Page 153 - Chemical engineering design

CHAPTER 4

Flow-sheeting

4.1. INTRODUCTION

This chapter covers the preparation and presentation of the process flow-sheet. The flow-
sheet is the key document in process design. It shows the arrangement of the equipment
selected to carry out the process; the stream connections; stream flow-rates and composi-
tions; and the operating conditions. It is a diagrammatic model of the process.
The flow-sheet will be used by the specialist design groups as the basis for their designs.
This will include piping, instrumentation, equipment design and plant layout. It will also
be used by operating personnel for the preparation of operating manuals and operator
training. During plant start-up and subsequent operation, the flow-sheet forms a basis for
comparison of operating performance with design.
The flow-sheet is drawn up from material balances made over the complete process
and each individual unit. Energy balances are also made to determine the energy flows
and the service requirements.
Manual flow-sheeting calculations can be tedious and time consuming when the process
is large or complex, and computer-aided flow-sheeting programs are being increasingly
used to facilitate this stage of process design. Their use enables the designer to consider
different processes, and more alterative processing schemes, in his search for the best
process and optimum process conditions. Some of the proprietary flow-sheeting programs
available are discussed in this chapter. A simple linear flow-sheeting program is presented
in detail and listed in the appendices.
In this chapter the calculation procedures used in flow-sheeting have for convenience
been divided into manual calculation procedures and computer-aided procedures.
The next step in process design after the flow-sheet is the preparation of Piping
and Instrumentation diagrams (abbreviated to P & I diagrams) often also called the
Engineering Flow-sheet or Mechanical Flow-sheet. The P & I diagrams, as the name
implies, show the engineering details of the process, and are based on the process flow-
sheet. The preparation and presentation of P & I diagrams is discussed in Chapter 5. The
abbreviation PFD (for Process Flow Diagram) is often used for process flow-sheets, and
PID for Piping and Instrumentation Diagrams.

4.2. FLOW-SHEET PRESENTATION
As the process flow-sheet is the definitive document on the process, the presentation
must be clear, comprehensive, accurate and complete. The various types of flow-sheet are
discussed below.

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