The Chemical Engineering Department at the College of Engineering, University of Baghdad, held a PhD dissertation examination titled:
Integration of Oxygen Separation Process and Ozonation via Microbubbles Technique for Thiourea Removal from Aqueous Solutions
By the student “Baseem H. Al-Sabbagh” and supervised by Prof. Dr. Ahmed Faiq Al-Alawy. The examination committee consisted of Prof. Dr. Wadood T. Mohammed as Chairman and the membership of Prof. Dr. Mahmood Kh. Hommadi, Prof. Dr. Asrar A. Al-Obaidi, Prof. Dr. Rasha Habbeb Salman, and Ass. Prof. Dr. Farah T. Jasim. The thesis was accepted after conducting a public discussion and listening to the student’s defense. The thesis was summarized as follows:
The aim of study:
The thesis aimed to treat water contaminated with sulfur-containing organic compounds (such as thiourea) using microbubble-assisted ozonation. The research addressed the separation of oxygen from ambient air using pressure swing adsorption (PSA) technology in order to employ it as a source for ozone generation. Subsequently, an ozone monitoring device was constructed to measure the amount of reacted ozone, with the objective of determining the optimal ozone dose required for the reaction. The effects of key operating parameters—such as temperature, pH, pollutant concentration, and ozonation time—were investigated, in addition to studying the reaction kinetics. The study also aimed to develop mathematical models describing oxygen separation and to simulate the mechanism of ozone dissolution in water.
Abstract:
Due to the lack of access to urea – a relatively safe chemical used as fertilizer -, farmers in Iraq tend to use thiourea instead. Thiourea resists the natural degradation if leaked to the groundwater which many village communities depend on this source of water especially in summer when drought strike is accompanied with shortage of water supply. In this study, the removal of thiourea by oxidation with ozone microbubbles was conducted to reveal its efficiency in the degradation of water contaminated with it.
A small-scale laboratory pressure swing adsorption (PSA) unit was built for concentrated oxygen generation and investigated for the best operating conditions. Key parameters such as oxygen recovery, air factor, and bed size factor are necessary to measure the performance of the PSA process. These are affected by product flow rate, PSA cycle duration, required oxygen purity, applied pressure, and type of adsorbent used. Li-LSX zeolite at a given applied pressure, was used as the main adsorbent for nitrogen in this study under different operating conditions. Results show that the best duration time for both the pressurization and depressurization cycle was found to be 12 seconds for all flow rates that is ranging from 1 lpm up to 10 lpm. It was found that oxygen recovery would increase directly from 0.05 up to 0.27 with product flow rate range 1-8 lpm respectively. However, after 8 lpm, a slight decrease in the recovery value was reported. Air factor changes inversely with product flow rate, shows sharp increase at low flow rates and levels off approaching a constant value of 15 when flow rate become more than 8 lpm. Bed size factor of 400 kg.d/tO2 was found to be the optimum value to obtain oxygen concentration >90%.
To increase the accuracy of ozone measurements, a low-cost in-line ozone monitor based on the principle of ultraviolet absorption was built and tested for different ozone for concentrations up to 300 mg h-1 and flow rates up to 5 lpm using air as a feed source. A widely available T5 ultraviolet (UV) tube was used as a UV source and two UV light absorption cells were made to act as reference and measuring cells. The output of the two cells was used to calculate the ozone concentration using Beer-Lambert’s law. To correlate the output readings of the ozone monitor with those obtained using the iodometric titration method, a correction factor of 1.5117 was determined and applied. The results demonstrate a strong linear correlation ≥0.99 between estimated and measured ozone concentration values. Relative errors less than 10% were observed for ozone concentrations ranging from 400 to 5000 ppb, while a relative error up to 16% was reported for concentrations below 400 ppb. The developed monitor offers a cost-effective alternative to expensive ozone monitoring systems for standard applications, with a total construction cost under $100.
Based on the application of ozone microbubbles, affecting factors such as pH (2-11), temperature (10-50°C), thiourea concentration (50-400 mg/l), applied ozone dose (2-13 mg/l), and ozonation time (0-25 min) were investigated.
An ozone dose of 2.7 mg ozone/mg thiourea was found to be the best value to achieve excellent removal of thiourea with lowest possible ozone loss. With ozonation time of 10 min, a removal efficiency of more than 98% of thiourea can be achieved and complete destruction can be obtained with longer durations. pH values of acidic to natural values 2-7 were found to be excellent for the ozone reaction with thiourea. A little decline in the removal efficiency was observed under basic conditions. Temperature has a little effect on removal efficiency up to about 30°C for the ozonation duration of 10 min with a little decline was observed at higher temperatures. The ozonation reaction was found to be coupled by chemical kinetics and mass transfer limitations. Increasing ozone dose or treatment time shows increase in removal efficiency in general.
At low thiourea concentrations, the reaction with ozone proceeds as a first order reaction. While at high concentrations, the reaction was found to proceed as a zero-order reaction.
The study also demonstrates numerical simulations for both oxygen separation using MATLAB and dissolution of ozone bubbles in liquid water using COMSOL Multiphysics. For nitrogen adsorption model, both experimental and simulation results exhibit similar proportion values for nitrogen concentration at flow rates below 4 lpm and below 6 sec. Increasing superficial gas velocity beyond 0.05 m/s result in shorter residence time and reduced gas separation efficiency.
Ozone dissolution model shows that a bubble size of 650 microns dominates the ozone dissolution process at kla value of 0.55 min-1 with relative error of 31.57% between experimental and simulation results.
