N.A. Ramaiah Deccan Sugar Institute, Pune, India. and

Transcription

1 Processing DETERMNATON OF FACTORS RELATNG TO AUTOMATON FOR BOLNG OF LOW GRADE MASSECUTES N PLANTATON WHTE SUGAR NDUSTRY N.A. Ramaiah Deccan Sugar nstitute, Pune, ndia and G.N. Acharya and P. Kapoor Central Electronic Engineering Research nstitute, Pilani, ndia ABSTRACT Temperature, ac resistivity and viscosity at different stages of boiling of low grade massecuites in plantation white sugar factories, were determined. These factors could be correlated to brix, purity and supersaturation of massecuites. A simple method was evolved for measuring on-line brix and purity of massecuites during boiling in pans, without the need of drawing out samples and analysing; the same was found helpful for microprocessor based pan boiling and for reduction of time and steam consumption. NTRODUCTON Automation in pan boiling improves crystallisation and exhaustibility of molasses and reduces steam consumption. The same has been achieved in raw sugar factories, by determining conductivity, viscosity, boiling point elevation, or dielectric constant and by correlating the same with supersaturation during boiling of low grade massec~ite.~~ 2, 3, 5,8 Data are scanty on such parameters relating to plantation white sugar industry. The present paper describes studies made, for the first time, for obtaining information which would enable one to design and operate equipment for automation in low grade pan boiling in plantation white sugar factories. A microprocessor based pan boiling system has been evolved and is here described. EXPERMENTAL The experiments were carried out in sulphitation sugar factories, situated in tropical and sub-tropical areas, and crushing Co varieties of cane. By installing probes in low grade pans, as shown in Fig. 1, data were obtained relative to temperature, resistivity (reverse of conductivity) and viscosity. Temperature was measured with PRT 101 platinum resistance thermometer sensor; to avoid drift in temperature readings, high stability dc current was passed through the sensor. The sensor provided both voltage and current outputs, which after application, gave on-sight digital display or long distance transmission. Keywords: Automation, pan control, microprocessor, low grade massecuites

2 N.A. RAMAAH, G.N. ACHARYA AND P. KAPOOR 699 i t was shown earlier (Kapur4) that measurement of ac resistivity, instead of conventional conductivity, bore a definite and reliable relationship with the properties of low grade massecuites. Fig. 2 gives the description of the device for measuring the resistivity of massecuites. The system consisted of two stainless steel electrodes (E) which were energised, connected serially with a resistance R,, by an amplitude stabilised sine wave oscillator (OSC) of 10 khz, at which frequency the dielectric effects were prominent. The equivalent circuit of the cell can be represented by bulk resistance R, in series with a capacitance C, formed by the liquid layer at the electrode surfaces (shown by dotted lines, Fig. 2). The output containing only the resistive component (R), was measured. From the observed value of resistance R, the resistivity ( p ) was obtained from R = p d/a where a is the electrode area and d is the distance between the two electrodes.

3 700 PROCESSNG Viscosity was measured with the help of a rotational viscometer consisting of a cylinder rotating over a hollow shaft suspended by a spring to an inner shaft; this was rotated at a constant r/min. Changes in viscosity cause angular disljlacement between the two shafts; the displacement is electrically sensed. n pans where mechanical circulators were provided, the viscosity was estimated by measuring the current of stirrer-motor; this was compared with actual viscosity measured. Data on brix, pol and purity of massecuites were determined by conventional methods." Supersaturation of media of low grade pans at different stages of boiling, was determined by drawing samples and analysing the same for brix, pol, purity, ash, etc., and by employing a method described earlier for the purpose (Ramaiah6, Mollers). EL- FGURE 2. Equivalent circuit for measuring the resistivity of low grade massecuites.

4 N.A. RAMAAH, G.N. ACHARYA AND P. KAPOOR 701 RESULTS Experiments were conducted in ten sulphitation factories - five in tropical, and five in sub-tropical, zones of ndia, in the months of December-March of crushing campaigns and n these factories, different varieties of cane such as CoC671, Co7704, CoJ64, Co1148, Co419, Co1158 and Co740, were in use. n each factory, more than 25 sets of results were recorded at different stages of boiling of low grade massecuites. Variation in basic data, particularly in resistivity, viscosity, etc., at a given temperature of massecuites, was within + 2% which was considered inconsequential. No attempt was made therefore, to present the voluminous data collected and only averages of the results appear here. The same was found adequate for developing a device for automation of low grade pans. TABLE. Viscosity at different stages of boiling of low grade massecuites Temperature 71 "C Brix Viscosity (Poise) FGURE BRlX Dependence of resistivity on brix.

5 702 PROCESSNG The following ranges of resistivity were recorded in different zones of pan operations: Zone Resistivity (ohm cm) Under-saturated 360 Saturated Supersaturated above 480 Metastable ntermediate Labile above 1400 FGURE4. TEMPERATURE (CO) Dependence of resistivity on temperature of low grade massecuites.

6 N.A. RAMAAH, G.N. ACHARYA AND P. KAPOOR 703 t was found that resistivity was a function of brix (Fig. 3) and of temperature (Fig. 4), as was to be expected. Data on viscosity were collected in different stages of pan boiling. Attempts were made to collect results at a constant temperature. n practice the temperature in a pan is governed largely by vacuum and at times of regular and uniform crushing and smooth running the temperature-variation was only about t- 0.5 OC; only then were data collected. A typical set of viscosities recorded in different stages of boiling of massecuites is given in Table. The viscosity of low grade massecuites in the plantation white sugar factories was in the range of 30 poise to 1200 poise, when the brix of massecuite varied from 50 to 95 (71 OC). Fig. 5 shows the relationship between the viscosity and resistivity of low grade massecuites, at different purities. At a given temperature, purity (P) had a distinct and significant correlation with resistivity ( P ) and viscosity (V); the same could be expressed by an equation: P = (a, P+ b,) 4 (al P+ bl)v + (a2 f + b2)v2... (1) where a,, al, a2 and b,, bl and b2 are polynomial constants. By regression analysis, the values of the constants were determined. From Eq. (), on-line brix and purity could be computed from measured values of resistivity and viscosity; the results are given in Table 11. RESSTVTY (OHM. CM) FlGURE 5. Relationship between the resistivity and viscosity of low grade massecuites.

7 704 PROCESSNG TABLE. Computation of brix and purity from measured values of viscosity and resistivity Polynomial coefficients -a, = 55.88, a, = 6.115, a, = 0.32, b, = b, = and b, = (A) Computation of brix Brix Viscosity Actual Computed Error % (B) Computation of purity Purity Resistivity Actual Computed Error % t should be noted that the correlation between the computed and actual values was within ~fr 0.2%. These results indicate that one could measure, with reasonable accuracy, the on-line brix and purity of low grade massecuites by reference to the measured values of resistivity and viscosity.

8 N.A. RAMAAH, G.N. ACHARYA AND P. KAPOOR 705 DSCUSSON During the initial part of the pan boiling operation, viz. concentration of the medium, introduction of the seed, etc., the resistivity data alone provided accurate control of the system (see Fig. 6). After the pan was "cut" and subsequently boiled to develop the grain size from 0.15 to 0.25 mm., control of brix and purity were found necessary; this could be achieved by realising Eq. (1) on a microprocessor based monitoring system with the measured values of resistivity and viscosity. Fig. 7 shows the block diagram of microprocessor based pan boiling system developed. t consisted of input sensor block, microprocessor based monitory system, output console and valves. The functioning of the system was, in brief, given below. The input sensor blocks consisted of sensors/transducers for resistivity, viscosity and temperature. The electrical signals of 4-20 ma were sent to the sensor microprocessor based pan system which was designed around ntel 8085, an 8-bit microprocessor along with a peripheral device such as 2716-EPRON, RAM and 8255 PP, etc. The analogue signals were interfaced through a 16-channel 12-bit successive approximation analogue to digital converter which loaded the input data on a memory 8231 APU; this operated on the data and computed brix and purity/supersaturation. Low grade pan boiling with the control of on-line brix and purity data, obtained from the data on resistivity and viscosity, gave very useful results. Boiling pans with the aid of a microprocessor resulted in 15% to 20% reduction in time and about 20% reduction in water consumption as compared to manual operation; this appeared significant in terms of over-all energy conservation. CONCLUSON The successful boiling of low grade massecuites requires control of purity/supersaturation and brix. This could be achieved by measuring ac resistivity, viscosity and temperature. When these results were processed through a microprocessor, online data on brix and purity of low grade massecuites at different stages of boiling CUT TME (HOUR) FGURE 6. Resistivity - time profile during boiling of low grade massecuites.

9 706 PROCESSNG could be had with reasonable reliability and accuracy. These factors are of considerable significance for automation of pan boiling. r- 1 PAN MONTORNG SYSTEM L J FGURE 7; Block diagram of microprocess based pan boiling system. REFERENCES 1. Batterham, R.J., Frew, J.A. and Wright, P.G. (1973). The use of Boiling Point Rise for the control of Pan, Proc. Queensland Soc. Sugarcane Techn Batterham, R.J., Frew, J.A. and Wright, P.G. (1974). Control of vacuum pans, SSCT Proc. 15, Foster, D.H. and Wright, P.G. (1962). Control of crystallisation in Vacuum Pan, Proc. SSCT: Kapur, P. and Patil, V.L. (1982). Low grade massecuite boiling using resistivity as a reference variable, nt. Sug. Journ. 84: Moller, G.R. (1984). Recent developments in pan boiling automatics, nt. Sug. Journ. 86: Ramaiah, N.A. and Murari, K. (1982). Solubility of Sucrose in presence of salts, Proc. Sug. Techn. Assn. (ndia) G.l System of Technical Control for Cane Sugar Factories in ndia, Publication of Sugar Techn. Assocn. of ndia (1964). 8. Tayfield, D. J., Rein, P.W. and Prome, S.R. (1980). The application of a microprocessor based system for automatic pan boiling control. Proc. of 54 Ann. Congr. South African Sugar Techn. Assocn.,