Lemoine, Romain olivier
(2008)
Hydrodynamics, Mass Transfer and Modeling of the Toluene Oxidation Process.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
The equilibrium solubility (C*), Critical mixing speed (NCRE) and (NCRI), Induced gas flow rate (QGI),volumetric liquid-side mass transfer coefficient (kLa), liquid-side mass transfer (kL), gas-liquid interfacial area (a), gas holdup (åG), Sauter mean bubble diameter (dS), and the bubble size distribution of N2, O2 and air in liquid toluene and three mixtures of toluene, benzaldehyde and benzoic acid, aimed at simulating the continuous liquid phase toluene oxidation (LPTO), were measured in a 4-liter ZipperClave surface aeration (SAR), gas inducing (GIR) and gas sparging (GSR) reactors operating under wide ranges of mixing speed (N) (800-1200 rpm), liquid height (H) (0.171-0.268 m in the SAR and GIR), superficial gas velocities (UG) (0.000-0.004 m/s in the GSR), temperature (T) (300-453 K) and pressure (P) (1-15 bar). These parameters were also measured in a 1-ft diameter, 10-ft high bubble column reactor (BCR) under various pressures (P) (2-8 bar), gas velocities (UG) (0.06-0.15 m/s).The solubility values of N2, O2 and air in liquid toluene and the three mixtures were calculated using a modified Peng-Robinson equation of state. (kLa) data were determined using the transient physical absorption technique. The bubble size distributions as well as the Sauter mean bubble diameters were obtained from the photographic method and the gas disengagement technique in the agitated reactors and bubble column reactor, respectively. In the agitated reactor, the gas holdup values were measured through the dispersion height measurement technique, and the manometric method using two differential pressure (dP) cells was employed in the bubble column reactor. From the gas holdup, Sauter mean bubble diameter and kLa experimental values, a and kL were calculated under various operating conditions. NCRE and NCRI as well as aWave were estimated by analyzing the videos taken with an on-line high-speed Phantom camera through the reactor's Jerguson windows. In the GIR, QGI was determined using a highly sensitive Coriolis mass flow meter. The Central Composite Statistical Design and analysis technique was used to study the effect of operating conditions on these hydrodynamic parameters.At constant temperature, the equilibrium solubilities (C*) of the three gases in all liquids used appeared to increase linearly with pressure and obey Henry's Law, however, the values exhibited minima with increasing temperature. The C* values were found to increase with increasing gas molecular weight, and decrease with the addition of benzaldehyde and benzoic acid to pure toluene. A dimensionless form of Arrhenius-type equation, in which the activation energy was dependent of temperature, was developed to predict Henry's law constant for the three gases in toluene and mixtures with a regression coefficient > 99%.In the SAR, increasing N, T or decreasing H increased aWave, åG, a, kL and kLa, and decreased dS and NCRE, whereas increasing P, decreased aWave, åG, a, kL and kLa and had no effect on dS and NCRE. In the GIR, increasing N or decreasing H increased QGI, åG, a, kL, kLa and dS and decreased NCRI. Also, increasing T increased and then decreased QGI, åG and a; increased kL and kLa; and decreased dS and NCRI. In addition, increasing P decreased slightly QGI and åG but did not affect a, kL, kLa, dS and NCRI under the operating conditions used. In the GSR, increasing N, T and UG increased åG, a, kL and kLa. Also, increasing N and T, or decreasing UG decreased dS. The addition of benzaldehyde and benzoic acid to pure toluene was found to significantly affect the hydrodynamic parameters (dS and åG), in the GSR and GIR, especially at low temperature due to formation of froth, which led to the enhancement of kLa. The hydrodynamic and mass transfer parameters obtained indicated that the behavior of the SAR was mainly dependent on kL, whereas those of the GSR and GIR were strongly affected not only by kL, but also by a. In the bubble column reactor, under the operating conditions used, kLa, a and åG values were found to increase with increasing gas superficial velocity and pressure, whereas dS and kL values appeared to decrease with pressure and increase with superficial gas velocity. The effect of gas nature on the hydrodynamic and mass transfer parameters was found to be insignificant, whereas the effect of addition of benzaldehyde and benzoic acid to pure toluene, aimed at mimicking the actual continuous liquid-phase toluene oxidation process, appeared to have a strong impact on both parameters due to froth formation.Empirical, statistical and Back-Propagation Neural Network (BPNN) correlations were also developed to predict the hydrodynamic and mass transfer parameters obtained in this study in the agitated reactors (ARs) and bubble column reactor (BCR) along with a large data bank of literature data (7374 data points in ARS and 3881 data points in BCRs). These correlations were then incorporated in calculation algorithms for predicting both hydrodynamic and mass transfer parameters in ARs and BCRs. Using these algorithms, two comprehensive models, including the effects of mass and heat transfer, hydrodynamics, and kinetics were developed for bubble column reactors (BCRs) and series of gas sparging reactors (GSRs) to simulate the commercial Liquid-Phase Toluene Oxidation (LPTO) process. An intrinsic kinetic rate equation for the toluene oxidation was also developed using literature data. The effects of the reactor diameter (DC), reactor height (H), and superficial gas velocity (UG) or mixing speed (N) on the LPTO process performances (toluene conversion, benzaldehyde selectivity and yield) were investigated in a BCR and a cascade of GSRs. The pressure and temperature at the inlet of the reactors were set at 1.0 MPa and 420 K; the feed gas to the reactors was a mixture (50/50 by mole) of oxygen and nitrogen; and the liquid feed was toluene containing Co catalyst and a NaBr promoter at concentrations of 0.22 wt% and 1.76 wt%, respectively. The heat of reaction was removed from both reactor types using water in cooling pipes, representing 2% of the reactor volume; and the gas was sparged into the reactors through a multi-orifices gas distributor with an open area, representing 10% of the reactor cross-sectional area. The model predictions showed that under the operating conditions used, toluene conversion of about 12%, a benzaldehyde selectivity of 40% and a benzaldehyde production in the range of about 1500 tons/year could be achieved using a superficial gas velocity of 0.1 m/s in the BCR (10-m height, 2-m Inside diameter) and 0.002 m/s in the series of 5 GSRs (2-m inside diameter, and 2-m liquid height). The BCR selected was found to operate in the kinetically-controlled regime whereas the 5-GSRs appeared to operate in a regime controlled by both gas-liquid mass transfer and reaction kinetics. Thus, due to its attractive economics in addition to the mechanical constraints of GSRs, the BCR seems to be the reactor of choice for the commercial-scale LPTO process.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
9 September 2008 |
| Date Type: |
Completion |
| Defense Date: |
10 March 2005 |
| Approval Date: |
9 September 2008 |
| Submission Date: |
7 March 2005 |
| Access Restriction: |
5 year -- Restrict access to University of Pittsburgh for a period of 5 years. |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Chemical Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Absorption; Bubble Column Reactor; Entrainment; Gas Holdup; Gas-Inducing Reactor; Gas-Liquid Interfacial Area; Gas-Sparging Reactor; Hydrodynamics; Mass Transfer Coefficient; Neural Networks; Sauter Mean Bubble Diameter; Solubility; Statistical Experimental Design; Surface Aeration Reactor; Toluene Oxidation Process; Volumetric Mass transfer Coefficient |
| Other ID: |
http://etd.library.pitt.edu/ETD/available/etd-03072005-164841/, etd-03072005-164841 |
| Date Deposited: |
10 Nov 2011 19:32 |
| Last Modified: |
15 Nov 2016 13:36 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/6444 |
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