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Because of the variety and complexity of modern processes ,it is not practicable to cover the entire subject matter of chemical engineering under a single head .The field is divided into convenient ,but arbitrary ,sectors.This text covers that portion of chemical engineering known as the unit operations. an economical method of organizing much of the subject matter of chemical engineering is based on two facts: (1) Although the number of individual processes is great, each one can be broken down into a series of steps, called operations ,each of which in turn appears in process after process；（2）the individual operations have common techniques and are based on the same scientific principles. For example, in ,most processes solids and fluids must be moved; and tasks such as drying, size reduction, distillation, and evaporation must be performed. The unit operation concept is this: by studying systematically these operations themselves-operations that clearly cross industry and process li

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