SULPHITATION EQUIPMENT

7.1. The sulphitation apparatus for cane juice clarification is designed taking into account the following important factors.

1. Variation in lime and sulphur dioxide doses should be possible keeping in view the requirements of trie juice for optimum clarification.

2. The reactants must be thoroughly mixed.

3. Mixing in the tank should be thorough so as to avoid formation of pockets of over alkalinity or over acidity.

4. The retention of juice in reaction vessels has to be 10-20 minutes for completion of reaction.

5. Capacity of the reaction vessels should be determined on the basis of retention time.

7.2. Batch sulphitation—This has become obsolete these days. Usually there are three MS tanks, each provided with S0² connection and a lime dosing vessel attached as well as connected to it. The treatment vessels are fitted with mechanical stirrers and when one vessel is being filled, the other is being discharged while the third is receiving treatment of S0² being bubbled at the bottom. After the required lime dose is fed and the desired pH of 7.2 - 7.3 is reached the S0² valve is closed and the treated juice is ready to be discharged. Because of the human element involved in controlling the reactions during the batch operation the results of treatment are not uniform and properly controlled.

Factories which converted their batch operations into continuous ones have obtained improved results¹².

7.3. Continuous Sulphitation—

7.3.1. In continuous sulphiters a number of designs have been evolved in the past 50 years or so, based on following considerations.

1. Two reactions tanks, interconnected, are installed, juice entering in one, and leaving the other.

2. Usually stirrers are provided

3. The reactants are normally fed in one tank while the other tank serves for completion of reaction, or rectification of pH if required.

CLARIFICATION 129 4. The total capacity of the equipment is based on juice retention of 10-20 min.

5. The flow of juice, lime and S0² have to be uninterrupted,

6. The gas distribution should generate small size S0² bubbles for efficiency of reaction.

7. Stirring and S0² bubbling should generate high circulation in the juice.

8. Lime used is of low brix (10°-15°) to facilitate quicker formation of CaS0³ and maintenance of optimum pH.

Some features of important designs of the continuous sulphiters are described later.

However, major components of system are (a) S0² destruction (b) lime dosing arrangement, (c) tank design (4) pH control, which need to be dealt with first.

7.3.2. SO², distribution—The aim in designing S0² distribution has to be to ensure maximum absorption of S0² and the existing designs could be classified as follows—

(i) In a simple arrangement the gas pipe is connected to the bottom of conical sulphitation tank, gas distribution being achieved by perforated plate placed over it.

In one design the S0² and juice are led into the tank through the same pipe thus achieving presulphitation of a short duration. Baffles provided inside the tanks and stirrer should take care of circulation inside the sulphiter.

(ii) In one design a cast iron baffle is located over the gas entry from bottom of the tank which distributes the gas inside the juice mass thereby setting up a current. This arrangement aids circulation of juice in the tank while at the same time dividing the gas stream in a cylindrical path. A serrated cone is connected to the SO^z gas pipe entering the tank.

(iii) In yet another design the S0² gas pipe enters the tank at the bottom and is connected to an inverted cone the top of which has perforations for distribution of gas or is covered with a perforated plate.

(iv) S0² can also be bubbled through a circular coil or a cross type distributer with large perforations.

An ideal distributor would be one that will bubble gas in finely divided state and will fulfill following requirements—

(i) It should be possible to clean the distributor easily if and when sublimation occurs.

(ii) The gas is divided into a number of streams with definite passages.

(iii) Size of gas bubbles is as small as possible for quick absorption.

In view of above a satisfactory distributor will consist of a pipe with enlarged opening or an inverted cone type. Covered with removable plate provided with holes large enough to resist choking. According to Marches¹³ ten seconds contact is enough for gas to be absorbed completely when the juice pH is 7.4 - 8.3 and temperature is 75°C.

7.3.3. Lime Dosing—The lime must be fed into the tank in the area of maximum circulation preferably near the S0² gas inlet, without in any way disturbing the circulation of juice.

Thus in some designs the lime pipe is connected to the sulphitation tank at a suitable point at the bottom or top of the tank whereas in some sulphiters lime pipe enters t h e bulk of juice. In general if the lime feeding is at the top of the juice column, the juice withdrawal is from the lowermost point of the sulphiter and vice versa.

The lime proportioning devices presently used can be broadly classified into t w o categories (a) Mechanically operated (b) Electrically operated or with electronic controls.

As pointed out earlier the lime vessel receives milk of lime continuously, the overflow therefrom delivering the surplus lime back to the lime storage tank. The lime tank is divided into two compartments, one for receiving lime and the other for maintaining a constant level through which lime connection to sulphitation vessel is given a n d provided with an overflow pipe. The inlet can be from bottom. As regards the proportioning of lime or regulating its flow into the tanks a number of variants are adopted based on (a) continuous feeding (b) manual regulation of dose. Some of these are described below.

(1) Lime nozzle—In this nozzle an adjustable conical pin is fitted and the flow of milk of lime can be varied according to the position of pin, whereby the opening of the nozzle is regulated. It is desirable to calibrate the flow at different positions of the conical pin and check the flow during running, off and on. A scale with markings and a lever for adjusting the pin enable control of the flow from the platform of the sulphitation station.

(2) Nielsen plug cock—This cock is fitted to a small milk of lime tank in which constant level is maintained. The plug cock consists of a hollow plug with an opening through which lime flows out into the sulphiter. The inlet to the plug is through an aperture, the area of which can be adjusted by turning the plug. This side opening of the plug is in contact with the outlet of the milk of lime vessel. The operation of the plug can be mechanised and regulated from the platform of the sulphitation tank.

(3) Merry go round bucket feeding—This arrangement consists of a revolving horizontal wheel of merry go round type to which are attached buckets at equal spacings. The wheel revolutions can be varied by means of a variable speed pulley through which a motor provides the drive. The buckets move in a circular box with two compartments. In one compartment the buckets are filled by milk of lime coming from storage tanks while in the other the buckets are tilted by a special device and emptied. This compartment is connected to the sulphitation tank. Variations in r p m and size of bucket bring about variations in the dose of lime.

(4) In yet another design lime quantity is controlled by the flow of either the incoming or outgoing juice from the system.

(5) In one old design at Ugar14 the flow of milk of lime was regulated by the flow of juice.

To the Duplex pump which delivered the mixed juice to the sulphitation unit was connected the device for delivering the lime to the sulphitation unit whereby the lime quantity would be proportional to the speed of pump and the juice quantity. This was later replaced by another device consisting of constant level lime tank with slotted partition and lever operated gate.

CLARIFICATION

131

(6) In some designs the flow of milk of lime is determined by a float placed over the juice receiving tank. The juice flows through a weir and the float will rise or fall depending on the flow of juice. The lime tank is also provided with a V notch and receives lime continuously. Lever connected to the juice float tilts the tank thereby increasing or decreasing the V notch level.

All the above methods, with all their ingenious mechanisms fail to take into account the ultimate goal of maintenance of the desired ^PH of the treated juice, unless suitably connected to pH control mechanism.

(b) Electrical Method—Attempts have been made since last three decades to use proper pH indicator for the treated juice. Initial attempts were based on the measurement of pH electrometrically employing antimony electrode. The antimony electrode however failed to register correct pH due to coating of the same. Later on when in fifties the glass electrode which can withstand high temperature was developed it was possible to continuously measure pH in industrial practice. Mechanical devices of controlling lime could be operated more efficiently with the help of pH indicators.

In sixties however pH controllers for juice clarification were developed wherein the milk of lime flow was controlled automatically in response to change in pH. The pH meter transmitted signals to an electrical system which started or stopped the flow of milk of lime.

7.3.4. Tank Design—In most of the designs two tanks are provided with a total capacity of 10-20 minutes hold up and fitted with stirring mechanism. One of these tanks receives the juice and lime and SO² continuously while the other one serves for correction and completion of reaction, sending out the treated juice continuously to a receiving tank. The stirrers running at about 50-100 r.p.m. are designed to aid circulation, in conjunction with the suitably placed baffles. There are some installations of sulphitation tanks without mechanical stirrers, wherein the circulation is expected to be brought about by the upward flow of SO² gas and the baffle plates, whereas in one design the recirculation of a part of treated juice into the sulphiter brings about forced circulation; however, this is likely to disturb the flocs once formed.

Circulation and mixing are aimed at bringing about turbulance and setting up currents without leaving any dead pockets of juice. It is in the centre of intense mixing that the reactants should be introduced and not in zones of smooth flow.

Yet another important factor in the design of the reaction tanks is the prevention of short circuiting of juice whereby the incoming juice finds shortest path to the outlet of the tank.

This has been efficiently tackled by providing an inner cylindrical tank from which juice is withdrawn and which seals the outgoing treated juice from the incoming raw juice in some designs.

7.5. pH Controller work in Indian Industry—In view of the importance of pH control in clarification, attempts have been made since five decades to introduce automation in the dosing of chemicals employed. In juice defecation automation being restricted to lime dosing only was introduced very early but in sulphitation, automatic control of both the lime and SO, could not be established early though regulation of lime dose on the basis of

final pH of treated juice was introduced in Java and in a few factories in India notably Ravalgaonl⁵ and Sakharwadi⁶ An ideal system of pH control shall incorporate following features—

(i) Since the results of clarification are dependent to a large extent on the amount of lime used, it is of utmost important mat the requisite lime dose is fed to juice, all the time.

In other words, the lime dose has to be given in the proportion which gives the desired results of clarification.

(ii) S0² gas should be regulated for maintaining final pH of treated juice at 7.2 - 7.3.

(iii) It should be possible to regulate lime or S0² so as to maintain the proper pH for either preliming or presulphitation. Regulation of S0² valve at the sulphitation tank posed problems as it would lead to disturbing the working of sulphur burners which lacked combustion control arrangements. This led to control of lime valve, maintaining S0² gas valve fully open in the initial stage of development of automation of sulphitation¹⁷. In recent years Central Electronics Engineering Research Institute of Pilani, have after considerable work and factory trials, established microprocessor based pH control system for regulating both lime dose and S0² gas blown in the treatment tank.¹⁸ The saiient features of this MAPCON system, operating successfully in a number of sulphitation plants in India are as under—¹⁹

(i) The control system is useful at three points in the treatment tank for regulating preliming, liming in the tank and S0² gas.

(ii) The sampled juice is cooled to 30°-40°C by heat exchangers before glass electrode is immersed in the same to prevent scale deposition on the electrode.

(iii) Glass calomel system of pH measurement is installed in which the reference electrode is kept in KC1 tank. At every sampling point glass reference electrode assembly is maintained.

(iv) The pH transmitter gives digital display of pH.

(v) Lime feed control valve is described earlier. A conical plug moves up and down the opening of a small pipe submerged in a small lime tank, for adjusting flow of lime to juice tank. The valve movement is controlled by independent stepper motor.

(vi) S0² control valve is also designed on the same principle as above, in which rubber bellow is used to separate S0² from the moving mechanism and the parts in contact with S0² are fabricated of non-corrosive material. In short stepper motor driven valves have to be introduced for control of reactants.

(vii) Juice flow stabilisation system based on the installation of pneumatic control valve in the delivery line of the weighed raw juice maintains constant known supply of juice to sulphitation unit. This enables maintenance of lime dose in proportion to juice.

(viii) In the central console units are housed all the electronic units. The set points are fed to micro-computer which performs corrective action in the event of deviation of actual pH from set point. The stepper motor drive is controlled by this unit.

CLARIFICATION 133

As regards control of S0² generation, this has been developed and introduced in some Indian factories with good results. This burner design includes (i) sensing of temperature of molten sulphur, sulphur vapour, S0² gas outlet and cooled S0² gas (it) use of secondary air in combustion chamber, (Hi) temperature controller for sulphur burner.

(ix) automatic control of rate of sulphur burning to meet the requirement of sulphitation tank by regulating the air supply to burner.

The Mapcon system in conjunction with automatic operation of sulphur burning is reported to be satisfactorily operating in many factories and has improved the performance of sulphitation.

7.6. Sequence of application of different agents—In juice sulphitation, lime, S0² and heat are the main agents used in the treatment and different modes of their application have been tried or suggested. In general following methods are followed:

(i) Simultaneous liming and sulphitation of juice at about 70°C maintaining more or less neutral condition with respect of pH.

(it) Preliming the raw juice at 70°C to 7.2 pH followed by simultaneous liming and sulphitation.

(iii) Shock liming to about 10 pH followed by neutralisation by S0². The juice which is preheated to about 70°C remains in high alkaline condition for only 8-10 seconds.

(iv) Presulphitation of juice heated to 65°-70°C, the pH being 5 or so. Alternatively presulphitation of cold juice to 3.8 - 4 pH followed by liming and sulphitation.

Needless to state the final pH of treated juice has to be 7.2 - 7.3 and the treated juice is heated to boiling point before being sent to settler, in any method of sulphitation that is followed.

There are different methods relating to sequencing of application of lime or S0², which have been tried and sometimes conflicting results have been reported on account of probably variations in the composition of cane juice. Nevertheless broadly the effects of variations in treatment can be summarised as under—

(i) Preheating of juice prior to treatment with lime and S0², to 65-70°C is essential for avoiding microbiological development and loss arising therefrom.

(ii) Presulphitation of juice leads to better settling characteristics of mud in the case of immature cane.

(iii) Preliming of juice heated to 80°C leads to improvement in settling in the case of juice containing high amount of polyphenols.

(iv) In some areas shock liming to pH 10.5 for 10 seconds followed by normal sulphitation has given good results in respect of colour of juice, settling and non sugar elimination.

(v) Simultaneous liming and sulphitation of juice has given good results in Deccan tract and this is by and large followed in this region.

In the light of the experience accumulated over the years it is evident that each factory has to decide its method of juice sulphitation which requires that—

(0 In the crushing compaign laboratory experiments have to be carried out to determine the composition of juice in different periods of the season.

(it) Experiments should be conducted on (a) the optimum dose of lime

(b) method of sulphitation to be followed

(c) The initial preheating temperature of juice in the factory for attaining the desired results.

(Hi) P²0⁵ content in raw juice should be frequently estimated and minimum required dose for good clarification decided by trials.

(ID) The design of the sulphitation equipment should permit any of the variations in the mode of application of lime and S0² i.e. preliming, shock liming, or presulphitation.

(v) Automation of lime dosing and S0² on the basis of desired pH at different points will be of great help in controlling the process conditions.

view of the various designs prevailing in the industry, each with its own special characteristics it is felt that a number of variants could be developed taking into account the juice characteristics and local requirements of factories. However, there are certain basic factors which need to be taken care of while designing a system of juice sulphitation in any modern plant These are as under—

(1) Thorough mixing of reactants with the juice is to be ensured by installing efficient stirring mechanism and baffles.

(2) Sealing the zone of treated juice from the incoming raw untreated juice as well as the reactants is necessary to obtain uniform treatment of the juice.

(3) Sample of juice for pH control should be drawn from this zone (treated juice) allowing a mininum time lag.

(4) The treated juice leaving the sulphitation tanks contains flocs of precipitates which should not be disturbed when it is pumped to the clarifier through the heaters. A 'screw' pump would be preferred from this angle.¹⁹

(5) The system should possess in-built flexibility of operation with respect to the sequence of liming and sulphitation (preliming or presulphitation etc.) since during different periods of the season the juices may respond to different treatments.

(6) The system should take care of capacity increases as well as increase in lime dose upto at least 25%. In fact it may even be necessary to enhance the sulphur and lime

(6) The system should take care of capacity increases as well as increase in lime dose upto at least 25%. In fact it may even be necessary to enhance the sulphur and lime