Method for carbonation

Abstract

The present application discloses a method for carbonation with CO.sub.2. The method now disclosed describes the use of a static or dynamic mixer to react the CO.sub.2 with the incoming of nation liquor to whom Ca(OH).sub.2 was previously added and readily starts the precipitation of tiny carbonate crystals. This solution can be advantageously used to compensate the deficit of CO.sub.2 in the carbonation process. This method for carbonation can be applied for example in the sugar refining industry.

Claims

1. A method for carbonation comprising the following steps: mixing affination liquor and Ca(OH).sub.2 on a first mixed vessel to provide a first mixture containing affination liquor and Ca(OH).sub.2, wherein a residence time in the first mixed vessel is less than 2 minutes; adding CO.sub.2 to the first mixture coming out of the first mixed vessel to provide a second mixture; feeding the second mixture to a static or dynamic mixer to mix the second mixture therein, wherein the static or dynamic mixer is fluidly connected to the first mixed vessel; downstream of the static or dynamic mixer, sending the second mixture to at least one first carbonator where flue gas containing CO.sub.2 is injected thereinto, wherein the static or dynamic mixer is fluidly connected to the at least one first carbonator; downstream of said at least one first carbonator, sending the second mixture to at least one second carbonator where flue gas containing CO.sub.2 is injected thereinto to obtain a filtration liquor; and filtering the filtration liquor.

2. The method of claim 1, wherein the CO.sub.2 used for providing the second mixture is pure.

3. The method of claim 1, wherein the CO.sub.2 used for providing the second mixture is impure.

4. The method of claim 1, wherein the first mixture contains between 0.6 to 0.8% of Ca(OH).sub.2 by weight.

5. The method of claim 1, wherein a residence time of the first mixture in the first mixed vessel is lower than two minutes.

6. The method of claim 1, wherein the pH of the first mixture is between 9.6 and 10.3.

7. The method of claim 1, wherein said at least one carbonator comprises three carbonators and flue gas containing CO.sub.2 is injected into each of the three carbonators.

8. The method of claim 1, wherein flue gas containing CO.sub.2 is injected into said at least one first carbonator until a pH therein first reaches 9.5.

9. The method of claim 1, flue gas containing CO.sub.2 is injected into said at least one second carbonator until a pH therein first reaches 8.0-8.5.

10. The method of claim 1, wherein a food grade flocculent is added to the first or second mixture or to the filtration liquor.

11. The method of claim 10, wherein the food grade flocculent is hydrolyzed polyacrylamide.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention.

(2) FIG. 1 is a schematic of a typical carbonation system including two stages using flue gas from boilers.

(3) FIG. 2 is a schematic of the inventive carbonation system.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present application describes a method for carbonation with CO.sub.2, which can be applied as example in the sugar refining.

(5) In this method, pure CO.sub.2 or mixtures of CO.sub.2 can be used advantageously to compensate the deficit of CO.sub.2 in the carbonation process, due to the fact that there is sometimes low concentration CO.sub.2 in the flue gases. This will allow the sugar mill to fine tune the process regarding CO.sub.2 balance and will bring carbonation back into control.

(6) The CO.sub.2 used can be pure or impure, for instance coming from a CO.sub.2 tank or from the flue gases of any of the boilers or a lime kiln or a CO.sub.2 concentration device, for example amine scrubber, membranes, etc.

(7) There are three ways to introduced CO.sub.2 in the process in order to achieve this goal: 1. in the flue gases; 2. in either stages of the carbonation; 3. in the liquor before the carbonation process and after Ca(OH).sub.2 addition.

(8) Option 1 will be limited by the efficiency of carbonation, which is very poor since flue gases contain about 90% inert gases and the bubbling system inside creates very coarse bubbles which will create the stripping of the CO.sub.2 added to the flue gas. In option 2, it is possible to consider adding CO.sub.2 inside the carbonators via a recirculation loop with a pump and a static mixerhowever the CO.sub.2 will have to be added at a pH lower than the incoming liquor to carbonation and as soon as the recirculating liquid is sent again to the carbonator, stripping will occurthus reducing the efficiency of carbonation.

(9) The method now disclosed describes the use of option 3 as it uses a static or dynamic mixer to react the CO.sub.2 with the incoming affination liquor to whom Ca(OH).sub.2 was previously added and readily starts the precipitation of tiny carbonate crystals. Thus the yield of use of CO.sub.2 will be very high, even if the crystals formed are very small, i.e. the crystals have a dimension smaller than the filter holes diameter.

(10) If impure CO.sub.2 is used, the inert gases contained will not react with Ca(OH).sub.2 even after the mixer. In this case the inert gas bubbles will continue in the liquor current and will be degassed in the carbonators.

(11) The next stages of carbonation will be preferably conducted with flue gases inside the carbonatorsso that higher residence time and lower partial pressure of CO.sub.2 will let calcium carbonate crystals continue to grow and thus entrap more of the liquor impurities. For lower partial pressure of CO.sub.2 on this application it is understood that it is a pressure between 6 KPa and 12 KPa.

(12) This crystal growth is critical to get a good filterability of the liquor. If needed, a food grade flocculent like for instance an acrylamide-acrylic acid resin, such as for example hydrolyzed polyacrylamide, can be added to increase the aggregation of the crystals and improve filterability.

(13) By this proposed way the sugar mill will be much less dependent on the availability of CO.sub.2 containing flue gases and can adapt the carbonation process to the amount of impurities present in the raw sugar. This will mean that the industrial can add higher amounts of Ca(OH).sub.2 if he needs to remove more impurities, since this higher amount will be compensated by the extra CO.sub.2 added after Ca(OH).sub.2 addition.

(14) The method comprises the following stages: Mixture of the affination liquor and the Ca(OH).sub.2, which can be comprised between 0.6 to 0.8% of Ca(OH).sub.2 as CaO is added on liquor solids, in a first agitated vessel; At this point, the pH of the mixture is higher than 11. At this high pH, occurs degradation of the hexoses present, to degradation products of strong colour. In order to avoid this degradation reaction, residence time in the vessel must be reduced to less than 2 minutes; CO.sub.2 is added to the mixture obtained on the previous step; The mixture is passed through a static or dynamic mixer in order to promote the carbonation reaction between the CO.sub.2 with the lime till a pH comprised between 9.6 and 10.3 obtained; the mixture can be divided in more than one first stage carbonators, where flue gas containing CO.sub.2 is injected and bubbled through the mixtures till a pH of 9.5; the mixtures are then sent to a second stage with at least one carbonator where the mixture is once again injected with flue gas containing CO.sub.2 till a pH of 8.5 to 8.0; the liquor obtained proceeds to filtration.

(15) The CO.sub.2 is added just before the mixer, since the pH of the mixture is higher on that moment, more than 11, which favours a fast and complete reaction of CO2 with Ca(OH).sub.2, in comparison with the first step of carbonation with injection of flue gas containing CO.sub.2, where the pH is approximately 9.5, and the second step of carbonation with injection of flue gas containing CO.sub.2 where the pH is approximately 8.5 to 8.0.

(16) The technology is of course not in any way restricted to the embodiments described herein and a person of ordinary skill in the area can provide many possibilities to modifications thereof as defined in the claims.

(17) The preferred embodiments described above are obviously combinable. The following dependent claims define further preferred embodiments of the disclosed technology.

(18) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(19) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(20) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

(21) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(22) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(23) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(24) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.