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Condebelt drying for pulp and paper industry

The Condebelt drying process was patented in 1975. The Condebelt drying process was originally created to increase drying rates of paper. Condebelt drying produces approximately 10-15 times higher drying rates than conventional cylinder drying. These higher drying rates are achieved by higher contact temperatures, higher pressing between the hot surface and paper and relatively low thermal resistance between steam and paper in the Condebelt drying process.

According to Retulainen (2001), three features of the technology warranted further investigation after the first pilot tests:

1) Improved strength and other quality characteristics
2) Increased drying rates
3) Possible energy savings

Despite being patented in 1975 and being proven to have several advantages over conventional cylinder drying, the Condebelt process and machinery are still in the development phase. As such, the information on Condebelt drying is scarce.

Introduction top

A lab-scale Condebelt dryer is illustrated in Figure 1. The hot top platen is stationary in vertical direction, while the cold bottom platen is movable. The bottom platen is driven by a piston underneath the bottom plate. The piston is powered by a hydraulic power unit. The press pulse and thermal oil heating of the top and bottom platens are software controlled. Experiments can be performed under atmospheric pressure or under vacuum in the chamber.

illustration © climatetechwiki.org

Figure 1: Lab-scale Condebelt dryer (click image to enlarge) Source: Timofeev, Ilomaki & Kuusela, 2004

Feasibility of technology and operational necessities top

In the Condebelt process, the paper web sits atop two permeable wires and is fed into an extended nip between two smooth steel belts, as seen in Fig. 2. While the upper steel belt is heated by high-pressure steam at temperatures of 110–160°C, the lower steel belt is cooled by circulating water at around 80°C. The top side of the web is in direct contact with the upper steel belt, while the wire side of the web contacts a fine wire with a coarse wire underneath. Vapor generated from the web passes through the wires and condenses on the lower steel belt. Water condensed on the surface of the lower belt or in the interstices of the wires is removed with a doctor or by applying vacuum. A web dried under such unsymmetrical conditions will show distinct two-sidedness. However, for certain grades such as linerboard and corrugating medium, two-sidedness may not be of great concern.

illustration © climatetechwiki.org

Figure 2: Condebelt drying process (click image to enlarge) Source: Lae Lee, Jung Yuon & Min Jung, 2000

The essential feature of the Condebelt process is the use of the cold belt as a condensing belt.

The conventional cylinder dryer uses a large ventilation system. This is not required in the Condebelt drying process, as evaporation energy is transferred to the cooled belt by means of vapor diffusion and condensation inside the closed drying chamber.

illustration © climatetechwiki.org

Figure 3: Effect of Condebelt Drying on Core Board (300–550 g/m2)(click image to enlarge) Source: Retulainen, 2001

The Condebelt drying process enhances web consolidation, which results in an increase in paper density and strength as seen in Figure 3. Condebelt-dried sheets have extremely good smoothness on the side which is in contact with the hot metal surface.

Status of the technology and its future market potential top

The development of Condebelt from idea to a commercial product took twenty one years. The first patent was issued 1975 and the start-up of the first mill scale installation was 1996. Two commercial Condebelt dryers are employed today on board machines (Lee, 2002; Retulainen et al., 1999). The first Condebelt dryer was installed in Finland in 1996 and second -in South Korea in 1999. In both cases the new drying system was successfully applied for drying of board grades. Strength properties of board were improved in comparison with sole cylinder drying and drying capacity was substantially increased. In spite of good results in commercial use, difficulties were experienced sometimes such as spot delamination of the web and limited energy transfer to the cooling water. The Condebelt drying remains a relatively new drying process in the paper industry. It is evident that more research is needed for better understanding of heat and mass transfer phenomena inside the dried sheet and inside the drying chamber. More knowledge will help to maximise all the process benefits, as well as to identify its limitations.

The results obtained from the first mill scale applications have clearly exceeded the early expectations put on the paper properties. Retulainen (2001) concludes that while the technology has not become a success story yet, it has the capacity and advantages to become one. 

How the technology could contribute to socio-economic development and environmental protection top

Retulainen (2001) posits that the experiences with Condebelt drying so far have confirmed that the main advantage of the Condebelt dryer lies in the improved linerboard properties. This improvement is illustrated in Figure 4.

illustration © climatetechwiki.org

Figure 4: Improvements in compression strength due to Condebelt drying: Edge crush test (ECT) values of corrugated board and Box compression strength (BCT) values of corrugated containers.(click image to enlarge) Source: Retulainen, 2001

 

 

The Korean installation has shown that the corrugated containers made of Condebelt dried board show remarkable improvement in edgewise and box compression strengths (Retulainen, 2001). The improved compression strength of the board has made it possible to substitute conventional triple wall board with double wall Condebelt dried board. This is a considerable advantage for the converter. The better printability and dimensional stability has been verified both by pilot scale tests (Retulainen et al., 2000) and mill experience. The experience indicates that the advantages can be best utilized within a integrated company containing pulp, board and converting operations. Many of the potential advantages proposed by the laboratory and pilot scale tests (Retulainen, 1997), such as reduction of basis weight, using higher yield pulp, increasing the amount of recycled pulp, or producing completely new board grades have hardly yet been utilized in practice. 

Financial requirements and costs top

According to Retulainen (2001), the first Condebelt installation, using a narrow machine, already showed that the energy and maintenance costs of the Condebelt dryer were similar to conventional cylinder drying. Retulainen (2001) argues that with a wider machine the energy consumption would be clearly better, resulting in lower energy costs.

Moreover, Retulainen (2001) notes that the technology provides the possibility to reduce raw material costs by using lower quality raw material or using lower basis weight board due to the increased strenght and performance of the end product. In addition, the faster drying rates of the Condebelt process might result in financial advantages as more paper and pulp can be dried within the same time period.

However, more investigation and research is required to establish the financial aspects of the Condebelt drying process.

Clean Development Mechanism market status top

The Condebelt drying process still requires more research and development before it can be actively implemented in the form of CDM projects. For more information on the CDM baseline establishment and latest news see: http://cdm.unfccc.int/

References top

Timofeev, O.N., J. Ilomaki, J. Kuusela (2004). Effect of process parameters on paper temperature in condebelt drying. Drying 2004 - Procceedings of the 14th International Drying Symposium (IDS 2004) Sao Paolo, Brazil, 22-25 August 2004, Vol. B, pp. 1327 -1334. Document can be found at: www.feq.unicamp.br/~ids2004/volB/pp%201327-1334.pdf  

Lae Lee, H., H. Jung Yuon & T. Min Jung (2000) Improvement of linerboard properties by Condebelt drying. Tappi Journal peer reviewed . paper. Document can be found at: http://www.tappi.org/Bookstore/Technical-Papers/Journal-Articles/TAPPI-JOURNAL/Archives/2000/July/Improvement-of-linerboard-properties-by-condebelt-drying-TAPPI-JOURNAL-July-2000-Vol-837.aspx

Retulainen, E., (2001). Key development phases of Condebelt: Long journey from idea to commercial product. Drying Technology, 19: 10, 2451 — 2467. Document can be found at either DOI: 10.1081/DRT-100108248 or URL: http://dx.doi.org/10.1081/DRT-100108248

Retulainen, E., Kangas, J., Kunnas, A. and Oittinen, P., 2000, Printability of Condebelt dried linerboard. The 54th Appita Annual Conference Melbourne, Australia. 3–6-April 2000. Proceedings, Volume 2, 3A22 pp. 403–408.