Presented at LimeCon December 2012

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LIME HYDRATION PROCESS DEVELOPMENT

O. Collarini (MD Cimprogetti S.p.A) – Dalmine – Italy Introduction Lime is one of the materials most widely used by man since thousands of years, but it is with the fast development of the industry in the 20th century that increasing quantities and different qualities of quick lime and hydrated lime have become needed to satisfy the new request arising from different industrial segments. In particular for hydrated lime, relatively newer industrial uses, such as Flue Gas Desulphurization, have required new product characteristics: from enhanced average fineness, to higher surface area and pores volume. All the above, as a result, has generated the need to evolve and partly redesign hydration equipment and their ancillary components in order to meet the product quality demand from one side and the emissions standards from the other side. In the next part of this presentation we shall look into the solutions to be adopted for producing a finer product from the hydration process.

The hydration reaction Hydration reaction is chemically simple but it is strongly exothermic, with a heat generation of approx. 490 BTU/lb. Rapid hydration is prone to produce the finer particles, as hydrate crystals are given less chances to agglomerate, however the most rapid reaction is not necessarily the best condition. In principle a typical high reactive lime reaction develops in three different phases (see fig. 1), which we have defined as follows:

Kinetic Transitory Diffusion

Kinet

ic

Transi

tory

Diffusi

on

Reaction develops in three different phases:• Kinetic• Transitory• Diffusion

Fig.1 – Hydration reaction phases

The Kinetic phase is normally very short (less than 10 seconds), and shows a sharp temperature increase, which can be as high as 50% of the total temperature raise.

The duration of the second phase, Transitory, (often less than one minute) may change in connection with the size of the lime lumps fed to the hydrator, and shows a visible bend of the temperature raise. In the last phase, Diffusion, the temperature raise sharply again until it flattens quickly to show the end of the reaction. In fig. 2, you may see a graphical explanation for the behavior shown before. Magnified quick lime particles surface is represented, to show what happens after the initial contact with water. In the first few seconds the reaction starts very strongly due to unobstructed contact between lime and water. Shortly after, a first layer of partly hydrated lime is generated on the surface and acts as a shield to the quick lime layers underneath as it tends to remain near the particles surface. Water penetration is therefore delayed. When calcium hydroxide crystals are progressively formed in their final shape, they start to separate, water penetration is improving and the reaction trend is resumed.

Lime particlesLime particles

The hydration mechanismThe hydration mechanism

KINETICKINETIC

TRANSITORYTRANSITORY

DIFFUSIONDIFFUSION

Fig. 2 – hydration mechanism The other factor to be considered, is the development of the reaction in function of the quick lime lumps size. We have in facts taken one quick lime sample and have ground it to three different sizes: C: +2.0 -4.0 mm. (exposed area 1x) B: +1.0 -2.0 mm. (exposed area 2x) A: +0.5 -1.0 mm. (exposed area 4x) As you can see in fig.3, the finer size (A), due to its 4 times higher contact surface, initially reacts more strongly than the other two sizes, but in turn it also shows a much more visible bend at the end of its Kinetic phase, as the localized high temperature developed on the particles surface, make them less reactive, as a final result.

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Time [s]

A[0.5,1 mm]A[0.5,1 mm]

B[1,2 mm]B[1,2 mm]C[2,4 mm]C[2,4 mm]

Exposed areas for a given quantity

-A : 4 X

-B : 2 X

-C : 1 (BASE VALUE)

Exposed areas for a given quantity

-A : 4 X

-B : 2 X

-C : 1 (BASE VALUE)

Reaction vs.quick lime lumps size

Fig.3 – Reaction vs. quicklime lumps size

New hydrators features Experience says that the rate of temperature growth in the reaction vessel (hydrator) has a major impact on the finished product characteristics.

Fig. 4 - Hydrator Cim-Hydrax-TG

It is also apparent that finer sizes are reacting so quickly that the generated heat cannot be distributed/removed as fast as it would be necessary to avoid particles “burning” and the consequent delay in hydration (unless chemical additives are used to control the reaction development). Therefore in the quest for finer particles production, an appropriate gradation must be selected in function of the available quicklime. The effectiveness of the water distribution and product mixing in the first stage of hydration has also a notable importance. Though the phenomena indicated above were well known from decades, a real break through was achieved when we were requested to meet the FGD lime quality standards with raw hydrate of good fineness directly out of the hydrating machine. In this light new design featuring a more effective mixing and water distribution along with adequate instrumentation and a computerized control system to follow the reaction more closely, were developed.

With the past generation hydrators, water was fed at one point near the quick lime inlet mouth and the 1st stage mixing shaft was often rotating at a slow speed.

Cim-Hydrax basic model 1985 - 1995Cim-Hydrax basic model 1985 - 1995

LimeLime WaterWater

Fig. 5 – Hydrator Cim-Hydrax – basic model Further stages of the hydrators provided further mixing for some more time (normally up to 15-20 minutes) in order to assure a complete reaction before discharging the material. As you can see in fig. 6, in a modern hydrator, the lime is still fed in a single point at the hydrator inlet, on the contrary water is fed through a multi-point water distribution system over the first stage length.

Cim-Hydrax-TG model after 1996Cim-Hydrax-TG model after 1996

LimeLime WaterWater

ThermocouplesThermocouples

Fig.6 – Hydrator Cim-Hydrax-TG

Water is fed at relatively high pressure (6 bar) and feeds specially designed nozzles which assure a good coverage of the entire chamber section. Several thermocouples are also fitted in each stage in order to monitor in better detail the temperature profile of the reaction itself. As a result, water distribution among the several spray points can be adjusted in function of the characteristics of the quick lime utilized and the resulting temperature profile.

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Cim-Hydrax-4G for special applicationsCim-Hydrax-4G for special applications

Multipoint water injection

Twin shafthigh-speed 3rd stage

Fig. 6 – Hydrator Cim-Hydrax-4G, designed for special applications The first stage design has evolved from a slow rotating (12-15 rpm) single shaft equipped with large paddles, to a twin counter-rotating shafts arrangement, featuring overlapping short paddles, able to reach a speed up to 50-60 rpm ( see fig. 7). A frequency converter allow the adjustment of the rotation speed to match the best operating conditions determined on a case by case basis.

1st stage Cim-Hydrax-TG1st stage Cim-Hydrax-TG

Fig. 7 – 1st stage of hydration process An adjustable weir at the end of the 1st stage permit to regulate the depth of the lime bed in function of the raw material characteristics, so that more agitation and/or more holding time can be provided when necessary. The 2nd stage is also featuring a twin counter-rotating shafts arrangement, but the rotation speed is slower than in the 1st stage, while the chamber volume is nearly double, as the hydrated lime require a much larger volume. In this chamber the reaction is normally completed and the paddles design has been studied to improve the turbulence. Also in 2nd stage an adjustable weir is providing a regulation of the lime bed depth (See fig. 8).

Fig.8 – 2nd stage of hydration process The 3rd stage, in the past also called “seasoning chamber” has also evolved thanks to the wider availability of high reactive limes (see fig. 9). It may be equipped with a twin counter-rotating shafts configuration, permitting speeds up to 50 rpm for an optimum product ventilation at the end of the reaction. In this latter case, paddles are installed with a reduced pitch in order to allow a greater agitation, but still keeping the same product speed throughout the chamber.

Fig. 9 – 3rd stage of hydration process

Another aspect of modern hydration plants design is the absolute need to minimize the particulate emissions. Today 50 mg/Nm3 is generally an accepted value, but 20 mg/Nm3 or even 10 mg/Nm3 are new limits, progressively introduced in several countries. Wet scrubbers cannot meet the new standards as hydration process water would not be enough and the use of surplus water with consequent generation of a polluted liquid effluent is generally not an acceptable option.

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A specially designed bag-house has been developed by Cimprogetti in the early nineties and finally adopted from the year 1996 onwards, after several years of trials with different configurations and different filtering media (see fig. 10).

Fig.10 – Cim-Zeropoll filter

A short consideration must be given also to the Cimprogetti highly performing control system CIM-LCPS® (See fig. 11).

Fig. 11 – Cim-Issy 32 control system

It is the essential complement to Cimprogetti hydrators and classifiers and can boast the specific advantage that all solutions and features adopted and/or learned by the Cimprogetti commissioning specialists worldwide, are continuously incorporated in the newer releases. In a few words it is a highly specialized, user friendly software, developed by people who operate hydrators. All meaningful process parameters are recorded and interact to maintain stable operation conditions. Start-up and shut-down operations are also automated. Finally, the progress outlined in this presentation, is also the result of a large number of field tests with the Cimprogetti pilot hydrator, executed over the years, in collaboration with several prominent Lime Producers worldwide.

Cim-Pilot is an exceptionally flexible little machine (100 kg/hr capacity) which is able to reproduce substantially all operating conditions one can imagine, with or without the use of additives to control the reaction and the specific surface area of the hydrated lime. (See fig. 12).

Fig. 12 – Cim-Pilot hydrator

Is the mission accomplished? The table below (provided courtesy of Singleton Birch Co. –UK), shows the fineness of raw hydrated lime produced by two 15 mtph Cimprogetti hydrators.

CIM-Hydrator model Design 1.2.1 Design 2.2.1

Raw hydrate specifications

Residue on 90 µm sieve 31.2 % 19.1 %

Residue on 1 mm sieve 28.7 % 15.0 %

Residue on 2 mm sieve 18.6 % 7.9 %

Residue on 4 mm sieve 6.1 % 1.6 %

Hydrator design 1.2.1 features a wet scrubber and a single shaft 1st stage. Hydrator design 2.2.1 features a bag-house for wet fumes dedusting (Cim-Zeropoll) and a double shaft hi-speed 1st stage, with multipoint water injection. Both hydrators are fed with the same +0 -5 mm high reactive quicklime, produced in vertical twin shaft regenerative kilns. As one can see, the hydrator design 2.2.1 is giving a distinctively finer product, right at the outlet of the hydrator itself. This will allow both an higher quantity of chemical grade hydrated lime to be made available after dynamic separation of the particles, or a better efficiency in Sulphur particles capture, whenever such raw hydrate is used directly in FGD scrubbers.

Fumes outlet

Water dosing

Hydrated lime outlet

Quicklime inlet

Control board

& data logging