COMPARATIVE STUDY ON THE USE OF SULFAMIC ACID,SULFURIC ACID AND HYDROCHLORIC ACID AS

DESCALANTS FOR BRINE HEATERAND HEAT RECOVERY TUBES1

Anees U. Malik, Ismaeel Andijani and Abdul Salam Al-Mubayaed*

*(Chemistry Lab, Desalination Plant, Al-Jubail)

Research & Development CenterSaline Water Conversion Corporation (SWCC)

P.O.Box 8328, Al-Jubail 31951Kingdom of Saudi Arabia

INTRODUCTION

Scale deposition in the heat exchanger tubes is the most undesirable yet unavoidableproblem in desalination plants. The scaling can lead to serious reductions in heattransfer across heat exchange surfaces with alarming consequences for plantperformance and efficiency. Even by using a ball cleaning system for heat exchangertubes in additive treated plants, scale formation occurs after many years of operationresulting in lowering of the production rates and/or higher energy costs plus the costsinvolved in retubing. Therefore, periodic cleaning of the affected parts of heatexchanger is an essential activity of the maintenance of desalination plant. Scaleremoval by acid cleaning is commonly employed in many types of plants. For removalof calcium carbonate and/or magnesium hydroxide scales, circulation of an inhibitedacid solution (1% HCl, H2SO4 or HSO3 NH2) through the scaled system was found to be appropriate [l]. 3% hydrochloric acid with 0.5% corrosion inhibitor has been usedsuccessfully as a descalant in heat transfer tubes of a 100 ton/day flash evaporator pilotplant [2]. Japan Titamum Society [3] has reported that the H2SO4 solution with IBITinhibitor No. 570S and HCl solution with IBIT No. 2S prevent the corrosion andhydrogen absorption of titanium and copper alloys. Sulfamic acid solution with IBITNo. 570S has been recommended for the descaling of MSF desalination plant.Inhibited sulfuric acid or sulfamic acid has been used as a descalant in heat exchangertubes in Al-Jubail and Al-Khobar desalination plants. Adding an inhibitor to the acid isessentially to diminish its corrosive effect on metals [4]. Inhibitor is a substance whichretards corrosion when added to an environment m small concentrations. IBIT

1 Issued as Technical Report No. SWCC (RDC) - 36 in March 1995.

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corrosion inhibitor is usually used in acid cleaning for copper-base alloys and had beenadded to sulfuric acid or sulfamic acid for cleaning the heat exchanger tubes. Thisorganic material shows its inhibiting action by surface adsorption forming a thin filmon the metal surface, with no significant reaction with the substrate [5]. It has beenobserved that one particular type of inhibitor effective in one system may not beeffective in another. The modes of adsorption of an inhibitor are dependent on:chemical structure of the molecule, chemical composition of the solution, nature of themetal surface and the electrochemical potential at the metal solution interface.ARMOHIB 28 inhibitor has been effectively used with hydrochloric acid for cleaningfouled demisters (SS 316L wire mesh pad) without any deterioration of the demistermetal while the same inhibited acid solution has been found corrosive for cupronickelalloys [6].

Recently, recommendations have been made for scales removal of the heat exchangertubes of the 46 units of Al-Jubail desalination plants by acid cleaning. This motivatedR&D Center to carry out a comparative study of the performance of sulfamic, sulfuricand hydrochloric acids with and without addition of IBIT inhibitor as descalant.Corrosion behavior of cupronickel alloys and titanium in these media was also studied.

EXPERIMENTS

Materials

Titanium (Grade-2) and 70-30 and 90-10 cupronickel alloys were obtainedcommercially in sheet forms. The flat specimens were used during the experimentswithout any further heat treatment. Industrial quality sulfamic acid in powder form andcorrosion inhibitor (IBIT) in liquid form were obtained from Al-Jubail desalinationplant.

Aqueous solutions of hydrochloric acid, sulfamic acid and sulfuric acid of 2%concentrations were prepared with addition of IBIT corrosion inhibitor.

Techniques & Procedures

Immersion Test:

Polished coupons of titanium metal, 70-30 and 90-10 cupronickel alloys of 60x40x2mm dimensions were immersed in one liter each of 2% hydrochloric, sulfamic orsulfuric acid solutions. Recommended amount (3 ml in 1 liter) of IBIT inhibitor wasadded to each of the acid solutions kept at room temperature under dynamic conditions.

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In these aqueous acid solutions, coupons were immersed for 6, 24 and 72 hrs. Aftertaking out from the acid solution, coupons were cleaned, dried and weighed.

Descaling Experiments:

Scales from flash chambers, water boxes and heat exchanger tubes (Fig 1 and 2) wereobtained from Al-Jubail desalination plant for analysis and to study the descalingperformance of hydrochloric acid, sulfamic acid and sulfuric acid. Pieces of CaCO3 andMg(OH)2 scales were added in solutions containing different concentrations of theseacids at room temperature. Under dynamic conditions addition of pre-weighted scalesto acid was continued for few hours till further dissolution of the scales was stopped.By substracting the amount of undissolved scales from the initial weight of the scales,amount of scales dissolved in the acid was determined.

Analysis of the Scales

Scale deposits from earlier stages of demister, flash chamber, brine heater tubes, heatrecovery tubes, heat rejection tubes and water boxes were chemically analysed byAtomic Absorption Spectrophotometer (AAS). X-ray diffraction analysis was used toidentify the different constituents of the scales.

RESULTS & DISCUSSIONS

Immersion Tests

Tables 1 and 2 summarize the results of immersion test of titanium and 70-30 and 90-10 cupronickel alloys carried out in 2% acid solution inhibited with IBIT corrosioninhibitor at room temperature and under dynamic conditions. In all acid solutions,coupons did not show any remarkable change in weight, and therefore, in order to findout metal dissolution in ug, chemical analysis of test solutions was carried out by AAchemical analysis data (Table-2). For short duration (6 hours), the corrosion rates ofcupronickel coupons are the highest, this is followed by a decrease in the corrosion rateas the duration of immersion time increases, This may be due to gradual formation of apassive film on metal surface. Corrosion of all the tested alloys in the three acid mediawere found very low (<0.1 mpy) (Fig. 3a, b and c). It was also observed that 70-30cupronickel is more corrosion resistant than 90 -10 cupronickel and titanium is the mostcorrosion resistant metal in these acid media.

For titanium coupons, no perceptible weight change and no significant change in thesurface morphology were observed in all test solutions during all 3 immersion times.As a result of immersion of cupronickel alloys in sulfamic acid inhibited with 0.3%

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IBIT, uniform reddish color was noticed over the entire 90-10 cupronickel couponsurfaces (Fig. 4a) while the existence of yellowish layer was found on 70-30 cupronickelcoupon surfaces (Fig. 4b). The different colours of 90-10 and 70-30 surfaces aftertreatment are attributed to the different original colours of the alloys. The same resultswere reported for sulfuric and hydrochloric acid solutions inhibited with 0.3% IBIT(Fig. 5 and 6). Therefore, addition of 0.3% IBIT in either of the acids (hydrochloric orsulfuric acids) is sufficient to protect the metal surface.

Descaling Test

Amount of scales [CaCO3 and Mg(OH)2] removed per acid volume versus acidconcentration have been plotted in Fig. 7. The results indicate that for a given acidconcentration, the amount of Mg (OH)2 dissolved is always lower than CaCO3 for alltypes of acid solution except in case of sulfuric acid. Hydrochloric acid appears to bethe best descalant for CaCO3 followed by sulfamic acid and then sulfuric acid. ForMg(OH)2 scales. sulfuric acid is the best descalant followed by hydrochloric acid andthen sulfamic. For 2% acid solutions, following data of dissolution of scales wereobtained.

Acids Dissolving Amount of Scales Dissolved (g/liter)the Scales CaCO3 Mg(OH)2HCl 33 17H2SO4 1 23HSO3NH2 10 6

The amount of different scales dissolved in an acid can be calculated from the followingchemical equations:

Mg(OH)2 + 2HC1 −−> MgCl2 + 2H2O CaCO3 + 2HC1 −−> CaC12 + H2O + CO2

Every 2 moles of HCI acid (73 g) consume 1 mole of Mg(HO)2 (58g) orCaCO3 (100g). This simple calculation shows that due to difference in molecularweights of scaling species, weight to weight, in any acid CaCO3 should bc dissolvedmore than Mg(OH)2. However, in H2SO4 very small amount of CaCO3 is dissolved.This has been attributed to the formation of adherent insoluble white product, CaSO4,during descaling process. The layers of CaSO4 which keep on building on the CaCO3scale surface slow down the reaction between CaCO3 and acid initially and finallyceasing it completely as per the followmg reaction:

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CaCO3 + H2SO4 −−> CaSO4 + H2O + CO2

The CaSO4 scale layer is very adherent and is not easily removed chemically ormechanically.

The X-ray diffraction analysis of the scales from heat transfer tubes indicate thepresence of CaCO3 and Mg(OH)2 as the major components, Results of the chemicalanalysis of the scales formed on heat transfer tubes of higher to lower temperaturestages indicate that there is an increasing trend in Ca++ contents and a decreasing trendin Mg++ contents (Table 3). A plausible explanation of this observation emerges fromthe results of a study which showed that the alkaline scales in distillation plant arepredominantly CaCO3 below about 80-85°C but at higher temperature, alkaline scale ismainly Mg(OH)2 [7].

CONCLUSIONS

(1) Hydrochloric acid appears to be the best descalant CaCO3 on cupronickelsurfaces followed by sulfamic acid. The use of sulfuric acid as a descalant forCaCO3 has an adverse effect due to the building up of highly adherent CaSO4layer on CaCO3 scales.

(2) For Mg(OH)2 scales, sulfuric acid is the best descalant followed by hydrochloricacid and then sulfamic acid.

(3) The studies indicate that 2% HCl with 0.3% IBIT may be the best descalerconsidering its highest removal rate for CaCO3 scale and comparatively highereffectiveness with Mg(OH)2 scale than that of sulfamic acid.

RECOMMENDATIONS

In view of the more efficient descaling and cost effectiveness, hydrochloric acid appearsto be more promising alternative to sulfamic acid. It is therefore, recommended to use2% HCl solution with 0.3% IBIT inhibitor (3 ml/lL HCl) at ambient temperature.While using HCl as a cleaning solution, addition of inhibitor in required amount,standard acid cleaning procedure and post acid cleaning procedure are to be strictlyfollowed.

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REFERENCES

1.2.

3.

4.

5.

6.

7.

Hanburym, Hudgkiess and Morris, Desalination Technology “93”.Sato and Hamada, Prevention of Scale and Corrosion on Flash Evaporators,“Nuclear Desalination”, International Atomic Energy Agency. Vienna, 1969.Counter measures Against Deposit of Scales and Oceanic Lives - Light GaugeTitanium Tubes for Seawater Desalination Plants - Part 3, Q&A PracticalApplication, Japan titanium Society, November 1994. p 14.Metal Hand Book, Vol. 13, Corrosion, Specific Industries and Environments,American Society of Metals, 1987. p 14.C. C. Nathan, Corrosion Inhibitor, Betz Laboratories, Inc. National Associationof Corrosion Engineers, Houston, Texas, NACE Publications, 1973A. U. Malik, I. N. Andijani, N. A. Siddiqi, S. Ahmed and A. S. al-Mobayaed,“Studies on the role of sulfamic acid as a descalant in desalination plant. Proc.VI Middle East Corrosion Conference, Bahrain, 24-26 Jan. 1994 pp 65-78.A short course on Desalination Technology, King Abdulaziz University, Jeddah,KSA, 1980, p 142.

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Fig. 1: --___ ..____ -.-(3\---,L

Figure 2. A section of titanium tube of brine heaterfouled with CaCO, scales.

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(a) (b)

Figure 4. Cupronickel (a) 90-10 (b) 70-30 coupons exposedto 2% sulfamic acid + 0.3% IBIT for 6 hrs showingno change in the surface.

Figure

(a) (b)

5. Cupronickel (a) 90-10 (b) 70-30 coupons exposedto 2% HC1+0.3% IBIT for 6 hrs showing nochange in the surface.

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Figure 6. Cupronickel (a) 90-10 (b) 70-30, coupons exposedto 2% H2SO4 + 0.3 IBIT for 6 hrs. showing no changein the surface

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