Flow through a convergence. Part 2: Mixing of high viscosity ratio polymer blends

Dispersive mixing of high viscosity ratio blends was studied in a converging flow using a capillary rheometer equipped with dies having different entry profiles. Three inhomogeneous bimodal polyolefin blends, with stress‐dependent viscosity ratios ranging from 8 to 450, were used in this work. Such magnitudes of viscosity ratio indicate that the dispersed droplets can be mixed only in an elongational flow field. The mixing efficiency was found to be dependent on both the profile of the convergence and flow rate. At the lowest flow rates, the dispersive mixing efficiency was very low, but it increased with an increasing flow rate until a profile‐dependent maximum. This maximum mixing efficiency was observed prior to fracture of the matrix material, after which the efficiency decreased. Stress and deformation fields within different profiles were estimated by numerical simulation using the K‐BKZ equation, and the results were used to interpret experimental results. The dispersive mixing efficiency was found to be proportional to the maximum elongational stress within the converging section, and to the length of the region where the critical conditions for elastic fracture of droplet material were met. It is shown that the dispersive mixing mechanism in high viscosity ratio blends is mainly dictated by elastic fracture of the droplet, and hence is applicable over a wide range of polymer blending processes.     

Flow through packed bed reactors: 1. Single-phase flow

Single-phase pressure drop was studied in a region of flow rates that is of particular interest to trickle bed reactors . Bed packings were made of uniformly sized spherical and non-spherical particles (cylinders, rings, trilobes, and quadralobes). Particles were packed by means of two methods: random close or dense packing (RCP) and random loose packing (RLP) obtaining bed porosities in the range of 0.37–0.52. It is shown that wall effects on pressure drop are negligible as long as the column-to-particle diameter ratio is above 10. Furthermore, the capillary model approach such as the Ergun equation is proven to be a sufficient approximation for typical values of bed porosities encountered in packed bed reactors. However, it is demonstrated that the original Ergun equation is only able to accurately predict the pressure drop of single-phase flow over spherical particles, whereas it systematically under predicts the pressure drop of single-phase flow over non-spherical particles. Special features of differently shaped non-spherical particles have been taken into account through phenomenological and empirical analyses in order to correct/upgrade the original Ergun equation. With the proposed upgraded Ergun equation one is able to predict single-phase pressure drop in a packed bed of arbitrary shaped particles to within ±10% on average. This approach has been shown to be far superior to any other available at this time.     

Experimental demonstration of the reverse flow catalytic membrane reactor concept for energy efficient syngas production. Part 1: Influence of operating conditions

In this contribution the technical feasibility of the reverse flow catalytic membrane reactor (RFCMR) concept with porous membranes for energy efficient syngas production is investigated. In earlier work an experimental proof of principle was already provided [Smit, J., Bekink, G.J., van Sint Annaland, M., Kuipers, J.A.M., 2005a. A reverse flow catalytic membrane reactor for the production of syngas: an experimental study. International Journal of Chemical Reactor Engineering 3 (A12)], but compensatory heating was required and problems related to the mechanical strength of the powder-based YsZ catalyst and the steel filter were reported. Therefore, in Part 1 the performance of a Rh-Pt/Al₂O₃ catalyst with improved mechanical strength and porous Al₂O₃ membranes with excellent temperature resistance was tested in an isothermal membrane reactor. For this purpose a novel sealing technique was developed that could withstand sufficiently high pressure differences and temperatures. Very high syngas selectivities close to the thermodynamic equilibrium could be achieved for a considerable period of time without any increase in pressure drop and without any decrease in syngas selectivity. Using the Rh-Pt/Al₂O₃ catalyst, several experiments were performed in a RFCMR demonstration unit and the influence of different operating conditions and design parameters on the reactor behaviour was investigated. It is shown that very high syngas selectivities (up to 95%) can be achieved with a maximal on-stream time of 12 h, without using any compensatory heating and despite inevitable radial heat losses. In Part 2 a reactor model is discussed that can well describe the experimental results presented in this part.     

Motions of a porous particle in stokes flow: Part 1. Unbounded single-fluid domain problem

Exact solutions in closed form have been found using the eigenfunction-expansion method for various linear and quadratic flows of anunbounded incompressible viscous fluid at low Reynolds number past aporous sphere with a uniform permeability distribution. The linear flows considered here are a simple shear and an axisymmetric uniaxial straining flows and the quadratic flows include a unidirectional paraboloidal and a stagnation-like flows as typical representations. The theoretical analysis determines a general motion of a freely suspended particle in the prescribed mean flow at infinity. Then the solutions are expressed in terms of fundamental singularity solutions for Slokes flow which will be applied to examine the motion of a porous sphere in the presence of a plane fluid-fluid interface in the forthcoming part of the present paper.     

Enhancement Factor for Gas Absorption in a Finite Liquid Layer. Part 1: Instantaneous Reaction in a Liquid in Plug Flow

An approximate analysis of gas absorption with instantaneous reaction in a liquid layer of finite thickness in plug flow is presented. An approximate solution to the enhancement factor for the case of unequal diffusivities between the dissolved gas and the liquid reactant has been derived and validated by numerical simulation. Depending on the diffusivity ratio of the liquid reactant to the dissolved gas (γ), the enhancement factor tends to be either lower or higher than the prediction of the classical enhancement factor equation based on the penetration theory (E_i,pen) at Fourier numbers typically larger than 0.1. An empirical correlation valid for all Fourier numbers is proposed to allow a quick estimation of the enhancement factor, which describes the prediction of the approximate solution and the simulation data with a relative error below 5 % under the investigated conditions (γ = 0.3–4, E_i,pen = 2–1000).     

Knudsen diffusion in powders. Part I Critical examination of a gas diffusion relationship used in Knudsen flow permeametry

B. V. Derjaguin's treatment of gas diffusion in a porous medium under conditions of Knudsen flow is discussed. A semi-theoretical value obtained for the constant β in the general equation is tested experimentally. In opposition to previous conclusions, molecular diffusion seems best interpreted in terms of elastic collisions against the pore walls. The structure of the pore system is shown to be the controlling parameter of the gas flow, and the novelty of the present treatment consists in ways to take the pore structure into account.     

Properties of elongational flow injection-molded polyethylene. Part 1: Influence of mold geometry

Injection molding of polyethylene was carried out using molds that introduce axial elongational flow during the filling process, As compared with conventional injection-molding methods, the present technique yields polyethylene materials having higher mechanical strengths (σ ≈ 150 MPa). The σ-value of the moldings can be systematically monitored by adequately varying the elongational flow through modification of the mold geometry. Geometrical features of the volume containing the melt before injection as well as geometry of the die, the extruder, and the inlet influence the overall elongational flow, and thus, the mechanical strength of the moldings.     

Effect of die temperature on the flow of polymer melts. Part I: Flow inside the die

The effect of die wall temperature on the flow of polymer melts in circular capillary dies was studied. At constant flow rates, it was found that die wall temperature had a greater effect on the pressure drop than melt temperature. A capillary die with two circular channels with different diameters was designed to simulate the profile extrusion. Changes of wall temperature varied the flow rate ratio between the two channels. An implicit finite difference method was used to simulate the velocity and temperature profiles inside the die. Values predicted by this model matched well with experimental data for both dies.     

Developing a zero-AOX shrink-resist process for wool. Part 1: Preliminary results

The efficiency of the proteolytic enzyme papain in conferring shrink-resistance to wool tops and woven fabrics has been enhanced by pretreatment of the wool with lipase/sodium monoperoxyphthalate/sodium sulphite. This process may be considered as a zero-AOX shrink-proofing treatment. The wool samples treated with this system show excellent shrink-proofed properties. Infrared spectroscopy and X-ray photoelectron spectroscopy showed that the Buntë salt together with low concentrations of cystine monoxide and cystine dioxide are formed in the course of the reaction. Studies of the surface of the treated wool using scanning electron microscopy have shown the complete absence of wool scales. A mechanism is proposed for shrink resistance in the lipase/sodium monoperoxyphthalate/papain treatment.     

Pulverized coal flame structures at elevated pressures. Part 1. Detailed operating conditions

The tests and simulations in this study characterize the chemical structure of pressurized pulverized coal flames, particularly (1) how the O₂ in simulated near-burner flame zones is apportioned among the various fuel components; and (2) the burner operating conditions and mechanisms that most strongly affect flame structure. CFD simulations resolved the structures of flames of a subbituminous and two hv bituminous coals for stoichiometric ratios (SR) from 0 to 1.8 for pressures from 1.0 to 3.0 MPa. The structures of all flames were largely determined by the accumulation of particles in the turbulent boundary layer on the flow tube wall. Gaseous fuel compounds always ignited first on the wall at the burner inlet, and this flame propagated toward the flow axis to form a 2D parabolic flame surface. Within the core, residual gaseous fuels, soot, and char may have eventually reached their ignition threshold and burned in a premixed mode. Residual CO, H₂, and char burned in the near-wall region after the volatiles flame had propagated deeper into the core.Whether or not the flame closed on the centerline was mainly determined by pressure and SR. Inlet conditions that formed closed flames at a lower test pressure eventually sustained open flames at progressively higher pressures. The impact of decreasing SR was qualitatively similar, due to the lower heat release rates for progressively lower SR. As the pressure is increased, flame ignition and, by association, flame stability will become more problematic due to the greater thermal capacitance of air streams at progressively higher pressures.     

Smoldering wave propagation mechanism. I. Critical conditions

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 1, pp. 16–20, January–February, 1993.