New standards in the production of dry materials
from liquids and suspensions

T‌he CD Dryer is an indirectly heated contact dryer. The drying process works as follows: The liquid is applied via a pump and feed pipes directly to the double-walled rotating discs arranged vertically in a pack. The discs are heated from inside by means of saturated steam. The direct heat transfer to the liquid product ensures very fast evaporation of the water and accordingly excellent efficiency as heat losses are minimized. After one disc rotation, scrapers attached to the discs remove the dried solids and feed the dried materials via a product discharge to the next process step – in the form of granulate, films or flakes, depending on the starting product. Solids from concentrated salt solutions can, for example, be obtained as fine-grained powder. With the help of downstream conveyors, the product can be transported further or, with the use of a silo or another container, collected directly.

The operating principle of the CD Dryers differs fundamentally from conventional disc dryers in which the discs are submerged in the wet material. They are mainly used only for the reduction of the water content of liquid, paste or bulk products, for example, to save on transport costs. The CD Dryer offers much more diverse possible applications and opens up great potential in a wide range of applications.

Many applications with small space requirement

One branch in which the CD Dryer obtains excellent drying results is, for example, the treatment of organic and inorganic industry and special waste-water. A large part of the industrial waste water comes from the foodstuffs segment: abattoirs, breweries, lemonade factories, distilleries and dairies produce large volumes of it and profit accordingly substantially from lower waste disposal costs thanks to product treatment with the CD Dryer. Examples of inorganic waste water that can be concentrated or dried with the CD Dryer are landfill leachate, bentonite and kaolin slurries, waste water loaded with tin as well as tungsten slurries.

Another area in which the CD Dryer can be used for extremely cost-efficient drying is the processing of a wide range of vegetable and animal substances such as algae suspensions, beer yeast, distillation residue, fish water or gelatines. Moreover, the CD Dryer dries all pumpable, non-caramelizing substances that do not contain any solid constituents and do not foam strongly. Accordingly, it can be used in almost all processing branches from the chemical industry through the ceramics industry to waste management, waste-water treatment  and breweries.

In recycling and reprocessing, efficient and simple contact drying is becoming increasingly important thanks to its comprehensive advantages. These benefits include cost reduction for the disposal of liquid from waste materials and the recovery of valuable solids from suspensions/solutions.

A medium-sized CD Dryer (model CD-908) was put into operation in Northern Spain. This dryer is used to recover a high-quality salt from a suspension. The disc dryer has a heat exchange area measuring 8 m² and processes 700 kg liquid per hour with a water content of around 85 %. Water evaporation with this compact dryer amounts to a remarkable 540 kg/h. Here the CD Dryer impresses with its simple and rugged principle of operation. The optional process solution (chamber filter press and an additional dryer downstream) was not acceptable for energy reasons and on account of the higher investment costs, but it is not necessary thanks to the introduction of the CD Dryer. With the help of disc contact drying, the product can be dried to final moisture content of 20 – 30 %, and accordingly it can be further processed immediately.

Another customer uses the CD Dryer especially for processing a liquid loaded with microorganisms. The liquid is dried from around 95 % water content to < 5 % residual water content in the solid product. Primary goal of this treatment is the significant cost saving for product disposal. Instead of several hundred litres of waste-water per hour that must be treated in a treatment plant, only a few kilograms solids are recovered and disposed of. In addition, the transport and handling of a dried powder incurs lower cost than liquid transport of the product.

In both practical applications, the CD Dryer technology scores points especially with its extremely compact set-up. The machine housing of the disc dryer appeals with its appearance and is used to house the necessary components. e.g. the disc drive, the steam/condensate fittings, the bearings as well as the switch cabinet in one machine unit. As a result, it is easy to transport the dryer and install it without any great difficulty or cost at the production site. Moreover, the housing protects the dryer operative from any rotating and hot components.

A major influence on the small dryer footprint is the design of the disc pack. In comparison with heat exchangers like cylinders/rollers, despite providing identical heat exchange surface area, the disc design reduces the space requirement of the core component by up to 60 %. Despite this compactness, when service or maintenance is required, the dryer can be easily and conveniently accessed through the hinged doors of the technical compartment and the service openings on the rear of the machine.

In all these application areas, the new CD Dryer presents an interesting alternative to conventional roller dryers, which are generally used to dry solids dissolved or suspended in liquids. The space requirement for the discs in the CD Dryer is up to 60 % smaller than that of the rollers of a comparable roller dryer with same heat transfer area. The compact design has another advantage: if needed, the dryer can be moved quickly and simply as one unit to another site. The forklift pockets needed for transport are located below the process chambers. The CD Dryer offers the widest flexibility in production.

Disc pack as the technical heart

The heart of the CD Dryer is the pack of discs. The hollow discs are heated from the inside by means of saturated steam with up to 5 bar pressure. This enables efficient contact drying of the liquid products. Depending on the application or product characteristics, the disc material and the coating of the discs can be adjusted to handle e.g. severely corrosive or even abrasive products. Also depending on the application, the discs can be made of normal steel or stainless steel, and in line with requirements, a special coating can be applied against corrosive and abrasive wear.

Diameter and number of discs are oriented to the required throughput rate. Two options are available: Discs with a diameter of 900 mm or 1300 mm. Both disc diameters are available in three housing sizes. For up to 4, 8 or 12 discs (900 mm diameter) and for up to 8, 12 or 16 discs (1300 mm diameter). If higher throughput rates are required, thanks to this platform design, additional discs can be added to the CD Dryer – up to the maximum possible number of discs for the specific type.

The rugged scrapers for removing the dry material from the heated discs align themselves automatically for optimum positioning on the sides of the discs. For the scrapers, depending on the specific properties of the products to be handled, various stainless steel, plastic or ceramic materials are available.

Whether the production process is running as required can be seen at one glance with the CD Dryer. The hood made of laminated safety glass enables easy monitoring of the process during dryer operation. In opened position, it also allows comfortable access to the processing chamber for cleaning or inspection or adjustment of the blades. The two technical chambers – on the left the steam chamber, on the right the condensate chamber – are fitted with inspection glass as well as having integrated lighting for visual checks. For maintenance work, the large hinged doors enable good access to the technical chambers. The current machine or process data can be intuitively displayed or changed on a touch panel.

Energy-efficient drying

With the CD Dryer, Allgaier is committed to disc drying technology. Compared to drying in roller dryers, it offers crucial advantages: the space required by the central disc pack is much smaller than that for a roller dryer. This is shown by the following example: for a heat transfer area of 24 m², a conventional roller dryer needs 7.7 m² of space. A CD Dryer needs just 3.0 m².

Moreover, the process of scraping off the dried products is comparatively complicated in a roller dryer. The blades have to be very precisely adjusted over a length of several metres. Assembly and maintenance are accordingly time- and cost-intensive. In the CD Dryer, several discs work parallel. Each disc has its own short scraper that aligns itself automatically and is easy to fit and maintain.

And when it comes to energy efficiency, the CD Dryer scores further points. The thin-walled, rugged discs of the CD Dryers guarantee ideal thermal conductivity and therefore a high specific evaporation rate per square metre heated dryer surface with a short dryer preheating time. In comparison, with convective drying processes in air operation or with gas circulation, the CD Dryer is a straight contact dryer and does not require any air supply for heat input as heat is supplied to the wet product in direct contact with the heated surfaces. The direct contact between of dryer and liquid lowers the energy consumption in the drying process considerably. Moreover, with the CD Dryer only small – usually dust-free – waste air streams are produced during the drying process. The low volume of vapours formed is discharged with a high water vapour load. As a result, comparatively low heat losses are incurred with the dryer exhaust air.

The reduced quantity of exhaust air compared with convective drying systems has a positive effect on the investment and running costs. Downstream equipment – such as cyclone, filter, washer and exhaust air fan – can be designed on a smaller scale and therefore realized at a much cheaper cost.

Established technology reinterpreted and exclusive on the European market

The CD Dryer technology has been widely established especially in Asia for decades. Allgaier Process Technology produces and markets the CD Dryer exclusively for Europe and has adapted the completely technology to meet European CE conformity criteria. The design was completely reworked and presented for the first time in its innovative form at ACHEMA 2018 in Frankfurt. Accompanying the new dryer design, the safety standards were increased massively so all rotating and warm machine components are installed in the housing. The intuitive dryer control with a touch panel with remote maintenance option and process visualization during operation with the new design meet the highest standards. The CD Dryer has therefore impressed in the ACHEMA Innovation Award and was included on the short list for this prize.

With an experienced partner and its own extensive tests at the test centre in Uhingen Allgaier Process Technology offers a unique contact dryer for treating solids-containing liquids. It demonstrates exceptional robustness when handling load fluctuations and changes in the composition of the suspensions and solutions. With tests on test-centre scale, the suitability of the CD Dryer for drying a wide range of waste water concentrates has been analysed and confirmed. For design of industrial CD Dryers, tests on pilot scale are conducted at the test centre. With the tests, the specific evaporation rate per m² disc surface, the achievable dryness of the solids produced as a function of the disc speed (dwell time), the optimum steam pressure for heating and the associated optimum surface temperature of the discs for efficient drying and the achievable quality of the dried materials are determined. The results of the tests enable reliable design of the industrial-scale dryers. For this purpose, a pilot plant is available at Allgaier‘s test centre in Uhingen for prospective and existing customers to conduct tests with real products.