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COMPARATIVE STUDY ON THE USE OF SULFAMIC ACID, SULFURIC ACID AND HYDROCHLORIC ACID AS DESCALANTS FOR BRINE HEATER AND HEAT RECOVERY TUBES1

Anees U. Malik, Ismaeel Andijani and Abdul Salam Al-Mubayaed* *(Chemistry Lab, Desalination Plant, Al-Jubail) Research & Development Center Saline Water Conversion Corporation (SWCC) P.O.Box 8328, Al-Jubail 31951 Kingdom of Saudi Arabia INTRODUCTION Scale deposition in the heat exchanger tubes is the most undesirable yet unavoidable problem in desalination plants. The scaling can lead to serious reductions in heat transfer across heat exchange surfaces with alarming consequences for plant performance and efficiency. Even by using a ball cleaning system for heat exchanger tubes in additive treated plants, scale formation occurs after many years of operation resulting in lowering of the production rates and/or higher energy costs plus the costs involved in retubing. Therefore, periodic cleaning of the affected parts of heat exchanger is an essential activity of the maintenance of desalination plant. Scale removal by acid cleaning is commonly employed in many types of plants. For removal of calcium carbonate and/or magnesium hydroxide scales, circulation of an inhibited acid 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 used successfully as a descalant in heat transfer tubes of a 100 ton/day flash evaporator pilot plant [2]. Japan Titamum Society [3] has reported that the H2SO4 solution with IBIT inhibitor No. 570S and HCl solution with IBIT No. 2S prevent the corrosion and hydrogen absorption of titanium and copper alloys. Sulfamic acid solution with IBIT No. 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 exchanger tubes in Al-Jubail and Al-Khobar desalination plants. Adding an inhibitor to the acid is essentially to diminish its corrosive effect on metals [4]. Inhibitor is a substance which retards 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 been added to sulfuric acid or sulfamic acid for cleaning the heat exchanger tubes. This organic material shows its inhibiting action by surface adsorption forming a thin film on the metal surface, with no significant reaction with the substrate [5]. It has been observed that one particular type of inhibitor effective in one system may not be effective in another. The modes of adsorption of an inhibitor are dependent on: chemical structure of the molecule, chemical composition of the solution, nature of the metal surface and the electrochemical potential at the metal solution interface. ARMOHIB 28 inhibitor has been effectively used with hydrochloric acid for cleaning fouled demisters (SS 316L wire mesh pad) without any deterioration of the demister metal while the same inhibited acid solution has been found corrosive for cupronickel alloys [6]. Recently, recommendations have been made for scales removal of the heat exchanger tubes of the 46 units of Al-Jubail desalination plants by acid cleaning. This motivated R&D Center to carry out a comparative study of the performance of sulfamic, sulfuric and 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 obtained commercially in sheet forms. The flat specimens were used during the experiments without any further heat treatment. Industrial quality sulfamic acid in powder form and corrosion inhibitor (IBIT) in liquid form were obtained from Al-Jubail desalination plant. 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 60x40x2 mm dimensions were immersed in one liter each of 2% hydrochloric, sulfamic or sulfuric acid solutions. Recommended amount (3 ml in 1 liter) of IBIT inhibitor was added 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. After taking 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) were obtained from Al-Jubail desalination plant for analysis and to study the descaling performance of hydrochloric acid, sulfamic acid and sulfuric acid. Pieces of CaCO3 and Mg(OH)2 scales were added in solutions containing different concentrations of these acids at room temperature. Under dynamic conditions addition of pre-weighted scales to 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, heat recovery tubes, heat rejection tubes and water boxes were chemically analysed by Atomic Absorption Spectrophotometer (AAS). X-ray diffraction analysis was used to identify 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 9010 cupronickel alloys carried out in 2% acid solution inhibited with IBIT corrosion inhibitor 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 find out metal dissolution in ug, chemical analysis of test solutions was carried out by AA chemical analysis data (Table-2). For short duration (6 hours), the corrosion rates of cupronickel coupons are the highest, this is followed by a decrease in the corrosion rate as the duration of immersion time increases, This may be due to gradual formation of a passive film on metal surface. Corrosion of all the tested alloys in the three acid media were found very low (<0.1 mpy) (Fig. 3a, b and c). It was also observed that 70-30 cupronickel is more corrosion resistant than 90 -10 cupronickel and titanium is the most corrosion resistant metal in these acid media. For titanium coupons, no perceptible weight change and no significant change in the surface 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 coupon surfaces (Fig. 4a) while the existence of yellowish layer was found on 70-30 cupronickel coupon surfaces (Fig. 4b). The different colours of 90-10 and 70-30 surfaces after treatment are attributed to the different original colours of the alloys. The same results were 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 or sulfuric acids) is sufficient to protect the metal surface. Descaling Test Amount of scales [CaCO3 and Mg(OH)2] removed per acid volume versus acid concentration have been plotted in Fig. 7. The results indicate that for a given acid concentration, the amount of Mg (OH)2 dissolved is always lower than CaCO3 for all types of acid solution except in case of sulfuric acid. Hydrochloric acid appears to be the best descalant for CaCO3 followed by sulfamic acid and then sulfuric acid. For Mg(OH)2 scales. sulfuric acid is the best descalant followed by hydrochloric acid and then sulfamic. For 2% acid solutions, following data of dissolution of scales were obtained.

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

The amount of different scales dissolved in an acid can be calculated from the following chemical equations: Mg(OH) 2 + 2HC1 + 2HC1 CaCO 3 --> MgCl 2 + 2H 2 O --> CaC1 2 + H 2O + CO 2

Every 2 moles of HCI acid (73 g) consume 1 mole of Mg(HO) 2 (58g) or CaCO3 (100g). This simple calculation shows that due to difference in molecular weights of scaling species, weight to weight, in any acid CaCO3 should bc dissolved more 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 CaCO3 scale surface slow down the reaction between CaCO3 and acid initially and finally ceasing it completely as per the followmg reaction:

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CaCO 3 + H2SO 4

-->

CaSO 4 + H 2O + CO 2

The CaSO4 scale layer is very adherent and is not easily removed chemically or mechanically. The X-ray diffraction analysis of the scales from heat transfer tubes indicate the presence of CaCO3 and Mg(OH)2 as the major components, Results of the chemical analysis of the scales formed on heat transfer tubes of higher to lower temperature stages indicate that there is an increasing trend in Ca++ contents and a decreasing trend in Mg++ contents (Table 3). A plausible explanation of this observation emerges from the results of a study which showed that the alkaline scales in distillation plant are predominantly CaCO3 below about 80-85°C but at higher temperature, alkaline scale is mainly Mg(OH)2 [7]. CONCLUSIONS (1) Hydrochloric acid appears to be the best descalant CaCO3 on cupronickel surfaces followed by sulfamic acid. The use of sulfuric acid as a descalant for CaCO3 has an adverse effect due to the building up of highly adherent CaSO4 layer on CaCO3 scales. (2) For Mg(OH)2 scales, sulfuric acid is the best descalant followed by hydrochloric acid and then sulfamic acid. (3) The studies indicate that 2% HCl with 0.3% IBIT may be the best descaler considering its highest removal rate for CaCO3 scale and comparatively higher effectiveness with Mg(OH)2 scale than that of sulfamic acid. RECOMMENDATIONS In view of the more efficient descaling and cost effectiveness, hydrochloric acid appears to be more promising alternative to sulfamic acid. It is therefore, recommended to use 2% 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 strictly followed.

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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 Gauge Titanium Tubes for Seawater Desalination Plants - Part 3, Q&A Practical Application, 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 Association of Corrosion Engineers, Houston, Texas, NACE Publications, 1973 A. 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 heater fouled with CaCO, scales.

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(a)

(b)

Figure 4. Cupronickel (a) 90-10 (b) 70-30 coupons exposed to 2% sulfamic acid + 0.3% IBIT for 6 hrs showing no change in the surface.

(a)

(b)

Figure 5. Cupronickel (a) 90-10 (b) 70-30 coupons exposed to 2% HC1+0.3% IBIT for 6 hrs showing no change in the surface.

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

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