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Example 6 – Recycle Gas Stream

Problem

This is the same problem as in Example 5 except that half of the gas outlet from the cyclone goes through a fan to gain 1.8 kPa static pressure and then is recycled back to the dryer’s gas inlet.

Solution:

To model this problem, at first a tee is required to split the gas outlet of the cyclone into two gas streams, one supposed to be discharged into the atmosphere, the other recycled back to the dryer’s gas inlet. You might think that the recycled gas stream can go through a fan to gain the required static pressure (1.8 kPa) and then be mixed through a mixer with the fresh hot gas stream coming out of the heater and introduced into the dyers gas inlet. The possible flowsheet is displayed in Figure 6a.

Figure 6a

Such a configuration of the flowsheet can not get the problem solved since it is necessary to know inlet gas conditions to solve the dryer’s outlet gas stream. However, a portion of the dryer’s gas outlet will be recycled as part of the dryer’s gas inlet. This is a nonlinear problem. To solve such a nonlinear problem it is necessary to assume some initial guess values for the recycled stream so that the dryer’s gas inlet is known as to solve the dryer’s gas outlet. This can be achieved by an auxiliary logical unit operation called Recycle.

The constructed flowsheet with the auxiliary recycle is displayed in Figure 6b. With the recycle the flowsheet can be solved through multiple loops of iterations by initially guessing the necessary values of the recycled stream and modifying the guessed values in each loop of iteration until the differences of the guess values between two consecutive loops are small enough to be negligible.

Figure 6b

The solution is shown in the global editor as follows

Figure 6b