Wooden or polypropylene drums?

Globalisation of the leather industry brings the tanner many tough challenges such as making tanneries as environmentally friendly as possible, sustaining higher than ever quality standards, optimising space, saving energy and reducing running costs. 'Environmentally friendly' sounds simple but involves many very different issues. It means a reduction of polluting effluent, thus saving treatment costs; it means energy saving and it means a high standard of articles for demanding customers. In this context new elements of drum development have been added to aid this new generation of tanners. Drum structure To achieve the optimal mechanical effect, some modifications were introduced to the recycling system so as to achieve a high turbulence liquid flux. In figure 1 we can see an optimised polypropylene drum, compared with figure 2, a wooden drum layout. The first advantage of polypropylene is its particularly smooth surface, high chemical corrosion resistance even to peroxides at normal liming temperatures and excellent mouldability, allowing a design of high performance recycling systems. Outer size with respect to volume Beacuse the polypropylene drum wallsare thinner they can hold more float than wooden drums with similar outer dimensions. This increase may vary from between 11.6% in a 4m x 4m drum up to 17.8% in a 2.5 m x 2.5 m drum, as can be seen in figure 3. Analysing the problem physically and theoretically and with the data actually available, there is no advantage in thermal insulation. The thermal conductivity constants are of the same order: Iroko wood: k = 0.18 Kcal. m / hr .m².°C polypropylene: k = 0.19 Kcal . m / hr .m².°C The only data available refers to the same condition, ie dry. Reality shows a different result since polypropylene, being impermeable, will remain dry. Obviously wood will absorb humidity until equilibrium is reached. Using a thickness of 75 mm for the wooden drum and 20 mm for polypropylene one, and using the following calculations: Q = kADt/Sp, where Q = heat dissipated after one hour, k = thermal conductivity constant, A = area, Dt = thermal gradient, Sp = drum wall thickness. It was found that the heat dissipated in one hour was 1154 Kcal/hr for Iroko wood and 4570 Kcal/hr for polypropylene. This calculation was applied for a 3 x 2 drum, loaded with 7 cubic metres of water at a temperature of 55°C. To verify these calculations, a practical test of the system was carried out. Two drums of comparative outer dimensions were filled with water at 65°C up to 40% of the useful capacity. The temperature of both empty drums was 23°C and external temperature 22.8°C. After running 10 minutes the temperature came down to 51.4°C in the polypropylene vessel and 51.2°C in the wooden one. The temperature was measured every hour and then plotted against time, figure 4. The result shows the opposite conclusion of the theoretical considerations, confirming the evident fact of a much higher thermal conductivity constant of the wet Iroko wood (in between the water and dry wood value). New studies are being carried to determine a reasonable thermal conductivity constant for wet wood to be applied in thermodynamic drum calculations. Materials deterioration Another interesting point to consider is materials resistance and deterioration. Two retanning drums which worked under similar conditions for five years were compared by microphotography with the same materials in new vessels. The results can be seen in figures 5 to 8. In these images can be seen how wood deteriorates with time, figures 5 and 6. After long use, the surface became rougher and thus more abrasive to the skins. In most cases and especially if the drum is used with low or no float, the grain layer may be seriously damaged. Figures 7 and 8 show the effect of processing on a polypropylene vessel. After five years, the inner walls in a polypropylene drum remain flat and smooth. Some minor scratches can be seen on the surface. Chemical stability One interesting advantage of polypropylene respect to wooden drums is its chemical stability. Polypropylene is stable against hydrogen peroxide, sodium hydroxide, sodium sulfide, calcium hydroxide, organic acids, ammonia, and hydrochloric acid at maximum concentrations. While it is stable to sulfuric acid, polyethylene may be slightly attacked by Oleum. This high stability to acids and bases allows for the elimination of dye stains from the vessel by using highly concentrated caustic products. This is normally required when the tanner needs to dye different colours in the same vessel. Future possibilities are seen for highly oxidative liming processes, were Inox vessels have been discarded, and wood is destroyed after short process periods. Chemicals exhaustion To achieve comparable mechanical action due to speed respect to inner dimensions and load, the following calculation was performed using the equation s_D = (42.2/Ø) x -√2(√(ωL/π x L)) where s_D = optimal drum speed, Ø = internal diameter of the drum, ωL = drum's load in ton and L = drum's inner width. All this supposes that at a same certain percentage of the optimal speed, two drums will have a comparable mechanical action. Using this calculation it is possible to plot a 3D graph where it is easy to see the variables of an ideal drum, figure 9. Once the speeds to achieve the same mechanical action theoretically were calculated, different comparisons during bulk processing were made. The first test was carried out for chrome exhaustion during rechroming, after a ten minutes process and a 12 hours process, without basification, figure 10. With the Iroko wood drum, around 56% of the chrome was absorbed, while in the polypropylene drum the value is about 65%. Again, a higher temperature in the polypropylene vessel was seen. This effect may be attributable to temperature conservation, the increased turbulent flux of the improved vessel or, more likely, to both effects combined. For this reason a similar test during vegetable retanning was done, where the temperature was kept lower. This time the first sample was taken after a five minutes run, and the next 20 minutes later. The results are shown in figure 11, where again a higher exhaustion in the polypropylene vessel is seen. This time exhaustion went from approximately 62% of the tannins in the wooden drum to about 84% in the polypropylene one. In this case the effect may be attributable to the highly turbulent flux and not to the temperature, which was similar in both vessels and which in so short a period of the sampling, remained stable. Conclusions: Polypropylene drums showed many advantages in the long term with respect to wooden vessels. In the short term they showed a higher versatility, combining the cleaning advantages of the Inox drum to the thermal properties of wood coupled with higher chemical resistance. The high turbulent flux recycling system applied to this vessel could only be done because of the mouldability of this material. Finishing the drum is finer especially with the recycling holes improving the flow of small shavings and cuttings and reducing the risk of staining when changing colours. Most of the trials were carried on in tanneries working both in wooden and polypropylene drums and following similar processes. Because it is not always possible to achieve the same load, for these trials purpose, speed in the polypropylene drum was adjusted to achieve similar mechanical action as described in the section devoted to chemicals exhaustion. Acknowledgments Eng. Ferdinando Ridolfi for his contribution on thermodynamics references and Eng. Sergio Cantini for the drums internal design. Addresses 1. Italprogetti Engineering s.r.l. , L.no Pacinotti 59/a - 56020- San Romano, PI, Italy 2. Genesis Ecotec s.r.l. , Via Lombardia 43 - 56029- Santa Croce sull'Arno PI, Italy