On-line estimation of colour, turbidity and ph values in sugar refining process

Transcription

1 Indian Jurnal f Engineering & Materials Sciences Vl. 3, June 1996, pp On-line estimatin f clur, turbidity and values in sugar refining prcess R Gvindaraj & P E Sankaranarayanan Ccnual Hccunic-, Fnginccring Research Institute (Madras Centre), CSIR Campus, Taramani, Madras , India Received 15 May 11)1)5; revised J3 February II)I)() Measurement f clur and turbidity f sugar juice in the refining prcess is imprtant twards maintaining the clarity f the juice fr further prcessing. Internatinal Cmmissin fr Unifrm Methds uf Sugar Analysis (lcumsa) recmmends the absrptin f light in 420 and 560 nm wavelengths, accuunts fur culuur and the absurptin at 720 nm wavelength and accunts fr turbidity f the sugar juice. Hwever, the clur f the juice is als dependent n its value. At present, sme ff-line prcedures are fllwed in the industry t estimate these parameters. Hence an n-line Persnal Cmputer based clur and turbidity measuring system has been designed, develuped and field tested. A mathematical relatinship has been established between culur, turbidity and values f the sugar juice, based n the results f the n-line system. In sugar industry, ne f the principal functins f the sugar refining prcess is t prduce a refined sugar essentially free f amber-brwn clur f raw sugar. The clur cntrl during sugar refining, t a large extent, is based n the clur f the slutin I. Hwever, clur depends n the f the sugar juice btained frm the sugar cane. Apart frm clur, turbidity is anther factr that has t be taken int cnsideratin. Turbidity is usually cnsidered t be the cludiness r haziness in the sugar slutin that will nt settle in a reasnable time. Measurement f clur and turbidity f the sugar juice in the clarificatin prcess is very imprtant and has great bearing n the subsequent prcessing f the factry. Offline instruments are being used in the industry t measure clur and turbidity based n the methd recmmended by the Internatinal Cmmissin fr Unifrm Methds f. Sugar Analysis (ICUMSA)2-4. The methd emplys the well knwn light transmissin r absrptin r scattering principle. Based n the ICUMSA methd an indigenus n-line PC based clur and turbidity system has been designed, develped and field tested in ne f the Sugar Mills in Suth India. This system measures the clur f the sugar juice in visible spectrum range nm. The results f the system trials are presented in this paper. Sugar Refining Prcess Sugar refining is a multistage prcess. The verall prcess may be divided int five areas, viz., (i) extractin f juice, (ii) purificatin f juice (clarificatin), (iii) evapratin, (iv) crystallizatin and (v) centrifuging r purging. The raw sugar juice extracted frm the sugar cane is acidic and turbid. The lime, sulphur dixide and heat treatment n the juice reduces the acidity and maintains the value at 7.0. The measurement is very imprtant during additin f lime and has strng dependence n the clur f the juice in the subsequent clarificatin prcess. The primary bject f the clarificatin is t remve frm the juice the maximum quantity f impurities at the earliest pssible stage in the prcess cmmensurate with ther cnsideratins, such as clarity and reactin f clarified juice. In 5eneral, sufficient lime is added t neutralize the rganic acids present in the juice, after which the temperature is raised t 95 C r ahve. This lime and heat treatment frms a heavy precipitate f cmplex cmpsitin, part lighter and part heavier than the juice, which cntains insluhle lime salts, cagulated albumin, and varying prprtins f the fats, waxes, and the gums. Finally, the turbid free clarified juice which cntains ;;, f slids ges t next stage f prcessing. ICUMSA Methd fr Clur and Turbidity Measurement The ICUMSA methd specifies the measurement f light intensities f light transmitted (Of absrbed) at wavelengths f 420 nm in the vilet

2 GOVINDARAJ & SANKARANARAYANAN: SUGAR REFINING PROCESS 97 and 720 nm in t~ red part f the visible spectrum respectively accunting fr the clur and turbidity f the sugar juice-", Als ICUMSA, in ne f its methds, recmmends the measurement f clur at 560 nm". Zerban et a/,8, demnstrated that absrptin readings at 560 nm, crrected t unit cncentratin and cell length, are equivalent t visual clur as defined by the American Natinal Standards Assciatin and the Natinal Bureau f Standards. Technique t cntrl Juice Clur A technique fr cntrlling the JUIce clur based n its value has been prpsed. A relatin between juice clur and the has been established. When a beam f mnchrmatic light enters an absrbing medium (sugar juice) at right angles t the plane surface f the medium, the rate f decrease in radiant pwer with the length f light path thrugh the medium is prprtinal t the radiant pwer f the beam". If the intensity f the light beam incident n the surface f the absrbing medium f cncentratin c be 1 0, and I be the intensity f the transmitted beam after passing thrugh the medium f path length b, then by Lambert-Beer's law.... (1) where a is the absrptin factr f the medium which depends n the wavelength f the incident light. Let 1'. be the transmittance f the sugar juice relative t a clear liquid say, pure water, The absrptin relatin fr the juice can be given as 10g(1/1'.)= - abc i.e. -lg( T.)= - abc Let As= abc... (2) When a mnchrmatic surce is used, the absrptin factr a is cnstant. Since the measurement cell used in the system is f fixed length, b is als a cnstant. Therefre, the nly variable factr in the abve equatin is the cncentratin c f the juice, which depends n the slid matter present and its value. Hence, the abve equatin can be written as... (3) where K, = ab and let this be called thee absrptin cnstant fr a given cncentratin. Results and Discussin The PC based n-line clur and turbidity measuring system develped in this Centre was installed and field tested in a sugar factry. The installatin was very near t the sugar juice clarifier. A by-pass line was taken frm the clarifier s that the clarified juice flws thrugh the measurement cell'" prvided in the sensr fr enabling the clur and turbidity measurements t be dne simultaneusly. Bth the transmissin and absrptin readings in the visible spectrum range Table I-Absrptin values f the clarified sugar juice in the visible spectrum range nm at 10 nm intervals, when =6.98 Wavelength Absrptin Wavelength Absrptin r ~ 0.4 c.e /1.0 « ~~~~_~~~_~ Wavelength, nm -.. Fig. I-Absrptin vs wavelength curve fr the clarified sugar juice in the spectrum range nm ,

3 98 INDIAN 1. ENG. MATER. set, JUNE nm at 10 nm intervals were measured and displayed n the PC mnitr. Als the transmissin and absrptin graphs were drawn and displayed. The value f the juice entering int the clarifier was als nted frm the instrumentatin available in the factry. Table 1 shws ne set f absrptin readings f the juice in the ttal visible spectrum range f nm, taken during the trial. The graphical plt f wavelength versus absrptin is shwn in Fig. 1. Thugh, this system can prvide absrptin readings ver the full visible spectrum, the prcess chemist's interest is nly in the absrptin readings at 420, 560 and 720 nm. The absrptin values f the clarified juice measured with the system at 420, 560 and 720 nm with values are shwn in Table 2. The absrptin values f the juice in the range f 420 and 560 nm have been fund t depend n the and als the turbid matter present in the medium. When turbidity is minimum the absrptin value at 720 nm is als minimum. It has been bserved during the trials, even thugh the clur f the juice was fund t be the desired ne by the prcess chemist, there was an Table 2-Absrptin values f clarified sugar juice at 420, 560 and 720 nm wavelengths A4211 A5~1I Anll : \ increase in the absrptin at 4020 nm. This has been due t the presence f turbid particles in the juice which increases the clur absrptin value at this wavelength. The absrptin values at 420 and 560 nm as shwn: in Table 2 are due t the change in clur and turbidity f the juice. The absrptin value at 420 nm, when there- is minimum absrptin at 720 nm (turbid free juice) is called the effective clur value f the sugar juice at 420 nm. This is the case fr 560nm als. The effective clur values f the juice at 420 and 560 nm can be arrived nly after separating the absrptin due t the turbid particles. The absrptin at 720 nm accunts fr the turbidity and any change in des nt affect the absrptin at this wavelength. The fllwing equatins have been established t find ut the effective clur absrptin values. Eq. (3) can be written fr the wavelength 420 nm as... (4) The cncentratin c is a functin f slids present in the juice as well as the value. Since the slid cntents f the clarified sugar juice is abut 10%, the change in the cncentratin f the juice is mainly due t the variatin in nly. Hence, the Eq. (4) can be written as... (5) where A ~20 is the effective clur absrptin value at 420 nm and K420 is absrptin cnstant at this wavelength. The A ~20 can be calculated frm the reprted values shwn in Table 2 using the fllwing equatin,... (6) where an is the minimum absrptin at 720 nm and this value is fund t be frm 'the Table 2. Frm Eqs (5) and (6), ne gets A420-(An-an)=K420 ()... (7) Using Eq. (6) the cnstant K 420 is calculated as frm readings f Table 2. Similarly the equatin- fr effective clur absrptin value at 560 m Can be written as A ~60 = K 5 bu () A ~60 = AS60 - (An AS60 - (An - ann) - an) = K 560 ()... (8)... (9)... (10) The cnstant K560 is the absrptin cnstant at 560 nmand has been calculated as

4 GOVINDARAJ & SANKARANARAYANAN: SUGAR REFINING PROCFSS 99 The effective clur absrptin values f the sugar juice independent f turbidity at 420 and 560 nm are cmputed using the Eqs (5)-(10) and Table 3 - Effective clur absrptin values f sugar juice at 420 and 560 run with the theretical, cmputed and measured values A*420 A *560 Theretical Cmputed Measured i '; are shwn in Table 3. The measured values and the theretical values fund using Eq. (5), are als given in Table 3. The graphical plt between the theretical values and the effective absrptiun values (420 and 560 nm) is shwn in Fig. 2. Als frm the tabulated readings, it is seen that when the increases, the absrptin als increases. Thus, in the clarificatin prcess, better clur f the sugar juice can be btained when the is cntrlled in the range The theretical values f btained are als used t cmpute near actual values f cmputed values. These cmputed values f are btained frm the theretical values f 7.5 =a " III J a. E 6.7 u Theretical Fig. 3-Plt f cmputed vs theretical 10r , c ē /1.D c( - 420nm nm 0.2 L I 0.1 L &...---'-----I._-'----'--~ '~ 6.1 ~ Th~retical Fig. 2- The relatin between theretical and absrptin f the clarified sugar juice at 420 and 560 run wavelengths 6 5~~~~~~~~~~ Cmputed Fig. 4- Trend relatin between measured and cmputed

5 w INDIAN J. ENG. MATER. SCI..IUNE 1996 using least squares methd. The relatin between theretical and the cmputed is a straight line as shwn in Fig. 3. Accrding t the mdel prpsed, the 'cmputed values have t agree well with the measured values f. The trend relatin, between the measured and the cmputed values is shwn in Fig. 4. The was measured in the factry when the sugar juice entered the clarifier, whereas the theretical values are based n the absrptin readings at 420 nm f the clarified juice. These readings are nt taken at the same pint and at the same time. There is bund t be sme time delay between the tw measurements and this accunts fr the scatter in the trend diagram. Fr cntrl purpses, the cmputed values wuld be useful. Cnclusins In this cmmunicatin, n-line measurement f clur, turbidity and f sugar juice in the clarificatin prcess f sugar refining is explained. A theretical relatin existing between clur, turbidity and f sugar juice has been derived. It can be seen frm the trend curve shwn, the cmputed based n the theretical mdel and the measured values, are fairly in agreement. This system will find extensive usage in all the sugar prcessing industries t cntrl juice clur thrugh cntrlling its value apprpriately. Acknwledgement The authrs wish t thank Mis Pnni Sugars and Chemicals Ltd., Erde, Tamil Nadu, India fr permitting them t cnduct the field trials f the n-line instrument develped as well as fr all the help they have prvided during the trials. References i Meade P & Chen C P, Cane sugar handbk, 10th ed (Jhn Wiley & Sns, New Yrk), 1977, Internatinal Cmmissin fr Unifrm Methds f Sugar Analysis, (Elsevier Publishing Cmpany), 1958,54. 3 Internatinal Cmmissin fr Unifrm Methds f Sugar Analysis, (~lsevier Publishing Cmpany), 1970, Internatinal Cmmissin fr Unifrm Methds f Sugar Analysis, (Elsevier Publishing Cmpany), 1974, Plews R W, Analytical methds used in sugar refining, (Elsevier Publishing Cmpany Ltd.. New Yrk). 1970,63. 6 Internatinal Cmmissin fr Unifrm Methds f Sugar Analysis, (Elsevier Publishing Cmpany), iruernatinal Cmmisin jr Unifrm Methds f Sugar Analysis, (Elsevier Publishing Cmpany), 1936,33. 8 Zerban et (11 Anal Chern, 23 (1951 \ 308. I) Willard H, Merritt L & Dean A, Instrtumental methds f analysis, 4th ed (East-West Editin, New Delhi), 1965, Gvidaraj R & Sankaranarayanan P E, Sme theretical studies n the measurement f clur and turbidity during refining f sugar, paper presented in Natinal Sympsium n Instrumentatin (NSI-18), Instrument Sciety f india, Tirupati, January 9-12,1994.