Crystallization rate manipulating

In order to retard the crystallization rate by manipulating thermodynamic terms, Naito suggests designing organic dyes with minimal changes in enthalpy for glass-crystal transitions (AH) and low equilibrium melting points (Tm), in addition to... 

Crystal habit (or morphology) is an important property for the industrial use of zeolites. Manipulating habit can be used to tailor the number of pores exposed to the surface of a crystal, as well as the length of pore channels. The shape of a crystal can vary because the different faces grow at different rates relative to one another. 

One approach which has resulted in experimental implementation is that of Randolph and co-workers f88-92 >. Using a simulation (21) Randolph and Beckman demonstrated that in a complex RTD crystallizer, the estimation of nuclei density could be used to eliminate cycling or reduce transients in the CSD. Randolph and Low (gg) experimentally attempted feedback control by manipulation of the fines dissolver flow rate and temperature in response to the estimated nuclei density. They found that manipulation of fines flow rate upset the fines measurement indicating that changes in the manipulated variable disturbed the measured variable. Partial fines dissolution resulting from manipulation of the fines dissolver temperature appeared to reduce CSD transients which were imposed upsets in the nucleation rate. In a continuation of this work Randolph et. al. < 921 used proportional control of inferred nuclei density to control an 18 liter KCl crystallizer. 

The number of inputs which are available for controlling crystallisation processes is limited. Possible Inputs for a continuous evaporative crystallisation process are, crystalliser temperature, residence time and rate of evaporation. These Inputs affect the crystal size distribution (CSD) through overall changes in the nucleatlon rate, the number of new crystals per unit time, and the growth rate, the increase in linear size per unit time, and therefore do not discriminate directly with respect to size. Moreover, it has been observed that, for a 970 litre continuous crystalliser, the effect of the residence time and the production rate is limited. Size classification, on the other hand, does allow direct manipulation of the CSD. 

For illustrative purposes, consider that the protocol for a cooling crystallizer can involve either natural cooling— cooling resulting from exposure of the crystallizer contents to a heat sink without intervention of a control system—or manipulation of cooling to reduce the system temperature in a specific manner. In both cases, the instantaneous heat-transfer rate is given by ... 

Slow solvent evaporation is a valuable method for producing crystals of high quality it is used primarily for small molecules. The rate of growth is manipulated by adjusting conditions (initial concentration, temperature, evaporation rate) so that crystals are obtained in a few days, not months or minutes. The rate of evaporation can be adjusted by covering the solution with a watch glass or a piece of film (aluminum or Parafilm ) with a number of small holes punched in it. The solution is then left to stand undisturbed. 

The manipulated variables for the optimization are the mass flux over the membrane (J) and the ultrasound utilization rju). Figure 7 shows the optimal trajectories of the manipulated variables. The results illustrate the strength of the task based design approach. In this case a very tight constraint on the growth rate has been imposed. It can be seen how both the flux and the utilization of ultrasound work together to maximize the crystal mean size. In the initial phase the ultrasound and flux are both high to create... 

The XPS measurements were carried out on a VG ESCALAB Mark II electron spectrometer. TPD data was also collected on the same system using VG SX 200 Mass spectrometer. The Ru(OOl) crystal was fixed on a liquid N2 cooled manipulator and its temperature was measured by chromel/alumel thermocouple spot-welded on the side. The crystal temperature could be varied from 95 to 1500 K. The crystal surface was cleaned initially by Ar-ion bombardment and then annealed at 1200 K for about 20 min before flashing to 1500 K. The Sm was deposited by resistive heating of Sm sample inside the preparation chamber. CO doses to the crystal surface were measured in Langmuir units (1L=10 Torr-sec). A linear heating rate of 5 K/sec was applied for TPD measurements. 

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