Crystallization as a mass transfer phenomenon

A simple model is assumed for the growth of crystals from solution. It is based on an assumption that the crystal growth depends mainly on two consecutive steps, i.e. diffusion and surface reaction. A calculation procedure is presented for the determination of rate constants for these steps. A newly proposed method was compared with a previous one for the case of the growth of CuSO₄·5H₂O crystals in a fluidized bed crystallizer. The obtained mass transfer coefficients were correlated by means of the Froessling equation.     

On the possibility of increasing the safety of nuclear weapons

All-Russian Research Institute of Experimental Physics, Arzamas-16, 607200. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 2, pp. 169–171, March–April, 1995.     

Structural control of rubber modified thermoplastic as produced by the mass process

Commercially important polyblends can be produced by a mass process which proceeds via the dispersion of a rubber solution phase. During the process several distinct stages can be identified and structural variation of the final polymer is achieved by controlled agitation. The relationship between the nature and structure of the dispersed phase and the mechanical properties of the polyblend are illustrated and discussed in the light of current theories of impact enhancement.     

Surface mass transfer processes using peat as an adsorbent for dyestuffs

The factors affecting the initial rate of Telon Blue (Acid) dye adsorption onto peat have been investigated. The surface mass transfer coefficients for the rate of dye removal from solution have been determined and correlated as the dimensionless mass transfer term Sh/Sc^0.33. The function Sh/Sc^0.33 has been correlated with respect to four variables, namely, agitation, initial dye concentration, peat particle size range and the temperature of the dye solution. The mass transfer term varies with T^5.5, d_p^0.13, c_o^?1.1 and R.P.M.^0.26; consequently temperature has the most pronounced effect on the mass transfer coefficient.     

Longitudinal diffusion and resistance to mass transfer as causes of nonideality in chromatography

The mechanisms of band broadening in linear, nonideal chromatography are examined. A development is presented of a rate theory for this process, wherein nonideality is caused by axial molecular diffusion; axial eddy diffusion; finiteness of transfer coefficient. The correspondence with the plate theory is given, so that the results can also be expressed in heights equivalent to a theoretical plate.The plate theory has been extended to the case of a finite volume of feed; the requirement for this feed volume to be negligible has been examined and a method is presented for evaluating concentration profiles obtained with a larger volume of feed.An analysis is given of experimental results, whereby the relative contributions to band broadening for various cooperating mechanisms could be ascertained.     

Evaporative analyser as a mass detector in the size-exclusion chromatography of coal extracts

The applicability of the evaporative analyser in the size-exclusion chromatography (SEC) of coal derivatives has been explored. The detector, in which the intensity is measured for the light scattered from the finely divided solute particles resulting from evaporation of droplets of chromatographic eluate, is mass responsive, sensitive, and linear for both narrow and gross coal-extract fractions. For molecular masses above 300 the mass response is almost uniform, and there are considerable advantages over the detectors commonly used in SEC. Application to lower molecular mass coal materials is limited by the evaporation of solute along with solvent in the detector.     

Mass transfer studies on multiple emulsion as a controlled mass release system

A novel type of multiple emulsions which contain a microemulsion in macrodroplets, was prepared by a two-step emulsification procedure. Mineral oil was used as the oil phase with a mixture of Aerosol OT and Span 20 as primary emulsifiers. A water-in-oil microemulsion was prepared by gradual addition of water in oil containing both these emulsifiers. This microemulsion system, when dispersed in an aqueous solution containing secondary emulsifier, produces water-in-oil-in-water (W/O/W) multiple emulsions. The release rate of solute dissolved in the internal aqueous phase was measured using the dialysis technique. A theoretical model describing the diffusion of a multiple emulsion system was developed, which predicts the half-life for 50% of the internal solute to diffuse to the external phase. Experimental results showed the stability of multiple emulsions improved significantly upon using a thermodynamically stable microemulsion as a primary emulsion and a polymeric surfactant as a secondary emulsifier. As a resull, half-life of these multiple emulsions is greater than that of conventional multiple emulsions.     

On the use of choking as a boundary for fast fluidizationsimulation of counter-diffusional mass transfer

Fast fluidization is widely regarded as a flow regime between the dilute and dense-phase transport regimes in upward flow of gas-particle suspensions where choking does not occur and the system is insensitive to changes in superficial gas velocity and solids flux. Among the attempts in recent years to predict the boundaries of the fast fluidization regime, the use of the concept of choking appears to be most popular. This paper examines the validity of the use of choking as a boundary for fast fluidization. It is shown that this approach arises from the confusion in the definition of choking. Alternative methods are discussed which would enablebetter prediction of the boundaries of fast fluidization.     

Carbon fibre cloth as an electrode material: electrical conductivity and mass transfer

The paper deals with carbon fibre cloth electrodes, employed in the fabrication of composite materials. The electrical conductivity of single bundles, groups of bundles and cloth is studied. Ohm's law applies to the electrical conduction in the direction of the fibre length, but bundle conductivity decreases with the number of fibres in the bundle. Electrochemical mass transfer between a flowing electrolyte and a cloth arranged as a flow-through or a flow-by porous electrode indicates the existence of double porosity in the material. Due to double porosity, only the external surface of the cloth works as an electrode. This finding shows that the material is not suitable for such an application.     

Development and experimental evaluation of aerodynamic lens as an aerosol inlet of single particle mass spectrometry

This study described a development and an experimental evaluation of an efficient aerodynamic lens inlet of the single particle mass spectrometry. Several key designing parameters and systematic factors were investigated for the whole lens system through a full numerical simulation. From many tests for various designs of the system, we showed that Mach number was not an independent parameter but interrelated well with flow Reynolds numbers and pressures upstream of the orifices. By manipulating the parameters, we showed for the first time a possibility that there exist a universal correlation between optimal Stokes number and a new factor incorporating the other dimensionless variables and a design parameter. The universality was confirmed by the full simulation results. We demonstrated that the new design of the system was capable of focusing ultrafine aerosols in the size range of 30–700 nm. At two different operating conditions, the formations of sub-millimeter beams of 30–300 nm NaCl aerosols are verified by light scattering imaging as well as microprobe observation of deposited aerosol beams. Finally, the measured sizes of aerosol beams agree reasonably well with those from the simulations as a function of particle size.     

Sugarcane bagasse as the scaffold for mass production of hierarchically porous carbon monoliths by surface self-assembly

The hierarchically porous carbon materials (HPCMs) with micro- and meso-porosity were prepared by surface coating and solvent evaporation-induced self assembly (EISA) using sugarcane bagasse as the scaffold. The triblock copolymer F127 and phenol-formaldehyde resin were used as mesostructural directing agent and carbon precursor, respectively. The microstructures in terms of morphology, pore texture, degree of graphitization, and thermal stability were characterized by scanning and transmission electron microscopy, nitrogen adsorption analysis, X-ray powder diffraction, and thermogravimetric analysis, respectively. The bulk morphology of HPCMs with hierarchically porous architecture can be retained after calcination at 1000 °C. Small and wide angel XRD patterns show 2-D hexagonal mesostructures and enhanced degree of graphitization. The specific surface areas of monolithic carbon materials are in the range 487-544 m² g⁻¹ with the micropore percentages of 66-67%. The thermal stability of HPCMs is enhanced by the strong interaction of hydroxyl groups of sugarcane bagasse with phenol-formaldehyde resins, and subsequently retains the highly ordered structures with pore size distributions centered at 0.5 and 3.2 nm. In addition, the bagasse-based HPCMs show good electrochemical property and the specific capacitances are in the range 190-234 F g⁻¹ at the scan rate of 5-50 mV s⁻¹. Results clearly show that the use of surface coating and EISA with sugarcane bagasse as the scaffold is an easy, effective and mass productive strategy for fabrication of HPCMs.     

Synthesis of small molar mass perdeuterated polyethylpropylene (d-PEP) as an auxiliary for neutron studies

Summary An inert non-crystalline cryoprotectant oil was needed as a non-incoherent scattering matrix for neutron studies of solvent containing crystalline materials. This led us to prepare perdeuterated polyethylpropylene (PEP) with a molar mass around 1100 g/mol. On route to this material an improved method for the preparation of isoprene-d₈ starting from calcium carbide and acetone-d₆ was devised. Subsequent anionic polymerization using t-butyl lithium as initiator resulted in an oligomeric perdeuterated polyisoprene (PI) with an average chain length of approx. 15 monomer units and Mw/Mn: 1.1. This polymer was deuterated with D₂ and Pd/C (10%) in cyclohexane to yield the desired perdeutero polymer (PEP-d). The polymer fulfill the important properties that it does not crystallize at low temperature and has a low neutron scattering efficiency. Received: 15 September 1999/Revised version: 3 November 1999/Accepted: 3 November 1999     

Gas chromatography/tandem mass spectrometry as an analytical tool for the identification of fatty acids

Structures of fatty acids present at very low quantities in mycobacteria are difficult to determine. A commonly used strategy is to introduce heteroatoms into functional groups by chemical means before subjecting them to gas chromatography/tandem mass spectrometry (GC/MS/MS) analysis. Routinely used methods give very low abundance diagnostic ions leading to ambiguities in structural conclusions. GC/MS/MS associated with electron capture ionization of pentafluorobenzyl esters was used to study very complex mixtures of fatty acids fromMycobacterium fallax andM. aurum. The charge-remote fragmentation of fatty acid carboxylate anions was used for structure determination at the nanogram level of a large number of unsaturated, branched, and cyclopropane-containing fatty acids. Some of them have not been observed previously in these Mycobacteria. On the basis of these studies, biosynthetic pathways of unsaturated, branched, and cyclopropane-containing fatty acid are proposed.     

Identification of natural diacylglycerols as the 3,5-dinitrophenylurethanes by chiral phase liquid chromatography with mass spectrometry

This study reports a facile identification of the molecular species of enantiomeric diacylglycerols by combining chiral phase high-performance liquid chromatography with positive chemical ionization mass spectrometry. For this purpose the 3,5-dinitrophenylurethane (DNPU) derivatives ofsn-1,2(2,3)-diacylglycerols are separated on an (R+)-naphthylethylamine polymer column (25 cm × 0.46 cm ID) using an isocratic solvent system made up of hexane/dichloroethane/acetonitrile (85∶10∶5, by vol) or isooctane/tert-butyl methyl ether/acetonitrile/isopropanol (80∶10∶5∶5, by vol). About 1% of the column effluent (1 mL/min) was admitted to a quadrupole mass spectrometer (Hewlett-Packard, Palo Alto, CA)via direct liquid inlet interface, and positive chemical ionization spectra were recorded over the range of 200–900 mass units. The DNPU derivatives of diacylglycerols yield characteristic [M-DNPU]⁺ and [RCO+74]⁺ ions for each diacylglycerol species from which the molecular weight and exact pairing of fatty acids can be unequivocally obtained. The characteristic ions appear to be generated in nearly correct mass proportions as indicated by preliminary quantitative comparisons. The abbreviations 14∶0, 16∶1, 18∶2, etc. represent normal chain fatty acids of 14, 16, 18, etc. acyl carbons and 0, 1, 2, etc. double bonds, respectively; 16∶0–18∶1, etc. represent diacylglycerols containing 16∶0 and 18∶1 fatty acids of unspecified positional distribution;sn indicates stereospecific numbering of glycerol carbons;sn-1,2-diacylglycerols andsn-2,3-diacylglycerols are enantiomeric diacylglycerols of unspecified fatty acid composition;rac-1,2-diacylglycerols are racemic diacylglycerols representing equal amounts ofsn-1,2-andsn-2,3-enantiomers;sn-1,2(2,3)-diacylglycerols are a mixture ofsn-1,2-andsn-2,3-diacylglycerols of unspecified proportion of enantiomers and unspecified fatty acid compisition and positional distribution; X-1,3-diacylglycerols are diacylglycerols of unspecified fatty acid composition and reverse isomer content.     

Liquid crystalline polymers as additives to enhance the device properties of low molecular mass liquid crystals

Magnetic and electric fields have been used to determine the splay (k₁₁) and bend (k₃₃) elastic constants in a series of polymer/monomer liquid crystal solutions using the well known Freedericksz transition technique. Measurements have been carried out as a function of concentration and temperature. The polymer used was a smectogenic polysiloxane side chain liquid crystal with both cyanobiphenyl and benzoate ester side groups. The monomeric solvent was the nematogen 4-n-pentyl-4′-cyanobiphenyl. All of the solutions studied were nematogenic up to a concentration of 40% w/w. It has been shown that k₁₁, k₃₃ and $\text{k}{}_{33}\text{k}{}_{11}$ all decrease with increasing polymer concentration and that at high enough concentrations $\text{k}{}_{33}\text{k}{}_{11}$ tends to become independent of temperature. The implications of these results are discussed in terms of the performance of the most common liquid crystal display, i.e. the twisted nematic device.