Ca2+ iyonlarının yenice üretilmiş CaCO3 tanecikleri üzerine etkisi

Thesis (Master)--Izmir Institute of Technology, Chemical Engineering, Izmir, 2019Includes bibliographical references ( leaves: 58-64)Text in English; Abstract: Turkish and EnglishThe objective of this study was to develop a method to synthesize CaCO3 nanoparticles from a chemical precipitation reaction under ambient and high supersaturation conditions. Equimolar CaCl2 and Na2CO3 solutions were reacted in a tubular reactor at a constant rate. The particles growth inhibition was attempted by dispersing the reaction mixture in a continuously stirred Ca(OH)2 solution. This procedure separated the nucleation phase from the growth inhibition process, and was conducted without pH and composition control. The possibility of impeding the CaCO3 particles overgrowth was explored at different precipitants and Ca(OH)2 concentrations. Their effects on the particles morphology, colloidal stability and specific surface area were studied. Although rapidlysettling particles were produced at precipitants concentration of 100 mM, colloidally stable CaCO3 nanoparticles were obtained at concentrations ≤75 mM. Additive Ca2+ ions, provided by the Ca(OH)2 solutions, inhibited the crystals growth by adsorbing irreversibly on the growth sites. The synthesized particles were as much as 95% smaller than those obtained when pure H2O was used instead. Ca2+ ions concentration and amount of precipitated particles were observed to be important factors for monodispersity and high growth inhibition. Monodisperse and stable nanoparticles were synthesized at low reactants concentration and/or precipitates volume. Vaterite phase was observed in the particles obtained when pure H2O was used as the growth-inhibiting solution. However, the presence of additive Ca2+ ions effected the crystallization of pure calcite, regardless of Ca(OH)2 or precipitants concentration, reaction mixtures retention time in the tubular reactor, volume of precipitates, and the growth-inhibiting solutions initial pH.Bu çalışmanın amacı, yüksek aşırı doyma ve normal ortam koşullarındaki bir çöktürme reaksiyonundan, CaCO3 nanotaneciklerini üretmeye yönelik metod geliştirmekti. Eşdeğer molar CaCl2 ve Na2CO3 çözeltileri sabit hızla, tüp şeklinde bir reaktörde reaksiyona sokulmuştur. Yeni oluşan partiküllerin fazla büyümesini engellemek için, reaksiyon karışımı devamlı karıştırılan Ca(OH)2 çözeltisine dağıtılmıştır. Bu yöntem, çekirdeklenme basamağını, kristal büyümesini engelleme prosesinden ayırmıştır. Kimyasal bileşim ve pH ayarlanmadan tamamlanmıştır. Üretilen taneciklerin aşırı büyümesinin engelleme olasılığı, farklı Ca(OH)2 çözeltisi ve tepkenlerin konsantrasyonlarında incelenmiştir. Partiküllerin morfolojisi, koloidal stabilitesi ve özgül yüzey alanı üzerindeki etkiler araştırılmıştır. 100 mM tepken konsantrasyonunda, hızlıca çöken partiküller üretilmesine rağmen, ≤75 mM konsantrasyonlarında kararlı CaCO3 nanotanecikleri elde edilmiştir. Ca(OH)2 çözeltisinden sağlanan katkı Ca2+ iyonları, kristallerin yüzeyine geri dönülemez bir şekilde tutunarak aşırı büyümelerini engellemiştir. Sentezlenen tanecikler, Ca(OH)2 yerine saf H2O kullanıldığında elde edilenlerden %95’e kadar daha küçüktür. Partiküllerin fazla büyümesinin engellemesi ve homojen boyut dağılımında üretilmesinde, katkı Ca2+ iyon konsantrasyonu ve Ca(OH)2 çözeltisine giren çökeltinin miktarı önemli faktörlerdir. Homojen dağılımlı ve koloidal kararlı kalsit nanotanecikleri düşük çökelti hacmi ve tepkenler konsantrasyonunda üretilmiştir. Ca(OH)2 yerine saf H2O kullanıldığında, taneciklerde vaterit polimorfu elde edilmiştir. Ancak, Ca(OH)2 veya tepkenlerin konsantrasyonuna, reaksiyon karışımını reaktörde tutma süresine, Ca(OH)2 çözeltisinin başlangıç pH’ına ve çökelti hacmine bakmaksızın, katkı Ca2+ iyonlarının varlığı saf kalsit fazlarının kristallenmesine etki etmiştir

Pressure-controlled formation of discontinuous clogs in tapered microchannels

In suspension flows through microchannels with parallel walls, rigid particles form clogs that grow continuously in the upstream direction. However, introducing a slight taper to channel walls leads to a qualitatively different clogging mechanism. Clogs of rigid particles do not grow continuously in these tapered pores. Instead, new clogs form upstream of pre-existing clogs, truncating their growth, and thereby creating multiple distinct clogs within a channel. We refer to this novel phenomenon as discontinuous clogging. Here, we investigate its features by analyzing the dimensions and locations of discontinuous clogs in parallel tapered pores. Measurements reveal the discontinuity of clog growth depends strongly on flow driving pressure and particle volume fraction. Increasing volume fraction increases clogging frequency and positions the clogs upstream, in wider regions of the channels. Two regimes of driving pressure appear to exist as the discontinuous clogs are observed to become dramatically longer above a critical pressure. Interestingly, these long clogs are located downstream, towards the channel outlet, at the lowest volume fraction. However, they are increasingly located upstream as volume fraction increases. The dependence of clog location on pressure and volume fraction lends insight into bridging mechanism. Particles arriving simultaneously to a given location can span the channel width to form a bridge, which happens easily at higher volume fractions. Permanent clogs form when driving pressure is lower than the force the bridges can sustain. As driving pressure increases, however, it can overcome the force chains, preventing formation of new permanent clogs, and pushing particles further downstream

Flow dynamics through discontinuous clogs of rigid particles in tapered microchannels

Abstract Suspended particles flowing through complex porous spaces exhibit clogging mechanisms determined by factors including their size, deformability, and the geometry of the confinement. This study describes the clogging of rigid particles in a microfluidic device made up of parallel microchannels that taper from the inlet to the outlet, where the constriction width is approximately equal to the particle size. This converging geometry summarizes the dynamics of clogging in flow channels with constrictions that narrow over multiple length scales. Our novel approach allows the investigation of suspension flow dynamics in confined systems where clogs are formed both by sieving and bridging mechanisms simultaneously. Here, flow tests are conducted at constant driving pressures for different particle volume fractions, and a power-law decay which appears to be peculiar to the channels’ tapered geometry is observed in all cases. Compared to non-tapered channels, the power-law behavior shows flowrate decay is significantly weaker in tapered channels. This weaker flowrate decay is explained by the formation of discontinuous clogs within each channel. Micrographs of the clogged channels reveal clogs do not grow continuously from their initial positions around the channels’ outlet. Rather, new clogs spanning the width of the channel at their points of inception are successively formed as the cake grows toward the inlet area in each microchannel. The results show changes in particle volume fraction at constant driving pressure affect the clogging rate without impacting the underlying dynamics. Unexpectedly, analyses of the particles packing behavior in the microchannels, and post-clogging permeability of the microfluidic devices, reveal the presence of two distinct regimes of driving pressure, though only a small portion of the total device volume and channels surface area are occupied by clogs, regardless of the particle volume fraction. This novel investigation of discontinuous clogging over multiple particle diameters provides unique insights into additional mechanisms to control flow losses in filtration and other confined systems

Contributory roles of concentration and pH in CaCO3 growth inhibition by additive Ca2+ ions

CaCO3 particles grow excessively upon chemical precipitation in the absence of impurities or growth inhibitors. Additive Ca2+ ions have been shown to preferentially adsorb on CaCO3 precipitates, effectively inhibiting their growth and promoting the crystallization of pure calcite without an observable intermediate phase. This phenomenon can be adapted towards the synthesis of small calcite particles from a conventional chemical precipitation method. Complementing such effort, this study discusses the influence of additive Ca2+ ions concentration and solution pH on the extent of CaCO3 growth inhibition. Equal volumes of equimolar CaCl2 and Na2CO3 solutions were mixed in a tubular reactor at a constant flowrate. The precipitates were continuously dispersed in Ca(OH)2 solution, where Ca2+ ions irreversibly adsorb on their surfaces. Compared to conditions where additive Ca2+ ions are absent, this method can produce more than 90% decrease in particle size. The results show the degree of growth inhibition increases as the concentration of additive Ca2+ ions increase. However, it is limited by increasing volume of precipitates. This study also reveals an unusual role of media pH. Here, growth inhibition that leads to the synthesis of monodisperse submicron CaCO3 particles is only observed in high alkaline pH conditions. This is due to the hydration of additive Ca2+ ions in low pH conditions. While additive Ca2+ ions adsorb on CaCO3 precipitates in pH conditions above the isoelectric point (pH ≈ 9), their ability to limit CaCO3 growth diminishes when pH < 12

Modeling of an activated sludge process for effluent prediction—a comparative study using ANFIS and GLM regression

In this paper, nonlinear system identification of the activated sludge process in an industrial wastewater treatment plant was completed using adaptive neuro-fuzzy inference system (ANFIS) and generalized linear model (GLM) regression. Predictive models of the effluent chemical and 5-day biochemical oxygen demands were developed from measured past inputs and outputs. From a set of candidates, least absolute shrinkage and selection operator (LASSO), and a fuzzy brute-force search were utilized in selecting the best combination of regressors for the GLMs and ANFIS models respectively. Root mean square error (RMSE) and Pearson’s correlation coefficient (R-value) served as metrics in assessing the predicting performance of the models. Contrasted with the GLM predictions, the obtained modeling results show that the ANFIS models provide better predictions of the studied effluent variables. The results of the empirical search for the dominant regressors indicate the models have an enormous potential in the estimation of the time lag before a desired effluent quality can be realized, and preempting process disturbances. Hence, the models can be used in developing a software tool that will facilitate the effective management of the treatment operation