Cheaper air drying by absorption techniques

Dry, temperature-controlled air is essential in many pharmaceutical manufacturing processes, from tableting to storage. Kathabar's Koen Egberts explains how using absorption air conditioning can reduce costs

Many processes in pharmaceutical manufacturing require air temperature and humidity to be controlled, and contaminants to be removed effectively. Absorption air conditioning not only offers a cheaper method for dehumidification, but it also removes gases and solid particles from the air. Virtually all microorganisms that are present in the intake air are killed by the system. Last but not least, absorption air conditioning offers a long-lasting solution with low energy costs.

Many different air conditioning systems are on the market, of which the best-known is condensation dehumidification. In the two stage process, air is first cooled down to the temperature at which the water vapour present in the air starts to condense (100% relative humidity). Subsequently, the air is heated again to the desired air temperature. In this system, the dew-point temperature, at which moisture starts to condense, is determined by the temperature of the cooling fluid, and this requires a substantial amount of energy.

A less well known, and therefore more infrequently employed, method for dehumidification air is absorption air-dehumidification. Here, the air to be dried is blown through a unit containing a spray of a water-attracting absorbent liquid. As the absorbent liquid removes moisture from the air, humidity decreases. At the same time, the wet, hygroscopic liquid removes most microorganisms and dust particles from the air. Moreover, the majority of microorganisms are destroyed.

absorbent liquid

Most absorption systems use a salt solution as the hygroscopic liquid. Even dissolved in water, the salt is able to absorb moisture. The temperature and concentration of the salt solution determine the hygroscopic action of the system. In view of properties such as evaporation temperature and hygroscopic action, lithium chloride and calcium chloride are the most commonly used salts. To protect the absorbent liquid causing corrosion, the units are manufactured entirely from synthetic materials. An absorption dehumidifier can supply air with a relative humidity ranging from 20 to 50% RH, and dew points as low as ;30°C can easily be achieved. Dehumidifiers based on the liquid absorption principle are available under the trade name Kathabar.

The principle of operation is illustrated in Fig. 1. In the conditioner, a hygroscopic salt solution is pumped around and sprayed into the dehumidifier (A). Humid air (either from the atmosphere or recycled air) passes into the dehumidifier. This air comes into close contact with the hygroscopic salt solution spray, which absorbs the moisture present in the air. Dry air leaves from the top of the unit. The salt solution, including the absorbed moisture, collects in the lower part of the unit. By subsequently cooling the salt solution, the air is cooled and dried simultaneously. Drip catchers at the air outlet of the dehumidifier ensure that the air stream does not contain any salt-solution particles.

The second part of the system is the regenerator (B). To ensure a stable concentration of salt in the dehumidifier, the absorbed moisture has to be evaporated. Therefore, part of the (diluted) salt solution is pumped to the regenerator. Here, the hygroscopic salt solution is pumped and sprayed around again. At the regeneration side, water is evaporated by heating the salt solution. A minor secondary air stream passing through the regenerator absorbs this moisture and takes it outside.

The concentrated salt solution then returns to the dehumidifier. As shown in the process flow diagram, the cold (diluted) salt solution from the dehumidifier meets the warm salt solution in the regenerator. A heat exchanger placed between these flows will preheat the cold solution before it enters the regenerator. The warm solution will give off heat, and will thus cool down before it is used in the dehumidifier again.

low energy consumption

The main factor determining the temperature of cooling liquid required in absorption dehumidification systems is the desired air temperature. The temperature of the cooling fluid should generally be 5 to 7°C below the desired air temperature. If, for instance, an air conditioner is set at 25°C, 20% relative humidity, the system needs a cooling liquid that has a temperature of approximately 20°C. Thus the system can typically operate on inexpensive cooling liquids with a relatively high temperature, such as water from a river, well, sea, or cooling tower.

Absorption air dehumidification systems have a considerably lower energy demand than condensation air conditioning ones. The energy costs of a Kathabar system are usually about half that of an equivalent conventional condensation air conditioning system.

Extensive research carried out by the research foundation of Toledo University in Ohio, US, shows that virtually all germs and microorganisms are killed in an absorption dehumidification system. The hygroscopic salt absorbs moisture from the air and, in addition, virtually all water contained in any microorganisms present. This means that living organisms such as bacteria and moulds are killed.

germicidal action

The salt's germicidal action does not wear off, unlike a bacteria filter, which has to be replaced or cleaned regularly. This is a further advantage in processes or spaces requiring sterile air. In addition to the system's germicidal action, it also removes gases and solid particles from the air, resulting in a reduced emission to the environment.

The system enables the complete recycling of air. Any unwanted particulates that may be present are scrubbed from the air by the salt solution.

Such a dehumidifier can also be used as humidifier. By simply adding demineralised water or condensed water to the salt solution, the air can be humidified hygienically during the cold, dry winter months.

In view of the current environmental and hygiene requirements, a system that uses little energy and produces minimal emissions is becoming increasingly necessary. The absorption dehumidification system, sometimes called liquid dehumidification, meets present-day requirements. A comparison of the energy used by the Kathabar air absorption system and a conventional condensation dehumidification air conditioning system follows.

For a capsule manufacturing facility, a volume of 10,000m³ of air at 24°C, 8.4 grams/kg should be dehumidified per hour to 17°C, 2 grams/kg.

The Kathabar system provides a constant air humidity and temperature all year round, irrespective of outdoor air conditions or fluctuations in indoor climate. Air temperature and air humidity are regulated simultaneously in the unit. The Kathabar dehumidifier is suitable for dehumidifying large volumes of air, and can dry air directly to 17°C, 2 grams/kg. The system requires 100 kW cooling capacity.

cooling capacity

Alternatively, with a condensation dehumidification system, 20,000 m³hr of air can be cooled down to the absolute humidity 2 grams/kg. The cooling liquid temperature should be below ;12°C. This would create a frost problem on the cooling coil. The cooling capacity needed amounts to 165 kW. After condensation, the air is reheated to 17°C.

Table 1 shows that the cooling capacity required by a condensation dehumidification system is almost 65kW more than that required by an absorption dehumidification system. A conventional system requires a cooling liquid temperature of ;12°C. However, the absorption system only needs a cooling-liquid temperature of +6°C.

The heating energy required for regeneration is much less than the reheating required in the conventional system. The application of absorption dehumidification systems therefore results in savings in energy costs as well as in a considerable saving in capital outlay for cooling equipment.

sterile air space

The absorption dehumidification system provides a dry, cool, virtually sterile air space. The salt solution kills virtually all germs, viruses, yeast and moulds. Conversely, the 'wet' cooling cells in a conventional system may be a source of bacterial or mould growth. Moreover, at the temperatures mentioned, cooling cells may ice up.

The production lines in the manufacture of hard capsules, pills, tablets, coatings, powders, sterile fillings and soft capsules may involve a variety of different processes. But they do, however, have one thing in common: in manufacture or storage, dry, cool and sterile air is essential for all of them.

In the manufacture, storage and processing of these products, absorption dehumidification systems have obvious applications. The absorption dehumidification system prevents the product from coagulating, breaking down, or from being susceptible to mould or bacterial growth as a result of the product absorbing moisture.

Moreover, the system prevents undesirable condensation of humid air on the product, which would result in a reduced shelf-life. Hygroscopic products are kept dry in the conditioned processing environments. Powder flows freely in conveyer systems. Gelatin capsules do not get sticky, and are easier to handle and fill. Hygroscopic tablets can be fully protected from moisture during manufacturing and packaging, increasing shelf life. For the filling of parenteral products, an absorption dehumidification system ensures a sterile air environment.

Vacuum cuts compressed air costs

Compressed air is a vital service in a chemical plant, being used for control and instrumentation, conveying of powders and granules, and in biochemistry for fermentation. From the 1950s to 1970s, most chemcial plants were equipped with ‘heatless" desiccant dryers to bring compressed air to the dryness requred for chemical processing, usually a pressure dewpoint of ;40°C. Although simple and reliable, these are very wasteful of energy as they use at least 14.5% of the compressed air flow to regenerate the desiccant beds. A less energy-hungry solution is the vacuum heat regenerated dryer such as the one from Zander, using heat to regenerate the beds. Ambient air is heated and sucked over the saturated desiccant bed by vacuum pump, evaporating moisture and leaving the bed dry for the changeover. The heat source can be electricity, superheated water, steam or gas. Desiccant life under normal conditions would be 5;7 years with the Zander system, as the temperature used in the drying process of about 100°C is half that in some other such systems. Running costs can be a little as a quarter of those for a heatless system. Running costs can be reduced further by fitting a dewpoint meter control that measures the outlet dewpoint of the dryer and prevents the vessels changing over until the one adsorbing moisture reaches its optimum saturation. Cycle time can also be doubled if the dryer loading is less than the specification, whether as a result of lower inlet temperature or flow. An additional advantage of these dryers is their ability to dry the compressed air to dewpoints below ;70°C. Contact: Zander, Tamworth, UK;
tel +44 1827 260056;
fax +44 1827 261196.