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Flow sheet Development

Once a need has been identified the chemical engineer creates one or more solutions to

meet this need possibly entailing different feeds and different intermediates to obtain the

products. Since these potential solutions generally require quite different process steps andunique operating conditions, the chemical engineer must establish separate flow sheet or road

maps for each solution. Preliminary process synthesis for each solution begins with the designer

creating flow sheets involving just the reaction, separation, and temperature change and pressure

change operations and selecting process equipment in task integration step. Only those simplified

flow sheets that provide a favorable gross profit are retained, the others are rejected. Thus

detailed work on any potential process is avoided when the projected cost of the feed materials

exceeds those of the products and by products. These various steps are described in detail in chap

4. Where both the traditional flow sheet development based on algorithmic approaches are

addressed

To evaluate the most promising flow sheet alternatives, the design engineer generally develops

base case designs for each of these alternatives. This is accomplished by creating a detailed

process flow sheet with a listing of steady state material and energy balances and a designation

of major equipment items. The material and energy balances are generally performed with the

use of computer aided process simulator. Attempts are made to improve the design of the process

units and to achieve more efficient process integrations through the use of separation train

synthesis heat and power integration and second low analysis. The results from using these

algorithmic methods permit the design engineer to compare the base case with other promising

alternatives and often identify flow sheets that should be developed with or in place of the base

case design.

After the detailed process flow sheet has been completed an assessment of the controllability of

the process is generally made beginning with the qualitative synthesis of control structures for

the entire flow sheet. Measures are available that can be used before final sizing of equipment to

assess the ease of controlling the process and the degree to which the design is resilient to

possible process disturbances. Once control systems are added to the process dynamic

simulations can be carried out to confirm the earlier projections.

Computer Aided DesignComputing hardware and software have become indispensable tools in process and plant

design. the capabilities provided by computers for rapid calculations large storage and logical

decisions plus the available technical and mathematical software permit design engineers to

examine the effect that various design variables will have on the process or plant design and to

be able to do this more rapidly than was required in the past to complete a single design by hand

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calculation. The emphasis of the design engineer has therefore shifted from problem solving to

one of conceiving examining and implementing alternative solutions that meet a specified need.

Chapter 5 provides a brief introduction to computer software useful for process and plant design

and to approaches in design that take advantage of the capacities of computers appropriate

software.

Process simulators are often useful in generating database because of their extensive data banks

of pure component properties and physical property correlations for ideal and non-ideal

mixtures. When such data are unavailable simulation programs can regress experimental data

obtained in the laboratory or pilot plant for empirical or theoretical curve fitting. The computer

aided process simulator is also a useful tool in preparing material and energy balances.

Spreadsheet software is another tool used by all chemical engineers because of the availability of

personal computers ease of use and adaptability to many types of sign problems. This software is

especially useful for mass and energy balances approximate sizing of equipment cost estimating

and economic analysis steps of process design. It is less useful for more detailed equipmentdesign since the complex algorithms usually necessary for this step are somewhat more difficult

to incorporate into a spreadsheet.

Cost Estimation

As soon the final process design stage is completed it becomes possible to make accurate

cost estimations because detailed equipment specifications and definite plant facility information

are available. Direct price quotations based on detailed specifications can then be obtained from

various equipment vendors. However as mentioned earlier no design project should proceeds tothe final stages before costs are considered and cost estimates should be made throughout all the

early stages of the design even when complete specifications are not available. Evaluation of

costs in the preliminary design phases is referred to as predesign cost estimation. Such estimates

should be capable of providing a basis for company management to design whether further

capital should be invested in the project.

The chemical engineer (or cost engineer) must be certain to consider all possible factors when

making a cost analysis. Fixed costs, direct production costs for raw materials, labor,

maintenance, power and utilities must all be included along with costs for plant and

administrative overhead, distribution of the final products, and other miscellaneous item.

Chapter 6 presents many of the special techniques that have been developed for making

predesign cost estimations. Labor and material indices standard cost ratios, and special

multiplication factors are examples of information used in making design estimates of costs. The

final test as to the validity of any cost estimation can come only when the completed plant has

been put into operation. However if the design engineer is well acquainted what the various

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estimation methods and their accuracy, it is possible to make remarkably close cost estimations

event before the final process design gives detailed specifications.