Basic process design elements

A process plant is a classification of factory which transforms materials in bulk. The feedstock and products may be transported by pipeline or conveyor, or in discrete quantities such as truckloads or bags, but they are recognized by their bulk properties. Examples of process plants are oil refineries, sugar mills, metallurgical extraction plants, coal washing plants, and fertilizer factories. The products are commodities rather than articles.

The plant consists of a number of the following.

• ‘Process equipment’ items, in which material is transformed physi-cally or chemiphysi-cally, for example crushers, reactors, screens, heaters, and heat exchangers. The process equipment is required to effect the physical and chemical changes and separations necessary to produce the desired products, and also to deal with any unwanted by-products, including waste, spillage, dust, and smoke.

• Materials transport and handling devices, by which the processed materials and effluents are transferred between the process equipment items, and in and out of the plant and any intermediate storage, and by which solid products and wastes are handled.

• Materials storage facilities, which may be required to provide balancing capacity for feedstock, products, or between process stages.

• ‘Process utilities’ (or simply ‘utilities’), which are systems to provide and reticulate fluids such as compressed air, steam, water, and nitrogen, which may be required at various parts of the plant for purposes such as powering pneumatic actuators, heating,

cooling, and providing inert blanketing. Systems to provide process reagents and catalysts may be included as utilities, or as part of the process.

(Note: All of the above four categories include items of mechanical equipment, namely machinery, tanks, pumps, conveyors, etc.)

• Electric power reticulation, for driving process machinery, for performing process functions such as electrolysis, for lighting, for powering of instrumentation and controls, and as a general utility.

• Instrumentation, to provide information on the state of the process and the plant, and, usually closely integrated to the instrumentation, control systems.

• Structures (made of various materials, including steel and concrete), which support the plant and equipment in the required configuration, enclose the plant if needed, and provide access for operation and maintenance.

• Foundations, which support the structures and some plant items directly, and various civil works for plant access, enclosure, product storage, and drainage.

• Plant buildings such as control rooms, substations, laboratories, operation and maintenance facilities, and administration offices.

In addition there are inevitably ‘offsite’ facilities such as access roads, bulk power and water supplies, security installations, offices not directly associated with the plant, and employee housing; these are not considered to be part of the process plant.

A process plant is fundamentally represented by a process flowsheet.

This sets out all the process stages (essentially discrete pieces of process equipment) and material storage points, and the material flows between them, and gives corresponding information on the flowrates and material conditions (chemical and physical). This information is usually provided for:

• the mass balance case, in which the mass flows will balance algebraically;

• a maximum case, corresponding to individual equipment or material transport maxima for design purposes (these flows are unlikely to balance); and

• sometimes, by cases for other plant operating conditions.

For thermal processes, the mass balance may be supplemented by a heat and/or energy balance.

The process flowsheets represent the process rather than the details of the plant. The latter are shown in ‘P&I’¹ diagrams, which depict all

1 Pipeline and instrumentation, although sometimes described as process and instrument-ation; but P&I has become an accepted international multilingual expression. Some engineers use ‘mechanical flow diagrams’, which do not show much instrumentation, and ‘control and instrumentation diagrams’, which focus as the name implies, and no doubt such presentation is appropriate for certain applications; but P&IDs usually suffice.

Fig. 2.1 Flowsheet with mass balance

plant equipment items, including their drive motors, all pipelines and valves (including their sizes), and all instruments and control loops.

Utility flowsheets and diagrams are often presented separately.

Plants may operate by batch production, in which the plant processes a quantum of feed per cycle, and stops at the end of each cycle for removal of the product and replacement of the feed. Alternatively, plants may operate continuously, 24 h per day, without stopping; and there are hybrid plants, or hybrid unit operations within plants, which are described as semi-continuous, in that the internal operation is cyclical but the cycles follow continuously, one after the other, with little operator intervention.

The critical performance factors for a process plant – the factors which determine its fitness for purpose and its effectiveness (and against which its designers’ performance is measured) – include the following.

• Feedstock transformation as specified. Product characteristics should be within a specified range corresponding to feedstock characteristics within a specified range, and capacity (throughput) should be within the range required for feed and for product. From the feed and product capacity may be derived the recovery, or yield of product per unit feed. Alternatively, the recovery and input or output may be stated, and the output or input respectively may be derived.²

• Cost of production, often expressed per unit of feed or product. The cost components include capital amortization and interest, plant operators’ salaries, maintenance materials and labour, purchased utilities, process reagents, insurance, etc. There may also be fees payable to process technology licensors. The capital cost component is often quoted separately as a stand-alone criterion.

• Plant reliability and availability. Reliability is the predictability of plant operation as planned, whereas availability is the proportion of time for which the plant is in a condition whereby operation (to acceptable standards) is possible. Availability may be less than 100 per cent because of planned outages for maintenance, for example one 3-week shutdown per 2-year cycle, or because of shutdowns caused by lack of reliability, or (invariably to some degree) both.

• Safety of construction and operation. This is assessed at the design stage by formalized hazard analysis for the process and by hazard

2 For some plants, there may also be a specification linking the feedstock or product specification (or grade) to the capacity and/or the recovery.

17 Fig. 2.2 P&I diagram

and operability (Hazop) study of the plant design. It is assessed during construction and operation by audit of the presence and efficacy of various safety features and constructional and operational practices, and it is reported historically by accident and loss statistics.

• Environmental impact, and its acceptability by legal, social, and ecological consideration.

• The plant life. Plant maintenance practices and costs are presumed for the purposes of economic analysis, and hopefully in practice, to be such as to keep the plant operational within specified performance levels over the intended life of the plant.

The last four factors clearly have an effect on, and their economic impact should be included in, the cost of production. However, they are important design and evaluation factors on their own, and may also have an effect on product marketability, or in some cases whether the plant is permitted to operate at all.

The time taken to build and commission the plant, and get it into full commercial operation, is equally a factor which significantly impacts on the planned and actual cost of production over the plant life. It may also have an important effect on the marketing, and hence economic value, of the product.