Continuous reactor

Continuous reactors (alternatively referred to as flow reactors) carry material as a flowing stream. Reactants are continuously fed into the reactor and emerge as continuous stream of product. Continuous reactors are used for a wide variety of chemical and biological processes within the food, chemical and pharmaceutical industries. A survey of the continuous reactor market will throw up a daunting variety of shapes and types of machine. Beneath this variation however lies a relatively small number of key design features which determine the capabilities of the reactor. When classifying continuous reactors, it can be more helpful to look at these design features rather than the whole system.

Reactors can be divided into two broad categories, batch reactors and continuous reactors. Batch reactors are stirred tanks sufficiently large to handle the full inventory of a complete batch cycle. In some cases, batch reactors may be operated in semi batch mode where one chemical is charged to the vessel and a second chemical is added slowly. Continuous reactors are generally smaller than batch reactors and handle the product as a flowing stream. Continuous reactors may be designed as pipes with or without baffles or a series of interconnected stages. The advantages of the two options are considered below.

The rate of heat transfer within a reactor can be determined from the following relationship:

where:

From a reactor design perspective, heat transfer capacity is heavily influenced by channel size since this determines the heat transfer area per unit volume. Channel size can be categorised in various ways however in broadest terms, the categories are as follows:

Industrial batch reactors : 1 – 10 m²/m³ (depending on reactor capacity)

Laboratory batch reactors : 10 – 100 m²/m³ (depending on reactor capacity)

Continuous reactors (non micro) : 100 - 5,000 m²/m³ (depending on channel size)

Micro reactors : 5,000 - 50,000 m²/m³ (depending on channel size)

Small diameter channels have the advantage of high heat transfer capacity. Against this however they have lower flow capacity, higher pressure drop and an increased tendency to block. In many cases, the physical structure and fabrication techniques for micro reactors make cleaning and unblocking very difficult to achieve.

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