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9 (2007) 3137www.elsevier.com/locate/powtecPowder Technology 17Effects of temperature on the scaling of calcium sulphate in pipes
Tung A. Hoang a,b,, H. Ming Ang a, Andrew L. Rohl b,c
a Department of Chemical Engineering, Curtin University of Technology, PO Box U 1987, Perth 6845, WA, Australiab Nanochemistry Research Institute and A.J. Parker CRC for Hydrometallurgy, Curtin University of Technology, PO Box U 1987, Perth 6845, WA, Australia
c Interactive Virtual Environments Centre (IVEC), Technology Park, Kensington WA 6151, Australia
Available online 25 November 2006Abstract
Calcium sulphate scaling is a serious problem encountered in many industrial and domestic applications. Supersaturation has been proven to bethe major driving force of scale formation, but the solubility of calcium sulphate changes with temperature. The main purpose of this work is toinvestigate the effects of temperature on the formation of calcium sulphate scales in pipes, using a pipe flow system. Various levels ofsupersaturation of the calcium sulphate solution have been employed at different temperatures. Results indicated that higher temperature produceda large increase of scale amounts and a significant decrease of induction periods. Many forms of hydrated calcium sulphate were created at hightemperature. The relationship between deposited scale mass and temperature was deduced from experimental data. From the relationship betweeninduction period and temperature activation energies of the surface nucleation were estimated to be in the range of 42 to 48 kJ mol1. 2006 Elsevier B.V. All rights reserved.
Keywords: Calcium sulphate; Scale deposit; Pipe flow system; Temperature effects1. Introduction
Scaling or the accumulation of materials depositing on thesurface of equipment is a complicated phenomenon, whichsignificantly affects a wide range of industrial processes, withserious technical and economic consequences . Calciumsulphate is frequently encountered both in nature and in industry. It is also the most unwelcome scalant in the production ofoil and gas, in water cooling systems and in hydrometallurgicalprocesses .
Early studies on gypsum scaling control mainly focussed onthe kinetics of scale formation [11,12] but later investigationsemphasized the influence of external factors [8,9,1322]. Asystematic study of the effects of various process parametersand the efficacy of some inorganic and organic additives incontrolling the formation of calcium sulphate in pipes was firstundertaken by researchers of the Department of ChemicalEngineering and A.J. Parker CRC for Hydrometallurgy atCurtin University of Technology, WA. [23,24]. Using a selfdesigned pipe flow system and systematically altering theprocess parameters they found that the scaling of calcium Corresponding author.E-mail address: email@example.com (T.A. Hoang).
0032-5910/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2006.11.013sulphate was affected significantly by the supersaturation of thesolution, run time and flow rate .
Calcium sulphate precipitates in many different solid phases:dihydrate (or gypsum), hemihydrate and anhydrous althoughgypsum is the most common one at ambient temperature[25,26]. The solubility of all forms of calcium sulphate changeswith increasing temperature (Fig. 1). The driving force forcrystallisation depends on the supersaturation level of thesolution. However, activation energy is another important factorthat needs to be considered.
2.1. Equipment setup
The pipe flow system consisted of two glass vessels, onecontaining CaCl2 solution and the other Na2SO4 solution. Thevessels were placed in a water bath with a heating unit to controltemperature. The solutions were transported through the testsection at the same flow rate by using a double-line peristalticpump (Fig. 2). The two solutions were mixed at the inlet of thetest section, which consisted of two stainless steel tubular units.Eight pairs of stainless steel semi-annular coupons were insertedinto the tubular units, serving as scaling surface. Each semi
Fig. 1. Solubility of calcium sulphate in water as a function of temperature. (Datafrom Linke, Marshall and Slusher, Silcock, ).
32 T.A. Hoang et al. / Powder Technology 179 (2007) 3137annular coupon had a length of 3 cm and an internal diameter of1.3 cm. The mixed solution coming out from the test sectionwas collected into a waste container. The test section andconnection hoses were covered with insulating material to keepthe temperature constant.
The conductivity of the output solution was measured usinga Yokogawa Model SC82 conductivity meter, which has twooperational ranges from 0 to 20 S/cm and from 0 to 200 mS/cm. The meter was equipped with an auto range function and anautomatic temperature compensation.
Calcium chloride dihydrate (CaCl2U2H2O), AR grade, andsodium sulphate (Na2SO4), AR grade, were from Chem SupplyPty Ltd., South Australia.Fig. 2. Diagram of a pipe flow system.2.3. Procedure
Equimolar solutions of calcium chloride and sodiumsulphate in predetermined concentrations were placed in awater bath until they reached the required temperature. Thesesolutions were pumped and mixed together before going intothe test section. After a predetermined time, the pump wasswitched off and the test section disconnected. The semi-annular coupons were withdrawn out of the units and placed inan oven at 60 C overnight, then cooled down to roomtemperature and weighed. The difference between the massesbefore and after the experiment was the mass of scale deposit.The concentration of the solutions, the run time and thetemperature were altered to investigate their effects. To estimatethe induction period, a 50 mL aliquot of the existing solutionwas taken every 2 min during the run and its conductivity wasmeasured immediately. The induction period could be deter-mined by drawing the best fit lines through the two sections ofthe conductivity-time curve and reading the time correspondingto their intersection.
3. Results and discussion
3.1. Effect of temperature on scale formation
The effect of temperature is shown in Fig. 3. Generally,within the investigated temperature range from 20 C to 60 C,the higher the temperature, the more scales formed. Tempera-ture seemed to significantly promote the scaling of calciumsulphate. For a 0.075 M solution of calcium sulphate, a smallrise of temperature from 20 C to 30 C tripled the scaleamounts formed after 3 h from 0.0212 kg/m2 to 0.0603 kg/m2.At 40 C, scale mass increased to 0.1599 kg/m2, almost 8 timesas much as that at 20 C. At 50 C and 60 C, the scales wereformed so abundantly that it started blocking the flow after 2 h.In contrast, the solubility of gypsum does not change muchwithin this temperature range (Fig. 1). It slightly increases from2.02 g/L at 20 C to 2.08 g/L at 30 C and 2.10 g/L at 40 C,Fig. 3. Effect of temperature on the scale formation. Flow rate=30 mL/min., Ca2+
concentration=0.075 M, run time=3 h.
Fig. 4. Correlation between log (scale mass) and temperature. Ca2+
concentration=0.075 M, flow rate=30 mL/min., run time=3 h. Fig. 6. Comparison of scaling rates at different temperatures. Ca2+
concentration=0.05 M. Flow rate=30 mL/min: a. 20 C, b. 25 C, c. 30 C,d. 40 C.
33T.A. Hoang et al. / Powder Technology 179 (2007) 3137then reduces to 2.07 g/L at 50 C and 2.01 g/L at 60 C. Theseresults indicate that solubility change was not the only cause ofthe increase in scaling rate. Higher temperatures providedenough energy to the molecules to overcome the activationenergy of the precipitation reaction and sped up the transport ofscale components from the bulk solution to the surface.
To determine the correlation between scaling and temperature,logarithm of scale mass is plotted against inverse temperature(Fig. 4). The curve is linear, indicating an exponential rela-tionship between the scaling mass and temperature. An equationcould be calculated from the graph using Microsoft Excel.
logscale mass 4047:1 1T
with the Pearson product moment correlation coefficientR2 =0.9995.Fig. 5. Plots of scale amounts against time at various temperatures. Flowrate=30 mL/min. Ca2+ concentration=0.05 M. Temperature: a. 20 C, b. 25 C,c. 30 C, d. 40 C.From Eq. (1) it can be seen that the higher the temperatureor the lower 1 /T, the larger log (scale mass), and accordinglyscale mass increased by an exponential factor when temper-ature rose.
scale mass CT :104047:11T 2
where CT is a constant.Since secondary nucleation is not significantly affected by
temperature, it could be concluded that the primary nucleationdominated the scaling mechanism. This is consistent withresults from other researchers, who reported that in the precipi-tation of sparingly soluble substances, secondary nucleationeither did not occur  or did occur only to a small extent[31,32]. The number of crystals formed by secondary nucleationduring precipitation was substantially lower than those resultingfrom primary nucleation .
3.2. Relationship between scaling rate and temperature
Plotting the amounts of calcium sulphate formed after theinduction period at different temperatures against time showsthat the curves are parabolic indicating scale amount is aquadratic function of run time (Fig. 5).
To obtain the value of average scaling rate, the scaleamount deposited after every hour was divided by time.Plotting average scaling rate against run time produces aTable 1Rate constant at different temperatures Ca2+ concentration=0.05 M, flowrate=30 mL/min
Temperature k(kg/m2.min2) R2
40 C 1.4215106 0.998630 C 1.19