Mix it up, dry it out

Processes which consume energy are attracting increasing attention from engineers. In these energy-conscious times, operators are anxious to cut their fuel bills. This might not so be difficult when electricity is involved, but when the operation adds energy to materials, it can be tricky to make sure that the energy is being used as efficiently as possible.

Drying processes are a case in point here. The only way to evaporate solvents is to add energy to the wet mixture. But equipment designers are now finding ways of minimising the amount of energy required - while at the same time adding functions to their equipment to improve the properties of the final product.

Combining more than one process operation into a single piece of equipment - process intensification - is a growing trend, and dryers have not been excluded. Mixing and blending are the most common operations to be combined with drying - Krauss Maffei Process Technologies, Hosokawa Micron and 3V are among the companies which market mixer-dryers, which have been established in the process market for several years.

Other trends include the use of vacuum pumps to reduce the pressure inside the drying chamber. This reduces the boiling-point of the solvents being driven off, reducing the amount of energy needed to dry the product - an attractive prospect in these energy-conscious times. It also assists in the processing of temperature-sensitive compounds, so vacuum dryers are very common in the pharmaceutical industry.

Vacuum specialist Rietschle, for example, recently installed vacuum pumps onto a new dryer at Genzyme's manufacturing facilities in Haverhill, Suffolk. Part of a major expansion at the plant, the dryer is used in the production of Renagel, a drug used by dialysis patients to control the level of potassium in the blood. Its active ingredient, sevelamer hydrochloride, is heat-sensitive, and the dryer helps preserve its activity.

Filtration can also be combined with drying, with companies such as 3V Cogeim and Charles Thompson leading the field. Filter-dryers take in wet suspensions and discharge dry, free-flowing powders. As they carry out both operations inside a single sealed vessel, they are suitable for processes where safety containment is an issue.

Filter-drying is a multistage batch operation. In Charles Thompson's machinery, for example, wet slurries are pumped into the filter-dryer and filtered under nitrogen pressure - with agitation if necessary - then washed several times, with a filtration step between each washing, before being smoothed, compressed and dried.

Like several makers of dryers, Charles Thompson has recently introduced microwave technology to its equipment. Often combined with vacuum drying, microwave drying is considered to be the best available technology for drying powders. The microwave radiation evaporates solvents from throughout the volume of the the product granulates. The energy heats free liquid on the surface of the particles, but if the material has a low 'loss factor' - ability to absorb microwaves - the energy can travel straight through the material as well, heating up and evaporating solvent that has been trapped in the material matrix.

Microwave drying is generally used only in the pharmaceutical industry. Drug active ingredients tend to have low loss factors, while the solvents used in their production - water or low molecular weight alcohols like methanol, ethanol or isopropanol - absorb microwaves readily. It's the fastest of all the drying processes, and gives a final product with no lumps or wet spots. Moreover, it uses a sealed process vessel, making ideal for products which are toxic or highly active. Its downsides - it can only be used for relatively small batches, and the equipment needed is expensive - tend not be important considerations in the drugs industry, where production batches are small and the value of the products high.

Other techniques come into play to develop continuous dryers. For example, Wyssmont's Turbo dryer uses a system of stacked trays which rotate around a turbo-fan to produce a consistent, uniformly-dried product. The wet product is introduced at the top of the vessel, falling onto the first tray.

After one revolution, a stationary wiper pushes the product onto the next tray down in the stack, where it is mixed and levelled to distribute the remaining moisture, before being wiped onto the next plate, and so on down the stack. The dryer can use almost any source of heat, including waste gas from other operations in the plant.