Synthesis and evaluation of phosphate-free antiscalants to controlCaSO4·2H2O scale formation in reverse osmosis desalination plants

Polysuccinimide (PSI) of various molecular weights (MW) was synthesized from aspartic acid. The PSIs were hydrolyzed with NaOH to obtain sodium polyaspartates. Partial hydrolysis of a PSI gave the copolymer poly(SI-cosodium aspartate). PSIs were treated with one equivalent of cyclic amines: pyrrolidine, piperidine, and azepane to afford the corresponding polyaspartamides in excellent yields. Treatment of PSI with 0.5 equivalents of the cyclic amines led to the formation of 1:1 copolymer poly(SI-co-aspartamide) which on basic hydrolysis gave the corresponding 1:1 copolymer poly(sodium aspartate-co-aspartamide). The antiscalant behavior of the newly synthesized compounds was evaluated at various concentrations (2.5 ppm– 10.0 ppm) for the inhibition of gypsum scale formation. The antiscalant evaluation experiments were performed by measuring conductivity and using concentrated brines. The study revealed the performance of low MW antiscalants to be superior to their high MW counterparts. Thus, a polyaspartate is found to be a better antiscalant than its corresponding copolymer of SI/aspartate, i.e. a partially hydrolyzed PSI. Derivatives of 7-membered cyclic amine azepane performed better than their 5- and 6-membered counterparts. At the same degree of polymerization, performance with decreasing order of effectiveness was found to be: polyaspartate N poly(aspartamide-co-aspartate) N (SI-co-aspartate) N poly(SI-co-aspartamide) N polyaspartamide. © 2014 Elsevier B.V. All rights reserved.

In comparing the efficacy of the inhibitors, the performance criterion proposed by Reitz [34] can be adopted. In RO desalination, the residence time of the saline water inside the permeators is less than 15 min. If, according to Reitz [34], a scale inhibitor at a certain dose level can keep a scale-forming salt in solution for at least 15 min (i.e., if it can help achieve an induction period of 15 min), it can be considered as an effective inhibitor at that dose level against that particular scaling. Based on the Reitz [34] criterion, the results indicate that almost all of the synthesized green additives in the present study are very effective with only 10 ppm dose concentration against precipitation of gypsum at 40 °C and 3CB brine concentrations except 5-100. Hydrolyzed derivatives are more efficient than unhydrolyzed PSI-derivatives. Antiscalant 3-100 was found to be the most effective with very high induction time at a 10 ppm dose concentration. Antiscalants 3-100, 4-100, 12-100 and 10-100 can be used as effective antiscalants for CaSO4·2H2O even at a concentration of only 2.5 ppm in brackish water RO plant. It is planned to evaluate the most promising antiscalants using a minimum size commercial RO unit with two-stage membrane configuration according to the procedure adopted by Butt et al. [36]. Actual brackish feed water will be used with appropriate filtration system and results will be reported in next publication. Acknowledgments The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum and Minerals (KFUPM) for funding this work through project No. 08WT76-04 as part of the National Science, Technology and Innovation Plan. The laboratory facilities provided by the Chemistry Department and Center for Refining & Petrochemicals, Research Institute, KFUPM are also acknowledged.