Carbonation Process in Sugar Refinery and Raw melt decolourization process

The objective of Carbonation Process

a) The objective of these processes is to remove the impurities that cause turbidity in raw melt liquor.

b) Carbonation is generally applied to melt liquor in the refinery ahead of any decolourising process.

c) Carbonation has a good effect on sugar liquors. It is also purification process, enabling a colour reduction of about 40% to 50% together with a reduction in ash content of about 20-25%.

d) The addition of lime and Carbon Dioxide in the sugar liquor produces Calcium Carbonate precipitates that adsorb the impurities and coloring matter present in the sugar liquor. Less soluble salts such as sulphates, colour bodies, starch, and many other impurities are entrapped by calcium carbonate.

e) These impurities are subsequently removed in the succeeding filtration process.

Reactions in the Carbonation process

a) This carbonation process involves two distinct steps.

b) The first step involves the formation of a voluminous and gelatinous precipitate by reaction of calcium and CO₂.

c) The second step involves the conditioning of the precipitate to improve its filterability.

Ca (OH)₂ + CO₂ = CaCO₃ + H₂O

d) The impurities are both absorbed by, and enmeshed in, the conglomerated particles of the calcium carbonate precipitated by the reaction of the carbon dioxide and calcium hydroxide.

e) This calcium carbonate entraps wax, gums, polysaccharides, colorants, and ash and also destroys the invert sugars. This precipitate that contains most of the impurities from the sugar solution is removed in the process in the succeeding filtration processes.

Carbonation Process Description

a) The process consists of adding slurry of calcium hydroxide into the raw melt solution in a liming tank to increase the pH to 10.5 – 11.

b) The liming tank is equipped with an agitator to keep the liquor and lime solution mixture in suspension and for better mixing.

c) Generally the carbonation process consists of two no’s of carbonators working in series as 1^st carbonator and 2^nd

d) The limed liquor from the liming tank flows transfer to the first carbonator tank where scrubbed Carbon Dioxide gas drawn from the Boiler flue gas after dilution is admitted to react with lime in the limed liquor to form calcium carbonate.

e) The CO₂ addition will be controlled according to the carbonated liquor pH.

f) In 1^st carbonated liquor tank pH is maintained about 9.5 – 9.6 and an amount of 75 -85 % of carbon dioxide gassing is carried out in the first carbonator tank.

g) The carbonated liquor passing from 1^st carbonator to the second carbonator. The pH of 2^nd carbonated liquor tank comes is 8.2-8.4 and amount of 15 – 25 % of carbon dioxide gassing is carried out in the second carbonator tank.

h) Carbonated liquor from the second carbonator tank flows by gravity into the carbonated liquor buffer tank.

i) Each carbonator is equipped with Richard tubes for better mixing of CO₂ with the liquor facilitates the online cleaning of nozzles and avoids jamming, which results in better colour reduction in the carbonation system.

j) Boiler flue gas is used as a source of CO₂ requirement for this carbonation process and the concentration CO₂ gas is about 12-14% in scrubbed flue gases.

k) The carbonated liquor is heated to 85⁰C in tubular heaters and filtered in Membrane pressure filters to separate the calcium-carbonated precipitate from liquor.

l) Clarified melt from the carbonation system will be subjected to 2^nd decolonization process.

Flow diagram of carbonation Process in Sugar Refinery for raw melt clarification.

Main/Ancillary Equipment Operation and Specification for Carbonation process

1. Liming Tank

a) A liming tank with a stirrer and located adjacent to the top of the 1^st carbonators is provided to ensure a more uniform and thorough mixing of raw liquor and milk of lime.

b) The amount of milk of lime added to the raw liquor is controlled by adding a proportionate volume of milk of lime to the flow of raw liquor going to the liming tank.

c) The ratio of raw liquor and milk of lime can be adjusted to obtain the desired pH.

d) pH sensor is also installed on the limed liquor sampling box to measure the actual pH of the lime liquor leaving the Liming tank.

e) Lime consumption/ton of sugar will be a maximum 7 to 10 kg or the amount of lime used is on the order of 0.5 to 0.8% CaO on melt.

f) Milk of lime is generally prepared at a concentration of about 10 to 15^o

2. First and Second Carbonation Tanks

a) Carbonator is also called saturator or reactor

b) Total liquid volume contained in the both saturators is generally sized to give an overall residence time of liquor in the system of about 1 hour.

c) Longer residence times in the reactor generally give higher color removal and better maturity level of precipitate for filterability, but above 1 hour the benefit is much reduced, and losses of reducing sugars are increased.

d) Normal liquor retention time of 30-40 minutes will be maintained in each carbonation tank depending upon the requirement.

e) Liquid height in the saturator vessel is normally limited according to the head the CO₂ gas pumps. Generally, limited to about 2.5 m.

f) The gas distribution system shall be distribute the gas uniformly over the cross-sectional area of the saturator, produce a large number of small bubbles, and ensure sufficient agitation in the vessel.

g) The right degree of mixing is obtained with superficial gas velocities in the range 10 to 15 which should be considered in assessing the geometry of the saturator vessels.

3. CO₂ Gas Production

a) Boiler flue gas is one of the major sources for gas production for this process.

b) The content CO2 % in flue gas depends on fuel being fired in the boilers

i) Coal firing boilers having around 10 to 12%

ii) Bagasse firing boilers having around 12 to 14%

iii) Gas-firing boilers having around 7 to 8%

c) Generally, a two-stage scrubbing system is required for the extraction CO₂ from the flue gas.

d) In the first stage, the flue gas is washed with hot water to remove particulate matter, and in the second stage, the flue gas is washed with a soda ash solution to remove the SO₂ down to a level of about 5ppm. In bagasse fired boiler the SO₂ content in flue gas is very low.

e) Materials of construction of scrubber system need to be chosen to withstand corrosive conditions if the flue gas contains SO₂.

f) Most of the sulphur dioxide is removed in the first scrubber, but the soda ash wash is necessary to protect the pipes and gas pumps from excessive corrosion.

g) Boilers flue gas is one of the major sources for gas production for this process.

h) The content CO₂ % in flue gas depends fuel being fired in the boilers

i) The CO2 requirement is 50 to 100 m³/ton of melt at 8 to 12 % concentration. But the design figure will be 150 m³/ton of melt at STP.

j) The pumping pressure of CO₂ will be in the range of 0.7 to 1.0 kg/cm²(g)

4. Heat exchanger

Plate-type heat exchangers or tubular heaters are used for heating melt upto 80 ⁰C to 85 ⁰

5. Filtration

a) The separation of precipitated calcium carbonate impurities from carbonated liquors is a prime importance factor to get quality of liquor in the Carbonation process.

b) Filterability of liquor is depends on the maturity and crystals grown of calcium carbonate precipitate.

c) In global terms, an installed filter area requirement of 15 m² of filtering area per tonne of melt per hour is required.

d) In terms of volumetric flow rate, generally 0.1 m²/m³ will be acceptable.

e) A filtration rate for carbonatation mud will be in the range of 0.6 to 0.9 m³/m²/hr.

f) Types of filters