Investigation of the surfactant distribution in oil-in-water emulsions during the crystallization of the dispersed phase via nuclear magnetic resonance relaxometry and diffusometry.

The crystallization of melt emulsions is of great interest to the food, cosmetic, and pharmaceutical industries. Surfactants are used in emulsions and suspensions to stabilize the dispersed phase; thus, questions arise about the liquid-liquid and solid-liquid interfaces of the droplets or particles and the distribution of the surfactant in the different phases (continuous and dispersed phase, interface). Nuclear magnetic resonance relaxation and diffusion measurements revealed that the internal and rotational mobility of surfactant molecules at the liquid-liquid interface decreases with increasing droplet sizes. Additionally, solid-liquid interfaces have fewer surfactants than liquid-liquid interfaces as a result of the desorption of the surfactant molecules during the crystallization of the droplets. Relaxation rates of surfactant molecules in aqueous solution as single molecules, micelles, and at the liquid-liquid and solid-liquid interface are analyzed for the first time.

Contact-mediated nucleation in melt emulsions investigated by rheo-nuclear magnetic resonance.

Increasing the efficiency of disperse phase crystallization is of great interest for melt emulsion production as the fraction of solidified droplets determines product quality and stability. Nucleation events must appear within every single one of the µm-sized droplets for solidification. Therefore, primary crystallization requires high subcooling and is, thus, time and energy consuming. Contact-mediated nucleation is a mechanism for intensifying the crystallization process. It is defined as the successful nucleation of a subcooled liquid droplet induced by contact with an already crystallized droplet. We investigated contact-mediated nucleation under shear flow conditions up to shear rates of 457 s-1 for a quantitative assessment of this mechanism. Rheo-nuclear magnetic resonance was successfully used for the time-resolved determination of the solids fraction of the dispersed phase of melt emulsions upon contact-mediated nucleation events. The measurements were carried out in a dedicated Taylor-Couette cell. The efficiency of contact-mediated nucleation λsec decreased with increasing shear rate, whereas the effective second order kinetic constant kcoll,eff increased approximately linearly at small shear rates and showed a linear decrease for shear rates higher than about 200 s-1 . These findings are in accordance with coalescence theory. Thus, the nucleation rate is optimal at specific flow conditions. There are limitations for successful inoculation at a low shear rate because of rare contact events and at a high shear rate due to too short contact time.

Direct DME synthesis on CZZ/H-FER from variable CO2/CO syngas feeds.

Catalyst systems for the conversion of synthesis gas, which are tolerant to fluctuating CO/CO2 gas compositions, have great potential for process-technical applications, related to the expected changes in the supply of synthesis gas. Copper-based catalysts usually used in the synthesis of methanol play an important role in this context. We investigated the productivity characteristics for their application in direct dimethyl ether (DME) synthesis as a function of the CO2/CO x ratio over the complete range from 0 to 1. For this purpose, we compared an industrial Cu/ZnO/Al2O3 methanol catalyst with a self-developed Cu/ZnO/ZrO2 catalyst prepared by a continuous coprecipitation approach. For DME synthesis, catalysts were combined with two commercial dehydration catalysts, H-FER 20 and γ-Al2O3, respectively. Using a standard testing procedure, we determined the productivity characteristics in a temperature range between 483 K and 523 K in a fixed bed reactor. The combination of Cu/ZnO/ZrO2 and H-FER 20 provided the highest DME productivity with up to 1017 gDME (kgCu h)-1 at 523 K, 50 bar and 36 000 mlN (g h)-1 and achieved DME productivities higher than 689 gDME (kgCu h)-1 at all investigated CO2/CO x ratios under the mentioned conditions. With the use of Cu/ZnO/ZrO2//H-FER 20 a promising operating range between CO2/CO x 0.47 and 0.8 was found where CO as well as CO2 can be converted with high DME selectivity. First results on the long-term stability of the system Cu/ZnO/ZrO2//H-FER 20 showed an overall reduction of 27.0% over 545 h time on stream and 14.6% between 200 h and 545 h under variable feed conditions with a consistently high DME selectivity.

An X-ray diffraction analysis of crystallised whey and whey-permeate powders.

Amorphous whey, whey-permeate and lactose powders have been crystallised at various air temperatures and humidities, and these crystallised powders have been examined using X-ray diffraction. The most stable lactose crystal under normal storage conditions, alpha-lactose monohydrate, forms preferentially in whey and whey-permeate powders at 50 degrees C, provided sufficient moisture is available, whereas anhydrous beta-lactose and mixed anhydrous lactose crystals, which are unstable under normal storage conditions, form preferentially at 90 degrees C. Thus, faster crystallisation at higher temperatures is offset by the formation of lactose-crystal forms that are less stable under normal storage conditions. Very little alpha-lactose monohydrate crystallised in the pure lactose powders over the range of temperatures and humidities tested, because the crystallisation of alpha- and beta-lactose is considerably more rapid than the mutarotation of beta- to alpha-lactose in the amorphous phase and the hydration of alpha-lactose during crystallisation. Protein and salts hinder the crystallisation process, which provides more time for mutarotation and crystal hydration in the whey and whey-permeate powders.

Crystallization kinetics of amorphous lactose, whey-permeate and whey powders.

Amorphous lactose, whey-permeate and whey powders have been converted to their crystalline forms by exposure to air at various temperatures and relative humidities. The total time required for sorption, induction and crystallization of these powders was observed by following the time-dependent mass change of the powders during treatment. These experiments have shown that higher temperatures and relative humidities lead to shorter crystallization times. Lactose crystallizes within 1 min at an air temperature of 100 degrees C and relative air humidity of 80%, whereas whey-permeate and whey powders requires up to 5 min at the same set of conditions. Thus, as previously described, the presence of proteins and salts in the whey-permeate and whey powders reduces the crystallization rate. The rate constants and activation energies have been determined over a range of temperatures and humidities to enable the calculation of crystallization times for the design of an industrial process that crystallizes whey and whey-permeate powders. Finally, the crystallization rates found in this work are sufficiently fast to be applicable in an industrial process that crystallizes whey and whey-permeate powders.

Investigations on the semi-batch precipitation of silica.

The semi-batch precipitation process of silica from sodium silicate and sulphuric acid has been structured into a primary particle production stage, an aggregation process until the gel point is reached and finally a gel fragmentation and compaction process. The progress of aggregation was analyzed by photon correlation spectroscopy. The reaction limited aggregation kernel was found to be proportional to B'(t)(q(p - q))A/2, where A assumes the value of one. The quantity B'(t) varies between 3.4 x 10(-20) and 2.6 x 10(-20) 1/min and is only a weak function of time. The stability ratio W for the aggregation process decreases with increasing aggregate size. During the period after the gel point the gel fragmentates. The gel clusters have an initial fractal dimension of approximately 1.8 which rapidly increases to 2.4 and stays roughly constant until the end of the process. The specific surface area decreases from 480 to 300 m2/g. The primary particles have a uniform size of 22.7 nm.

Morphology and internal structure of barium sulfate--derivation of a new growth mechanism.

Particle formation is the decisive step to control crystal morphology. Besides the classical primary processes, nucleation and molecular growth, the particle size can also increase by aggregation. The special case of self-assembled aggregation leads to the formation of highly ordered particles which often possess a porous internal structure. In the experiments of these studies the particle formation of barium sulfate has been investigated. SEM analysis shows a large variety of growth forms including plate-like, star-like, and spherical particles, whereas TEM exposures reveal the porous internal structure at all investigated supersaturation levels. The pore size and the volume fraction can be influenced by changing the supersaturation ratio. By means of a fast sampling technique in combination with cryo-TEM analysis it has been shown that the particles at the early stages of growth shortly after the beginning of nucleation consist of many small nanocrystallites which have aggregated in a highly ordered manner. The diffraction pattern indicates many small-angle grain boundaries, whereas the particles at the end of the precipitation process are monocrystalline. This leads to the conclusion that barium sulfate grows according to a self-assembled aggregation mechanism followed by a fast recrystallization process.