K.D.P. Nigam

Static Mixers in the Process Industries—A Review

This paper summarizes the field of static mixers including recent improvements and applications to industrial processes. The most commonly used static mixers are described and compared. Their respective advantages and limitations are emphasized. Efficiencies of static mixers are compared based both on theory and experimental results from the literature. The operations, which can benefit from the use of static mixers, are explored, namely, mixing of miscible fluids, liquid–liquid and gas–liquid interface generation, liquid–solid dispersion and heat transfer. Design parameters governing the performance of the various mixers in these applications are reported. The key parameters needed for the selection of a suitable mixer are highlighted.

Behaviour of solid particles in viscous non-Newtonian solutions: settling velocity, wall effects and bed expansion in solid-liquid fluidized beds

Abstract Settling velocities of particles in viscous non-Newtonian pseudoplastic solutions were measured. Spherical particles of glass and stainless steel were used. Aqueous solution of carboxymethyl cellulose (CMC) in the concentration range of 0.2 to 2 wt.% was used as the liquid phase. The settling velocities were measured in nine different columns with internal diameter in the range of 9 – 200 mm. The wall effect was investigated by varying the ratio of particle diameter to column diameter in the range of 0.05 to 0.78. Experiments were also performed for settling of particles in viscous Newtonian solutions of glycerol. The particle Reynolds number was varied in the range of 2 × 10−3 to 150. A correlation was obtained for the wall effect as a function of diameter ratio and the particle Reynolds number. Bed expansion was investigated in the case of solid-liquid fluidized bed and using non-Newtonian liquids Two glass particles of 1.65 and 3.1 mm were fluidized using the aqueous solutions of CMC. The bed was of 76 mm internal diameter. The expansion characteristics have been satisfactorily correlated.

Residence time distribution from a continuous Couette flow device

Abstract This paper describes an experimental study of residence time distribution (RTD) by pulse response analysis in a continuous Couette flow device with rotating inner cylinder and stationary outer cylinder. The experiments were performed under conditions of (a) negligible and (b) significant influence of molecular diffusion and (c) Taylor vortex flow. Diethylene glycol and water were used as the test fluids with congo red dye and potassium permanganate solution as tracers respectively. A unique RTD described by an analytical expression was observed for experiments with low axial Reynolds number under the condition of negligible influence of molecular diffusion. For most experiments performed under conditions of significant influence of molecular diffusion and Taylor vortex flow regime over the ranges 0

Hydrodynamics and mixing in highly viscous pseudo-plastic non-newtonian solutions in bubble columns

Abstract Gas hold-up structures in highly viscous pseudo-plastic carboxymethyl cellulose (CMC) solutions were measured in 0.10, 0.20, 0.38 and 1.0 m diameter batch bubble columns. A correlation for the gas hold-up based on a large number of data and covering a wide range of viscosities and column diameters is presented in the churn turbulent regime. A flow transition chart for the churn turbulent and slug flow regimes is also presented for highly viscous pseudo-plastic solutions. Transient conductivity measurements were carried out in 0.20 and 0.38 m i.d. bubble columns with CMC solutions to measure mixing time. A “tanks in series with interstage recirculation” model was used to predict mixing times in the churn turbulent regime. Reasonably good agreement was found between the predicted and experimental values of mixing time.

Extraction of nickel from spent catalyst using fresh and recovered EDTA

This study investigates the possibility of recovering nickel from spent catalyst (NiO/Al(2)O(3)) used in the fertilizer industry. EDTA (ethylenediaminetetraaceticacid) di sodium salt was used as a chelating to extract the Ni after which sulfuric acid was added to obtain NiSO(4). The dechelation process takes 5-6h to break the complex and EDTA which was recovered as H(4)EDTA acid in the solid form and solution contains NiSO(4). The objective was to evaluate the nickel removal efficiency of EDTA and reusability of recovered EDTA. The parameters affecting nickel recovery were EDTA concentration, time of chelation, catalyst to liquid ratio (s:l), mixing speed, pH and catalyst particle size. The extraction was up to 95% under the following conditions: 0.8M concentration of EDTA, solid to liquid ratio 1:50 (g/ml), particle size 100 microm, pH 10, 10h of chelation time, 700 rpm and 100 degrees C. Up to 95% of the EDTA could be recovered without losing significant activity.

Liquid distribution studies in trickle-bed reactors

Liquid distribution plays an important role in determining the reactor performance in a trickle-bed reactor. Radial liquid distribution was studied in a trickle-bed reactor with five different shapes and sizes of catalyst packing in a uniformly distributed liquid inlet. The liquid tends to flow preferentially along the existing filaments where the porosity is high. The introduction of gas flow into the liquid–solid system smoothens the liquid distribution to some extent due to the competition between liquid and gas phases for the interstitial pore space.

Optimum gas sparger design for bubble columns with a low height-to-diameter ratio

The mixing time and fractional gas hold-up have been measured in a bubble column 1 m in diameter using nine sparger designs. It has been shown that an optimum design of sparger (for either mixing time or gas hold-up) is possible for columns having a low height-to-diameter ratio and at low superficial gas velocities. The optimum design also depends on the physical properties of the system. The optimum design of the sparger has been shown to be different for mixing time and gas hold-up.

The Kenics static mixer: new data and proposed correlations

New results for wall-to-tube heat and mass transfer in a Kenics-type static mixer allow a unification of extent data. Nearly universal correlations are presented. They allow detailed comparisons with open tube designs. Heat transfer enhancement is greatest in deep laminar flow. Even there, the Kenics mixers exhibit modest improvements when compared on the basis of equal pressure drop. Existing designs are not yet optimized for heat transfer and studies at lower aspect ratios for the mixing elements are recommended.

Modelling of a coiled tubular chemical reactor

Abstract First order reactions in laminar Newtonion and non-Newtonion fluids in coiled tubes have been analyzed for high range of Dean numbers (NDe≤250). Process parameters which govern the secondary flow and the reaction parameter on the performance of coiled tube chemical reactor are systematically examined. It is interesting to observe that the phenomenon of covective diffusion with reaction in coiled tube can be simulated. Numerical calculations show the effect of process parameter modelling the rection parameter (α), dimensionless axial distance and power law index (n). The performance of the coiled tube reactor is compared with plug flow and laminar flow reactors.

RECENT DEVELOPMENTS ON HYDROPROCESSING REACTORS

The importance of hydroprocessing has been accelerated due to the imposition of stringent environmental regulations on petroleum products to achieve future goals of sustainability and cleaner environment, and the more demanding situation of middle distillates (mainly transportation fuels) against the availability of heavier and high sulfur crudes. Trickle-bed reactors (TBRs) are mostly used in hydroprocessing. Other types of reactors, such as moving bed reactors and ebullated-bed reactors (EBRs) are also used in the hydroprocessing. All of the above mentioned reactors are operated with cocurrent up or downflow of gas and liquid. TBRs have been most commonly studied over the last 30 years. The design and scale up of TBR is still not well understood, especially with respect to hydrodynamic parameters. Though the EBRs are gaining importance because of their flexibility in dealing with most types of heavy feedstock and the ease with catalyst replacement during operation, there are very limited studies available in literature for the design and scale up of the reactor. Recently, countercurrent operation in a structured catalytic packed bed is gaining importance for hydroprocessing in order to obtain higher conversions. The structured packed column, in which the catalyst particles are enclosed within