Charles Okorafor

Professor of Chemical Engineering


Presently I have tried to expand my research activities which has led to my co-supervising a PhD thesis with City University Graduate School, New York.Within the last year, (my graduate students and I ) have looked at the following projects and are in process of being published in peer-reviewed chemical engineering journal.


Methyl Acetate Synthesis through Micro-reacting System

Methyl acetate is one of the most important industrial solvents due to its properties. It has a low boiling point and high miscibility with other organics. These characteristics contribute to its versatile use. It acts as a good solvent for cellulose nitrate and acetate, oils and other esters. It is the primary compound in acetic anhydride. It is also used as solvent in lacquers, paints and dyes. It is not very stable in the presence of strong bases and acids, thus it is easily hydrolyzed to acetic acid and methanol.

The synthesis of methyl acetate by the esterification reaction of methanol and acetic acid is very difficult. This is due to equilibrium restrictions in the reactor and the formation of methyl acetate- methanol and methyl acetate- water minimum boiling azeotropes, in the separation system. Presently methyl acetate can be produced by two methods:

  1. Conventional process which utilizes multiple reactors      in which one of the reactants is fed in excess amount,--to achieve high      conversion--, coupled with series of vacuum and atmospheric distillation      units.
  2. Reactive distillation in which a reaction section is      incorporated into a distillation column. As the reaction proceeds, the      methyl acetate is removed as distillate since it is more volatile than the      other compounds in the system.

Since the initial commercialization of reactive distillation in the late 1980s, many studies have been undertaken to understand the process and develop better design methodologies. It has been shown that reactive distillation has many advantages over the conventional process in methyl acetate synthesis. Two major ones are (a) reduced capital and operating costs due to higher conversion, and (b) reduced extent of parallel and consecutive reactions resulting in higher selectivity. A major drawback that limits its wider application is the requirement that, the chemical reaction has to have significant conversion at distillation temperature.

In general, the liquid phase reaction of acetic acid and methanol in the presence of sulfuric acid catalyst makes methyl acetate. Due to the present trend for safe, environmentally friendly and miniaturization process, an alternative synthesis method is being suggested,-the micro-reacting system-..

In this study the effectiveness of a micro-reacting system for the synthesis was investigated. The micro-reacting system consists of a micro-reactor and micro-pump. The micro-reactor is made up of plates with functionally distinct channels in the submillimeter range. Such a scale is known to provide high surface – to- volume ratio, ultra-fast mixing, extremely efficient heat transfer and of course narrowly distributed residence time. It can be designed as a plug-flow reactor and operate continuously at steady state. Under these conditions, the reactants are mixed at the beginning of the reaction micro-channel. This allows for a good control of the reaction start as well as measurement of thermal effects immediately the reactants mix, without any perturbation due to mixing.  In this study a conversion factor of about 90% was achieved.  Thus, the reversible reaction behaved as an irreversible.

The question becomes what really transformed a typical reversible reaction into an irreversible one?   We speculate that the high surface to volume ratio, the minimal residence time and the ultra-fast mixing may have affected the kinetics of the reaction.  We are embarking on the investigation of the reaction kinetics to help explain this high conversion factor.


The second ongoing project deals with the kinetics and production of calcium hydroxyapatite that can be used for bone and teeth fillings

Calcium phosphate exists in many forms and each has its respective application in biology, medicine and dentistry. One of these forms that is largely used in biomedical applications is hydroxyapatite, [(HAP)-- (Ca10 (PO4) 6(OH)2 ]. The main interest in this form is due to its similarity to the inorganic component of bones and teeth. Literature published on HAP shows the wide varieties of synthesizing techniques of this compound and many parameters that affect the final product. The system is highly sensitive to preparative solution conditions that lead to the production of the non-stoichiometric hydroxyapatite (ns-HAP). Another problem related to the precipitation of hydroxyapatite is the ease of foreign ion incorporation in the lattice of the crystal, which will delay or inhibit the formation of stoichiometric hydroxyapatite (s-HAP). All previous studies showed that amorphous calcium phosphate form usually (ACP) precedes the formation of hydroxyapatite (HAP). Several methods had been developed to prepare HAP, including dry process, precipitation, hydrolyzation of calcium phosphate, hydrothermal synthesis, spray pyrolysis, freeze-drying, sol gel technique and electrochemical deposition.

In this project the effect of temperature, pH and reactant addition rates on the kinetics and formation of HAP crystallization produced by hydrothermal method in a batch and semi-batch modes of operation. The hydroxyapatite particles are prepared by chemical precipitation from an aqueous solution. Calcium chloride and sodium phosphate monobasic monohydrate were used as reagents. Different concentrations of the starting materials are prepared. The reaction of HAP formation is as follows:

5CaCl2 + 3NaH2PO4 +7NaOH à  Ca5(PO4)3OH + 10NaCl + 6H2O

In order to determine the kinetics of HAP formation, the above process is repeated at different temperatures. Five temperatures are studied:  25, 40, 55, 70° C.  The effect of pH is also considered at values of 7, 8, 9, 10 and 11. The concentration of CaCl2 and NaH2PO4 are kept at 1.0M so that when they are mixed the Ca/P ratio will be more than 1.67. The mixture will be set in various agitator speeds and sampled at different intervals. The values of Ca/P ratio at different temperatures and reaction times will be measured by chemical titration because we do not have an induced plasma atomic spectroscopy. The phosphorus content can be determined spectrophotometrically with a UV-VIS spectrophotometer after mineralization of sample in a mixture of concentrated hydrochloric and nitric acids. This method consists in forming a yellow color phosphorus–vanadium–molybdenum complex and a photometric measurement of absorbance at a wavelength of 430 nm. Calcium content is determined with a complexometric titration method using disodium versenate. The method involves dissolving a sample in nitric acid, precipitation of phosphates as bismuth(III) phosphate(V) BiPO4, and then determining the calcium content by complexometric titration with disodium versenate (EDTA) in the presence of a mixed indicator (calcein and thymolphthalein).

The components of the dry samples will be analyzed by FTIR spectrum. From FTIR spectrum I will be attempt to confirm the transition of octacalcium phosphate (OCP, Ca8H2(PO4)6·5H2O, Ca/P =1.33) formed at the beginning of the reaction, which will quickly transform  to amorphous calcium phosphate (ACP, Ca3(PO4)2H2O, Ca/P = 1.5) due to its instability. Other aspects of the study will investigate the relationship between the formation ratio of HAP and reaction time under different temperature and the relationship between the time required for forming pure-phase HAP and temperature. The order of the reaction will be determined according to the conversion ratio at different times, and the activation energy will be calculated by using the Arrhenius equation.

  1. A just completed studied developed an optimal control design strategy for complex chemical plants.  An eight step approach was formulated.  The artcle is under review for publication.

Since the late 1970s, research studies in the chemical process industries (CPI) have focused on improvement in energy efficiency, cost efficiency and reduction in environmental impact of chemical processes.  These have led to highly integrated processes resulting in increased complexity of the plants.  This has necessitated also changes in designing of process control for the plant.   Thus, the control engineer does not have the luxury of setting up overall control structure for the plant by summing up individual unit operation control configuration as was the case previously.  A holistic approach presently known as the ‘plant-wide control’ (PWC) is now in vogue.

By looking at processes as a whole, the degree of freedom to pair control and manipulative variables increases and correspondingly finding an optimal control design has become more difficult.

The genesis of the plant-wide control (PWC) studies can be traced to the pioneering work of Buckley1who developed process control structure based on throughput (material) balances. A major boost in PWC studies in the past twenty years can be referenced to the formulation of ‘real’ process plant control problem2, commonly known as the Tennessee Eastman (TE) problem.. The TE problem made it possible for PWC researchers to test their various methodologies.  Since then many other ‘real’ problems have been studied.  Suraj Vasuderan, et al3 has a table of ‘real’ processes that have PWC developed and tested up to 2009. These PWC methodologies can be classified as either heuristic4, mathematics-based5, or integrated framework6. The integrated framework (IF) consists of heuristic coupled with simulation (steady state and dynamic) studies.  All these approaches result in different ways of achieving decentralized PWC structure to address all the major process control problems such as the effect of recycles and energy integration.

Dynamic theory, constrained optimization and systems are incorporated in one form or another to develop a PWC structure under the mathematics-based approach.. Unfortunately, this approach is very difficult to formulate for a complete process plant resulting in very intensive computation..  Many studies employing the mathematics-based approach try to achieve design by reducing the complexity of the formulation and solution computation.  The branch and bound (BAB) algorithm7 produces a global ranking of all possible inputs and outputs pairing. This results in an efficient stabilizing control producing decentralized proportional controllers.  In a modified branch and bound (BAB)8 method, the branch pruning involve both upward (the subset size gradually increasing) and downward direction simultaneously.  According to this approach, the method greatly reduces the total number of subsets, resulting in efficient handling of large-scale processes.  In another study9 the optimal controller gain matrix from the solution of the output optimum control problem is split into feedback and forward parts.  From these parts, a decentralized PWC system structure is formulated.  This is known as model predictive control (MPC). A relationship comprising relative gain array (RGA), internal model control (IMC) concept and cost coefficients computed from Parseval’s theorem is solved by mixed integer linear programming (MILP)10 . This result is then used to develop PWC structure.

Our approach involves the implantation of eight steps to achieve an optimal control design.

My Research