Sulphitation Part I
Sulphitation processes are subject to almost as many modifications as simple defecation. The variations may include the following:
- modifications of the sequence of addition of lime and SO2 (liming first, sulphiting first, simultaneous addition of lime and gas, fractional procedures);
- temperature modifications (sulphiting cold or hot, stepwise heating); and
- addition of reagents (batch, continuous, with either manual or automatic control). Obviously these variables permit a large series of combinations, and only the most commonly used are outlined here.
The cold raw juice is pumped through a tower or box with a counter-current of SO2 to absorb as much gas as possible (acidity 3.0-4.0 ml 0.1 N alkali for 10 ml of juice; pH 4.0 or below). Liming to slight acidity (pH about 6.5) is followed by heating, settling, and decanting as in the defecation process. Evaporation to a thin syrup follows, and the syrup is settled for 6-24 h before vacuum pan boiling. One boiling, yielding a near-white sugar that is heavily washed in the centrifugal, is frequently followed by a second boiling to a raw sugar. The "boil-back" molasses is allowed to settle for several weeks before it is placed on the market. The success of the process is largely dependent on the quality and price of this molasses.
Sulfitation can also be carried out by injecting SO2 (industrial liquid SO2 in cylinders) into the cold raw juice to a level of about 400 ppm SO2. This is for the production of raw sugar and A molasses. The A molasses is inverted to yield a sucrose-invert ratio of about 1:1, giving a total sugar of 65% at 80 Brix, with an SO2 level of 30-40 ppm.
Sulphitation After Liming
This process is termed alkaline sulphitation as opposed to acid sulphitation previously described. It uses about 8 gal (30 litre) of 26 Brix milk of lime per 100 gal (378 litre) of juice giving a large excess of lime. Sulphitation is then carried out to about pH 7.5 producing a heavy precipitate that may be removed with settling and decantation. Heavier liming (10-12 gal, 38 - 45 litre), will result in a precipitate that permit filter-pressing. After evaporation the syrup is cooled and sulphited to slight acidity (pH 6.5). Treating diffusion juice with lime and then sulphitation decreases the colour of syrup, raw sugar, and refined sugar by 25% 46% and 35% respectively The filterability is improved and molasses purity is lower, giving better sugar recovery
Hot sulphitation serves to reduce the solubility of calcium-sulphite, which is more soluble at low temperatures, the minimum solubility is at about 75°C (167 °F). The juice is first heated to this temperature then sulphited and limed boiled, and settled. Harloff's process is a hot treatment procedure in which the juice is heated to 75 °C and the lime and SO2 are added simultaneously in such a way as to maintain the reaction acid to phenolphthalein and alkaline to litmus (pH about 7.4-7.8), except toward the end, when a quantity of lime is added to attain a strongly alkaline reaction (pH 10+), after which the sulphitation is completed to neutrality to litmus (pH about 7.2). As in all other similar processes, the juice is finally brought to boiling temperatures in juice heaters and settled.
Continuous sulphitation means the continuous addition of SO2 and lime to the constantly flowing stream of juice. Marches shows many different procedures with diagrams indicating construction details, methods of lime and gas addition, baffles to ensure proper circulation and other details.
Many of the continuous liming processes may have different fractional procedures, but are not in general practice.
Sulphitation of Syrup
Sulphiting the syrup leaving the evaporators gives a sugar of higher and more regular quality than juice sulphitation alone. The syrup density is lower than in ordinary defecation processes, 55 Brix against 65 Brix or higher Sulphited syrup is usually maintained at a distinct acid reaction, pH 6.1 - 6.5.
Control of Temperatures and Reactions
Good circulation and thorough mixing both of the lime and of SO2 are very important A bent circulation baffle devised by Thompson gives the best results in cylindrical sulphitators Avoidance of high alkalinities at high temperatures or for extended periods is recommended for the same reasons as in defecation control: such high alkalinities result in decomposition of reducing sugars and in colour formation. Poor mixing of lime and juice may produce local over-liming. Temperatures above 75 °C are detrimental and some prefer not to exceed 70 °C until the final pH adjustment is made, to give a clarified juice to the evaporators of pH 6.9-7.0.
Generally the mixed cold juice is sprayed into tall vertical cylindrical tanks, 4ft (1.2 m) or more in diameter and possibly 15 ft (4.5 m) high, fitted for the upper two-thirds with a series of hardwood grids made of 2 x 4 ft (0.6 x 1.2 m) timbers set on edge. The juice enters the top of the tower in a spray and falls through the wooden grillwork, where it encounters the rising current of SO2. Either the flow of gas through the system is induced by an air ejector or the SO2 is under pressure. The sulphitated juices are drawn from the conical bottom of the tower at a pH of 3.8-4.0, limed in a separate liming tank to pH 6.5-6.8, then heated to boiling and settled.
Continuous sulphitation can be carried out in cylindrical sulphitators holding a fixed volume of juice. Heated juice (75 °C) flows through the tank continuously, while the milk of lime is added constantly to the entering juice and a continuous pressurised flow of SO2 into the liquid near the bottom of the tank supplies the needed circulation. The supply of gas is kept constant, and the lime addition is regulated by a controller. In actual practice, the juice is pre-limed before entering the sulphiting tank, generally to neutrality, then is maintained near the neutral point by the sulphitation-lime addition.
Zozulya et al. describe a new sulphitator which comprises a vertical tank with a feed-line at right angles to the top of the side wall. The juice is fed into the feed-line through a perforated disc and comes into contact with SO2 gas metered through a valve at right angles to the liquid stream. An internal cyclone at the top of the tank acts as exhaust gas-liquid separator and as supplementary mixer for the incoming gas and the juice. Performance data of this new design show results superior to the conventional spray type with better gas utilisation and decolourisation.
Sulphitation with Bentonite
A process employing colloidal bentonite combined with sulphitation was developed in Argentina for the production of direct-consumption white sugars, especially with juices of deteriorated or frozen cane. Bentonite is a clay, and the material selected is sold in Argentina under the trade name Clarigel. The advantages claimed are lower sulphur and lime consumption, much greater removal of organic non-sugars, better boiling properties of syrups and molasses because of reduced viscosities, and less scaling of evaporators.
Sulphur Stoves or Burners
The production of SO2 occurs when sulphur is burned in a current of air. Older-type stoves operate intermittently; modem burners provide for the addition of sulphur without interruption of the burning.
In any type of sulphur burner the air supplied to the furnace should be dry, because moisture in the air will cause the formation of sulphuric acid, obviously detrimental to piping, and soon, and can be especially serious if it reaches the juice. The drying agent is generally quicklime spread on trays, and it should be replaced before it becomes saturated with water, about every 8 h.
Rotary sulphur burners use induced draft. Mechanical feed ensures continuous operation. Best results are obtained with sulphur of high purity (99.6-99.9%). The sulphur melts by its own heat of combustion in the rotating cylinder, presenting a large surface for combustion as the sulphur drips through the air. Air is drawn in at an adjustable neck ring and anti-sublimation sleeve at the connection between the rotating drum and combustion chamber, a cast-iron or brick lined compartment with baffles, where the oxidation of the sulphur and mixing with the diluting air are completed. A uniform gas (5-16% SO2) free of sulphuric acid is delivered to the sulphitators.
There are new methods of SO2 generation. The Swedish Celleco SBM-250 sulphur burner
has a burning capacity of 5 t/d but has a turn-down ratio of 20:1, or 250 kg/d. It is normally operated at 2.0-3.0 psig, but can also function effectively at 42 psig. A typical flow scheme for a modern SO2 generation plant is given
Liquid Sulphur Dioxide
Where transportation costs will permit, liquid SO2 offers many advantages. MeGinnis diagrams a system for the introduction of liquid SO2.
The method is comparatively troublefree and adapts itself readily to automatic pH control. A large reduction in sulphur consumption results; freedom from sulfuric acid, precise control of SO2 addition, and elimination of sulfur-burning equipment are other advantages.
Hydrogen peroxide has also been tried in sugar refining. and reduced white sugar color by 46% and ash by 20%.
Source: James C.P. Chen, Chung-Chi Chou Cane Sugar Handbook : A Manual for Cane Sugar Manufacturers and Their Chemists
Sulphitation Part II
The carbonated liquor after the first filtration still contains an appreciable amount of calcium in solution which has to be removed. This is done. by treating the filtrate with sulphur dioxide to form calcium sulphite precipitate. The latter is then separated from the liquor during a second filtration to produce a final clear liquor. Sulphitation is not an essential part of a carbonatation refinery, another Process such as ion-exchange can also be used to remove excess calcium.
Because sulphitation is only a minor operation in a carbonatation refinery, geared mainly to reducing excessive alkalinity to the neutral point, the amount used is relatively small and the apparatus sometimes a bit crude. especially the sulphur burner. The equipment in use in our refineries to perform liquor sulphitation consists of:
- the sulphur burner for production of SO2 gas
- a tower for contacting liquor and SO2
or a venturi system of contacting, such as the Quarez sulphitator.
Sulphur burner, Production of SO2 gas:
The combustion of sulphur is required to produce sulphur dioxide, because the reaction takes place in the gaseous state between sulphur vapour and oxygen, according to the formula:
S + O2 → SO2 + 293 kJ
The reaction is exothermic and the combustion gas has an SO2content of 6 to 16%. A simple type of sulphur burner is normally used being of the stationary type and quite suitable for the light sulphitation of liquor required. Ideally the design and operation of sulphur burners require that some important points be recognised, in particular:
- Keep to a minimum the formation of SO3 which will react with moisture in the air to produce sulphuric acid. The cooling of SO2 gas to below 200°C is essential, the production of SO3 then being negligible (5). The, drying of the air of combustion is also required to prevent the formation of H2SO4.
- Prevent the sublimation of sulphur, which can cause blockages and impair SO2 production by controlling the furnace temperature to less than 300°C.
- The air flow should be kept constant and controlled.
- A regulated supply of sulphur should be provided, if possible.
The points mentioned above are not easy to control in the type of furnace in use, but then the operation is not critical enough to warrant a more complex approach.
The Sulphur Tower
As the name implies this is a tower containing splash trays, stacked on top of one another and designed to create a continuous passage for the liquor from the top to the bottom, while the SO2 gas travels up the tower. The liquor is broken into droplets in falling from one splash tray to the next. The gas is drawn up the tower by suction from a fan and the exhaust fumes are dispersed into the atmosphere. Reaction takes place as the SO2 conies into contact with the liquor. The sulphited liquor, with calcium sulphite precipitate in suspension, exits the tower at the base into a small seal tank, since the tower is under slight vacuum.
The Quarez sulphitation system consists of a holding tank, a circulating pump, a venturi and sulphur furnace to produce SO2 The level in the tank is kept constant by means of an overflow. Liquor in the holding tank is circulated by the pump and a certain amount is forced through an injector creating a vacuum, which causes the SO2 gas to be sucked in and mixed. The rest of the liquor by-passes the injector by means of an adjustable valve, the setting of which controls the amount of gassing and the final pH of the liquor.
This system is in operation in Pongola and the data available on the installation is given here:
|Refined sugar throughput||22||tons/h|
|Tons Brix in Raw Melt||25||tons/h|
|Volume of liquor to Sulphitation||20||m3/h|
|Number of circulations||15||/h|
|Capacity of circulating pump||300||m3/h|
Sulphitation is carried out to a pH of 7.0 and even at 6.9 – 6.8; but a lower pH than this will result in inversion of sucrose and must be avoided. It is therefore of paramount importance to reliably control the final pH set point. This is generally done by varying the proportion of SO2 gas to liquor by measuring liquor pH.
Filtration of the sulphited liquor should take place at or near 85°C to take advantage of the decreasing solubility of calcium sulphite at high temperatures as well as lower viscosity. A heat exchanger of the shell and tube type is normally used for this purpose. The amount of calcium sulphite precipitate is much less than the carbonate precipitate and less filtering surface is required.
Disposal of Sweet Water
The cake from the primary and secondary filters is sent to sludge filters for sweetening-off. The sweet water should be returned to process that is C and B sugar melting, B and C, pan movement water, etc and preferably not back to the raw sugar refinery melter, on account of colour and ash increase in refinery melt.
- Koster, K C (1988) : From TSD- Internal Report, De-bottlenecking of GH Refinery
- Murray, J P and Rungass, F M (1975) Proc SASTA 49; 90 - 93
- Bennett, M C (1974) Int. Sug J. 76, 40 -44; 68 73
- Lamusse, J P (1988) Proc SASTA 62, 51 - 55
- Honig, P (1953) Principles Of Sugar Technology Vol I pp 646 653
- Wiehe, H F and Pillay, J V (1986) Summaries of talks at the Colloquium On Refining (1985). SMRI Tech Rep 1432, p 6.