Volume 56, Nº 5 (2016)

The basic principles of the spinning of polysulfone hollow fiber membranes by the dry-jet wet spinning process, where the polymer solution is extruded through an air gap between the spinneret and coagulation bath by the free fall spinning method, have been discussed. The main distinctive feature of the method is that the deformation of both the extruded polymer solution and the nascent hollow fiber (spinneret drawing) is due to the action of the gravity force alone without applying an external tensile force. Published data on the effect of the shear rate of the spinning solution at the spinneret outlet and nascent fiber drawing in the air gap on the structure and permeability of the hollow fiber membranes have been analyzed. The main factors affecting the spinneret drawing and dimensions of the hollow fiber membranes formed by free fall spinning have been experimentally revealed using polysulfones of different molecular weights. The factors are the dope composition, approaching ratio and viscosity, the air gap length, the temperature and the feed rate of the dope and the bore fluid, coagulation power of the bore fluid with respect to the dope. It has been found that as the spinneret draw value increases, the hydraulic permeability and the rejection coefficient of the resulting fiber generally change in a nonmonotonic manner: the pure water flux of the hollow fibers passes through a maximum and the rejection coefficient, through a maximum or minimum. The behavior is caused by the fact that the pore structure of the hollow fiber membranes is formed during uniaxial drawing and, in some cases (at increasing bore liquid flow rate), during biaxial deformation. When an external mechanical force is applied to the forming hollow fiber, an increase in the fraction of interconnected pores and the transition from cylindrical to slitlike pores are possible, which results in an increase in the hydraulic permeability of the fiber walls. A further increase in the spinneret draw ratio results in the reverse process, a decrease in the membrane matrix porosity due to the orientation and collapse of the pores, yielding a decrease in the flux and an increase in the rejection coefficient. By blocking the process of hollow fiber shrinkage through an increase in the bore fluid flow rate (increasing the internal diameter of the hollow fiber), it is possible to enhance the effective porosity of the fiber walls without substantial change in pore size, i.e., a transition from a system with isolated or partly isolated pores to a system of interconnected pores. A sharp increase in the hydraulic permeability of the hollow fiber membranes without a substantial change in their rejection is supposedly caused by this structural change.

The structure and local mechanical properties of the surface of polysulfone(PS-100)- and polyacrylonitrile( PAN-100)-based source ultrafiltration membranes and ones modified with thin films of polyvinylpyridine (PVPyr) or latex particles applied by the Langmuir-Blodgett (LB) method and by the formation of a PVPyr/sodium polystyrene sulfonate polyelectrolyte complex have been studied using atomic force microscopy. It has been found that the application of a PVPyr layer on the PS-100 membrane results in a twofold decrease in the membrane pure water flux from 250–360 L/(m² h), while a substantial increase in the rejection coefficient is observed: the nominal molecular weight cut-off limit is reduced from 100000 to 10000 Da. For the PAN membrane a 10% increase in water permeability was found, and the rejection coefficient for PVP K-30 increases from 55 to 65%. Modification with latex particles results in a sharp decrease in the membrane performance. Comparative analysis of the topography and the local elasticity modulus of the samples indicates almost complete blocking of the porous structure of the source membranes in the case of their modification with latex particles, as well as the destruction of the PVPyr layer with the formation of globular structures upon contact with water. The surface energy of the samples after the modification increases. Layer-by-layer applying of PVPyr and sodium polystyrene sulfonate results in a twofold decrease in the pure water flux of the PAN-100 membrane and in an increase in the rejection coefficient due to the formation of a polyelectrolyte complex. In addition, for the PAN/PVPyr/PSS membrane a substantial decrease in the degree of fouling after filtration of a model calibrant solution was detected. It was shown that depending on the type of the source matrix and the polymer used for the surface modification by the horizontal precipitation (LB) method, it is possible to produce membranes with a predetermined pore size, mechanical and transport properties; at the same time, the density and permeability of the modifying layer are determined by the physicochemical properties and the surface structure of the source membrane.

The pore space structure of MGA-95 and ESPA membranes has been investigated by the smallangle X-ray scattering (SAXS). The pore radii as the parameters forming the skeleton of the pore space of the reverse-osmosis composite membranes have been established. Three types of scattering pores (sphere, disc, and cylinder) are found in the region of the X-ray scattering vector of 0.171 < s < 0.538 nm⁻¹, and their average radii of gyration have been determined. The pore structure of the MGA-95 and ESPA membranes, which can be described in terms of the self-affine fractal concept, has been revealed. In the region of the scattering vector of 0.245 < s < 0.342 nm⁻¹ (the average pores with 9.2 < r <12.8 nm) the pore structure corresponds to the “stacked sphere” model with the long straight channels with D = 1.2 and D = 1.3 fractal indices. The pores with r~20 nm form the sinuous channels with the fractal indices of D = 2.1 and 2.8, respectively. Scattering at 0.391 < s < 0.538 nm⁻¹ (5.8 < r < 8.0 nm) occurs on compact monopores with the smooth surface and a fractal index of D = 3.9 (D = 4, Porod regime).

The thermal gas treatment of a reservoir is a promising technology for oil recovery from kerogencontaining rocks of the Bazhenovo Formation. It is expected that this treatment can form a kerogen pyrolysis area, which will advance in the direction of filtration flow. The pyrolysis will result in an increase in rock porosity. An analogue of this process from the viewpoint of hydrodynamics can be melting of part of the material of the matrix of a permeable rock by heat due to the filtration stream having a high temperature. The paper presents results of an experimental study on the melting-front propagation dynamics for a part of the framework material during filtration of a hot viscous fluid through the material. The formation and propagation of the melting front as melted “fingers” and the phenomenon of rim formation from the molten matter at melting front are described.

The possibility to achieve a high degree (up to 90%) of blood plasma exchange in a single-stage, three-channel, countercurrent convective mass exchange apparatus (CMEA) has been confirmed with the use of a model suspension as an example. It has been shown that CMEA is more efficient than a plasma filter at high (over 50%) degrees of plasma exchange.

By using as an example the catalytic dehydrogenation reactions of propane and n-butane in a membrane reactor, it has been shown that the effects of the palladium membrane thickness, temperature, feedstock space velocity, and sweep-gas flow rate on the process behavior are interrelated. Therefore, a change in one of the process conditions can disrupt the balance between the rates of H₂ formation in the catalytic reaction and hydrogen removal through the membrane, which is required to increase the yield of the desired product. To maintain the balance of the rates when one of the conditions is changed (for example, the thickness of the membrane is reduced to increase the productivity of the membrane reactor), detailed optimization of the temperature and the feedstock l and sweep-gas flow rates consumption is required.