In the previous two sections, we focused on problems associated with the design and economics of a new chemical process, where there was freedom to select equipment. In this section, we explore problems associated with an existing chemical process. Two important factors that must be understood in dealing with existing equipment are:

1. Changes are limited by the performance of the existing equipment.
2. Any changes in operation of the process cannot be considered in isolation. The impact on the total process must always be considered.

      Over the 10 to 30 years or more a plant is expected to operate, process operations vary. A plant seldom operates at the original process conditions provided on the design PFD. This is due to:

1. Design/Construction: Installed equipment is often oversized. This reduces risks resulting from inaccuracies in design correlations, uncertainties in material properties, etc.
2. External Effects: Feed materials, product specifications and flowrates, environmental regulations, and costs of raw materials and utilities all are likely to change during the life of the process.
3. Replacement of Equipment: New and improved equipment (or catalysts) may replace existing units in the plant.
4. Changes in Equipment Performance: In general, equipment effectiveness degrades with age. For example, heat transfer surfaces foul, packed towers develop channels, catalysts loose activity, bearings on pumps and compressors become worn, etc. Plants are shut down periodically for maintenance to restore equipment performance.

      The material provided in this section involves several categories of performance problems:

Predictive Problems: An examination of the changes that take place for a change in process or equipment input and/or a change in equipment effectiveness.

Diagnostic/Troubleshooting Problems: If a change in process output (process disturbance or upset) is observed, the cause (change in process input, change in equipment performance) must be identified.

Control Systems Problems: If a change in process output is undesirable or a change in process input or equipment performance is anticipated, compensating action that can be taken to maintain process output must be identified.

Debottlenecking Problems: Often, a process change is necessary or desired, such as scale-up (increasing production capacity) or allowing for a change in product or raw material specifications. Identification of the equipment that limits the ability to make the desired change or constrains the change is necessary.

      This section introduces the basic principles by which existing equipment and processes can be evaluated, operated, controlled, and subjected to changes in operating conditions. This material is treated in the following chapters:

      The basic structure of performance problems is considered in the context of an input, an output, and a system.

      Tools needed to analyze performance problems, such as ratios, limiting resistances, and base cases, are presented.

      The performance of single pieces of equipment is analyzed for changes in process conditions, flowrates, utility flowrates, and degradation of equipment. It is assumed that the equipment has been designed and built and that the physical parameters of the equipment cannot change.

      The performance of multiple pieces of equipment is analyzed. It is shown how a change in one unit affects the performance of another unit.

      Evaluation of the performance of different types of reactors is illustrated. The choice of process conditions to change selectivity is addressed.

      Using examples from earlier chapters in this section, it is shown how a deviation in output from a piece of equipment can be controlled by altering an input. This is different from, and complementary to, what is treated in a typical process control class.

      A series of case-studies are presented to introduce the philosophy and methodology for process troubleshooting.