M.M. Hassan

Separation of methane—nitrogen mixtures by pressure swing adsorption using a carbon molecular sieve*

A 60%-40% and a 92%-8% methane nitrogen mixture were separated in a two-bed pressure swing adsorption (PSA) unit using a carbon molecular sieve (CMS) adsorbent. The CMS adsorbent used in this separation is a dual resistance type comprising a barrier resistance on the crystal surface and diffusion resistance in the crystal interior. This dual resistance is modelled using the linear driving force (LDF) PSA model. The parameter Ω of the LDF model, which relates the theoretical and experimental results, was found to be significantly different from that for conventional diffusion-controlled processes. This difference is attributed to the presence of a barrier resistance in the CMS.

Rate and equilibrium sorption parameters for nitrogen and methane on carbon molecular sieve

Abstract Sorption parameters for nitrogen and methane on carbon molecular sieve (CMS) have been obtained using chromatographic, volumetric and gravimetric techniques. The equilibrium parameters-Henry constants, heats of adsorption and Langmuir constants-are consistent with reported literature values. Measurements of adsorption rates using a volumetric apparatus give results inconsistent with the diffusion hypothesis. The phenomena can be explained by the presence of a barrier resistance along with a diffusion mechanism. Analysis of results shows that the resistance measured by the chromatographic method is consistent with the resistance of the combined barrier resistance-diffusion model. The latter two resistances can only be obtained from the volumetric results. The general equation for additivity of resistances of Haynes and Sarma (AIChE J (1973) 19 1043) for breakthrough curves on a chromatogram is modified to account for the barrier resistance.

Adsorption equilibria and rate parameters for nitrogen and methane on Maxsorb activated carbon

Equilibrium and kinetic parameters for methane and nitrogen on a new, high specific area active carbon Maxsorb are reported. Volumetric and chromatographic methods are used to measure the pure component adsorption isotherm and the effective mass transfer coefficient for each gas. The adsorption isotherms at 300 K, measured up to a pressure of 550 kPa, are approximately linear for both methane and nitrogen on Maxsorb. The equilibrium separation factor is 3.0 in favor of methane. The mass transfer resistance is observed to be very low for each sorbate. The equilibrium and kinetic parameters are input in the mathematical model of binary breakthrough experiments using an axial dispersion model. The theoretical and experimental breakthrough curves are observed to be in excellent agreement.

Effects of inhibitors on nitrification in a packed-bed biological flow reactor

The individual effect of trivalent arsenic, hexavalent chromium and fluoride on nitrification is studied under continuous load in a packed bed biological flow reactor. The results show that Michaelis-Menten rate expression gives the best representation of nitrification data in the absence of inhibitors. However, in the presence of inhibitors, the system follows a non-competitive mode of inhibition with the following rate expression: αi=VmaxSKS+SKiSKi+I. The values of Vmax and Ks are estimated as 1.466 mg l−1 min−1 and 2.349 mg l−1 respectively. The inhibitor constant Ki is evaluated as 273 mg l−1 for trivalent arsenic, 56 mg l−1 for hexavalent chromium and 1185 mg l−1 for fluoride.

Theoretical Analysis of a Packed-bed Biological Reactor for Various Reaction Kinetics

Abstract The performance characteristics of a packed-bed biological reactor have been analysed taking into consideration the diffusional resistance of the biofilm. The model equations are solved by the method of orthogonal collocation from the transient to the steady state condition for various reaction kinetics of practical significance, namely Michaelis-Menten kinetics with and without substrate inhibition. The effect of various process variables of physical importance are investigated parametrically. It is found that the substrate conversion increases with increase in the Peclet number, the mass transfer coefficient, the surface area of biofilm and the film thickness. However, the substrate conversion is not affected by the film thickness after it has exceeded a certain value. Furthermore, analyses for individual kinetics corresponding to the three classic modes of non-competitive, competitive and anti-competitive inhibition have been presented. The results show that inhibitor concentration decreases the substrate conversion and that the extent of the effect is a maximum for the non- competitive case and a minimum for the anti-competitive case.

Modeling of Axial and Recycle Backmixing Effects in a Biological Packed Bed Loop Reactor

Abstract A theoretical model for a packed bed biological loop reactor is presented by considering both external and internal resistances for the general case of Monod kinetics. Numerical solutions have been obtained by the method of orthogonal collocation for a wide range of saturation parameters to cover the two limiting cases of zero-order and first-order kinetics. The numerical solutions for the limiting cases were found to show good agreement with the analytical solutions. The effects of recycle ratio and axial dispersion on the performance of the reactor were studied parametrically. The results show that for low recycle ratios the conversion increases with decrease in Peclet number for first-order and Monod type kinetics; for zero-order kinetics, however, the conversion is independent of both Peclet number and recycle ratio. The effectiveness factor profiles along the length of the reactor were compared for Monod kinetics. It was found that an increase in recycle ratio tends to flatten the profile. The effect of axial dispersion on the concentration profiles at higher recycle ratio was found to be negligible. The dynamics of how steady state conditions are achieved in the reactor is also presented.

Chromium(VI) inhibition in multi-substrate carbon oxidation and nitrification process in an upflow packed bed biofilm reactor

Abstract The performance of an upflow packed bed biofilm reactor has been analyzed mathematically under multi-substrate limitation of carbon oxidation and nitrification reactions while subjected to inhibition of a continuous dose of Cr(VI). For a fixed inlet concentration of 150 mg l−1 of NH4+-N and 250 mg l−1 of acetate and varying the inlet concentrations of Cr(VI), the results show that the toxicity of Cr(VI) to both organic oxidation and nitrification processes increases rapidly with increase in its inlet concentration. The toxicity of Cr(VI) has been found to be much pronounced when nitrification alone is considered to take place in the reactor. The concentration profiles within the biofilm show that oxygen is a limiting component near the inlet and at the middle of the reactor. This necessitates the need for ample supply of oxygen at these locations within the reactor as major portion of oxidation of both acetate and NH4+-N takes place there. The model predictions show a close agreement with the available experimental data for a variety of operating conditions.

Analysis of non-isothermal tubular reactor packed with immobilized enzyme systems

Abstract The dynamic and steady state performance of a non-isothermal tubular reactor packed with spherical encapsulated enzyme particles has been modeled in terms of different dimensionless transport and kinetic parameters. The dynamic concentration profile for an initially substrate-free reactor reaches a maximum before achieving steady state. The steady state dimensionless bulk substrate concentration, unlike the temperature, progressively decreases along the reactor bed. On increase in the external mass transfer coefficient K L and Biot number Bi m for mass transfer, the concentration profile decreases more steeply. The simulation study shows that the biocatalyst particles may be considered isothermal. The exit substrate concentration decreases with increase in Peclet number Pe m for mass transfer, i.e. backmixing effects, indicating that a plug flow reactor will have a higher overall conversion than a perfect mixer. The dynamic bulk temperature rises more rapidly near the reactor inlet with increase in the Peclet number Pe h for heat transfer, i.e. thermal backmixing effects. The external resistance to mass and heat transfer becomes negligible above a critical value of K L and external heat transfer coefficient h . The bulk substrate concentration, unlike the temperature, decreases with increase in the dimensionless heat α of reaction. For typical Michaelis-Menten kinetics, the exit conversion and temperature will be limited between those for zero- and first-order kinetics.

Effects of enzyme microcapsule shape on the performance of a nonisothermal packed-bed tubular reactor

The effects of enzyme microcapsule shape (spherical, cylindrical and flat plate) on the performance of a nonisothermal, packed-bed reactor have been modeled as a function of Biot number and Peclet number for mass and heat transfer (Bi m , Bi h , Pe m and Pe h ), and dimensionless heat of reaction α. Under the given simulation conditions, only higher values of Bi m and Bi h (>2.5) confirm the influence of microcapsule shape on the reactor performance such that the axial and overall conversion and bulk temperature decrease as follows : spherical > cylindrical > flat plate. In terms of the shape-independent modified Biot number, Bi* = Bi/{(n + 1)/3), this order is retained for 2 < Bi* < 8. The influence of increasing Pe m , Pe h , and α on conversion and bulk temperature also follows the above order. For the flat plate, the exit conversion and temperature are not influenced by Pe m and Pe h , that is, mass transfer and thermal backmixing effects, respectively. On the other hand, for the spherical and cylindrical microcapsules, overall backmixing effects are negligible only beyond a critical value of Pe m (∼7) and Pe h (∼1.75). The conversion and bulk temperature increase with the increase in α, independent of the microcapsule shape. The spherical and cylindrical microcapsules, unlike the flat plate, cannot be considered isothermal.

The order of micromixing and segregation effects on the biological growth process in a stirred-tank reactor

Abstract Depending on the hydrodynamic conditions, a stirred tank reactor may be divided into two micromixing environments: maximum mixing followed by complete segregation (case 1), or vice versa (case 2). The Ng—Rippin two-environment model simulates case 1, whereas the Fan reversed two-environment model covers case 2. The micromixing concepts of Danckwerts and of Zwietering have been applied to both models in terms of the degree of segregation J to evaluate the influence of the order of micromixing—segregation effects on biological growth processes. The model predictions for both endogeneous and exogeneous cell metabolism show that case 2 gives more substrate conversion and cell production than does case 1, for the same extent of micromixing, particularly at low dilution rates. At high dilution rates, both models predict the same reactor performance, independent of the micromixing phenomenon. The substrate conversion and cell production decrease with increasing dilution rate, following a similar trend. Further, the effects of micromixing are found to be strong functions of dilution rate. At high dilution rates for case 2, the micromixing effects are pronounced only when the reactor approaches complete segregation. However, for case 1, the effects are appreciable when the reactor deviates slightly from perfect mixing. For some intermediate dilution rates, the Fan model, unlike the Ng—Rippin model, shows that the reactor output decreases linearly with increasing degree of segregation. Beyond a critical value of the dilution rate, the reactor output falls linearly with dilution rate for exogeneous cell metabolism (case 2). On the contrary, for case 1, the output decreases exponentially throughout the entire range of dilution rates.