has an impact on product layer diffusion. Beyond a conversion of around 9%, reaction rate drops significantly, which can be dueto pore closure.
Irabien consider the adsorption of sulfur dioxide on calcium hydroxideacting as a nonideal solid sorbent is the rate-limiting step. They use a parameterreferring to this nonideal behavior of the solid surface as independent of temperature butexponentially dependent on relative humidity. The authors obtained activationenergy of75 kJ/mol for the reaction.
All published work thus far indicates that relative humidity has the greatest impact onthe reaction rate between sulfur dioxide and calcium hydroxide. The relative humidity isin turn correlated with the moisture content of the solids. Additives that will modify themoisture content of the calcium hydroxide solids in equilibrium with a gas phase of agiven relative humidity would then be expected to enhance the reactivity of calciumhydroxide towards sulfur dioxide in FGD processes.
Organic and inorganic additives have been tested in spray dryer systems to improvethe
desulfurization
power
of
calcium
hydroxide
and
calciumcarbonatew.It seems that inorganic hygroscopic salts such as barium, potassium, sodium and calciumchlorides and also cobalt, sodium and calcium nitrates would be the most effective ones.Some researchers also consider sodiumhydroxide as an effective additive due to itsalkaline and hygroscopic properties.
Ruiz-Alsop and Rochelleindicated that deliquescence alone does notexplain
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thepositive effect of some salts. They contend that for an additive to be effective, it is alsonecessary that the hydroxide of the cation be very soluble, otherwise, the cation willprecipitate out as the hydroxide and the anion will form the calcium salt which could notbe hygroscopic. The effectiveness of a certain salt also depends on the relative humidity.This could be expected because when therelative humidity of the gaseous phase islower than the water activity in a saturated solution of the salt, it would not absorb waterand so, it would not enhance the calcium hydroxide reactivity. These researcherscontend that chlorides and sodium nitrate modify the properties of the product (half-hy-drated calcium sulfite) layer that is formed as the reaction takes place, therebyfacilitating the access of sulfur dioxide to unreacted calcium hydroxide, which remainsin the interior of the particle.
The scope of the present work is to quantify the influence on the reaction rate ofsulfur dioxide concentration, relative humidity, temperature and type and amount ofadditive. An empirical equation, which relates the reaction rate with these variables, hasbeen obtained and an apparent activation energy value for the reaction has also beendetermined from kinetic constants at different temperatures by using the Arrhenius plot.
2. Experimental section
This equipmentconsists of a continuous feeding and humidification system of a gaseous stream, afixed-bed reactor and an analytical system. The apparatus is operated with a personalcomputer using LabView software (National
7
Instruments), which allows programmingand control of the experimental conditions, namely, nitrogen and sulfur dioxide flowrates, humidification temperature and electric resistance heating of the pipes to avoidcondensations and also provides the experimental data acquisition, in particular nitrogenand sulfur dioxide flow rates, reaction temperature, pressure, relative humidity andsulfur dioxide concentration, vs. reaction time.
Simulated flue gas was obtained by mixing sulfur dioxide and nitrogen from separatecylinders in appropriate amounts using mass flow controllers Before mixing, pure nitrogenwas passed (by switching on valve 1 from thecomputer)through the humidificationsystemthat consisted of three cylindrical flasks with 200 ml of watereach submerged into a thermostatic bath. Each flask contains small glass spheres toimprove the contact between gas and water. After the humidification system, thetemperature and the relative humidity of the wet nitrogen were measured by using aVaisala HMP 235 transmitter .At the same location, the pressure was alsoMeasuredto calculate the flow rate of water vapour generated. Thewet nitrogen by-passed the reactor until the desired experimental conditionswerereached and then valve 2 was opened from the computer to allow thegaseous streamflow through the reactor. The bed was always humidified for 15 min while the sulfurdioxide analyser was set to zero. At this time, the desired flow of sulfur dioxide wasintroduced by a mass flow controller and the experiment began. Data generated duringthe experiment were stored in an EXCEL format computer file.
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The glass reactor, a jacketed Pyrex tube (450 mm height, 12 mm i.d.)with a porousplate to hold 1 g of dry calcium hydroxide (Probus, 99% purity and particle size smallerthan 0.05 mm in diameter)or calcium hydroxide–additive mixtures(all additives weresupplied by Fluka, 99% purity and particle size smaller than 0.05 mm in diameter) diluted with 8 g of silica sand (Merck; 0.1–0.3 mm in diameter)to assure isothermaloperation and to prevent channelling due to excessive pressure drop, was thermostatedby pumping a thermal
fluid
(water–ethyleneglycol
mixture)
from
an
external
thermostaticbath.The reacted flue gas is passed through a refrigeration systemin orderto remove water because it interferes with the SO2 analyser measurement.The output from the analyser was continu-ously collected by the computer for 1h (experiment time)and the concentration (ppm) of sulfur dioxide stored as a function of time (experimental curve). Each experiment wasconducted in the same manner except a reactive solid was substituted for the
9
10 g ofinert silica(‘‘blank’’ experiment) to obtain a reference flow curve. The reaction rate wascalculated as SO2 mol removed/h mol OH- from the area enclosed by the two curves (experimental and ‘‘blank’’). Some experiments were replicated to estimate the experimentalerror in reaction rate.
3. Conclusions
In this research, the quantitative influence of sulfur dioxide concentration, temperature,relative humidity and the type and amount of the three inorganic additives on thereaction rate between calcium hydroxide and sulfur dioxide have been determined.
The SO2 concentration (0–3000 ppm)was shown to have no significant
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