**5. Nomenclature**

82 Mass Transfer in Chemical Engineering Processes

Bearing in mind that the outside air atmospheric conditions can present a significant variation throughout the day, it is usefull to define a dynamic control for the desiccant rotor operation. For instance, supposing a steady increase of 30% in outside air relative humidity, how much would be the required increase in P\*, so as to obtain a constant humidity at the process air stream outlet? Figure (15) shows the results for different increasing values for the regeneration temperature. It can be seen that for T = 60°C, an increase in 10% of the process air stream inlet will require the period of revolution to double, being unable to respond to a further increase of the relative humidity. Conversely, a higher regeneration temperature such as T = 100°C will only require a small increase in the period P\*, being able

> 60 70 80 90 Process Air Stream Inlet Relative Hum. (%)

A mathematical model for the heat and mass transfer on a hygroscopic material was developed, and resulting set of partial differential equations was solved using the finitevolume technique. The results showed that the process air stream outlet condition is strongly influenced by the regeneration temperature, as well as of the non-dimensional period of revolution. It was also shown that an increase on the outside air humidity can be easily handled by increasing the non-dimensional period of revolution, as long as a temperature of regeneration of at least 100°C is provided. The results for the humidity distribution along the desiccant felt show that the moisture removal capacity of silica-gel is limited, which opens an opportunity for the application of more selective materials. However, it shouldn´t be disregarded that a greater affinity to water vapour also implies a

Thi = 60C

Thi = 80C

Thi = 100C

8

Fig. 15. Required increase in P\*=10.0

**4. Conclusion** 

12

16

P\*

20

24

to respond to a relative humidity of process air stream inlet as high as 90%.


