Optimization of the Solution of a Dispersion Model

The study of the combination of chemical kinetics with transport phenomena is the main step for reactor design. It is possible to deviate from the model behaviour, the cause of which may be fluid channelling, fluid recirculation, or creation of stagnant regions in the vessel, by using a dispersion model. In this paper, the known general solution of the dispersion model for closed vessels is given in a new, straightforward form. In order to improve the classical theoretical solution, a hybrid of analytical and numerical methods is used. It is based on the general analytic solution and the least-squares method by fitting the results of a tracer test carried out on the vessel with the values of the analytic solution. Thus, the accuracy of the estimation for the vessel dispersion number is increased. The presented method can be used to similar problems modelled by a partial differential equation and some boundary conditions which are not sufficient to ensure the uniqueness of the solution.

RTD Measurement, Modeling, and Analysis of Liquid Phase of Three-Tube Industrial Pulp Digester

Residence-time distribution (RTD) experiments were performed to analyze an industrial scale three-tube series continuous pulping digester’s hydrodynamic performance. An impulse of radiotracer 82Br (γ energy source) was introduced at the inlet of the first tube. The radiotracer concentration in the liquid phase was traced at the outlet of each tube. The input behavior of the radiotracer converted to a non-ideal pulse tracer input for the second and third tubes of the digester. Numerical convolution is adopted to deal with the non-ideal pulse input of the radiotracer. A modeling procedure for determining the RTD from the outlet tracer concentration data is proposed. A plug flow component followed by axial dispersion model is considered, and is adjusted after its convolution with the inlet tracer concentration data to obtain the RTD of the individual tubes. The obtained RTD data are analyzed to explain the flow behavior, degree of dispersion, and flow abnormalities existing in the digester. The mean residence-time (MRT), and dispersion number are estimated for the model components for the three tubes. The vessel dispersion number is found to decrease from tube 1 to tube 3. Overall, the conversion of the highly dispersed flow regime into the plug-flow regime is observed in the whole digester.

Dispersion Number Identification in an Imbert Gasifier Under Parameter Uncertainty

The problem of identifying the dispersion number associated with the convective-radiative heat dispersion mechanism in an experimental gasification tubular reactor is addressed. The dependency of temperature response characteristics on the intensity and duration of a heat pulse input are characterized on the basis of a set of off-line experiments, finding that for the range of interest: (i) the coefficient of variation of the temporal temperature response depends almost linearly on the dispersion number, and (ii) the related function is robustly invertible with respect to model and experimental uncertainty. This results establishes the feasibility of identifying the key heat dispersion (inverse Peclet) number from a reasonable on-line heat pulse injection test.

Preliminary evaluation of the suspending properties of Brachystegia eurycoma gum on metronidazole suspension

The aim of this study was to evaluate the suspending properties of Brachystegia eurycoma gum on metronidazole suspension. The suspending properties of Brachystegia eurycoma gum (family leguminosae) were evaluated comparatively with that of compound tragacanth powder at concentration range of 2.5 – 10.0%w/v in metronidazole suspension. The following parameters were determined; sedimentation volume (%), viscosity, pH and re-dispersion number. The values obtained were used as basis for comparison of the suspending agents studied. Brachystegia eurycoma and compound tragacanth gums had a pH range between 4.7 to 4.9 and between 3.9 to 4.1 respectively which indicates that they are slightly acidic. Particles suspended with tragacanth gum at concentration ? 7.5%w/w redispersed easily than those formulated with the Brachystegia eurycoma gum at ? 10% w/w. It was observed that with increase in concentration of the gum the viscosity of the suspension increased correspondingly. For instance, at concentration of 2.5%w/w viscosities of the suspensions are 490 poise (Brachystegia eurycoma gum) and 603 poise (compound tragacanth gum) while at concentrations of 7.5%w/w their viscosities were 914 poise (Brachystegia eurycoma gum) and 1709 poise (compound tragacanth gum). There was a direct proportionality between viscosity of the gums at different concentrations and the sedimentation rate of the suspensions, as the viscosity of the gum increases, the rate of sedimentation of the suspension decreases. Brachystegia eurycoma gum at predetermined concentration can be exploited as an alternative excipient in the formulation of pharmaceutical suspensions of insoluble substances.DOI: http://dx.doi.org/10.3329/icpj.v3i11.20727 International Current Pharmaceutical Journal, October 2014, 3(11): 328-330

Numerical Simulation of Foamy Oil Stability Using Natural Gas Huff and Puff for Ultra-Deep Heavy Oil Reservoir

The nature of injected gas dispersion in oil distinguishes foamy oil behavior from conventional heavy oil behavior. Unlike normal two-phase flow, it involves flow of dispersed gas bubbles with pseudo single phase. This paper presents the results of a numerical simulation study of the stability of foamy oil created by liberation of dissolved gas during natural gas huff and puff process. Through the history matching of labs test conducted by three series of various core tubes in numerical simulation, foamy oil impactions on recovery were discussed based on vertical heterogeneous model. The effects on the stability of foamy oil flow behavior were investigated by mobility ratio, viscous to gravity ratio, layer permeability contrast, vertical to horizontal permeability ratio and the transverse dispersion number in the paper. The results show that foamy oil stability increases with higher oil viscosity, higher injection gas density. The oil recovery decrease with the mobility ratio and the layer permeability contrast, while the oil recovery increase with the vertical to horizontal permeability ratio. This work demonstrates that the transverse dispersion number should be used to assess vertical or microscopic sweep efficiency. The study indicates that foamy oil in porous media during production is unstable, but it will be huge potentials to apply natural gas huff and puff for ultra-deep heavy oil reservoirs.

Axial Mixing in a Novel Perforated Plate Bubble Column

This investigation reports the experimental and theoretical results carried out to evaluate the axial dispersion number for an air-water system in a novel hybrid rotating and reciprocating perforated plate bubble column for single phase and two phase flow conditions. Axial dispersion studies are carried out using stimulus response technique. Sodium hydroxide solution is used as the tracer. Effects of superficial liquid velocity, agitation level and superficial gas velocity on axial dispersion number were analyzed and found to be significant. For the single phase (water) flow condition, it is found that the main variables affecting the axial dispersion number are the agitation level and superficial liquid velocity. When compared to the agitation level, the effect of superficial liquid velocity on axial dispersion number is more predominant. The increase in superficial liquid velocity decreases the axial dispersion number. The same trend is shown by agitation level but the effect is less. The rotational movement of the perforated plates enhances the radial mixing in the section; hence, axial dispersion number is reduced. For the two phase flow condition, the increase in superficial liquid velocity decreases the axial dispersion number, as reported in the single phase flow condition. The increase in agitation level decreases the axial dispersion number, but this decreasing trend is non-linear. An increase in superficial gas velocity increases the axial dispersion number. Correlations have been developed for axial dispersion number for single phase and two phase flow conditions. The correlation values are found to concur with the experimental values.

Fractional analysis of arsenic in subsurface-flow constructed wetlands with different length to depth ratios

Arsenic (As) removal in subsurface-flow constructed wetlands (CW) planting with vetiver grasses was experimented by comparing between two different configurations; (i) deep-bed units (dpCW) with length to depth (L:D) ratio = 2 and (ii) shallow-bed units (shCW) with L:D ratio = 8; operating at hydraulic retention time (HRT) of 6, 9, and 12 days. The tracer study of CW units revealed that no effect of L:D ratio on dispersion number could be determined, but affecting to the effective volume ratio. Based on the data obtained from the pilot-scale experiments of CW units for 117 days, it is apparent that the dpCW could achieve relatively high As removals (52.9%, 59.2%, and 72.1% at HRT of 6, 9, and 12 days, respectively). Analysis of As mass balance showed that only 0.2–0.4% of As input was uptaken by vetiver grasses whereas the major portion was retained in the CW media (38.9–77.6%). Forms of the retained As was determined by sequential fractionation which could indicate As complexation with iron and manganese on the media surface of 31–38% and As trapping into the media of 42–52% of the total. No obvious difference of As fractions in bed of between dpCW and shCW units was observable.