ABSTRACT
Unmodified (UN), acid-treated (AT) and microwave-acid-treated (MAT) activated carbons were optimized for their solute removal efficacies by adjusting feed mixture compositions and process conditions. Acetaminophen, benzotriazole, and caffeine were used either individually or as binary/ternary mixtures in this study. The process conditions considered were the pH, adsorbent dosage, and type of adsorbent. Experimental responses such as total adsorbent loading (q total) and total percentage removal (PRtotal) were fitted with empirical models that had high adjusted R 2 (>0.95), insignificant lack of fit (p-value > 0.22), and high model predictive R 2 (>0.93). Mixture compositions of the feed were found to interact significantly not only among themselves but with process variables as well. Hence, adsorption optimization must simultaneously consider mixture as well as process variables. The conventional response surface methodology for mixtures, termed as ridge analysis, optimizes mixture compositions at specified values of process variables. An improved steepest ascent method which considers mixture and process variables simultaneously was developed in this work. This could track the path of steepest ascent toward globally optimal settings, from any arbitrary starting point within the design space. For the chosen adsorbent, optimal settings for feed mixture compositions and pH were found to change along this steepest ascent path. The feed compositions, pH, and adsorbent dosage identified for maximum adsorbent utilization were usually quite different from those identified for maximum total percentage removal. When both these objectives were optimized together, the most favorable compromise solutions for q total and PRtotal were, respectively, 264.1 mg/g and 43.4% for UN, 294.9 mg/g and 52.5% for AT, and 336.6 mg/g and 55.9% for MAT.
ABSTRACT
Multicomponent adsorption processes are affected by both mixture and process variables viz. feed composition, pH, adsorbent dosage, and adsorbent type. Optimization of multicomponent adsorption processes with multiple objectives is challenging. It is important to accurately identify possible solutions and select the compromise solution that best satisfies the different objectives. Conventional algorithms, when applied to multicomponent adsorption, were found to identify the Pareto front less accurately, thereby necessitating the need for a reliable method. The steep portion of the Pareto front was especially not captured satisfactorily by the different conventional algorithms such as pattern search (PS), Non-dominated Sorting Genetic Algorithm (NSGA-II), and Epsilon-Constraint (EC). This portion assumes importance, if the compromise solution occurs in its vicinity. To address these challenges, a novel bi-objective optimization technique termed as elliptical method (EM) was developed and described in this work. It involves an exhaustive search, provides a well distributed Pareto front, and clearly delineates the steep region. After validating with benchmark problems, EM was applied to batch multi-component adsorption. The two objectives optimized simultaneously were adsorbent loading and percentage removal of the different solutes. The Pareto front and the compromise solution involving the best combination of the two objectives were significantly superior in the elliptical method when compared to those obtained from typical algorithms including epsilon-constraint (EC) method. The Pareto front was also well defined by the elliptical method without discontinuities near the extreme and steep regions. The number of points found by EM in the steeper region for the grade II adsorbent was 10 times greater than those found by the EC method while the PS and NSGA could not delineate this portion. The average time taken (considering both adsorbents) for EM per solution was 0.17 s which was at least 30.6% faster than the other methods. The compromise solution with the elliptical method was superior to the other methods. For instance, with grade II adsorbent, the compromise solution from the elliptical method suggested operating conditions that led to a total adsorbent loading and percentage removal of 333.4 mg/g and 56.0%. On the other hand, pattern search gave 324.1 mg/g and 56.5%, whereas the NSGA-II method gave 321.9 mg/g and 53.3%. For this adsorbent, elliptical method's compromise solution was 50% and 20% closer in terms of the Euclidean distance to the utopia point than NSGA and PS methods, respectively. The elliptical method will facilitate reliable wastewater tertiary treatment taking into cognizance the utilization of the adsorbent as well as the percentage purity requirement.
