, throughout the year.

Some down time will be allowed for maintenance and, for some processes, catalyst regeneration.

The plant attainment or operating rate is the percentage of the available hours in a year that the plant operates (usually 90-95%)

Attainment% = (hours operated / 8760) × 100

Batch processes

designed to operate intermittently, with some, or all, of the process units being frequently shut down and started up.

used when some flexibility is wanted in production rate or product specifications.

Advantages of Batch Processing

1. Batch processing allows production of multiple different products or different product grades in the same equipment.

2. In a batch plant, the integrity of a batch is preserved as it moves from operation to operation. This can be very useful for quality control purposes.

3. The production rate of batch plants is very flexible, as there are no turn-down issues when operating at low output.

4. Batch plants are easier to clean and maintain sterile operation.

5. Batch processes are easier to scale up from chemist’s recipes.

6. Batch plants have low capital for small production volumes. The same piece of equipment can often be used for several unit operations

Drawbacks of Batch Processing

1. The scale of production is limited.

2. It is difficult to achieve economies of scale by going to high production rates.

3. Batch-to-batch quality can vary, leading to high production of waste products or off-spec product.

4. Recycle and heat recovery are harder, making batch plants less energy efficient and more likely to produce waste byproducts.

5. Asset utilization is lower for batch plants as the plant almost inevitably is idle part of the time.

6. The fixed costs of production are much higher for batch plants on a $/unit mass of product basis.

batch processing usually only makes sense for products that have high value and are produced in small quantities.

continuous production is favoured if the process is well understood, the production volume is large and the market is competitive

design work required in the engineering of a chemical manufacturing process can be divided into two broad phases.

• Phase 1: Process design

• Phase 2: Plant design

Phase 1: Process Design

• Covers the steps from the initial selection of the process to be used, through to the issuing of the process flow-sheets;

• Includes the selection, specification and chemical engineering design of equipment.

• In a typical organization, this phase is the responsibility of the Process Design Group, and the work is mainly done by chemical engineers.

• The process design group may also be responsible for the preparation of the piping and instrumentation diagrams.

Phase 2: Plant Design

Includes the detailed mechanical design of equipment, the structural, civil and electrical design, and the specification and design of the ancillary services

A project manager, often a chemical engineer by training, is usually responsible for the co-ordination of the project.

he activities of the different specialist groups involved in the design, the project manager will ensure that intermediate deliverables identified in the project plan are completed on time and that the project is kept close to the planned budget.

• The project design should start with a clear specification defining the product, capacity, raw materials, process and site location.

• If the project is based on an established process and product, a full specification can be drawn up at the start of the project.

For a new product, the specification will be developed from an economic evaluation of possible processes, based on laboratory research, pilot plant tests and product market research

The technical “know-how” for the process could come from the operating company or could be licensed from the contractor or a technology vendor.

• The operating company, technology provider and contractor will work closely together throughout all stages of the project.

On many modern projects, the operating company may well be a joint venture between several companies. • The project may be carried out between companies based in different parts of the world.