Water reduction approaches are very complex and bring to a progressive closure of the process water loop, which leads to accumulation of contaminants and may cause paper defects, deterioration of product quality and problems with tissue machine run-ability. The degree of water loop closure is strictly correlated with the negative impact that may affect the paper process and the capability to support it in an overall efficient system. In this context, it is crucial to establish the right equilibrium between advantages and disadvantages relevant to water consumption restrictions. In this article, we present the best practices adopted by TOSCOTEC, which ensure an overall plant fresh water consumption of less than 4 m3/ton paper. All values reported below refer to a typical Toscotec tissue plant AHEAD 2.2 with a max production capacity of 125 tpd. All water consumption figures calculated do not include fresh water use for chemicals preparation.
1.1 Machine showers
Toscotec has optimized the design of machine showers by reducing their water flow and pressure, expecially on lubrication showers and by preferring the use of CW or FCW over fresh water whenever possible.
Morover, the upgrade of their mechanical design (nozzle spacing and pitch, and orifice size) was aimed at reducing water consumption. In the chart below, optimized shower’s flows for Toscotec AHEAD 2.2 tissue machine plant are summarized:
| Tab. 1: Tissue machine shower requirement for 2.2 plant | |
|---|---|
| Shower requirement | Total flow (lpm) |
| Continuous filtered clarified water | 2645 |
| Discontinuous filtered clarified water | 160 |
| Continuous fresh water | 60 |
| Yankee coating spray boom | 8 |
In this configuration, Toscotec ensures a tissue machine water consumption lower than 2 m3/ton paper while guaranteeing optimal machine performances.
1.2 Suction roll sealing technologies
A traditional suction roll seal lubrication shower for a Toscotec AHEAD 2.2 tissue machine requires a fresh water flow of about 66 lpm to guarantee proper seal lubrication. Thanks to Voith’s new HydroSeal technology, this fresh water requirement has been reduced by approximately 87%. The required flow with HydroSeal is around 8 lpm. HydroSeal is a seal strip with a lubrication system designed to drastically lower water consumption in suction rolls installed in the forming and press section. Upon installation of the HydroSeal seal strips, the mill achieves 78% reduction in water lubrication for seals.
Fig. 1: Cooling water circuit configuration: open vs closed loop.
1.3 Cooling utilities management
In paper making plants, lubrication units, gearboxes and other utilities are usually cooled down by water.
Generally, cooling water can be managed through an open or a closed loop (Fig. 1).
Close loop approach is usually more energy demanding than an open loop one, due to the cooling unit energy consumption. There are several cooling units available on the market which have different operation concepts leading to different energy and water consumption figures. Chiller units have the highest energy consumption but do not need a constant make-up of water. A proper chiller unit allows to save water and reduce the annual water consumption up zero, but it requires an annual energy consumption of 307,979 kWh. Besides the energy and water evaluation, it is also important to underline that mill location plays an important role in the selection of the cooling unit.
1.4 Stock preparation
The following water-demanding utilities can be found in the stock preparation system:
- Agitator and pump seal lubrication.
- Hoses.
Selecting the right sealing type can make the difference in the water consumption of the overall plant. In general, gland packing seals require 5 to10 lpm which can increase if the sealing unit is not well regulated. In a typical tissue machine plant with a max production capacity of 125 tpd where pumps and agitators are equipped with gland packing, the required water seal is about 100 lpm which accounts for 30% of the entire incoming fresh water flow.
1.5 Additional water recovery with energy efficiency
In a tissue machine, the final dewatering is done through evaporation with the Yankee dryer and the hoods. The evaporated water, typically 1÷1.3 twater/tpaper, is available in the exhaust air flow in the vapour state. The heat recovery units that are installed on the line for various purposes (including building and process water heating) can recover more than 50% of the above-said water vapour by condensing it. The remaining water is usually wasted out. In order to reduce the overall water consumption of the tissue machine, Toscotec optimizes water condensation by installing additional heat exchangers to cool down exhaust air using atmospheric air as the cooling media. The energy consumption is approximately 15Wh electric/kg water. When it comes to heat pumps, energy efficiency and water recovery go hand in hand. The main function of heat pumps is to increase energy efficiency, but they also save water. Hood exhaust is cooled down causing high vapour condensation which releases energy; this thermal power is converted into steam by high temperature heat pumps. In this way, water recovery and steam generation together achieve a higher level of efficiency.
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Conclusion
The water consumption of today’s papermaking processes varies between 5 and 20 m3/ton paper. Toscotec has taken up this technological challenge and is actively contributing to the reduction of water consumption in tissue making through the development of new technologies and strategies which require less water consumption. In this article we summarized all the best practise and technological developments which allow to reach an overall plant freshwater consumption lower than 4 m3/ton paper.
Thanks to machine showers optimization and Voith’s new HydroSeal technology, it is possible to obtain a total tissue machine water consumption lower than 2 m3/ton paper. To this result, we have to add the stock preparation water consumption that can be lowered down to 2 m3/ton paper through cooling utilities strategies and seal selection.
