Continuous production

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Continuous production is a method used to manufacture, produce, or process materials without interruption. Continuous production is called a continuous process or a continuous flow process because the materials, either dry bulk or fluids that are being processed are continuously in motion, undergoing chemical reactions or subject to mechanical or heat treatment. Continuous usually means operating 24 hours per day, seven days per week with infrequent maintenance shutdowns, such as semi-annual. Some common continuous processes are the following:

• Oil refining
• Chemicals
• Synthetic fibers
• Fertilizers
• Pulp and paper
• Blast furnace (iron)
• Metal smelting
• Natural gas processing
• Sanitary waste water treatment
• Continuous casting of steel
• Rotary kilns for calcining lime or cement
• Float glass

Processes are operated continuously for practical as well as economic reasons. Shutting down and starting up many continuous processes typically results in off quality product that must be reprocessed or disposed of. Also, many tanks, vessels and pipes cannot be left full of materials because of unwanted chemical reactions, settling of suspended materials or crystallization or hardening of materials. Also, cycling temperatures and pressures from starting up and shutting down certain processes (line kilns, blast furnaces, pressure vessels, etc.) may cause metal fatigue.

In the more complex operations there are sequential shut down and start up procedures that must be carefully followed in order to protect personnel and equipment. Typically a start up or shut down will take several hours.

Continuous processes use process control to automate and control operational variables such as flow rates, tank levels, pressures, temperatures and machine speeds.^[1]

History

Many continuous processes of today were originally batch operations.

One of the oldest semi-continuous flow processes is the blast furnace for producing pig iron. The blast furnace is intermittently charged with ore, fuel and flux and intermittently taped for molten pig iron and slag; however, the chemical reaction of reducing the iron and silicon and later oxidizing the silicon is continuous.

Another early continuous processes was Oliver Evans'es flour mill (ca. 1800), which was fully automated.

Early chemical production and oil refining was done in batches until process control was sufficiently developed to allow remote control and automation for continuous processing. Processes began to operate continuously during the 19th century. By the early 20th century continuous processes were common.

Sources and further reading

• R H Perry, C H Chilton, C W Green (Ed), Perry's Chemical Engineers' Handbook (7th Ed), McGraw-Hill (1997), ISBN 978-0070498419

1. ^ Benett, Stuart (1986). A History of Control Engineering 1800-1930. Institution of Engineering and Technology. ISBN 978-0863410475.