Geothermics, Vol. 18, No. 1/2, pp. 17-24, 1989. 0375-6505/89 $3.00 + 0.00 Printed in Great Britain. Pe rgamon Press plc

LABORATORY STUDIES OF PbS SCALE FORMATIO}~ IN STEEL PIPES

N. ANDRITSOS and A.J. KARABELAS

Chemical Process Engineering Research Institute P.O. Box 19517, and Department of Chemical Engineering, University of Thessaloniki, GR 540 06 Thessaloniki, Greece

Abstract-Experimental results are reported on the deposition rates and the morphology of scales. The data are obtained under conditions that resemble the rather sudden PbS supersaturation occurring in geothermal brines. It is concluded that PbS scale formation is strongly influenced by both pH and concentration, in the range of very small concentrations of practical inter- est. For a fixed PbS concentration, appreciable scale forma- tion takes place within a limited pH range of about two units. Based on data interpretation, possible mechanisms of sulfide scale formation are suggested.

INTRODUCTION

As shown in a companion paper (Karabelas et al, 1988), lead sulfide is one of the main constituents of hard scale encountered in some geothermal plants handling high enthalpy fluids. This type of PbS-rich scale is observed in particular close to the primary fluid flashing point, where a substantial increase of pH occurs due to the rather sudden evolution of CO 2 and H2S. The pH increase and the concomitant reduction of PbS solubility result in the formation of colloidal sulfide and of hard scale. Uncovering the raechanism of this scale formation is, obviously, of paramount significance in devising methods to avoid or to mitigate the scaling problems in geothermal plants.

The purpose of this investigation is to study mainly the influence of pH, liquid velocity, PbS concentration, and residence time of the colloidal dispersion on scale formation in a pipe, under conditions that (partly) simulate the rather sudden PbS supersaturation in the brine due to flashing. This is achieved by mixing two liquid streams, containing sulfide and lead ions in stoichiometric ratio, at the entrance of a specially designed flow system.

In the first paper on this topic (Andritsos and Karabelas, 1988) two types of PbS deposits are recognized, i.e. "hard" and loosely bound "sludgy" depos±ts. The initial deposition rate of hard deposits is found to increase linearly with flow rate in the range of Reynolds numbers investigated. Of special significance is the strong influence of pH on both the aeposition rate and the type of deposits. Finally it is suggested, on the basis of preliminary evidence, that the mechanism of PbS scale formation is a combination of particle deposi- tion and crystallization. In this publication additional data are presented to elucidate mainly the effects of PbS concentration, of pH, and of fluid resi- dence time in the line, on the deposition rate and on the morphology of the scale.

SUMMARY OF EXPERIMENTAL PROCEDURES

The experimental arrangement used in this work is described in detail elsewhere (Andritsos and Karabelas, 1988). In brief, initial ueposition rates are obtained by measuring the mass of PbS deposited on specially designed

17

18 N. Andritsos and A. J. Karabelas

stainless steel or TEFLON coupons, in two test sections. The pipe length between these sections is 10 m. The inside diameter of the coupons is 13 mm. The two liquid solutions containing lead nitrate and sodium sulfide are indroduced 20 diameters upstream of the first test section. All coupons are cleaned first with detergent and then in an ultrasonic device, prior to each experimental run. Concentrated HNO 3 is used to control the pH o£ the final solution, which is moni[ored by a 632 Metrohm pH-meter.

Morphological characterization of the deposited PbS is performed using a JEOL-840 scanning electron microscope. Special stainless steel plugs, flush- mounted in several coupons, are used to obtain micrographs of the deposits. These plugs are rinsed with distilled water after their removal from the flow system, dried and finally gold sputtered for SEM observations. The absorbance of the various PbS sols is measured with a U-3200 Hitachi Spectrophotometer. The wavelegth of all absorbance measurements is 436 nm. Preliminary measurements of particle size distribution were made using the Photon Correlation spectroscopy technique.

RESULTS AND DISCUSSION

1. SEM Observations

Very useful information on the morphology of scale and on the mechanism of PbS scale formation is obtained by examining the SEM micrographs. Some of these observations are included in Table I and can be summarized as follows.

pH < 3.50 and Small Concentration. Initially very small particles, smaller than %20 nm, appear on the surface in a uniform spatial distribution. At low pH (<3.0), large cubic crystals grow, with time, by crystallization or re- crystallization at the expense of the smaller particles (Ostwald ripening). This is evident by comparing the micrographs (a) and (b) in Fig. I which correspond to pH=2.4 and show that a large part of the surface is not covered by PbS scale. At a somewhat higher pH (~3.1), all other conditions being the same, there is total surface coverage with much smaller crystals. This striking pH-effect is evident by comparing Fig. 1(b) with I(c) showing deposits formed over a period of 24 hours. The different scale morphology is very likely due to the rapidly decreasing PbS solubility with increasing pH (as will be further discussed below) which tends to drastically reduce Ostwald ripening and dis- solution of the small particles. Furthermore, as the micrograph in Fig. 1(d) shows, with increasing time some outward protruding clusters of crystals appear. They have a pyramid-like shape and they seem to be enlarged by indi- vidual crystal growth and possibly by small particle deposition and"cementing'~ In fact, in addition to the large crystals, one observes on these clusters some "arm"-like extensions consisting of relatively small particles. One additional observation is that, as the crystals grow to sizes greater than I ~un, some defects appear on them, especially at the edges and in some faces.

By comparing micrographs taken at t=5 min and t=20 min (pH=3.1, Table I.) one observes that in the latter case the substrate coverage is greater but not the size of the deposited particles. This seems to lend support to the mechanism of particulate deposition as opposed to direct crystallization onto the metal surface.

pH > 3.50 and Small Concentration. With increasing pH particle agglomerates on the surface appear to increase. Most of the deposited PbS mass is in the form of such agglomerates, consisting of tens or hundreds of small crystals, having an equivalent diameter up to 20 ~/n. Some large single crystals or small- size agglomerates are also visible on the substrate. The agglomeration may take place in part in the bulk of the liquid and in part on the substrate, as preliminary evidence suggests. The much smaller deposition rates observed in this region of pH values can partly be explained by the fact that such large agglomerates are easily removed by the turbulent flow.

"Sludgy" deposits (pH=3.1, Small Concentration). As discussed in Andritsos and Karabelas (1988) this term describes deposits, loosely-held on the sub- strate, which are removed from the coupons by rinsing with water. Micrographs of such deposits show that these are very large agglomerates consisting of tiny crystallites of indiscernible size (<0.05 ~m) . "Sludgy" deposits may also contain clusters of large crystals, very similar to the protruding clusters observed to grow on a scale-covered substrate. This is evident in micrographs of "sludgy" deposits collected on the 5th day of a 10-day run.

PbS Scale Formation in Steel Pipes 19

High PbS Concentration. Only limited SEM observations were performed on coupons from a 24 hour run with PbS concentration C=11 ppm and pH=2.4. The morphology of the scale is very similar to the run with C=3.5 ppm at pH=3.1 as indicated in Table I.

It is interesting to point out here that most of the qualitative features of the PbS scale formation discussed in this section can be also found in the work of Jackson (1968), on the growth of cadmium sulfide crystals from reacting solutions.

2. Effect of Time

(a) Fluid Residence Time A distinct characteristic of the PbS deposition is its variation with the

residence time of the liquid or the length of the pipe. This effect is depicted in Fig. 2a and 2b, where the dependence of deposition on the pH and on the PbS concentration are presented, respectively. Although tile residence time of the liquid between the test section is only 22 s, for a Reynolds number of 6000, remarkably different deposition rates are obtained. Only at low pH values are the deposition rates at the two test sections almost identical. Above a pH value of 2.6 the difference between the deposition rates at the two test sections gradually increases and at pH=3.3 the deposited mass at the 2nd Test Section is only 20% of the mass deposited at the Ist Test Section for the same period of time.

The above results can be fairly well interpreted with the aid of the scanning electron micrographs summarized in Table I. For pH values less than %2.6, or generally in the pH range where PbS is soluble, the scale formation is very likely due to direct crystallization on the substrate. There is probably transport of ions to the surface from the bulk of the fluid and a continuous growth of the PbS crystals on the surface. The condition of the fluid remains unchanged along the pipeline, as no substantial amount of PbS is lost. Indeed, even at the maximum deposition rate the loss of mass due to deposition along the pipeline does not exceed 4% of the material capable of forming scale. For pH values greater than ~2.6 a combination of crystallization and particle deposition appears to cause scale formation. The difference in deposition rates between the two test sections can be attributed to the following.

(I) At the Ist Test Section, the PbS colloidal sol is not well developed due to limited mixing time, and consequently direct crystallization on the surface may occur.

(2) Along the pipeline agglomeration of PbS particles takes place due to turbulent flow. Large deposited agglomerates are more easily detached by the flow than individual particles, so that the net mass of scale formed at the second test section is considerably lower than at the first one. The agglomeration is evident both from the presence of "sludgy" deposits on the second test section and from the different absorbance of liquid samples taken simultaneously from the two test sections. Both the initial and the maximum absorbance values of samples from the second test section are higher than those from the first one.

(b) Long Time Runs Figure 3a presents two "long" time runs with the same PbS concentration

(3.5 ppm) and pH and different liquid velocities. The morphology of the deposits in both runs is very similar. For both runs a noticeable change of the deposition rate occurs gradually for times greater than approximately one day. This deposition behavior may be explained by taking into account the morphological pattern of scale formation obtained from SEI4 micrographs. Initially, for the particular pH and concentration of these runs, the scale is formed uniformly on the substrate. However, after the substrate is almost covered with particles, further scale growth takes place in the form of clusters of PbS crystals, in certain sites, which may be easily detached by the flow. The observed presence of such clusters in "sludgy" deposits is an indication of this kind of detachment. It should be added here, that for both runs depicted in Fig. 3a as well as for a run at pH=2,4, C=11 ppm and Re=6000, with morphologically similar scale, the point of "saturation" of the substrate and the change of the deposition rate appears to occur approximately for the same deposited mass, namely 0.6 mg/cm 2.

20 \ . ~qndritsos at~ci A. ,1. Karabe/a,s

3. Effect of PbS Concentration

The effect of the sulfide concentration is examined in two ways [:~i<l "~ and 2c present the deposition rates o[ the "hard" and "sludgy" deposits fo~ ~ ~ constant pH value and liquid velocity. On the other hand, ~'ig. 3c depicts the influence of pH on the deposition rate for three different PbS concentrations~ For all runs in Fig 2b and 2c the pH was kept at 3!0.~. At low concentrations the deposition rates ("hard" deposits) increase almost linearly with concentration, but they drop very rapidly at concentrations greater than 4 ppK~

The declining rates of "hard" deposits are accompanied by increasing rates of the "sludgy" deposits, as depicted in Fig. 2c. Nevertheless, even if the rates of both "hard" and "sludgy" deposits were added, the tota~ deposition rate would not follow a linear increase with concentration. This deposition behavior with respect to concentration can be explained from the results presented in Fig. 3c. The maximum deposition rates fall within the short ph range where a dramatic change of the sol absorbance occurs, as indicated in Fig. 3b. This change of absorbance is apparently associated with the change of PbS solubility at that particular concentration. At pH values much lower than that of the absorbance change, no scale formation is possible due to complete PbS dissolution, while at higher pH values considerable agglomeration averts the formation of "hard" deposits, as discussed previously. It is of interest to note here that the width of the pH range where appreciable scale formation takes place is almost the same for the two concentrations studied. Moreover, the maximum deposition rate appears to be proportional to PbS concentration.

An implication of these experiments is that PbS solubility may play a more important role on scale formation than particle-substrate physicochemical interactions. The only evidence obtained so far on the latter type of inter- action is related to the adhesion forces between the deposits and the sub- strate. Deposits formed at a pH value of 3.5, where the PbS particles and the stainless steel substrate have opposite charges, are more difficult to remove than deposits at a pH=2.0. At this pH value, both PbS particles and the sub- strate have positive charges.

4. Effect of Substrate

The use of coupons made of TEFLON does not change any quantitative feature of the deposition curve for two PbS concentrations, namely 3.5 and ~ ppm. The maximum deposition rate occurs at the same pH value as for stainless steel, while the pH range of appreciable scale formation remains unchanged. However, consistently higher deposition rates are observed, in the order of ~0-30% for both the "hard" and "sludgy" deposits and for both concentrations examined, as illustrated in Flg. 2a.

CONCLUSIONS AND COMMENTS

The following general conclusions have been reached on the basis of available data - PbS scale formation is strohgly influenced by both pH and concentration.

For a given concentration, as the pH increases the growth of individual crystals tends to decrease, whereas the degree of particle agglomeration tends to increase.

- For a given PbS concentration, appreciable scale formation takes place within a limited pH range of about two pH units. The pH value of maximum deposition rate depends on concentration.

- The strong influence of pH on deposition rate appears to be exerted more through the modification of PbS solubility than through particle-substrate physico-chemical interactions. However, the latter may still play a significant role on the strength of particle-substrate bonding. One practical implication of the results obtained so far is that the main

approach of attacking the sulfide scale problem is likely to be the modificat- ion of pH. In fact, it appears that not only acidic but also basic pH may effectively inhibit sulfide scaling.

PbS Scale Formation in Steel Pipes 21

REFERENCES

Andritsos, N., and Karabelas, A.J. (1988) Deposition of colloidal lead sulfide in a pipe. International ~nferenceon Fouling and Cleaning of Process Plants, 25-29 July, Oxford.

Jackson, P.A. (1968) Growth of Cadmium Sulfide crystals from reacting solutions. J. Crystal Growth 3, 4, 395-399.

Karabelas, A.J., Andritsos, N., Mouza, A., Mitrakas, M., Vrouzi, F. and Christanis, K. (1988) Characheristics of scales from the Milos Geothermal Plant. Workshop on Deposition of Solids in Geothermal Systems, 16-19 August, Reykjavik.

ACKNOWLEDGEmeNT The authors gratefully acknowledge the support received by the Commission of European Communities (under contract No. 3N3G-0040-GR~ the Public Power Corporation and the General Secretariat for Res. & Technology.

Table I. Summary of SEM Observations at the Ist Test Section, for Re=6000 and C=3.5 ppm PbS

pH Run ~ dmin dmax Fig Remarks Time (~m) (~m) (bm)

2.4 2 hrs ~0.8 ~0.5 %1.0 la Almost uniform spatial and size distribution.

2.4 24 hrs ~4.0 %2.0 %6.0

3.1 5 min ~0.I <0.02 %0.20

3.1 20 min %0.15 <0.02 ~0.25

3.1 24 hrs ~0.80 <0.10 %1.5

3.1 3 days %1.0 <0.1 ~3.0

3.1 10 days %1.0 <0.10 %3.0

3.15" 24 hrs %0.40 <0.05 %0.8

3.45 15 min ~0.08 <0.02 %0.2

3.45 30 min %0.12 <0.02 ~0.2

3.45 60 min ~0.15 <0.02 %0.2

5.0 2 hrs ~0.5 <0.05 ~I.0

5.0 25 hrs ~0.5 <0.05 %1.0

Ib Large cubic crystals. Crystal interpenetration.

Almost uniform spatial distribution Evidence of 2-3 particle agglomeration

As above 2-5 particle agglomeration.

Ic Outward protruding clusters of crystals. Defect on crystals as they grow.

Id Enlargment of the clusters by crystal growth and possibly by particle deposition/cementing.

Considerable defects of crystals, especially at the edges.

Single particle, but mainly agglomerate deposition.

As for pH=3.1 and t=5 min.

2-8 particle agglomeration

The particle diameter increases. Particle agglomeration.

Basically deposited agglomerates up to 15 ~m in diameter. Single crystals also observed.

As above.

As for pH=3.1 and t=3 days. 2.4** 24 hrs ~I .0 <0.01 %2.0

* Observation at the 2nd Test Section. ** PbS Concentration 11 ppm.

(a)

pH=2.4,

t=2 hrs

(~)

pH=3.~,

t=24 hrs

(b)

pH=2.4,

t=24 hrs

(d)

pH=3.1,

t=3 days

Fig. I.

Morphology

of the deposits.

PbS Scale Formation in Steel Pipes 23

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24 ,%. A n d r i t s o s and A. .I. Karabelas

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