Munters continuous research and development of existing and proven techniques result in high performing Quantum™ desiccant drying wheel (rotor) - for demanding process drying applications.
The gentle and energy efficient drying of products requires ingenious system solutions. In this respect, solutions based on desiccant dehumidification rotors are increasingly replacing hot air systems, particularly when it is a question of sensitive products such as vitamins or active ingredients, or extremely thin layers, such as water-based paints for example.
An important application of the process drying is spray drying, which converts liquids into powder-based substances. In general, spray drying is operated by hot air in a temperature range between 150 and 300 °C. The substance to be dried is blown through a top-mounted spray lance into a spray tower and dried by the hot air. Due to its low relative humidity the process air has a high water extraction potential and provides a high degree of drying. The performance of a spray tower is limited by the amount of energy that can be supplied by the hot air. If an increase in performance is required there are two possibilities: either the temperature of the injected air is increased or the air humidity is decreased. For sensitive products such as milk or vitamins the injection temperature is reduced to 80 – 100 °C, in order to maintain quality, so that in essence production can only be increased by drier air. In this respect desiccant rotor based systems provide high efficiency and uniform process conditions.
Air drying
Air drying systems according to the Munters principle use a desiccant rotor for the drying of process air. From a functional point of view a rotor of this sort is a concentrator. The rotor consists of 86% honeycombed micro-porous substrate with a very large surface area on which a drying medium is deposited. As a desiccant material Munters uses a titanium stabilized silica gel (Ti-HPS) manufactured in-house. A rotor of this sort has an operational life of over ten years.
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The basic principle shows the design of a desiccant rotor for the drying technique. The rotor is divided into two segments; process air and regeneration air. A sealing system prevents mixing of the air flows.
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The rotor is divided by a sealing system into a 90° segment and a 270 ° segment. Process air is blown through the larger rotor segment (270 °). In this way the humidity is absorbed by electro-static attraction, i.e. water molecules accumulate on the rotor. The smaller segment has hot air from the counter flow blown through it, which activates the accumulated water molecules. These break loose and are conveyed to the outside by the desiccant or regeneration air flow. In order to achieve continual drying of the process air a drive motor turns the rotor 8 to 11 revolutions per hour.
Rotor output
A silica gel rotor removes about 5 to 8g water per kg air (g/kg) from the ambient air. The output depends on the loading of the rotor and the temperature level of the regeneration air. Fundamentally the lower the flow speed, the larger the rotor (diameter and depth in the flow direction) and the higher the energy input on the regeneration side (higher temperature and greater quantity of air) the higher the drying performance.
In Germany, a mean absolute humidity of the ambient air of 8g/kg is expected during the course of the year. For an optimal drying process the humidity of the process air should be about 2g/kg. If one assumes a rating for the ambient air of 32 to 35 °C and an ambient air humidity of 12g/kg, the result for the process air is a volume flow of 25,000 to 120,000 m³/h (according to the size and performance of the spray tower) with an injection air humidity of 2g/kg. For a dehumidification output over 8g/kg another technique must be employed.
Boost drying performance
If a higher drying performance (>8 g/kg) is required it is conventional practice to fit a pre-dehumidifier with a cold water cooler upstream of the rotor. The air is dehumidified by being cooled to about 10°C in the cooler and the air humidity condenses on the cold surfaces of the cooler. The condensate drips into a collecting pan and is drained away. The disadvantage of this system is that a cold water system is required or an additional water cooling unit must be used. Added to this there are the installation costs for the pipe network and the additional operating costs. In addition the condensate in the ventilator is considered to be a health risk. The pre-dehumidifier has one big advantage, however. This is the lower temperature required for the regeneration of the rotor – about 125 °C. This temperature can be provided by a factory steam network. As an alternative to pre-dehumidification, high-performance rotors may be used.
Quantum™ rotor, Munters High-performance rotor
The high-performance desiccant rotor dries the ambient air without wet pre-dehumidification to an absolute humidity value of 2g/kg, independently of the season and the ambient air conditions. The rotor is a development of the titanium silica rotor and uses a composite material of molecular sieve (zeolite, which adsorbs gases, vapors and dissolved substances with specific molecular sizes) and titanium silica gel as desiccant substance. The additional outlay of energy for the higher performance remains within limits and does not increase out of proportion. For the regeneration, however, an energy source such as natural gas or medium steam pressure is required.
During adsorption the air in the rotor is dehumidified by up to 14 g/kg and is heated concurrently to a maximum of 42K by the zeolite in the molecular sieve, so that a post heater for the process air can have smaller dimensions or be restricted.
From a hygienic point of view dispensing with the wet pre-dehumidification is a great advantage as no condensate has to be drained away and the periodic cleaning of the heat transfer surfaces of the pre-cooler is not required. The energy efficiency of the system can be increased still further using a rotor bypass. For favorable ambient air conditions the bypass reduces the load on the system fan by the lower pressure loss in the device.
Purge air lowers energy consumption
For processes with temperatures from 40 to 80 °C or processes that do not need to run within the hygienic area, process air pre-coolers may be used. In this respect it is recommended that there is an exact inventory and analysis of all fuels available on site. It’s very often the case that the use of cold well water (or cooling tower water) can positively influence the energy balance, in effect compensating for a design feature by acting as a pre-switching cooler for a high-performance rotor. The cooler makes the rotor more absorptive and the dry air outlet temperature is reduced. For fluidized bed dryers in particular, which can be used for all types of powder and instant products, a low process air outlet temperature is required. For these applications “Power Purge” a self-contained purge air process, is suitable.
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The “Power Purge” process from Munters uses purge air to transfer heat energy from the process air side to the regeneration side of the adsorption rotor.
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To begin with the purge air withdraws thermal energy from the part of the rotor that expels it from the regeneration sector at about 120 °C. The hot purge air is fed into the rotor segment that has just moved into the regeneration sector and releases the heat to the rotor again. The process increases the output of the rotor as the purge air cools the rotor segment for process air drying and makes it more absorptive. The energy consumption on the regeneration side of the system is reduced by 30% and the temperature increase in the dehumidification process by 20%, so that any post cooler can be of smaller dimension or supplied with less energy. The “Power Purge” process is also suitable for the retrofitting for current rotor systems from 1000 mm rotor diameter. For applications in the field of baby food or other hygienic-relevant areas the use of terminal hygiene filters is recommended.