Conductometric studies and its significance during cane sugar manufacture – The Catalysts Group

Conductometric studies and its significance during cane sugar manufacture

Gautam Kapoor and P. Sanyal

Introduction

The conventional mode of quality and quantity assessment is being attained by way of determining, Brix, Pol, Purity, pH and temperature parameters in sugar manufacturing process. It is evident that these parameters viz Brix and Pol in particular are based on calibration terms. The sugar house products contains non sugar and sugar part but the calibration is done against pure sucrose solution by accounting sugar part only. Consequently it fails to establish any linear relationship between dilution and Brix parameter (Sanyal et al., 2004) of sugar house products for not accounting the non-sugar part. As such the representative Brix and Pol readings of any sugar house products falls under approximation category.

Besides above, it amounts to dilution error as well as time consuming process of parameter determination. These parameters mainly accounts for sugar part of the material under test. On the above aspect, the present study views alternatively i.e. to measure the non sugar part for its control during the sugar manufacturing process at all intermediate steps. In fact, the quality is basic requirement to assess the production, however the input –output balance concerns for the product value in terms of quantity analysis.

The present study inputs technological aspect for controlling the sugar manufacturing process with non-conventional option considering conductivity measurement of non-sugar part and to compare the observation in terms of conventional parameters of sugar house products. The basic theoretical dependability has also been studied for evolving the methodologies.

Conductivity is the ability of material to conduct electrical current. It is evident that chloride, nitrate, sulphate all carry negative charges, while sodium, potassium, magnesium, calcium carry positive charges as available in case of cane juice. These dissolved solids concern the ability to conduct electricity. Other way measuring the conductivity indicates the amount of total dissolved non sugars (TDNS) in the material under test.

Conductivity is probably an equally important metric but not less than that of pH value in cane juice processing having around 80% water content. Secondly pH is not having a wide range value output, whereas, conductivity value is much in wider range and thus may prove better controlling parameter.

It is worth mentioning that the activity is a measure of the amount of ions chemically active in a concentration of ions in solution. In fact participation of ion in a chemical reaction is not only determined by the concentration but also the presence of other ions in the solution .In concentrated solution the activity of the ions is considerably less than the total concentration as in the case of syrup to massecuite. Hence the activity coefficient ‘F’ is the actual measure in terms of ratio between the active concentration and the total concentration (Soand, 2009) i.e.

‘F’ = Active concentration / Total concentration

Initially way back in 1926 and at later stage studies on ash determinations by conductometric method have been carried out by many workers (Todt, 1926; Prasad et al., 1999), but failed to find out a way for regular or ongoing approach in sugar industry. The literature reveals that conductivity has been the basis of calculation for a few controlling parameters in sugar industry but left half way.

Purity determination of sugar house products like massecuite /molasses in particular have been carried out by many workers (Ziesch,1930, 1931; Doss 1983; Ponant, 1977) and observed encouraging results .In the said approach the refractometric brix has been replaced by conductivity having linear relationship between two with in the ambit of critical temperature and Brix .One of the proposed formulas is given below

Y A(100  P)xe cxn

Where x = Brix / (100-Brix) and A, e, and n are constants. Further to this Doss (1983) has given a simpler calculation for purity measurement by observing electrical conductivity with better reproducibility. It is to be pointed out that the conductometric purity is very similar to ordinary purity values but found to be of reproducible nature, significant and precise.

Discussion

The basic approach of conductivity measurement has been tested for KCl solution. From Fig.1, it is evident that a linear relation always exits for conductivity versus dilution of the KCl solution under test, up to a certain level of concentration. The conductivity increases with higher concentration of KCl. Similarly, data obtained pertains to conductivity versus

Fig. 1. Conductivity verses concentration of KCl solution

concentration, with increasing order of cane juice concentration, pure sugar solution and molasses solution concentration, keeping in view the instrumental range for different products. These data are delineated in Fig 2, 3 and 4, respectively. In all cases, relation between conductivity (µ s/cm) versus concentration shows almost a linear nature and the trend follows the standard KCl behavior. This indicates that the sugar house product responds to conductivity parameter with better reproducibility and stability in the manner pure solution like KCl behaves. The reason of emphasis for

 

Fig. 2. Conductivity verses concentration of cane juice

 

Fig. 3. Conductivity verses concentration of sugar solution

this behavior, explains that non sugar part of sugar house products shows more indicative than the conventional parameters pertain to sugar part.

Further to this, temperature effect has also been tested by applying basic formula on standard condition of temperature 25OC. When measuring conductivity or total dissolved solids in other than standard conditions, certain corrections for these variations must be accounted for before going on to observe final values. Without applying the requisite correction for standard temperature, conductivity or TDS measurement at various temperature are meaningless because they cannot be compared. This part of consideration has been studied with observed data for implementation. The formula used is;

K25 = Kt  / [1+0.02 ( t – 25)]

Where Kt is the observed conductivity at given temperature ‘t’ to make the conductivity results at par of 25 OC for comparison purpose.The data is given in Table no 1, wherein the same was got confirmed by using conductivity meter of inbuilt temperature compensation system. This particular exercise has been carried out for cane juice as well as diluted molasses samples. The results are having similar trend with minor and negligible deviations. This clearly indicates that temperature effect can be minimized and able to bring it at par 25 OC for comparison purpose in case of juice and molasses etc. In the present study, the importance of conductivity parameter finds a favour in terms of wide range parameter neglecting the precise effect of temperature for step-to-step process comparison during sugar manufacture.

Further to this, for practical purpose, the effect of different doses of milk of lime (12 oBe solution) on conductivity of cane juice of 5 % solution has been determined by using temperature

Table 1. Temperature effect with compensation on conductance of juice and molases

Juice conductance

Sr. No. Temp.t. Kt conductivity in s/cm at corresponding temp. Conductivity is shown by applying temp. correction using TDS Scan-3 at Conductivity is shown by applying temp. correction using Thermo Orion 555 A at 25 °C
1. 43.8 340.0 247.0 250
2. 49.4 372.0 250.0 252
3. 55.4 392.0 245.0 251
4. 72.1 481.0 248.0 249
5. 75.6 493.0 245.0 248

 

Molasses

1. 42.0 1313 980 982
2. 47.7 1418 975 980
3. 55.4 1579 982 980
4. 66.3 1786 978 982
5. 72.0 1893 976 980

 

compensation based conductivity meter. The data has been taken at par of 25 OC. The graphical representation at Fig.5 shows a linear relationship with increasing order of milk of lime dose (Fig. 5). Such graphical trend can be evaluated for purposeful practical meaning. However, it is well-established that ionic interaction can alter the linear relationship between conductivity and concentration in some certain level of concentration. (John et al., 2006)

Further observations come out from Fig.6,7 and 8. wherein Fig.6 shows purity drop in cane juice samples i.e. less than 2 % against keeping time for three parallel sets. Whereas the non sugar plays its own role gets responsible for increases (Fig. 7) in conductivity with wide range around 50 % with respect to time for 20% (v/v) juice solution. The gist of Fig 6 and 7 has been represented in Fig8. The fall in purity is in other way a rise in non sugar i.e. conductivity of the medium. Hence, this clearly favors the conductivity parameter more paramount and informative. In comparison, the observation of purity determination involves instrumental and dilution error as discussed elsewhere (Sanyal et al., 2002).

Fig. 4. Conductivity verses concentration of molasses solution

Fig. 5. Conductivity verses concentration of milk of lime

Other way pH observation has been a point of concern for the reason of temperature effect. An enhancement of temperature can also cause an increase in the number of ions in solution due to the dissociation of molecules. In such change it reflects that conductivity data falls under wide range of deflection for particular case of weak acid / base. But since pH is a measure of the hydrogen ion concentration, a change in the temperature of a solution will be viewed by a subsequent change in pH. The sources of error to be reduced are temperature effects on electrode slope, isothermal point, thermal equilibrium, chemical equilibrium and membrane resistance.

These collective errors can be as high as 1.0 pH unit. Such small range may not provide better provision for overall process control (John et al., 2006).

 

 

 

 

 

Fig. 6 Relationship between purity drops of cane juice in different interval of time.

 

 

 

 

 

 

Fig .7 Showing relationships between conductivity of cane juice increases with respect to time.

 

 

 

 

 

Fig. 8. Conductivity increase as purity of cane juice decreases.

So far pH versus conductivity measurement are concerned in case of water quality assessment, it is obvious that pH is specific to hydrogen ions and it will not respond to other ions that may be present in the solution under test .For example, if iron sulphate is dissolved, the measurement of pH, the acid concentration would be unaffected. However, because of conductivity responds to all ions present, the measurement of conductivity responds to all ions present in the said solution. The measurement of conductivity will change with change of iron sulphate content, while the acid content remains constant. It is also important to consider that if the concentration of the acid is less than 0.5 %, pH or conductivity could be used. But if the acid concentration is greater, conductivity measurement becomes only alternative. It is for the technical reason that the hydrogen sensitive measuring electrode becomes saturated at high concentration of hydrogen ions. At the same time conductivity sensors are not hampered by high concentration of hydrogen ions. (John et al., 2006). With all the above basic technological aspects the control of the process shows a reproducible trend by way of measuring conductivity parameter.

Conclusion

In this presentation certain operating parameters like brix, pol, purity and pH have been critically discussed on, their technical qualities besides the optional parameter i.e. conductivity for regular and ongoing control of sugar manufacturing process. The application of conductivity measurement of sugar house products has been taken in very simple way to evolve quicker and easy method based on reproducible trends showing nature. Further work is in progress and shall be communicated for coming issue.

References

Doss KSG (1983). A new control figure for monitoring crystallization of sugar – conductometric purity. Proc. 47th Conv. Sugar Technology Association of India.M1-M6.

John J Barron, Colin Ashton, Leo Geary (2006). The effect of temperature on pH measurement .Technical Service Department, Reagecon Diagnostic Ltd. Shannon Free Zone, Country, clare, Ireland. 57th annual meeting of the International Society of Electrochemistry, Edinburgh.

John J Barron, Colin Ashton, Leo Geary (2006). pH versus conductivity measurement. Technical Service Department, Reagecon Diagnostic Ltd. Shannon Free Zone, Country, clare, Ireland .57th annual meeting of the International Society of Electrochemistry, Edinburgh.

Ponant J (1977). Conductrometric method for determining the purity of sugar solution. Science and Industry (Philips Scientific and Industrial Equipment Division, Eindhoven) 10: 21-23E.

Prasad Mahendra, Chandra R, Nigam GD (1989). Electrical conductivity control of calcium sulphite precipitation in cane juice clarification. International Sugar Journal 91: 49-51.

Prasad Mahendra, Tiwari L P, Sudhanshu Mohan (1999). pH metric and conductometric technique for controlling supply of stale cane. Proc. 61th Conv. Sugar Technology Association of India M161-M166.

Sanyal P, Nigam RB, Gupta Satish K (2002). Measure of clarification in cane sugar industry .Cooperative Sugar 33: 563-565.

Sanyal P, Nigam RB, Gupta SK, Mitra SK, Sanyal P (2004). Use of conductivity data for quality assurance in Plantation white sugar factory. Proc. 65th Conv. Sugar Technology Association of India M61 –M67.

Soand J D (2009). Assessment of sugar polarization using an electrical conductivity method. International Sugar Journal 111: 306-312.

Todt ZVD (1926) Conductivity ash. Facts about Sugar 21: 1090.

Zisch JH (1930). The conductivity of beet sugar syrup and a proposed rapid true purity method. Facts about Sugar 25:741-743.

Zisch JH (1931). Conductivity of beet sugar syrup- its releation to sucrose, rafiinose and ash. Facts about Sugar 26:299-301.

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