Chemical Engineering Design Project

A Case Study Approach, Second Edition

Martyn S Ray

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000162110
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

This new edition follows the original format, which combines a detailed case study - the production of phthalic anhydride - with practical advice and comprehensive background information. Guiding the reader through all major aspects of a chemical engineering design, the text includes both the initial technical and economic feasibility study as well as the detailed design stages. Each aspect of the design is illustrated with material from an award-winning student design project.
The book embodies the "learning by doing" approach to design. The student is directed to appropriate information sources and is encouraged to make decisions at each stage of the design process rather than simply following a design method. Thoroughly revised, updated, and expanded, the accompanying text includes developments in important areas and many new references.

PART I

TECHNICAL AND ECONOMIC FEASIBILITY STUDY

(References are included within each chapter. References for general information sources are in Section IV, and design books for various topics are included in Chapters 8 and 9.)

1. THE DESIGN PROBLEM

1.1 INITIAL CONSIDERATIONS AND SPECIFICATION

A chemical engineering design project does not follow a set of standard steps similar to the familiar undergraduate textbook problems, nor does it have a single ‘correct’ solution. The considerations in a design project are many and varied. The solution that is finally accepted is (usually) the ‘better’ solution (often based upon economic considerations) from several alternatives. The important feature of a design study is that decisions must be made at every stage, and compromises are frequently required. It is also necessary to reconsider various decisions in the later stages of a project, and to re-work solutions and re-evaluate proposals.

1.1.1 The Feasibility Study

The feasibility study for a design project involves mainly the technical and economic evaluation of the process in order to determine whether the detailed design (and subsequent construction) stages should be undertaken. There are many aspects that need to be considered, such as legal implications, political influences, etc., and these should be considered before the expensive detailed design stage commences.

An undergraduate design project is not unlike the work of a project engineering team in industry. There is a problem to be solved, there is a project manager (the lecturer-in-charge), a team of project engineers (the students, or perhaps one student), and various deadlines to be met. The initial task is to define the actual problem; depending upon the information provided by the lecturer (see later), the problem may be well-defined or it may be quite unclear initially exactly what is required. The first task in any project is to consider the options and alternatives and to gradually eliminate those which are not feasible or not required, and then evolve a plan and hence a project definition. The approach taken is often to try and define the project as quickly as possible in order to proceed with the “real work”. While time management is an essential attribute, the rush to eliminate options can lead to problems and revisions later in the project and it is worthwhile keeping the number of useful options open as long as possible. Students often complain initially that they do not know what is required or what is possible, the answer in both cases is everything! Consider all possibilities, and even options that don’t seem possible! Decide when decisions will need to be made, and at that time eliminate the least attractive options and eventually arrive at a feasible project definition.

1.1.2 Time Management

An essential ingredient of all successful projects is realistic time management. Once a project is defined it is necessary to identify the associated tasks, the required deadlines, and the resources available. Undergraduate projects have the same requirement; and without appropriate time allocations to specific tasks then the project will either not be completed on time or it will be an inferior piece of work. It is necessary to look to the end of a project at the beginning of the work, rather than focussing on the day-by-day or week-by-week tasks. An overall view is required in the early stages of a project proposal, and the essential/key tasks, decisions and bottlenecks need to be identified. Appropriate actions and planning can then be incorporated in the time-plan for the project. It is also necessary to be flexible and to modify the timing of the project stages as the work proceeds. However, it is the unforeseen and unexpected events that are usually most crucial!

1.1.3 Stages in a Design Problem

Many textbooks have described and analysed the design process (e.g. Ray, 1985), and whatever approach is taken there are several common steps. The following six steps in the design of a chemical process have been identified (see Ulrich, 1984):

1. Conception and definition.

2. Flowsheet development.

3. Design of equipment.

4. Economic analysis.

5. Optimization.

6. Reporting.

This is obviously rather cursory and in need of further consideration. The student would benefit from reading the introductory chapters in one or more of the popular texts on plant and process design (e.g. Peters and Timmerhaus, 1991; Ulrich, 1984; Baasel, 1990). The importance of a clear (but open) project definition and consideration of the constraints and limitations cannot be over-emphasised. There are many aspects and stages in the technical and economic feasibility study which preceed the detailed equipment design work. There is often the need for compromise, and for optimization of decisions at both the local (or individual problem) level, and in terms of the overall (or big picture) view of a project. Some of these appreciations come with experience and some can be gained by wider reading, and some by keeping an open mind! Further consideration of the stages in a project and the approach to design will be left to the student and the lecturer to explore.

1.1.4 The Search for Information

The first step in a design project is to try and identify (all?) the relevant information that is available. The second step is to identify the information required, decided after initial consideration of the problem. The aim should be for quality rather than quantity of information, i.e. a few useful/relevant references rather than hundreds of general references. Two possibilities exist:

(a) Not enough information is available — a search is required (see Sections IV and 2.3 for ideas regarding sources of information).

(b) Too much information is available — the task is to assess the reliability of conflicting information.

It is unlikely that all the required information is initially available, or that the data is sufficient to complete the project (except for university teaching assignments!). Remember: Published information is not necessarily correct!

The assessment and definition stage of a project is often either rushed, overlooked or postponed by eager students, such an approach usually leads to wasted time and effort later in the design. Make it a rule to know what you have and where you are going, rather than simply thinking that you know.

1.1.5 Scope of the Project

Undergraduate design projects are often well-defined, although this need not be the case. The design project set by the IChemE (UK) is for the design of a process for the production of a particular chemical...