Izumi Hirasawa

Effects of experimental conditions on the mechanism of particle aggregation in protein precipitation by polyelectrolytes with a high molecular weight

Abstract Lysozyme precipitation by polyacrylic acid (PAA), especially with a high molecular weight of 4×10 6 , where the polymer bridging effect of PAA is thought to be dominant in the particle aggregation of lysozyme precipitates, was carried out to investigate the relationship between the bridging effect of PAA and the operation conditions in a semi-batch reactor. The bridging effect of PAA was found to be affected by the experimental conditions, such as the mixing intensity, PAA dosage, and ionic strength of the solution, which resulted in a variation in the particle size of the lysozyme precipitates due to a change in the aggregation mechanism. The polymer bridging of PAA was enhanced by increasing the agitation speed and PAA dosage and decreasing the ionic strength, which produced a larger particle size. However, under conditions of either poor mixing or small PAA dosage, smaller particles were formed with similar sizes, regardless of the ionic strength, as the bridging effect of PAA did not contribute to the precipitate aggregation. Based on the correlation between the concentration of the lysozyme precipitates and the mean particle size generated during the PAA feeding time, the point of change in the aggregation mechanism was identified and the precipitate concentration at which the mechanism conversion occurred was shorten by increasing the mixing intensity.

Removal and Recovery of Nickel Ion from Wastewater of Electroless Plating by Reduction Crystallization

Wastewater treatment after electroless nickel plating runs has been known to be difficult and this has been a subject of worldwide concern. Discarding wastewater of electroless nickel plating into sea was prohibited under the London Dumping Treaty since 1996. We have studied on recovering nickel from wastewater by reduction crystallization. In this study, the characteristics of nickel ion removal and recovery were investigated in the process of reduction crystallization using a laboratory scale batch crystallizer (500 ml), in order to propose a new process for wastewater treatment from nickel plating. In the reduction crystallization, which can use hypophosphite ion as a reducing agent for nickel ion in the wastewater, nickel ion could be recovered as a form of nickel metal by seeding nickel powder having large specific surface area.

Deducing primary nucleation parameters from metastable zone width and induction time data determined with simulation

Metastable zone widths (MSZWs) and induction times were determined with numerical simulations on an unseeded aqueous solution. The simulated MSZWs and induction times were significantly affected by secondary nucleation caused by nuclei grown crystals. However, this secondary nucleation-mediated effect on the MSZWs and induction times was negligibly small at high cooling rates and at high supercoolings, respectively. The primary nucleation parameters kb1 and b1 in the rate expression B1 = kb1(ΔT)b1, where B1 is the nucleation rate and ΔT is supercooling, were deduced by applying mathematical Kubota models to the determined MSZWs and induction times, respectively. Correct primary nucleation parameters (i.e., the same values as those input for the simulations) were deduced only under the condition of neglected secondary nucleation. The simulation results suggest that, in actual experiments, the proper primary nucleation parameters b1 and kb1 can be deduced from the MSZWs and induction times if these were measured under the condition of neglected secondary nucleation. In addition, an experiment with a slow stirrer speed (low secondary nucleation rate) and with a high sensitivity detector (earlier detection of the MSZW and induction time) were recommended for the deduction of correct primary nucleation parameters.

Crystallization phenomena of magnesium ammonium phosphate (MAP) in a fluidized-bed-type crystallizer

In this study, experiments using a fluidized crystallizer with added seed, were performed, to observe the crystal growth and nucleation phenomena of MAP, by changing the degree of supersaturation. The average growth rate of suspended seed crystals correlated relatively well with the degree of supersaturation to obtain the quantitative correlation equation. Also the average crystal growth rate was in good accordance with the crystal growth rate of the small crystals which comprised the seed crystal. This showed that crystal growth was preceeded by the growth of individual crystal is acting as one single seed crystal. But over a certain degree of supersaturation, the surface condition of grown seed crystals was observed to become rough. Concentration of fines was recognized to correlate with the degree of supersaturation, and excess fines were produced over the critical supersaturation, which was the same as when the surface condition of crystals became rough. The critical supersaturation was considered to offer some design criteria for stable operation in the magnesium ammonium phosphate crystallization in the environmental field.

Modification of crystal growth mechanism of yttrium oxalate in metastable solution

In the seeded crystallization of yttrium oxalate from a metastable supersaturated solution, the growth mechanism was observed to become modified by variations in agitation and additive concentration. From the desupersaturation curve, the crystal growth rate was estimated and described using a power law of supersaturation (RT=KgΔCp). Without an additive in the metastable solution, the overall growth rate coefficient, Kg, increased with an increase in the agitation speed while the growth rate order, p, was almost independent of the agitation speed. When the additive sodium tripolyphosphate was used to inhibit crystal growth, the desupersaturation in the solution was substantially retarded and the growth rate order varied up to about 2.5 with an increase in the additive concentration. Using a two-step growth model, the kinetic parameter dependencies of the crystal growth of yttrium oxalate, such as the mass transfer coefficient, surface integration order, and surface integration coefficient, were estimated in relation to the agitation speed and additive concentration. The mass transfer coefficient increased with an increase in the agitation, whereas the surface integration coefficient was independent of the agitation speed. However, as the additive concentration in the solution increased, the surface integration coefficient was reduced while the mass transfer coefficient remained almost invariant. The change in the rate-determining step by the additive was examined using an effectiveness factor. It was shown that the effect of the additive on the surface integration followed a Langmuir type isotherm.

Aging characteristics of protein precipitates produced by polyelectrolyte precipitation in turbulently agitated reactor

Abstract The relationship between aging and aggregation mechanism of solid lysozyme precipitates produced by polyacrylic acid (PAA) with a high molecular weight of 4.0×10 6 was experimentally investigated in a semi-batch reactor. As feeding the PAA, the aggregation mechanism between lysozyme and PAA was found to be determined by the aging history, which was shifted from charge neutralization to polymer bridging with increasing the agitation level. The lysozyme precipitates generated through a high bridging effect of PAA at a high mixing speed were broken up with aging, i.e. additional agitation of the product suspension after the feeding time. In contrast, the particle size of the precipitates formed at a low mixing intensity through a poor polymer bridging increased due to aging-induced aggregation, except for the extremely long aging time. Based on the largest particle size of the precipitates after aging, the aggregate strength was clearly confirmed to be enhanced by the bridging effect. Regardless of the mixing level, the lysozyme precipitates exhibited a steady particle size when the Camp number approached about 5×10 5 by adjusting the aging time with the same agitation speed as that used during the PAA feeding. However, the steady particle size was not observed when the Camp number was controlled by the aging intensity for the fixed aging time. The precipitates underwent the phenomenon of infilling at a high aging intensity, which appeared to participate, along with the shear stress, in the particle fragmentation that occurred during the aging process.

Secondary nucleation-mediated effects of stirrer speed and growth rate on induction time for unseeded solution

Simulation of induction time for nucleation was performed under isothermal conditions for unseeded aqueous solutions of different concentrations. The effects of stirrer speed and growth rate on induction time were both explained with the proposed secondary nucleation-mediated mechanism. The effect of stirrer speed Nr was incorporated into the simulation with an empirical relation of kb2 ∝ Nrj, where kb2 is the coefficient in the secondary nucleation rate equation B2 = kb2 (ΔT)b2μ3, ΔT is supercooling, j and b2 are empirical constants and μ3 is the third moment of crystal size distribution. The simulated induction time decreased with an increase in stirrer speed at lower supercoolings, while it remained unchanged at higher supercoolings. Such action of stirrer speed was similar to that observed in the literature data. The growth rate effect was considered to be caused by a faster increase in the secondary nucleation rate via a faster increase in μ3. The simulated induction time decreased with an increase in crystal growth rate. This type of growth rate effect is completely different from the existing mechanism considering the time needed for invisible nuclei to grow to a detectable size.

Development of a process for the recovery of phosphorus resource from digested sludge by crystallization technology

Removal and recovery of phosphorus from sewage in form of MAP (magnesium ammonium phosphate) have attracted attention from the viewpoint of eutrophication prevention and phosphorus resource recovery as well as scaling prevention inside digestion tanks. In this work, phosphorus recovery demonstration tests were conducted in a 50 m3/d facility having a complete mixing type reactor and a liquid cyclone. Digested sludge, having 690 mg/L T-P and 268 mg/L PO4-P, was used as test material. The T-P and PO4-P of treated sludge were 464 mg/L and 20 mg/L achieving a T-P recovery efficiency of 33% and a PO4-P crystallization ratio of 93%. The reacted phosphorus did not become fine crystals and the recovered MAP particles were found to be valuable as a fertilizer. A case study in applying this phosphorus recovery process for treatment of sludge from an anaerobic-aerobic process of a 21,000 m3/d sewage system, showed that 30% of phosphorus concentration can be reduced in the final effluent, recovering 315 kg/d as MAP.

Effects of detector sensitivity and resolution on induction time reading

The effects of sensitivity and resolution of a nucleation detector on induction time were mathematically analysed. The induction time was defined here as the time at which the number density of crystals nucleated under isothermal conditions had reached the minimum detectable number density of crystals (N/M)det. The value of (N/M)det depends on the detector used, and therefore can be called the detector sensitivity. Meanwhile, the detector resolution is defined as the minimum detectable crystal size Ld. The equation for induction time tind was derived for a simplified case wherein secondary nucleation and concentration reduction were both neglected. For a more general case where such a simplification cannot be made, the induction time tind was calculated numerically. The sensitivity and resolution had effects on induction time. The induction time increased as the sensitivity decreased (i.e., the value of (N/M)det increased). It also increased with a decrease in the resolution (i.e., an increase in the minimum detectable size Ld). The effect of nucleus size L0 on induction time tind was also analysed. The analysis suggested that the induction time in an actual experiment, where the minimum detectable size Ld would be much larger than the nucleus size L0, was independent of nucleus size itself but was affected by the resolution Ld. The interfacial energy of a crystal nucleus in solution that is deduced from the widely used plot of ln tindvs. 1/(ln S)2 (S: supersaturation ratio) was pointed out to be questionable. The effect of agitation rate (or stirrer speed) on induction time was also discussed.

A population balance model for solvent-mediated polymorphic transformation in unseeded solutions

A new population balance model for solvent-mediated polymorphic transformation is presented. The model takes into account the secondary nucleation caused by nuclei grown crystals as well as the primary nucleation. Numerical simulation was performed for crystallization of a hypothetical enantiotropic dimorphic compound in an unseeded solution. In the simulation, particular attention was paid to the effect of secondary nucleation of the stable polymorph on the transformation time. The simulated transformation time decreased with an increase in the secondary nucleation rate of the stable polymorph. Reported experimental data on the effect of stirrer speed were explained by the secondary nucleation-mediated mechanism, in which the secondary nucleation rate is assumed to increase with an increase in stirrer speed. The effect of scale-up on the transformation time was also suggested to be explained by the secondary nucleation-mediated mechanism.