Introduction to Chemical Engineering Processes/Why use recycle?

What Is Recycle?Edit

Recycling is the act of taking one stream in a process and reusing it in an earlier part of the process rather than discarding it. It is used in a wide variety of processes.

Uses and Benefit of RecycleEdit

The use of recycle makes a great deal of environmental and economic sense, for the following reasons among others:

  • Using recycled materials lets a company achieve a wider range of separations

This will be demonstrated in the next section. However, there is a trade off: the more dilute or concentrated you want your product to be, the lower the flow rate you can achieve in the concentrated or dilute stream.

  • By using recycle, in combination with some sort of separation process, a company can increase the overall conversion of an equilibrium reaction.

You may recall from general chemistry that many reactions do not go to completion but only up to a certain point, because they are reversible. How far the reaction goes depends on the concentrations (or partial pressures for a gas) of the products and the reactants, which are related by the reaction stoichiometry and the equilibrium constant K. If we want to increase the amount of conversion, one way we can do this is to separate out the products from the product mixture and re-feed the purified reactants in to the reactor. By Le Chatlier's Principle, this will cause the reaction to continue moving towards the products.

  • By using recycled materials, it is possible to recover expensive catalysts and reagents.

Catalysts aren't cheap, and if we don't try to recycle them into the reactor, they may be lost in the product stream. This not only gives us a contaminated product but also wastes a lot of catalyst.

  • Because of the previous three uses, recycle can decrease the amount of equipment needed to get a process meet specifications and consumer demand.

For example, it may improve reaction conversion enough to eliminate the need for a second reactor to achieve an economical conversion.

  • Recycle reduces the amount of waste that a company generates.

Not only is this the most environmentally sounds way to go about it, it also saves the company money in disposal costs.

  • Most importantly, all of these things can save a company money.

By using less equipment, the company saves maintainence as well as capital costs, and probably gets the product faster too, if the proper analysis is is use to stop the wastage of the material