PERFORMANCE EVALUATION OF A MODIFIED POLYMALEIC ACID AS ANTISCALANT IN A REHEAT PLANT 1

Transcription

1 PERFORMANCE EVALUATION OF A MODIFIED POLYMALEIC ACID AS ANTISCALANT IN A REHEAT PLANT 1 Ata Yaseen Abdulgader, Ghulam M. Mustafa Nabil Nada Kheder Al- Hassani and Mohamed Khafagy Saline Water Conversion Corporation P.O.Box 8328, Al-Jubail , Saudi Arabia Tel: , Fax: rdc@swcc.gov.sa ABSTRACT In thermal distillation process, the cleanliness of heat transfer tubes affects the performance of the plant. Scale deposition on these surfaces increases resistance to heat flow and leads to plant performance deterioration. Scale is formed by deposition of seawater salts that go beyond their solubility limits. This formation of scale is inhibited mainly by addition of some chemicals (antiscalants) that have the characteristic to increase the threshold limits of saturated seawater salt solutions and/ or have the ability to distort the scale crystalline lattice so as not to adhere to heat transfer surfaces. Saline Water Conversion Corporation (SWCC) of Saudi Arabia being a large consumer of antiscalants, has been encouraging development in the field of scale inhibiting chemicals to improve their effectiveness and to reduce their costs. SWCC is offering manufacturers to test their newly developed antiscaling chemicals in its operating plants. In this regard, SWCC was offered to test one of the latest developed low temperature antiscaling chemical of polymaleic acid base in a reheat plant under its normal operating conditions. Multi-effect evaporator with thermo-vapor compression (METC) process is known for its simple morphology and high performance. Heat flux in METC is higher comparative 1 Proceedings of the Fourth Annual Workshop on Water Conservation in the Kingdom, Dhahran April,

2 to multi-stage flash (MSF) process, this explains why METC is plagued by scale formation, albeit the process operates at maximum top brine temperature(tbt) of 65 o C. Scaling in this type of evaporators can be controlled by proper design of feed distribution on the top of condenser tubes to avoid the formation of dry spots and also by selection of reliable antiscalant. Practical experience in operating METC reveals that acid cleaning every 6 to 12 months is required to keep scale build up within acceptable limits. This paper is presenting the performance of the said modified polymaleic acid in a reheat plant employing METC process. The trial test was carried out for about one year at a TBT of 65 o C. The evaluation test was based on the measured and calculated thermal parameters, production rate, GOR, and PR, as well as visual inspection of accessible heat transfer tube surfaces. The results of the evaluation showed that this chemical would be effective in controlling scale at a dose rate of 2 ppm for more than one year with no intermediate acid cleaning of the tubes being mandatory. INTRODUCTION Saudi Arabia has no option other than desalting seawater to provide potable water to most of its population. In densely populated areas, multistage flash (MSF) distillation process is commonly employed due to its simplicity, reliability, and huge capacity. In less populated areas where moderate sized plants are required, multi-effect distillation (MED) process can provide potable water at a reasonable rate which is comparable to the predominant MSF desalting method [Khan, 1980]. Multi-effect distillation was the first thermal process ( ) used to desalinate significant amounts of seawater. Even though during later stages of development (1953 onwards), multistage flash distillation became the most dominant and frequently used process (60%-70%) for large producing units, MED remains of importance and accounts for certain percentage (5-15%) of desalinated water production. Recent studies and developments have equipped MED with high efficiency and wide range of production capacities. The reason being many clients/ consultants are now showing interest in MED process. Such parties consider MED as 2

3 the cheaper and the more flexible process that provides higher plant performance compared to MSF. The source of energy for MED process is a relatively low temperature steam. Providing this energy requirement directly from a boiler will be costly and wasteful process. However, in a single purpose operation, coupling MED with vapor compression will yield a process that uses steam in more efficient manner compared to straight forward MED process [Darwish, 1995]. Steam utilization in MED can be improved by increasing the number of effects and/ or by coupling with a thermo-vapor-compression system. The increase in number of effects is restricted by the top brine temperature that has the maximum allowable limit of 75 o C with the use of presently available antiscalants. For an MED unit at a TBT of 75 o C, it is difficult to increase the number of effects above 14. Increase in effects from 4 to 12 can increase the gain output ratio (GOR) of multi-effect thermo-vapor compression (METC) system from 8 to 17 [Al-Najem et al., 1997]. In METC system the motive steam is used to compress some of the vapor generated in the last effect by the ejector. The compressed vapor along with the expanded motive steam from ejector is used as the main heat source. GOR of a 4-effect METC system is almost twice that of a conventional 4-effect boiling desalination plant [Hamed et al., 1996]. The METC system uses smaller heat transfer areas than both MSF and MED systems with the same GOR or steam requirement [Al-Najem et al. 1997]. Like MSF process, each effect in MED plant operates at a successively lower pressure, which is created by steam ejectors. In each effect, vapors are produced primarily by heat input and partially by flashing. In horizontal tube falling film evaporator type of MED, the first effect is heated by low pressure steam which is fed into the tubes. Preheated seawater feed is sprayed by nozzles over the tube bundles of every effect. Steam transfers its latent heat to the falling film of seawater thus forming vapor and gets condensed in the tubes forming a part of the product water. Concentrated brine and product water from one effect are cascaded to the next effect which is maintained at lower pressure where it flashes, while vapors are produced primarily by heat input and partly by flashing in each effect then directed to the tubes of the next effect and so on. Thus vapors from one effect are used as heat input to the next effect for heating and evaporating the preheated seawater feed. Brine from the last effect is rejected as 3

4 blowdown while vapors from the last effect are condensed in a heat rejection condenser where feed seawater serves as the coolant and gets preheated. A portion of this preheated seawater is treated with antiscalant chemicals and then sprayed through nozzles on the tube bundle of each effect. Even though, the top brine temperature in MED is low, scaling tendency of heat transfer tubes by deposition of seawater salts is a problem usually encountered in this process. It has been pointed out that higher scaling potential of MED compared to MSF at a given temperature is primarily due to the boundary layer characteristic of film boiling and flashing in the MED process compared to bulk and surface flashing in MSF process [Al-Sofi, 2001]. Controlling of scale in MED process can be achieved either by acid injection or by chemical additive (antiscalant) dosing [Nada, 1986]. The most common method of seawater pretreatment is the addition of additives such as phosphonates and synthetic polymeric scale inhibitors like polyacrylic acid, polymaleic acid, or polycarboxylates, i.e., homopolymers and co-polymers of maleic acid or acrylic acid [Al-Sofi, et al., 1989, Ruland, et al., 1995]. Phosphonates which significantly retard precipitation rate in aqueous system, has been recognized to form calcium phosphonate salts under certain conditions of hardness, ph and temperature. This phosphonate scale can be a troublesome deposit in itself, but the important event is the depletion of phosphonate thus drastically reducing scale control potential which may cause severe CaCO 3 scaling. Polymeric scale inhibitors, on the other hand, offer the over-all best performance because of their excellent hydrolytic and thermal stability. They have ability to form large number of co-ordinate bonds with calcium ion thereby preventing further crystal growth [Amjad and Pugh, 1995]. Many types of additives have been developed during last two decades. Some are good for low temperature and some for both low and high temperature operations. Studies on DSB(M), Belgard EV, Belgard EV2000, Belgard EV2030 and Sokalan PM 10I and evaluation of their performance in controlling scale formation have been conducted [Al-Sofi et al., 1995, Finan, et al., 1989, Shams El Din, 1988 & 1995, Abdulgader, et al., 1995, SWCC Report, 1995]. The effectiveness of scale inhibiting additives can be explained by two phenomena: the first is the threshold effect, and the second is the crystal distortion effect [Al- 4

5 Sofi, 1995 & 1999]. Threshold treatment stands for using a very small quantity of additive, which prevents the initiation of scale formation [Glater, et al., 1980]. Moreover, the presence of a scale inhibitor additive causes the distortion of the crystals which prevents the adhesion of individually precipitated particles or between such particles and the metal surface. After distortion a crystal which is normally flat (plate like) shaped takes a spheroidal shape which has less contact surface and so it rotates along the flow. It is pointed out that the scale which has been distorted by a scale inhibitor is easier to disperse, and that the normal flow of brine is sufficient to keep these smaller particles on the move until they are removed from the plant in the brine blowdown [SWCC / JICA Report, 1995]. This paper presents the results of the test conducted for performance evaluation of an antiscalant based on modified polymaleic acid in a reheat plant employing METC process. The main objective of the test was to evaluate the antiscalant chemical and to confirm the claim of the supplier that the chemical is better than or comparable to the presently used chemicals in SWCC MED plants. The trial test was carried out for about one year at TBT of 65 o C. The test evaluation was based on the measured and calculated thermal parameters, production rate, GOR, and PR, as well as visual inspection of accessible heat transfer tube surfaces. The results of this evaluation showed that this chemical would be effective in controlling scale at a dose rate of 2 ppm for more than a year without any intermediate acid cleaning. MATERIALS AND METHODS Material The evaluated low temperature antiscalant has the following characteristics: Appearance - pale yellow, liquid Solid content % Specific gravity at 20 o C ph at 20 o C - < 2.0 Solubility in Water - Soluble in all proportion Corrosion Behavior - Corrosive (prior to dilution/dosing) 5

6 Chemical Composition - Organic acids, Maleic acid, Phosphino carboxylic acid, 1-Hydroxyethlidene-1 and 1-Diphosphonic acid. Equipment The trial test was carried out at Al-Aziziah reheat plant. It is a multi-effect process plant of horizontal tube falling film evaporator type coupled with thermo-vapor compression by steam ejector. The plant consists of three distillers. Each distiller has 4 falling film effects and 2 heat rejection condensers. Figure 1 shows schematic process diagram of Al-Aziziah units. The unit design production is 1500 m 3 /day at a max. top brine temperature of 65 o C. Test Conditions Unit C was operated at full load during one year testing period with the following conditions: Steam pressure - 8 bar Concentration ratio Antiscalant dose rates ppm at the start of the test then reduced in four steps to an optimized value of 2 ppm. Dose Rate Optimization In the process of optimization and to confirm the effectiveness of the said antiscalant compared to other available antiscalants, dose rate was gradually reduced in steps even below the minimum dose rate achieved by the antiscalants currently in use. Plant operation after each step of dose rate reduction was followed by visual inspection of accessible heat transfer tubes of each effect. As this chemical was put to test for the first time in Al-Aziziah plant, special care was taken to confirm its credibility as an antiscalant. At the start, the chemical was tested at high dose rate of 4.5 ppm for as long as 6 months. Once the effectiveness of the chemical was confirmed, test duration were reduced down to as long as 8 weeks for testing dose rates of 3.5, 3.0, 2.5 and finally 2 ppm. Evaluation Criteria 6

7 Performance Evaluation of the antiscalant was judged on the following criteria. 7

8 Thermal performance calculation Antiscalant effectiveness can be tested by the ability of transferring heat from steam/vapor inside the tubes to the falling seawater film outside the tubes. As scale depositions on the outer surfaces of tubes resist heat transfer, hence their effect on plant performance can be taken as criteria. This was based on gain output and performance ratios as well as plant production rate as they are all related to heat transfer process. Monitoring these values and comparing them with the design values give a representative indication about scaling condition of heat transfer tubes. Visual Inspection Visual inspection gives a clear description of the condition of heat transfer surfaces and at the same time, supports any conclusion that can be drawn from the calculated and the measured values of thermal parameters. Inspection was performed at the end of each optimization step with reduced dose rate in order to evaluate the performance of antiscalant at that dose rate. However, in horizontal tube falling film design MED units such inspection is very much limited peripheral tubes as inner tubes are hardly accessible. Nevertheless, such limitations do in general pose real concerns, as peripheral tubes are considered to be quite representative of the entire bundle. Chemical Analysis All routine chemical analyses necessary to control operation were performed. The trend of ph, M-Alkalinity, and chloride of seawater feed, brine blowdown and product water were monitored throughout test period. Copper and iron contents in the above streams were also analyzed during test period. Blowdown concentration ratio (C.R.) were calculated by the ratios of chloride content in brine blowdown and seawater throughout the test period. The above analyses and calculations were essential to ensure that the plant was operated according to the preset test conditions. Samples of scale from evaporator wall and accessible tubes were collected and analyzed. RESULTS AND DISCUSSION 8

9 The unit was cleaned by acid before the start of the trial test. The unit was then operated at full capacity for the entire duration of the test. Motive steam pressure was kept at nominal pressure of 8 bar. The test was started with a dosing rate of 4.5 ppm followed by stepwise reduction in dose rate based on the recommendation made after visual inspection of the accessible distiller tubes at the end of each step. GOR and PR were calculated for each step of the test and plotted along with seawater and production flows as well as temperatures of seawater and in the first effect as shown in Figure 2. As steam supply pressure and temperature were constant throughout the test period, the change in GOR or PR of the unit is attributed to the fluctuation in seawater feed flow and temperature which could not be maintained at the specified full load values. The increase in seawater flow to the effects reduced GOR and PR. Similarly any decrease in temperature of seawater feed to the effects lowered the effect The test was started at the highest dose rate of 4.5 ppm and continued at this step of dosing rate for a period of six months (starting from June 23, and ending on Dec. 23, 97). During the first three months of this step, the values of measured production rates and calculated values of GOR and PR were found above the design values with no significant variations. However, during the last three months of this step, production rate and GOR started decreasing gradually below design values and remained uniform just above 55 m 3 /hr and 7 kg of distillate per kg steam compared to design values of 62.5 and 8, respectively as shown in Figure 2. The trend of production and plant performance values, which are found to decline from over design to less than design values during this step of the test, can be attributed to fluctuation in seawater feed flow and temperature. During two visual inspections each after three months of operation during this step revealed presence of little scale only at the ends of the heat transfer tube bundles which is considered to be quite usual in Multi-effect horizontal tube evaporators. Based on successful operating results of the first step, dosing rate was reduced to 3.5 ppm. The duration of this step was 6 weeks (starting from Jan. 2, and ending on Feb. 17, 98). Measured and calculated values of thermal parameters as shown in Figure 2, were less than the design values at start of this test step, thereafter, they increased gradually at a very slow rate. After a month of operation, the rate of increase picked up and the parameters jumped to 9

10 values near to that of design and stayed at that level up to the end of this step. Visual inspection at the end of this step revealed that there was no scale on heat transfer tubes except at the tube ends i.e., at tube plates. Further gradual reduction in dose rates were made based on the performance and visual inspection of the plant and the unit was operated at 3 ppm for a duration of 3 weeks as a third step, then at 2.5 ppm for two months as a fourth step and finally at 2 ppm for one and half months as a fifth step. Figures 2 shows that the measured parameters were fluctuating around the design values through out the test period thus leading to changes in plant production and performance. However, values of measured and calculated parameters were found over design values for most of the test period and the dip in values below design were mainly due to increase in seawater flow and decrease in seawater temperature i.e., during the winter season from Oct to Feb (within days 90 to 220 as shown in Figure 2). Figure 2 also shows that as the seawater temperature started recovering, the production as well as the performance of the plant moved above design values. This shows that the plant operated satisfactorily during entire antiscalant dose rate steps and that the antiscalant was performing well in keeping heat transfer tubes free from scale. Visual inspection at end of the test revealed that heat transfer tubes were clean except at the tube ends near to the tubesheets, which is considered normal in MED process due to improper distribution of seawater feed over tube bundles. Chemical analysis such as ph and M-alkalinity of feed, product and brine blowdown as well as loss of total alkalinity (LTA) indicated that the plant was working smoothly with no exceptional behavior during above test periods. Moreover, blowdown concentration ratio was kept constant at 1.4 through out these test periods. While copper and iron contents in product and blowdown were also found within acceptable limits. Figure 3 shows measured variations in the above chemical parameters with time. Chemical analysis of scale samples indicated that it consisted of calcium carbonate of different structures. Analysis also showed the presence of calcium sulphate and sodium chloride in heat rejection section. 10

11 CONCLUSIONS i) Evaluation test of the modified polymaleic acid as antiscalant in Al-Aziziah reheat plant at dose rates ranging from 4.5 to 2.0 ppm showed that there was no adverse scaling on the tubes. ii) Visual inspection at the end of each dose reduction step revealed no significant scaling reached from plant performance calculation. At the same time, it is confirmed that reduction in production rate, GOR, and PR below design values mainly during dose rates of 4.5 & 3.5 ppm were not due to any scale deposition but due to fluctuations in the temperature and the flow of seawater entering the effects. Visual inspection at the end of one full year of operation also unveiled that the plant could be operated for longer time without the need of taking it out of service for acid cleaning of the tubes. iii) It is, thus concluded that the modified polymaleic acid is capable in controlling scale in MED reheat plant at a dose rate of 2 ppm and top brine temperature of 65 o C, where the plant is expected to be operated for more than one year without the need of taking it out of service for acid cleaning of heat transfer tubes. RECOMMENDATIONS ) It is recommended to continue operation at the optimized dose rate of 2 ppm for a longer period (8-12 months) or until acid cleaning of the tubes becomes essential. (ii) Provided the long term operation of the recommended antiscalant is established successful at dose rate of 2 ppm, it is recommended to test the antiscalant at further reduced dose rate down to a minimum value at which controlling of scale is still attainable or scale formation stays within an acceptable limit. REFERENCES 11

12 1. Abdulgader A. Y., Sulami S., Imam M., Mustafa G. M., (1995), A report on Performance Evaluation of Belgard EV2030 at 112 o C TBT Operation at Al-Jubail RDC MSF Pilot Plant., SWCC Report, Saudi Arabia (Unpublished). 2. Al-Najem, N. M., Darwish, M.A. and Youssef, F. A., (1997), Thermovapor Compression Desalters: Energy and Availability Analysis of Single- and Multi-effect Systems, Desalination, 110: Al-Sofi M. AK., El-Sayed E. F., Imam M., Mustafa G. M., Hamada T., Tanaka Y., Haseba S., Goto T., (1995), Heat Transfer Measurement as a Criterion for Performance Evaluation of scale inhibition in MSF plants, IDA world Congress on Desalination and Water Science, Abu Dhabi, UAE, 3: Al-Sofi, M. AK., (1999), Fouling Phenomena in Multistage Flash (MSF) Distillers, Desalination, 126: Al-Sofi, M. AK., (2001), Seawater Desalination Research and Development Experience and Vision, KFUPM Fourth Annual Workshop on Water Conversation in the Kingdom. 6. Al-Sofi, M. AK., Khalaf S. and Al-Omran A., (1989), Practical Experience in Scale Control, Desalination, 73: Amjad Z. and Pugh J., (1995), Calcium Carbonate Revisited: A More Accurate Approach to Scale Growth and Inhibitor, IDA world Congress on Desalination and Water Science, Abu Dhabi, UAE, 3: Darwish, M. A., (1995), Desalination Process: A Technical Comparison, IDA world Congress on Desalination and Water Science, Abu Dhabi, 3: Finan, M. A., Smith S., Evans C. K., Muir J. W. H., (1989), Belgard EV - Experience in Scale Control, Desalination, 73: Glater J., York J. L., and Campbell K. S., (1980), Scale Formation and Prevention, Principle of Desalination edited by K. S. Spiegler, Academic Press, 164p. 11. Hamed, O.A., Zamamiri, A. M., Aly, S. and Lior, N., (1996), Thermal Performance and Exergy Analysis of A Thermal Vapor Compression Desalination System, Energy Conversion Management, 37: Khan A.H., (1980), Desalination Process and Multistage Flash Distillation Practice, Elsevier, 117p. 13. Nada, N., (1986), Operating Experience of MSF Units, Topics in Desalination, SWCC. 14. Ruland A., Buechner K.H. and Urooj A., (1995), Quantitative Determination of Polymeric Scale Inhibitors by Polyelectrolyte Titration, IDA world Congress on Desalination and Water Science, Abu Dhabi, UAE, 3: Shams El Din A. M., Mohammed R. A., (1995), Physico Chemical Studies on Belgard EV and Belgard EV 2000 Solutions Part II Potentiometric and Conductometric Acid 12

13 Base Titration, IDA world Congress on Desalination and Water Science, Abu Dhabi, UAE, 3: Shams El Din, A. M., (1988), A 700 Days Experiment with Belgard EV 2000 as Antiscale Agent in MSF Distillers, Desalination, 69: SWCC/JICA Report, (1995), The Project of the Seawater Desalination Technology in Kingdom of Saudi Arabia, a joint work, SWCC, Saudi Arabia (Unpublished). 18. SWCC Report, (1995), Performance Evaluation of Sokalan PM 10I at SWCC Al- Jubail Phase-II Unit No. 8 for 105 o C TBT Operation, SWCC, Saudi Arabia (Unpublished). 13

14 Figure 1, Typical 4-Effect Multi-effect Thermo-vapor Compression Process 14

15 SW Temp., o C Step-1 Dosing rate 4.5 Step ppm Step-3 3 ppm Step ppm Step-5 2 ppm SW Flow, m 3 /hr Design Seawater Flow = First Effect Temp. o C Production, m 3 /hr Design Production = Plant Performance Design GOR = 8.0 PR GOR Design PR = 6.715(Kg/2326 kj) Test Duration (Days) Figure 2, Variation with Time of the Measured and Calculated Parameters During the Test Period. 15

16 3 2.5 C. R. & LTA C. R. LTA BB ph Feed ph Product ph 8 ph BB M-Alk. Feed M-Alk M. Alk., ppm BB Fe Feed Fe Product Fe Fe, ppb BB Cu Feed Cu Product Cu 60 Cu, ppb /6/97 20/06/ Tests Duration in Days Figure 3, Chemical Analyses Results for the Entire Test Period 16