From plastics to sugar A Special Bulk Materials Heat Exchanger Succeeds in the Sugar Industry
Around three years ago Coperion Waeschle launched the Bulk-X-Change bulk materials heat exchanger for the indirect cooling and heating of bulk materials. Its main success has been in the plastics industry, but as this article shows, it is equally suitable for many other bulk solids heat transfer applications, e.g. sugar.
White crystal sugar tends to cake in storage silos when the temperature and moisture content are high. After centrifuges have removed most of the water, the sugar is therefore further dried and cooled, generally in drum dryers. In many plants, production increases have overloaded the drum dryers, leading to excessive moisture contents and temperatures when the sugar is discharged to storage. The sugar industry needs new cooling and drying equipment that can be retrofitted to existing plants. To ensure trouble-free storage, one solution is to retrofit a plate cooler downstream of the drum dryers to reduce the temperature of the sugar to ca. 30 °C.
Such a system is installed at a sugar plant in Brugelette/Belgium operated by Raffinerie Tirlemontoise, a subsidiary of Südzucker. For various reasons, however, the sugar industry is looking for alternatives to this device. Südzucker therefore decided to run long-term trials with the Bulk-X-Change bulk materials heat exchanger manufactured by Coperion Waeschle. In the autumn of 2007 this heat exchanger was installed in parallel with the existing plate heat exchanger at Brugelette.
How the heat exchanger works
In the Bulk-X-Change heat exchanger, the bulk solid flows slowly and gently downwards under gravity through an arrangement of vertical tubes, while cooling water flows through a shell surrounding the tubes. A discharge device below the exchanger—generally a rotary valve, screw or belt conveyor—discharges the material in a controlled manner. An important feature is the special way in which product is fed into the tubes. Each tube has a funnel-shaped inlet, and the tubes are arranged so that there are no horizontal areas between them to encourage product deposits. As a result, the product flows regularly and evenly into the tubes.
When a pressure-tight rotary valve is used as the discharge device, it is also possible to inject air into the discharge cone of the heat exchanger. The air flows upwards into the tubes, countercurrent to the flow of bulk material, and is released in the buffer space above the exchanger. The counterflow of air removes any residual moisture from the bulk material, prevents condensation on the walls of the tubes, and significantly improves heat transfer.
A fair comparison
The picture shows how the special bulk materials heat exchanger was installed in such a way as to ensure that it could be tested alongside the existing cooler under exactly the same conditions. An overflow (not visible in the picture) at the inlet of the plate heat exchanger served to draw sugar into the bypass line feeding the Bulk-X-Change, and the cooling water followed a similar arrangement. A rotary valve controlled the throughput of the special bulk materials heat exchanger. The speed of the rotary valve was adjusted until the temperature of the sugar was the same as that from the plate heat exchanger. The initial trial did not use countercurrent airflow. Measurements included the inlet and discharge temperatures of the sugar; the flowrate, inlet and discharge temperatures of the cooling water; and the mass flow through the bulk materials heat exchanger, which was measured by timing the flow of sugar into a bucket. The heat capacity of the sugar was measured using a differential scanning calorimeter.
The tested product was white crystal sugar with the following bulk material properties:
- average particle size (x50): approximately 0.5 mm;
- bulk density: 850 kg/m3;
- individual particle density: 1,600 kg/m3.
The chart shows a typical set of values recorded at a throughput of 1,037 kg/h over a 16-hour period. The table shows enthalpy balances for the sugar and the cooling water for a one-hour period taken from the chart. The difference between the two calculations is small and within the limits of accuracy of the measurements. The plant capacity at this point was approximately 51.4 t/h. The inlet temperature of the sugar was 46 °C and the average discharge temperature was around 30.5 °C.
The heat exchangers operated around the clock for four weeks without any intervention. The plate heat exchanger and the Bulk-X-Change then became fully plugged at almost the same time. Investigation showed that the moisture content of the sugar supplied to both coolers had been too high, due to insufficient drying in the upstream equipment. The root of the problem is those occasions when all the centrifuges discharge at the same time. This increases the load on the drum dryer, and as a result, sugar leaving the drum dryer has a higher moisture content than usual. If the cooling water temperature is low, indirect coolers of any design will then face the worst possibility of condensation on their heat transfer surfaces, followed by plugging. The fact that this happened to both heat exchangers at nearly the same time shows that both devices are equally sensitive to high moisture levels.
Re-starting the Bulk-X-Change was simple, but for sugar manufacturers it is clearly preferable if condensation does not occur at all. To cure the condensation problem, a countercurrent flow of air was therefore introduced into the discharge cone. With this modification, it proved very reliable, even under critical operating conditions such as very low cooling water temperature combined with low sugar inlet temperature.