Drying options: drumming home drying

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Published: Friday, 20 March 2009

Mathias Trojosky of Allgaier Process Technology discusses the pros and cons of rotary drum versus fluidised bed drying techniques

 
Allgaier’s drum dryer, System Mozer TK, is suitable for drying materials such as limestone and diabase. Courtesy Allgaier Process Technology GmbH 
It can often be difficult to choose the right drying system, and users are frequently faced with the choice between a rotary drum dryer or a fluidised bed, vibratory dryer. While the applications for these two dryer types can overlap somewhat, there are various selection criteria which favour one dryer over the other for a given application.

Drum dryers have been used for decades for drying many different types of goods and are widely used in the building, minerals and raw materials sectors. Fluidised bed dryers, on the other hand, are more commonly used during the production of chemicals, foodstuffs and pharmaceuticals.

Various niche applications have been developed for fluidised bed dryers, including: spray-granulation dryers, suspension and paste dryers, and fluidised bed dryers or coolers with heat exchangers integrated into the fluidised bed.

In the last twenty years fluidised bed dryers have also been installed in the building material and minerals industries. Meanwhile, drum dryers have been improved further to allow for hard-wearing solutions and energy efficient applications, particularly when used in the minerals industry.

Drum dryers

In drum dryers, the material to be dried is moved by the rotation of the drum and by the effects of vanes and blades mounted inside the drum. Past drum dryer models were in most cases constructed with a slight downward inclination in order to support the movement of the material.

However, more modern drums are almost always mounted horizontally. Lifting blades lift the moist material from the base of the drum and then allow it to fall, thus ensuring good contact between the moist material and the hot air.

In most drying applications, the material and the hot air flow parallel in the same direction through the drum. The moist material is transported through the drum by means of vanes, moving in both the same direction as the hot air and also at cross flow to the air.

Drying minerals

In the minerals industry, the drying air is in most cases heated with the aid of a gas or light-oil burner. The combustion gases are then mixed with ambient air to achieve average temperatures between 600-900°C. For materials that are less sensitive to heat, such as quartz sand, it is possible for the flame to burn inside the rotating drum.

In the case of heat-sensitive minerals, such as bentonite, clay, limestone, or recycled plastics or organic waste materials, combustion chambers are used to ensure that the combustion gases are mixed with ambient air before they enter the drum.

The outlay for heating drum dryers is relatively low. Modern burners have only relatively small fans for the combustion air, and in many cases it is not necessary to use fans to blow the heated air into the dryer or to install costly piping.

The moist drying air leaving the dryer is extracted with an exhaust fan, filtered in the bag filter to remove dust and exhausted via a chimney into the open air. The piping for the exhaust air is also relatively simple, since the air needs to be extracted only at a single point from the dryer housing.

Drum drying rates

Drum dryers are constructed for handling solids throughput at a rate of 5-10 tph. An advantage of drum dryers is that they are almost unaffected by variations in the moisture content of the drying materials, in the throughput or in the grain size of the material or by lumps or coarse grains in the incoming material. Even in the event of a power failure, operations can in most cases be resumed immediately when the power returns.

Drum dryers are also appropriate for handling fine solids, but are particularly suitable for coarse and very coarse bulk goods, and it is not absolutely necessary to adjust the amount of air when products are changed. Even if the supply of drying air fails, the material is still moved reliably through the drum. The dryer controller can be automated relatively easily.

Abrasive effect

The drum, plus its internal blades and vanes, are made from thick-walled steel. If the blades and vanes are designed correctly, even very abrasive materials can be dried in drum dryers. Care must be taken to ensure that the material to be dried slides as little as possible on the blades and vanes and that it falls onto a bed of material in the bottom of the drum.

In general, dryers have low specific heating-energy consumption rates if the process can be run with a high inlet air temperature. High gas temperatures result in low amounts of drying air and less heat losses with the exhaust air. For this reason, drum dyers which operate with hot gas temperatures of up to 900°C are very efficient, and are commonly used in the minerals industry. The exhaust air temperatures of drum dryers are generally higher than those of fluidised bed dryers.

Fluidised bed dryers

In fluidised bed dryers, good fluidisation of the drying material is a prerequisite for reliable transport and good ventilation of the material through the dryer. Drum dryers, in contrast, transport the drying material with the aid of vanes and blades in the rotating drum.

The fans which blow air into a fluidised bed dryer not only have to fluidise the material to be dried, but they must also overcome the pressure loss in the gas distribution plate, which must be high enough to ensure uniform distribution of the drying air over the entire cross-section of the dryer.

The electricity consumption of a drum dryer is thus relatively low and is about two thirds of the value for fluidised bed dryers.

Transport

The transport of the drying material is achieved by fluidising this material with an upward flow of drying air. If this is done correctly, the material assumes a pseudo-fluid state and flows out of the dryer at the same rate as moist material enters the dryer. The solid flows in a cross flow to the upward flow of drying air.

The outlet of the dryer is equipped with an adjustable weir with which the height of the fluidised bed can be adjusted, and this affects the retention time of the material in the dryer. In the ideal case, all particles in a fluidised bed are continually in contact with the drying air, which results in the best heat transfer conditions. High specific drying rates or relatively small dryer dimensions are possible.

Good vibrations

Since this ideal state can often not be achieved in practical applications, however, fluidised bed dryers are built as vibratory dryers in order to improve the fluidisation of the drying material.

Vibration ensures that even coarse particles which cannot be fluidised by the flow of air are transported better. But there are limits to this. Depending on the range of grain sizes and the apparent density of the material to be dried, the use of fluidised bed dryers is recommended only up to a grain size of about 6mm, with an absolute cut-off at grains of 8mm in size.

Coarse particles can accumulate in the dryer if these restrictions are not adhered to, as the grains cannot flow over the adjustable weir at the outlet of the dryer. If no adjustable weir is used (in order to prevent the accumulation of coarse particles), it is no longer possible to adjust the retention time of the material being dried by varying the height of the weir, or, in other cases, the poorly fluidised solid material is no longer ventilated correctly.

Optimal conditions

In general, the air distribution plates should be designed with a sufficiently high pressure loss in order to ensure good distribution of the drying air over the complete area of the dryer.

Therefore the fans for the drying air must generate a high pressure, otherwise the air will take the path of least resistance through the drying material; resulting in a poor air flow, particularly in places where the material is moist. For this reason, fluidised bed dryers have a higher specific consumption of electricity (about 150% of the energy consumption of a drum dryer).

Wherever possible, fluidised bed dryers should always be operated with solid materials comprising fine grains, since they are designed for this. If, for example, in the case of a product change, material with a considerably larger grain size enters the dryer, the fluidisation may collapse and efficient drying may no longer be possible.

Thanks to the fact that each particle of the drying material is continuously surrounded by air, sensitive moist solids or granulates from mixer granulators can be dried without damage in a fluidised bed. Due to the continuous lifting and dropping of the material, drum dryers cause a higher abrasion of the particles.

Just like drum dryers, fluidised bed dryers are normally regulated such that the temperature of the drying air leaving the dryer remains constant. Depending on the temperature of this air, a control loop adjusts the heat input to the incoming drying air and thus reacts to varying amounts of material or to varying moisture contents.

Because the temperature of the exhaust air is closely related to the temperature of the material at the end of the dryer, and since this determines the residual moisture of the material, it is possible to ensure constant residual moistures of the material being dried.

However, the specific fuel consumption per tonne of the drying material increases if the drying system is not operated at its design parameters. This may occur owing to a continuously low amount of material in the dryer, or if the material contains a low moisture content.

Since the amount of air entering a fluidised bed dryer must be kept constant in order to maintain the fluidisation of the material, the same amount of heat is lost with the exhaust air even if the dryer is operated at a reduced capacity. It is not possible to reduce the amount of air in order to maintain a high temperature of the drying air, as it is possible in drum dryers.

Fluidised bed dryers are particularly suitable for use in the chemicals industry and for the processing and recycling of materials which are easily damaged by high temperatures.

Which dryer?

Owing to the suitability of drum and fluidised bed dryers in a number of applications, the criteria which should ultimately determine which dryer you buy are:

● expected stability of the production conditions (throughput, moisture content, grain size)

● prices of the energy sources (electricity or gas)

● infrastructure at the installation location

● level of training and organisation of the operating personnel

● the services available at site of installation

The investments for a complete drum dryer or fluidised bed system, including the electrical control system, are often comparable. In some cases, the criteria are met by both systems. In most cases, therefore, the decision must be made on the basis of two or three of the main criteria which are of particular importance to the operator or the investor.

Contributor: Dr.-Ing. Mathias Trojosky, head of technology drying department, Allgaier Process Technology GmbH, Germany


Saving energy during drying

In cases where the drying material has to be cooled immediately, the use of a double-shell dryer for drying and cooling in a single unit is recommended. In a double-shell dryer the material is dried in the inner tube of the drum. The warm, dry material then flows from the inner tube into the outer shell, where it is moved in the opposite direction by suitably designed vanes and blades to be cooled by the incoming, cold ambient air.

With the System Mozer-TK, the dried goods can be cooled to about 55-60°C, which is sufficient for most applications in the building materials and minerals industry, and in particular in the production of mortar. If lower temperatures are required, a separate cooling drum, followed by a water-cooled contact cooler or a fluidised bed cooler can be used.

A particularly energy efficient drying and cooling drum is the System Mozer-TK Plus, which is used primarily for the drying and cooling of quartz sand. In contrast to the System Mozer-TK, the material is not cooled with ambient air. Instead, a certain amount of moist sand is mixed with the warm sand after this has passed through the inner tube of the drum.

The evaporation of the water in the moist sand cools the mixture, and the heat stored in the dry sand helps to dry the moist sand. In the TK Plus, the moist material is fed in from both sides.

Depending on the moisture content of the sand to be dried, 10-20% of moist sand is mixed with 80-90% of dry sand. This system permits the fuel consumption for drying to be reduced by about 15%.

In fluidised bed dryers, energy can be recovered by feeding the warm, filtered exhaust air from the cooling zone back into the drum as pre-heated drying air – particularly in cases where it is possible to achieve a good balance between the amount of drying air and the amount of cooling air. In the ideal case, all of the heat resulting from cooling of the dried material can be used for drying.

The energy consumption is then reduced to the amount of fuel which would be needed, without recycling of the exhaust air, to heat the ambient air to the temperature of the exhaust air from the cooler which is recycled.

In all variants of combined drying/cooling systems, the initial cooling of the dried material leaving the dryer results from the evaporation of the residual moisture. Further drying in the first part of the cooling zone results from the heat still stored in the dried material – rather like the drum dryer/cooler TK Plus.