Khim Hoong Chu

Fixed bed sorption: Setting the record straight on the Bohart–Adams and Thomas models

The Bohart-Adams model and the so-called Thomas model are commonly used in the modeling of fixed bed breakthrough curves in environmental sorption and biosorption research. Some authors fit the two equations to the same set of experimental breakthrough data and compare their modeling capabilities, implying that the two expressions are separate and independent models. This article clarifies unambiguously and demonstrates conclusively that the two models are in reality mathematically equivalent to each other and their parameters interchangeable. The salient features of the Bohart-Adams model and the genuine Thomas model are outlined here and their relationship delineated. The main difference between the Bohart-Adams and actual Thomas models lies in the form of the sorption isotherm assumed. The former assumes a rectangular (irreversible) isotherm while the latter assumes a Langmuir (favorable) isotherm. It is shown that, when the sorption isotherm is highly favorable, the actual Thomas model reduces to the Bohart-Adams model.

A Simplified Model and Similarity Solutions for Interfacial Evolution of Two-Phase Flow in Porous Media

For two-phase flows of immiscible displacement processes in porous media, we proposed a simplified model to capture the interfacial fronts, which is given by explicit expressions and satisfies the continuity conditions of pressure and normal velocity across the interface. A new similarity solution for the interfacial evolution in the rectangular coordinate system was derived by postulating a first-order approximation of the velocity distribution in the region that the two-phase fluids co-exist. The interfacial evolution equation can be explicitly expressed as a linear function, where the slope of the interfacial equation is simply related to the mobility ratio of two-phase fluids in porous media. The application of the proposed solutions to predictions of interfacial evolutions in carbon dioxide injected into saline aquifers was illustrated under different mobility ratios and operational parameters. For the purpose of comparison, the numerical solutions obtained by level set method and the similarity solutions based on the Dupuit assumptions were presented. The results show that the proposed solution can give a better approximation of interfacial evolution than the currently available similarity solutions, especially in the situation that the mobility ratio is large. The proposed approximate solutions can provide physical insight into the interfacial phenomenon and be readily used for rapidly screening carbon dioxide storage capacity in subsurface formations and monitoring the migration of carbon dioxide plume.

Batch Kinetics of Metal Biosorption: Application of the Bohart-Adams Rate Law

The most common rate model formulations for metal biosorption in batch systems assume that the uptake process is limited by surface reaction, with the so-called pseudo first order and second order rate equations being the most popular. However, from a modeling perspective the rate coefficients of these equations are of limited use as it is not clear how they can be utilized in the modeling of other process configurations such as fixed bed columns. As an alternative, we advocate a modeling approach predicated on the Bohart-Adams surface reaction rate law because it is of a form that can be readily integrated with the governing equations for batch and fixed bed systems, allowing explicit analytical solutions to be obtained. In this work we fit the batch solution of the Bohart-Adams rate law to kinetic data taken from the literature using a genetic algorithm. The resultant parameter estimates can be used in the fixed bed solution of the Bohart-Adams rate law to generate breakthrough predictions. Moreover, the batch Bohart-Adams rate law is compared with a general nth order surface reaction rate law using the Akaike information criterion approach.

Loss of cell components during rehydration of dried Rhodotorula glutinis and its implications for lead uptake

Microbial cells are routinely dried and ground before they are used in metal biosorption studies. In this work, a metal biosorbent was prepared by drying biomass of the yeast Rhodotorula glutinis in an oven at 70°C for 24 h followed by grinding. Two forms of the prepared biosorbent particles, washed and unwashed, were examined for their ability to remove lead from solution. It was found that the unwashed biosorbent exhibited higher lead uptake than the washed biosorbent. Analysis of the supernatant of washed cells incubated in water and that of unwashed cells incubated in lead solution revealed the presence of protein, carbohydrates, organic acids and inorganic phosphate. Overall, the washed and unwashed cells leached, respectively, 14.5 and 13.4% of their initial dry weight (100 mg). Acid‐base titration data revealed that the leached components contained several potential binding sites for metal cations with carboxyl and phosphoryl groups being particularly important. The higher level of lead uptake exhibited by the unwashed biomass was attributed to the fact that it leached smaller amounts of cell constituents with proton binding sites relative to the washed cells.

Modeling of Biosorption Processes

Biosorption entails the use of microbial or plant biomass, usually inactivated, to remove toxic metal ions in aqueous solutions. It is particularly effective in dealing with low concentration, high volume metal waste streams. Although biosorption processes have not yet been commercialized to any significant extent, they offer a promising area for future developments. This chapter presents several process models that can facilitate the design and analysis of batch and fixed bed biosorption systems.

Removal of Ammonia from a Biologically Treated Pesticide Wastewater by Struvite Precipitation

The aim of this study was to investigate ammonia removal from the effluent of a full-scale biofilm reactor treating pesticide wastewater through the formation of struvite (MgNH4PO4•6H2O). The requirement of magnesium and phosphate ions for precipitation was met by adding MgO and Na2HPO4. Different NH4+:Mg2+:PO43- molar ratios were tested in batch reactors to optimize the precipitation conditions. Ammonia was reduced to 25 mg/L from an initial concentration of 210 mg/L at the highest ratio tested (1:1.2:1.3), equating to 88% removal. However, the extent of ammonia removal was only slightly affected when the N:Mg ratio was reduced from 1:1.2 to 1:0.6; dropping to 83% from 88% on a percentage basis. Struvite precipitation can be easily incorporated within the existing post-treatment section of the full-scale plant, providing the pesticide manufacturing facility with a reliable backup process for its nitrification system.

Biopolymer Production by the Yeast Rhodotorula glutinis

In this work, exopolysaccharide (EPS) production by the yeast Rhodotorula glutinis was investigated. The results suggest that the synthesis of EPS in batch fermentations was enhanced by using a low C/N ratio to stimulate fast cell growth during the exponential phase and by adding glucose to stationary phase cultures to provide excess carbon for EPS formation. The apparent viscosity and molecular weight of EPS were found to decrease with increasing fermentation time due to the partial hydrolysis of EPS by the acidic fermentation broth (pH 1.8). Maintaining stationary phase cultures at pH 4 was found to be effective in minimizing acid hydrolysis and producing EPS with high molecular weight and apparent viscosity.

Simulation and Optimization of an O-Xylene Columns

For a distillation column under practical operation, the reflux ratio, feed location, flowrate of the top or bottom products are all very important parameters. Using the process simulation package ASPEN PLUS to solve mass balance, heat balance and phase equilibrium equations of a distillation column, the relationships between feed location and content of key components, feed location and heat loads, reflux ratio and content of key components, reflux ratio and heat loads, flowrate of the top product and content of key components, and flowrate of the top product and heat loads, can be determined. With the specifications for the content of key components as constraints, the optimal feed location, reflux ratio and flowrate of the top product can be determined. The o-xylene (OX) column in a xylene plant is optimized as a case study in this paper. Under the optimal conditions the heating utility for the reboiler can be reduced by 20%, OX can be produced by 0.8% more, and operating cost can be reduced by 2,336,000 yuan per year.

Cost‐Based Optimization of a Papermaking Wastewater Regeneration Recycling System

Wastewater can be regenerated for recycling in an industrial process to reduce freshwater consumption and wastewater discharge. Such an environment friendly approach will also lead to cost savings that accrue due to reduced freshwater usage and wastewater discharge. However, the resulting cost savings are offset to varying degrees by the costs incurred for the regeneration of wastewater for recycling. Therefore, systematic procedures should be used to determine the true economic benefits for any water‐using system involving wastewater regeneration recycling. In this paper, a total cost accounting procedure is employed to construct a comprehensive cost model for a paper mill. The resulting cost model is optimized by means of mathematical programming to determine the optimal regeneration flowrate and regeneration efficiency that will yield the minimum total cost.