The make-up mixer is a unit operation for which a known needs to be mixed in another process stream, where the total flow of the resulting stream is the target specification.
Consider the following example:
Here, a feed stream is separated with the aid of a solvent. The solvent is recovered, but as a result of the imperfect separations, part of the solvent is lost. So a solvent make-up stream needs to be set.
The flow specification is required on the recycle stream, after make-up, i.e. the stream that comes out of the mixer. As the inlet recycle stream is determined, this means that the flow of the make-up stream is unknown and needs to be calculated. In sequential flowsheets however, the flow of a feed stream cannot be specified directly by a unit operation. The make-up mixer is a utility unit operation to determine the flow of the make-up stream.
To accomplish this, we specify a make-up feed stream with a flow that is too high. This means that only part of the make-up feed stream will be used. A surplus make-up stream is exiting the mixer, as shown in the following example:
The surplus make-up stream will have the same pressure, temperature and composition as the make-up feed stream. Its flow is a result of the specification of total flow on the mixer's recycled outlet. The difference in flows between these streams is the required flow of the actual make-up feed stream, and will be reported by the make-up mixer.
The recycle outlet stream will have the same pressure and temperature as the recycle inlet stream.
The target recycle flow can be specified in molar flow units or mass flow units. You can specify the total target flow, or the target compound flow for one of the compounds.