Page 10 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4 Chapter 1 General aspects of process design

drying, filtration, or for that matter any number of “unit operations” and/or “unit processes.” Such
examples can be:

• Production of common salt involving crystallization, evaporation, filtration, etc.
• Effluent treatment in plants employing several processes required for removal of specific
pollutants present
• Coke oven plant that produces coke of different grades and by-products, starting from coal
• Catalytic hydro-desulfurization of naphtha that involves reactors and separation systems
• Air separation plant to produce oxygen and nitrogen involving compression, Joule-Thomson
cooling, liquefaction and distillation
• Distillery consisting of several processes like fermentation, distillation, etc. to produce the
saleable product “spirit.”
Large chemical complexes contain several interlinked plants/processes. Each plant produces
stream(s) that is either a marketable product or is feed stream to another plant within the complex.
There are multiple streams of raw materials, products and by-products within the complex. The
“chemical complex” as a whole produces “products” that are marketable. As examples of large process
complexes, one often talks about the Steel Plant, Fertilizer Plant, Petroleum Refinery, etc. For example,
in a petroleum refinery, crude oil is fractionated to several component streams in its Crude Distillation
Unit (CDU). Although the main operation, in this case, is “distillation,” the facility contains process
steps, such as preheating of crude oil in a heat exchanger network with various hot streams, heating of
crude oil in a furnace before it enters the distillation column, washing of some of the streams with
caustic solution and subsequently with water to remove sulfurous impurities and so on. Several process
plants like cracking units, hydrotreating unit, etc., in a refinery complex, further process the streams
from the CDU.
A process can be designed to operate in “batch mode” or in “continuous mode.” Batch processes for
the same annual capacity will require bigger equipment but offer greater control on the process. Batch
operations are preferred when the processing capacity is low, but the
required control over process parameters is stricter. Also, when the
product value is high compared to the investment in the plant
Batch vs Continuous equipment, the economical option is often a batch process. These
processes also offer greater variation and flexibility in the use of the
equipment and its operation. As an example, a batch dryer for drying
a specific product can be utilized for drying other products whenever
it is free. Widespread applications of batch processes in the pharmaceutical industry dealing with
relatively lower processing capacity and requiring stringent quality control, clearly illustrate these
facts. In contrast, a continuous plant will require smaller equipment for the same annual capacity. This
allows attainment of higher processing capacity for the same investment. Most large scale/high ca-
pacity industries like fertilizer, steel, petroleum refining, petrochemicals are continuous process plants
as it turns out to be the more economic option.
Some plants with low to moderate capacity use combinations of the batch, as well as continuous
processes. In these plants, the output streams from the batch process are stored and processed later in
the continuous processes. In some plants, the order of processing may be reversed. Use of continuous
process avoids interruptions in processing that entail spending energy to start and shut down the
process. Processing steps that take longer time or are difficult to control or require closer checks and
controls operate in “batch mode.”