Process reactors are used for commercial production applications in the range of 1,000 to 1,000,000 litres. They are sometimes called industrial-scale reactors. Process reactors are categorized as batch, continuous, or fed-batch, depending upon the way in which they process samples, products, and waste. Continuous process reactors can be many more times productive than batch reactors, and can be operated for long periods of time without being shut-down. Fed-batch reactors are more common, however, and can achieve high productivity.