Broadfoot, R. Sahadeo, P. and Muddle, C.P. Proc. Aust. Soc. Sugar Cane Technol., Vol. 24, 2002

Transcription

1 IMPACT OF MASSECUITE FEED PROPERTIES ON CONTINUOUS HIGH GRADE CENTRIFUGAL PERFORMANCE By R. BROADFOOT 1, P. SAHADEO 2 and C.P. MUDDLE 1 1 Sugar Research Institute, Mackay, Queensland 2 Sugar Milling Research Institute, Durban, South Africa r.broadfoot@sri.org.au KEYWORDS: Continuous Centrifugals, High Grade Massecuite, Purging. Abstract During recent seasons, the performance of continuous high grade centrifugals has been assessed for a wide range of processing conditions, which included varying the massecuite feed rates, wash water application rates and basket rotational speeds. The purging qualities of the feed massecuites were determined to have a major impact on the performance of the centrifugals in achieving the target polarisation of the sugar. The paper describes the techniques that have been used to define the effect of the massecuite feed properties and the extent to which fugalling performance is affected. Introduction The application of continuous centrifugals in Australian sugar factories for processing high grade massecuites to produce shipment raw sugar has to a large extent been pioneered by Australian researchers, sugar equipment manufacturers and sugar milling companies. The Bundaberg Sugar Company developed a hybrid continuous centrifugal based on componentry from a batch machine in the mid 1980s (Smith and Howard, 1986; Griffin and Borchardt, 1990). About the same time, early developmental work was undertaken by staff of BSES and the Department of Chemical Engineering at the University of Queensland. This work culminated in the first prototype continuous centrifugal being installed by North Queensland Engineers and Agents (NQEA) at Kalamia Mill in 1989 (Kirby et al., 1990). The STG Company further advanced the design and application of continuous centrifugals in the raw and refined sugar industries (Greig et al., 1995). The licence for the NQEA design has been obtained subsequently by Thomas Broadbent and Sons Limited who have progressed this development. Currently, approximately half of Australia s raw sugar factories have at least one continuous centrifugal producing shipment raw sugar from A or B massecuites. Compared with batch centrifugals, the continuous machines are less able to consistently produce sugar at the target polarisation required for specific brands of sugar, particularly for the higher pol sugars. Other disadvantages are the production of sugar of higher moisture content and with increased breakage or attrition of the crystal edges. Under most operational conditions, these aspects can be satisfactorily managed. During the past eight years, SRI staff have undertaken research into improving the operational performance of continuous high grade centrifugals, with emphasis on consistently being

2 able to meet the standards for high pol sugar production, while processing massecuites at acceptably high throughput rates (Broadfoot and Miller, 1998; Broadfoot and Muddle, 2001). This paper describes the outcomes of this research which define the impact that the quality of the feed massecuites has on the performance of continuous high grade centrifugals, how the purging qualities of the feed massecuites are defined and the procedures which may be used to improve the fugalling characteristics of the massecuites to so improve the performance of continuous high grade centrifugals. Experimental procedures for assessing centrifugal performance Properties of the feed massecuite (viscosity, crystal content, crystal mean size and coefficient of variation of the crystal sizes) impose a substantial influence on batch centrifugal performance (Broadfoot and Miller, 1996) and, as demonstrated subsequently, on continuous centrifugal performance. The procedure adopted (in general) to investigate the performance of continuous fugals was to undertake a series of trials on a single batch of massecuite and to limit the change in test conditions (e.g. wash water application rate, massecuite feed rate, rotational speed of the basket etc.) to an assessment of the effect of a single variable. For each series of fugalling trials, a sample of the strike massecuite was fugalled in SRI s laboratory basket (batch) centrifugal under standard conditions (spin at 465 G (2000 r/min) for 4 minutes, without wash water) based on the procedures of Broadfoot and Miller (1996). The purity of the separated sugar provides a measure of the purging qualities of the feed massecuite. This work correlated the purged sugar purity to the processing variables, which are in descending order of importance, mother molasses viscosity, centrifugal acceleration and coefficient of variation of the crystal sizes. From the data set, mean crystal size was determined to be not significant, and this may be attributed to the narrow range of mean crystal sizes encountered in typical Australian shipment massecuites. This test procedure has been designated the SRI purging characterisation test. For washing efficiency tests on a continuous centrifugal, the massecuite throughput rate and other conditions (excluding basket water) were held constant and the effect of changing the basket water addition rate was determined. In order to separate the effects of the purging characteristics from the washing performance, a test was undertaken in each series (where possible) with no water wash applied to the basket. For a given fugal installation, the sugar purity achieved for zero basket wash addition is dependent on the purging qualities of the massecuite, the massecuite throughput rate and the rotational speed. For the tests at zero basket wash water addition, a higher purity of sugar is produced at lower massecuite feed rates and at higher rotational speeds. Results of washing efficiency tests Washing efficiency tests were conducted for a range of processing conditions from several strike massecuites with markedly different purging qualities. These trials were undertaken on three different makes of continuous high grade centrifugals. In all cases, the characteristics of the washing efficiency relationship were similar. A washing efficiency correlation (Broadfoot and Miller, 1999) was fitted to the data from the washing efficiency investigations. This correlation is expressed in terms of the impurity%sugar

3 solids, which is equivalent to (100 sugar purity*). The form of the correlation is: I%SUG I%SUG NO WASH k1(1 e = 100 -k2w ) (1) where I%SUG = impurity%sugar solids for the imposed water%massecuite I%SUG NO WASH = I%SUG obtained in the factory fugal without wash water application w = wash water % on massecuite. The variable w is the total basket water added. K = coefficient defining the limit of removal of impurities at very high wash water addition rates. The larger the value of k 1 the greater the amount of impurities removed at the limit. k 2 = coefficient defining the rate of removal of impurities with basket wash water addition. The larger the value of k 2 the more readily impurities are removed with wash water addition. The coefficient k 2 is the coefficient which is most subject to error, particularly if there is insufficient data at low basket water application rates. While the massecuite throughput rate and basket rotational speed affect the washing efficiency behaviour, the effect of massecuite purging qualities is marked. The results of washing efficiency tests are described below for various massecuite types. The massecuite throughput rates and rotational speeds are typical of normal operation and are indicated for each data set. Table 1 provides data on the massecuite composition including mother molasses consistency, mean crystal size, coefficient of variation, the purged sugar purity, I%SUG NO WASH and coefficients k 1 and k 2. Massecuite type Table 1 Details of the massecuite composition and test results for different fugalling trials. Mill Massecuite purity Mother molasses consistency, Pa.s n Mean aperture, mm Coeff. of var n. Purged sugar purity* I%SUG NO k 1 k 2 WASH + A A A B Poor quality A C A D Preconditioned A High purity E massecuite *Sugar obtained from the SRI purging characterisation tests. + Refer Figures 1 to 4 for the specific massecuite feed rates and basket rotational speeds for the individual tests. * The purity of a sugar sample = polarisation 100/(100 moisture %)

4 Typical Australian A massecuites Figure 1 shows washing efficiency results for processing typical A massecuites produced by Australian mills. The two examples, from two mills designated Mill A and Mill B, show different purging characteristics for the feed massecuites Sugar purity Mill A Mill B Mill A: Rate 35 t/h Speed: 835 rpm Mill B: Rate 30 t/h Speed 775 rpm Fig. 1 Washing efficiency results for processing typical A massecuites produced by Australian mills. Increased quantities of basket water produce diminishing benefits in the removal of impurities from the surface of the sugar and a limit exists to the extent that basket wash water application can raise the purity of the product sugar. In practical terms, the results show it is difficult to achieve sugar purities greater than 99.5 (Mill A example) and 99.2 (Mill B example) by increased application of wash water only. Additions of wash water at rates greater than 5 6% for Mill A and 7 8% for Mill B produce minimal increase in the sugar purity. These general observations have been reported previously (Greig et al., 1995; Broadfoot and Miller, 1998; Thompson and Grimwood, 1997). An A massecuite of inferior purging quality Figure 2 shows a washing efficiency result for processing low purity A massecuites of inferior purging qualities (Mill C). These massecuites comprise mother molasses of higher consistency (viscosity) and crystals of small mean aperture and broad size spread.

5 Sugar purity Rate 32 t/h Speed 890 rpm Fig. 2 Washing efficiency result for processing low purity A massecuites of inferior purging qualities (Mill C). For this massecuite the limiting sugar purity was about 99.0 and the addition of increased wash water rates above about 10% on massecuite produced minimal increase in the sugar purity. Preconditioned A massecuite Investigations conducted in 1998 at Mill D demonstrated a substantial improvement in the fugalling performance by preconditioning A massecuites with water mixed into the massecuite feed at a dilution of about 0.8% on massecuite (Broadfoot et al., 1999). The reduction in the consistency of the mother molasses improved the purging characteristics of the massecuite (refer Table 1). Figure 3 shows results for an A massecuite and a preconditioned massecuite sourced from the same strike massecuite, under similar processing conditions of throughput rate and rotational speed (note reduced scale of sugar purity data).

6 Sugar purity Untreated A massecuite Pre-conditioned massecuite Rate 36 t/h Speed 820 rpm Fig. 3 Results for an A massecuite and a preconditioned massecuite sourced from the same strike massecuite, under similar processing conditions of throughput rate and rotational speed. The data show the untreated A massecuite reached a limiting sugar purity of 99.5 for a wash water application rate of 7.1% on massecuite. The preconditioned A massecuite produced sugar of slightly higher purity (99.55) at a basket wash water rate of 6.1% on massecuite. Specially produced high purity massecuite A massecuite of 96.5 purity was produced and processed through a continuous centrifugal. The washing efficiency relationship is shown in Figure 4 (note reduced scale of sugar purity data). This massecuite exhibited markedly superior purging characteristics compared to a typical Australian A massecuite (87 to 90 purity) and achieved a limiting sugar purity of 99.8 for a wash water application rate of 5% on massecuite. Of note, the consistency of the mother molasses was only 0.4 Pa.s n, which compares with mother molasses consistency values of 0.8 to 1.7 Pa.s n for typical Australian A massecuites.

7 Sugar purity Rate 35 t/h Speed 650 rpm Fig. 4 Washing efficiency results for a high purity massecuite. Review of washing efficiency results The examples of washing efficiency behaviour clearly demonstrate that the purging qualities of the feed massecuite have a strong influence on the results. In particular, the purging qualities affect the purity of sugar obtained for zero wash water application and, ultimately, the limit of sugar purity that can be obtained at high wash water application rates. The purging qualities also affect the minimum wash water application rate at which the limiting sugar purity is reached. From the small data set presented here it is not possible to define a model of washing efficiency. This is currently being developed in terms of the main processing variables of purging quality, wash water application rate and massecuite feed rate (refer Discussion). Other variables of lesser influence are rotational speed and method of wash water application. Mass fraction of molasses retained on the crystal surface Estimates of the mass fraction of the sugar which comprises the molasses film retained on the surface of the crystal were made based on the following assumptions: The composition of the retained molasses film is the same as the mother molasses in the feed massecuite. The purity of the crystal is The impurity/water ratio of the inclusions in the crystal is the same as the impurity/water ratio of the molasses film. The calculation procedure was provided by Greig et al. (1992) and Broadfoot and Miller (1995). The assumption that the retained molasses film on the crystal has the same composition as the mother molasses is a substantial simplification. In practice, the composition is not known but it

8 would be of higher purity and lower dry solids content than the mother molasses. The film would comprise some of the original mother molasses, dissolved crystal and water. The assumption has the advantage that it provides a readily usable methodology to estimate the purity of the product sugar, knowing typical values of retained molasses fraction and the composition of the mother molasses in the feed massecuite. In practice, the actual mass fraction of molasses would be slightly greater than the mass fraction calculated from the mother molasses composition. A large series of trials to investigate the effect of several variables on fugalling performance was undertaken at mills A, B and C (Broadfoot and Muddle, 2001). The effects of wash water addition rate, massecuite feed rate, wash water application method, rotational speed and screen specification (for Mill C) were investigated. Among these trials, the variations in massecuite feed properties among the different batch strikes were unavoidably present. Figures 5, 6 and 7 show the estimates of the mass fraction of retained molasses as a function of wash water application rate for the trials at Mill A, Mill B and Mill C respectively. The data in each plot are designated according to whether the purged sugar from the purging characterisation test in the SRI fugal was above or below an appropriate mid-range value for the tests. This delineated the effect of massecuite feed properties on the estimate of retained molasses among the scatter of data due to the influence of all the other variables Mass fraction Purged sugar purity <99.14 > Fig. 5 Mass fraction of sugar comprising retained molasses as a function of wash water application rate and purged sugar purity for the trials at Mill A. As expected, all three plots show the mass fraction of retained molasses decreasing from high values for zero wash water application to low values, typically less than 0.02, for high wash water application rates. For the trials at Mills B and C, the minimum value of retained molasses fraction (for a few tests only) was approximately The retained molasses mass fraction averages about 0.10 (range 0.06 to 0.16) for zero wash water application.

9 Mass fraction Purged sugar purity <98.8 > Fig. 6 Mass fraction of sugar comprising retained molasses as a function of wash water application rate and purged sugar purity for the trials at Mill B. Mass fraction Purged sugar purity <98.1 >98.1 Fig. 7 Mass fraction of sugar comprising retained molasses as a function of wash water application rate and purged sugar purity for the trials at Mill C. The data shown in figures 5, 6 and 7 demonstrate that a minimum mass fraction of molasses is retained on the surface of the sugar crystal, and this does not reduce further at very high wash water rates. This then defines the maximum purity of the sugar that can be produced, depending on the purity of the mother molasses (Greig et al., 1995)

10 The preferred method of operation of continuous fugals is to produce sugar at the target purity (target polarisation) to suit the marketing specification, through the use of the minimum application of basket water on massecuite. As one of the major limitations of continuous high grade fugals is the ability to consistently produce high polarisation sugar from a broad range of massecuite qualities, then the preferred operational position is to be able to achieve as low a retained mass fraction of molasses on the crystal as possible (i.e. the highest sugar purity). This then allows the target sugar purity to be produced at a reduced water application rate. For each series of trials the results show the strong effect that massecuite feed characteristics have on the retained mass fraction of molasses. Massecuites that inherently purge well achieve a lower fraction of retained molasses at high wash water application rates and, importantly, achieve a target level of retained molasses (corresponding to a target sugar purity) for a reduced wash water rate. Discussion The trials which have encompassed a wide range of massecuite qualities and fugalling conditions have demonstrated that the purging qualities of the feed massecuite have a major influence on the ability of the fugal to produce high purity sugar. The purging quality of the massecuite affects the combination of massecuite feed rate and the basket water application rate which can be employed to achieve the target sugar purity, as shown in the generalised relationship in Figure Good purging massecuite Sugar purity Effect of increasing massecuite feed rate Poor purging massecuite Basket water % massecuite Fig. 8 Generalised dependency of sugar purity on the massecuite purging quality, massecuite feed rate and basket water application rate. Processing of massecuites of good purging characteristics was shown in the trials to promote the following:

11 Ability to process at higher massecuite feed rates while still meeting the target sugar purity. Associated benefits through operation at higher massecuite feed rates are better coverage of the crystal bed on the basket, reduced attrition of the edges of the crystals, (perhaps) slightly reduced breakage as crystals are discharged from the top rim, a reduced increase in the grist of the product sugar relative to the grist in the feed massecuite, and reduced propensity to produce lumps of lower pol, finer crystals. Ability to reach the target sugar purity (at a nominated massecuite feed rate) with a reduced quantity of wash water. Associated benefits through operation at lower wash water application rates are the production of molasses of higher dry solids concentration (hence less evaporation required on the pan stage) and less disturbance of the crystal bed on the filtering screens. The smoother flow of crystal on the screens produces less attrition, perhaps reduced crystal breakage and less likelihood of lump formation. Ability to reach a higher sugar purity due to the reduced mass fraction of retained molasses (at a nominated massecuite feed rate and wash water application rate). Ability to achieve the target processing conditions (rate, sugar purity) at lower rotational speeds. In practice, the scope to reduce the rotational speed (from the norm of about 400 G at the top of the basket) and still meet the target processing conditions depends on the specific circumstances and would only likely be feasible for conditions of good/excellent purging massecuites, at moderate massecuite feed rates (refer the washing efficiency tests shown in Figure 4). In practice, the purging qualities of the feed massecuite can be improved through adoption of the following measures: (i) (ii) (iii) (iv) Boil massecuites of large, uniformly sized crystals. In particular, it is important to ensure that the massecuites do not contain substantial quantities of small crystals, e.g. size less than 0.2 mm. Produce well exhausted massecuites. For pans which are able to produce massecuites of high crystal content, the dry substance and viscosity of the mother molasses are reduced due to the transfer of sucrose from solution to crystal. Maintain the feed massecuite at a high temperature and avoid excessive cooling. Cooling of the feed massecuites increases the mother molasses viscosity. Condition the feed massecuites. There are several procedures which could be implemented to condition the massecuite prior to feeding into the fugal; all aim to reduce the viscosity of the mother molasses and all have to be carefully regulated to ensure that excessive quantities of crystal are not dissolved. These procedures include: Dilution of the feed massecuite with molasses or water in a controlled manner. Procedures which are currently being used include: lubrication of the pan drop massecuite either in the pan just prior to discharge or in the receiver, dilution in a separate conditioning device (Broadfoot et al., 1999). Injection of steam into the feed massecuite stream. [Care is needed with this

12 procedure that the massecuite/steam mixture is always vented to atmosphere. This is a serious safety issue]. Non-contact massecuite reheating. This procedure is difficult to implement costeffectively due to the high massecuite feed rate and the short residence time available for holding the heated massecuite without causing significant crystal dissolution. Conclusions Through extensive trials into the effects of different processing variables on the performance of continuous high grade centrifugals it has been demonstrated that the massecuite purging characteristics have a major impact on the fugalling performance and, in particular, in the ability of the fugals to produce sugar of high purity. For a given fugal installation, the purging characteristics of the massecuites have been shown to affect the washing efficiency relationships and the quantity of molasses retained on the crystal surface. The washing efficiency behaviour is defined most strongly by the impact of the purging characteristics of the feed massecuite on the purity of the sugar that is produced by the continuous centrifugal for zero wash water application. In order to define the effects of massecuite feed properties, SRI conducts a purging characterisation test on the feed massecuite in a laboratory basket (batch) centrifugal and also undertakes a trial in the continuous centrifugal at zero wash water application. The purging qualities of the feed massecuite can then be defined and taken into account adequately to determine the effects of the other processing variables on the fugalling performance. The supply of massecuites of improved purging qualities allows the continuous fugal to achieve the target sugar purity, while operating at increased throughput rate and/or reduced wash water application rate. Either change is conducive to improved operation of the fugal. Measures have been described which can be implemented to improve the purging qualities of the feed massecuite. Acknowledgments The assistance provided by the staff at the many sugar factories where the fugal trials were undertaken is appreciated. Funding assistance to the project was provided by the member mills of SRI, the Sugar Research and Development Corporation and the fugal manufacturers STG-FCB and Thomas Broadbent and Sons. The Sugar Milling Research Institute, South Africa provided substantial analytical resources to the trials undertaken in South Africa. Their assistance is appreciated. Special acknowledgment is made to the research input made by Prof. Ted White of the University of Queensland in the development of the continuous fugal technology for the Australian sugar industry. REFERENCES Broadfoot, R. and Miller, K.F. (1995). Fugalling characteristics of high grade massecuites. Final report, SRDC Project SRI19S, Sugar Research and Development Corporation, Brisbane. Broadfoot, R. and Miller, K.F. (1996). Fugalling characteristics of high grade massecuites. Proc. Aust. Soc. Sugar Cane Technol., 18: Broadfoot, R. and Miller, K.F. (1998). Performance assessment of continuous high grade fugals. Proc. Aust. Soc. Sugar Cane Technol., 20:

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