Dryer Selection: hasty decisions can be costly!

In a pharmaceutical or chemical process train, the drying of wet solids is perhaps the longest step. Not only that, it is also hard to scale up from a trial or test unit to the production scale equipment. Generally speaking, wet solid particles have 2 types of moisture, a surface or unbound moisture and an intrinsic or bound moisture. When the unbound moisture is being removed, the drying takes place at a ‘constant rate’ during which the amount of moisture removal is steady per unit time. Once all the unbound moisture is removed, the drying goes into whats known as the ‘falling rate’ period during which the bound moisture is being removed at gradually declining rates, and generally at greater levels of vacuum so as to keep the product temperature constant. Usually, heat is imparted to the wet cake in the presence of a partial vacuum, so that the moisture is removed at a relatively low temperature, usually in the 40-60 C range.

What makes the whole thing tricky is that, in some pharma-chemical drying processes, the material being dried cannot tolerate (or its chemical properties be adversely affected by) a sudden spike in temperatures. So, utmost care must be taken in selecting a dryer based upon

1. the product that is being dried, and

2. the process or equipment to be used,

and constantly monitoring the product temperature during the entire drying process.

• The product: perhaps of utmost importance is understanding how the moisture leaves the product. As such, if there’s product inconsistencies (such as widely varying particle sizes), then the drying may not be uniform throughout and the final result may also not be consistent. Another factor is the thermophysical properties of the solids and the liquid. Of course, the initial and final desired moisture content will impact the equipment selection, as will their toxicological properties.

• The process: will it be batch or continuous? Each has its pros and cons, and in some cases it may not be possible to use one or the other. The throughput capacity is another important consideration, as also the desired flexibility and the moisture removal load required. Finally, the nature of the upstream and downstream processes have also to be taken into consideration, and the entire line needs to be balanced as best you can.

Given the above considerations, there are a slew of different dryer types to choose from, and they vary greatly in both price, maintenance and operating costs. However, they also vary greatly in their efficacy for your particular process, and making your selection based solely on price will cost you dearly for years to come, in ways hidden and unseen, i.e., the cost of lost production for the entire process train.

Do confer with an impartial 3rd party before embarking on this journey. It may not immediately be evident but, the time and money will be well spent.