than that of conventional evaporation processes for the initial concentration and de-ashing of the gelatin liquor. INTRODUCTION. Gelatin is manufactured by ...
Desalination , 78 (1990) 279-286 Elsevier Science Publishers B.V., Amsterdam -
279 Printed in The Netherlands
Concentration and Purification of Gelatin Liquor by Ultrafiltration B. CHAKRAVORTY and D.P. SINGH Chemical and Biochemical Engineering Division, Thapar Corporate Research & Development Centre, Patiala - 147 001 (India)
(ReceivedMarch 7,199O) SUMMARY
Weak gelatin liquor has been concentrated to 15% and above from initial concentration of 5% dissolved solids by the application of ultrafiltration techniques. Applied pressures in the range of 220-350 kPa have been found to be optimum for the concentration as well as purification. An economic evaluation based on the experimental results shows that ultrafiltration is cheaper than that of conventional evaporation processes for the initial concentration and de-ashing of the gelatin liquor. INTRODUCTION
Gelatin is manufactured by extraction of proteinaceous materials from different raw materials such as demineralised bones (Ossein), cattle hides and pigskins. These raw materials, which are pretreated to remove non-collagenous components, are brought to a high temperature in contact with water at neutrality to obtain gelatin solution of moderate gel forming properties, low viscosity and poor colour. Mild acidification of the extraction liquor accelerates conversion of collagens into gelatin at moderate temperature. Heating the collagen in an alkaline medium likewise speeds up conversion, but at the same time it also promotes other degradation process, whereby the gel forming properties are impaired. For this reason, most of the industrial processes are based on neutral or acidic pH values. In the conventional process the gelatin liquor is produced in six to ten hours and has a concentration of 3-S% gelatin. This liquor is filtered and fed into a
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triple effect evaporator to concentrate up to 35% gelatin. Final processing involves drum drying to about 85-90% followed by crushing, testing, blending and packaging. In the triple effect evaporator about 80-85% of the water is removed. This energy intensive dewatering step is sensitive to increase in steam generation cost which is related to the cost escalation of coal or other similar fuels. A major fraction (40%) of the total steam consumed in gelatin mill is required for the evaporation. The cost escalation rate has been found to be much more in the case of steam than those of electric power. These economic factors encouraged us to examine alternative energy efficient processes for dewatering gelatin liquors particularly in the initial concentration stage where bulk of water is removed. The ultrafiltration (UF) process offers a great potential in dewatering of gelatin liquor as macromolecules are rejected by membranes while water along with inorganic molecules are permitted to migrate across the membranes. The UF separation process requires less energy for dewatering in comparison to that of evaporation as there is no phase change involved in the former. The molecular weight of gelatin falls in the range of 15,000 to 250,000 with an average of 50,000 to 70,000. The high molecular weight of gelatin makes it extremely suitable for better separation efficiency across the UF membrane at moderate to low applied pressure. Simultaneous purification of gelatin also takes place along with dewatering through removal of small inorganic molecules. The major advantages of UF include reduction in the volume of water which ultimately needs to be evaporated in the evaporators. Thermal degradation of protein molecules is expected to be lessened in this case, as shorter residence time is required in the evaporators. Modular arrangement of UF systems makes it relatively simple for incremental additions to the overall capacity of the mill. This paper attempts to examine viability of indigenous UF systems as an alternative to the expensive evaporation process for dewatering of the dilute gelatin liquor. EXPERIMENTAL Gelatin liquors of desired concentrations were prepared by redissolving technical grade gelatin flakes in warm water followed by filtration. The dewatering experiments were conducted on cellulosic as well as non-cellulosic membranes with an effective area of 0.182 m2 to 2.8 m2 and a nominal molecular weight cut off up to 20,000. Plate-and-frame and spiral wound configurations of UF membranes were used. The batch concentration experiments were conducted by recirculating the retentate through the UF system till desired concentrations were achieved. Heat exchangers were used in the feed
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tank for heating or cooling which also provided the means of temperature control within the system. UF performance in terms of permeate flux, rejection efficiency and quality of the retentate were investigated. Changes in the gelatin properties and loss of proteins in the UF treated liquor were evaluated. Concentration of gelatin solutions were measured gravimetrically as well as by specific gravity measurements. Viscosity of gelatin liquors were either measured on a Brook Field or on a Ostwald viscometer at a concentration of 12.5% at 40°C. Protein analysis were done following Loury’s method or calculated from nitrogen content. Quantitative analysis of nitrogen was done in Carlo-Erba elemental analyser. Ash contents were found out by gravimetric methods. RESULTS AND DISCUSSION
The effect of variation in the transmembrane pressure on flux is shown in Fig. 1. The results show our increase in flux for an increase in pressure up to the gel polarised condition[ 1,2]. Beyond the gel polarisation point, flux value remains unaffected on further increase in pressure. This indicates that the rejected proteins have formed a gel layer at the membrane surface which limits the flux. Thus for the gelatin liquor, flux becomes independent of pressure in the region of 350 kPa. Dewatering experiments on spiral wound modules were therefore conducted at this optimum applied pressure. However applied pressure
TRANSMEMDRANE
PRESSURE
Fig. 1. Effect of pressure drop on flux.
( K P,)
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was kept at 200 kPa in the plate-and-frame system where more open membranes were used. Further improvements in the efficiency of the process can be achieved through optimization of recirculation velocity, temperature and concentration of solutes. Higher flux values were obtained on increasing the recirculating velocity of the gelatin liquor. Increase in temperature improves the flux from 2 to 5% per “C[ 11. Advantage of higher temperature on flux could not be exploited beyond 40°C in this case, due to lower thermal stability of the cellulosic membranes. At any fixed temperature, recirculation velocity and applied pressure the flux value was observed to decrease with increase in concentration of gelatin liquor. A linear relationship between concentration and flux was observed up to a concentration of 13%. At higher concentrations the relationship changed from linear to slightly curvilinear as also reported by others[2,3]. UF is a mass transfer process where selective separation, based on size, takes place across a membrane which is kept under a pressure gradient. The separation efficiency of an ultrafiltration membrane is expressed as follows[2]:
Where Cp and CB are the concentrations of solute in the permeate and in the bulk of the feed respectively. In an ideal separation or concentration of gelatin CT would be 1 for protein and zero for other inorganic species. Under such ideal conditions purification (or de-ashing) of gelatin takes place simultaneously with the dewatering. Table I shows the separations achieved for protein macromolecules from water and inorganic species. It also gives an idea about the quality of concentrated gelatin reported in terms of viscosity of the retentate. The protein analysis of permeate and retentate from all the batches show a rejection of 0.97 (i.e. about 3% loss) independent of feed concentration. The loss of protein remained more or less unchanged, on both types of membranes having different molecular weight cut-off. Increase in the viscosity of the retentate before and after drying the concentrated gelatin liquor suggests that the gelatin lost is of poor quality. The loss could however be reduced by the use of tighter membranes at the cost of reduction in flux or an increase in operating pressure. Higher loss of protein has been noticed at operating pressures above 350 kPa which is accounted for by hydrolytic fragmentation of these macromolecules. It is, therefore, not advisable to increase operating pressure while using tighter membranes, as the overall loss is likely to be much more than 3%. De-ashification or purification of gelatin is an additional advantage of UF
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over evaporators. Table I shows the extent of de-ashification achieved in this work using indigenous as well as DDS system. While low molecular weight cutoff indigenous membranes reduced ash content by 22% the DDS system reached 45-50% de-ashification. The higher de-ashification in the latter is possibly due to larger pores in the membrane than that of the former. Various other factors controlling de-ashification is dealt in greater depth by Fane and his coworkers[4]. ECONOMICS OF ULTFL4FILTRATION The ultrafiltration of gelatin liquor for the preconcentration has been shown to be technical feasible. The appropriate incorporation of UF with evaporators ensures energy efficient removal of water from the weak gelatin liquor. UF removes major part of water leaving the rest for the evaporators. This section examines the economic advantages of UF - evaporator combination over that of the evaporators alone. The indigenous UF system has been compared with a triple effect evaporator for a 2 tons/day capacity gelatin mill. About 40 m3 or weak liquor (at 5%) is available for dewatering to raise the concentration to 35%. To achieve 35% consistency 34.3 m3 of water is required to be removed from the weak liquor. UF is proposed for the concentration of the liquor from 5% to 15% thereby removing 80% of the total water required to be evaporated. The remaining dewatering can be done by evaporation. The foregoing section, therefore, compares the economics of UF and evaporation for the removal of initial 80% of the water from the weak liquor. Tables II and III indicate the economic superiority of UF over that of the evaporators. In this role, UF could lead either to a reduction in overall cost for a new mill or provide an economically attractive means of increasing plant capacity for an existing evaporator. In the above economic analysis the cost penalty associated with incomplete rejection of gelatin has not been accounted for. To compensate the 3% loss, which is around Rs.l500/day for the plant under consideration, it is suggested to increase the cost of final product at least by the same margin. This is particularly justified as the quality of the UF concentrated gelatin has been reported to be superior than that of the existing product. Alternatively the process cost could be reduced by reusing the permeate in the extraction unit. Reuse of the permeate would reduce the fresh water consumption in the mill besides offering a second chance for the recovery of lost protein in the subsequent stages of dewatering.
285 TABLE II Economic analysis of dewatering by evaporation. Removal of 27 m3 of water/day Number of working days/year - 350 Capital investment on evaporators:
Basis
(i) (ii) (iii) operational cost (RsJyear) (1)
Steam
Cost - Rs. 25O/ton Steam economy - 2.4:
984,000
FbWtX
(2)
240 kWh/day Cost Rs. 1.25ikWh: Labollr Number - 10 Wage - Rs. 50/&y: Depnxiation Rate - 10%: Maintenance:
(3) (4) (5) Total cost
Rs. 800,000
105,000 175,000 80,000 50,000 1,394,OOO
:
Dewatering cost : Fts. 148/m3 TABLE III Economic analysis of dewatering by UP. Basis
(i) (ii)
(iii) (iv) opemtional uwugshear) 0
Removal of 27 m3 of water/day Number of working days/year 1350 Total membrane area required -151.2 m2 (on the basis of flux of 180 LMD) Capital investment on the UF system Membrane replacement (3 months life) Power 5 kWh/m2 Cost Rs. 1.25/kWh
Rs. 516,000 462,000
60,ooO
(3)
g (6) Total cost Dewatering
Number-7 Wage - Rs. 5Olday: Chemicals Depnxiation Rate - 10% Maintenance
122,500 100,000 52,000 25,000 821,500
cost : Rs. 87/m3
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CONCLUSIONS
Evaporation based conventional methods of dewatering gelatin liquors are energy intensive and sensitive to the frequent escalation of fuel costs. UF offers a cheap alternative to the evaporation based methods. Weak gelatin liquors can be effectively preconcentrated up to 15% followed by the final concentration in the evaporators. This combination allows old mills to increase evaporation capacity without additional investment in a new evaporator. Additional advantages of UF include product improvement through de-ashification and increase in viscosity. An economic analysis, based on experimental work using indigenous cellulosic membranes, indicates ample scope for UF in gelatin industries. Encouraged by the results obtained in this investigation, on-line plant trials have been planned to test the technology for commercial exploitation. ACKNOWLEDGEMENTS
The authors express their gratitude to the Director of Thapar Corporate Research and Development Centre for his kind permission to publish this work. Cooperation received from others during the course of this work are thankfully acknowledged. REFERENCES 1 2 3 4
W.F. Blat& A. Dravid and A.S. Michaels, in: Membrane Science and Technology, J. Flinn, (Ed.), Plenum, New York, 1970, p. 21. A.G. Fane and J.P. Friend, Dewatering of Gelatin Liquor by UltratXration, Chemeca 77, Canberra, Australia, September, (1977) 203-207. M.C. Porter, Concentration Polarization with Membrane Ultrafiltration, Ind. Eng. C&em.- Prod. Res. Develop., 11 (1972). A.R. Akred, A.G. Fane and J.P. Friend, in: Ultrafiltration Membranes and Applications, A.R. Cooper, (IX), Plenum, New York, 1979, p. 353.
