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Sodyum perboratın saf ve saf olmayan ortamlarda nükleasyon kinetiğinin incelenmesi

Investigation of nucleation kinetics of sodium perborate in pure and impure media

  1. Tez No: 14402
  2. Yazar: SİBEL TİTİZ
  3. Danışmanlar: Y.DOÇ.DR. G. YILDIZ YÜKSEL
  4. Tez Türü: Yüksek Lisans
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1991
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 74

Özet

ÖZET Son yıllarda yoğunlaşan sodyum perborat tetrahidratın büyüme ve nükleasyon hızları üzerine safsızlıkların etkisini inceleyen ça lışmalarda, incelenecek parametrelerin çokluğu nedeniyle, henüz çok kesin sonuçlara gidilememektedir. Bu nedenle, bu alandaki eksikliğin bir kısmını giderebilmek amacıyla, saf sodyum perborat çözelti- leriyle çalışılmış ve stokiometrik ve değişik reaktan oranları ile hazırlanmış sodyum perborat çözeltilerinde nükleasyon hızı ölçümle ri yapılmıştır. Çalışmamızda, sodyum perborat çözeltilerinin nükleasyon hızı na reaktan oranlarının ve safsızlıkların etkisi incelenmeye çalışıl mıştır. Nükleasyon hızı ölçüm deneyleri sonucunda nükleasyon hız mertebesi ve hız sabiti tayin edilerek sodyum perborat tetrahidratın saf ve saf olmayan ortamlarda nükleasyon kinetiği incelenmiştir. Ayrıca elde edilen sonuçlar, deneyler esnasında yapılan kimyasal ve fiziksel analizlerle de desteklenmiştir. Bu araştırma sonucunda, sodyum perborat tetrahidratın saf ortamdaki nükleasyon hız mertebesinin 5, hız sabitinin ise 3.5x10 5 değerini verdiği saptanmıştır. Nükleasyon hız sabiti, hız mertebe si ve metastabil bölge genişlikleri gözönüne alınarak en uygun sod yum perborat tetrahidrat kristal izasyonunun 25-50 g/l sodyum meta- borat fazlalığında yapılabileceği ortaya çıkmıştır. Yine çalışma sonuçlarına göre, çalışılan bölgede boraksın nükleasyon hız mertebesine etkisi olmadığı, buna karşılık hız sabi tini sodyum metaborata göre düşürdüğü görülmüştür. Hidrojen perok sit varlığında yapılan deneylerde, hidrojen peroksitin nükleasyona çok önemli bir etkisi olmadığı, aynı sonucun sodyum hidroksit var lığında yapılan çalışmalar için de geçerli olduğu söylenebilmekte- dir. Safsızlık olarak kullanılan magnezyum sülfat varlığında yapı lan çalışmalardan elde edilen sonuç, benzer yapıda ancak magnezyum sülfat içermeyen çözeltilerden elde edilen sonuçlara bir paralel lik göstermiştir. Bu nedenle, göz önüne alınan magnezyum sülfat safsızlık konsantrasyonu bölgesinde nükleasyon hızı üzerine bir etkinin olmadığı görülmüştür. vi

Özet (Çeviri)

INVESTIGATION OF NUCLEATION KINETICS OF SODIUM PERBORATE IN PURE AND IMPURE MEDIA SUMMARY In this study, nucleation kinetics of sodium perborate tetra- hydrate which is produced by reaction crystallization from sodium metaborate and hydrogen peroxide, is studied in pure and impure media. For this purpose rate of nucleations are measured using the experimen tal system proposed by Nyvlt. Excess of sodium metaborate, sodium tetraborate, hydrogen peroxide and sodium hydroxide are used at the impure part of this study. In addition to these experiments effects of magnesium sulfate to nucleation kinetics of sodium perborate tetrahydrate is also studied. Factors affecting the crystallization of sodium perborate tet rahydrate are numerous. Besides supersaturation ratio, mixing inten sity, crystallization temperature and effects of impurities which are normally encountered in all crystallization processes, ratio of reactants in the following equation: NaB02 + H202 + 3H20 -* NaB03.4H20 are also very important. In addition to this main equation, prepara tion of sodium metaborate from NaOH and borax by the following equa tion: Na2B40? + 2NaOH ?+ 4NaB02 + H20 imposes some additional parameters such as excess of borax and sodium hydroxide in metaborate solution. Excess of some reactants may result in different nucleation and crystal growth mechanism. Which in turn may influence the crystal size distribution and other physical pro perties. Besides some impurities which may exist in reactants, some che micals are added to hydrogen peroxide as stabiliser and inhibitors and these also may affect the crystallization kinetics depending on the kind and concentration. Similarly adding magnesium sulphate and sodium silicate to produce magnesium silicate in the crystal 1 i zer in order to stabilise the sodium perborate tetrahydrate affect the pheno mena depending on the concentration of magnesium silicate and excess of one of the components. Literature survey shows that present information about this subject is mainly in form of patent literature. Each patent deals vnwith only one property of the product taking only one parameter, disregarding the other parameters. As it can be expected many cont radictory results exist in the literature because of the complexity of the system. No systematic research is performed up to the present time in order to quantify the different effects. Recently some sci entific papers appeared in the literature. But, these are not suf ficient to explain the crystallization kinetics of sodium perborate tetrahidrate yet. For these reasons, in this study, nucleation kinetics of sodium perborate tetrahydrate is investigated by using extra pure components to see the effect of excess of reactants and magnesium sulphate. For preparating extra pure sodium perborate solution contains excess of reactants, extra pure reactants are prepared. Pure NaB02.4H20 is prepared from technical Na2B407.5H2Û and NaOH by double recrystallization using Na^CC^ in the first crystallization step in order to precipitate alkaline-earth metal cations. Pure NaBÛ2.4H2Û crystals is dissolved in distilled water before being used. Pure H2O2 is prepared from technical 50% H2O2 in two step distillation in all-glass apparatus. In the first step, H2O2 is totally distilled under vacuum to be freed from non-volatile matter. In the second step fractional distillation is carried out at 58-60 mm Hg and 96- 100°C to separate the volatile matter. By this method H2O2 in pure state is obtained. This is diluted before being used. Fig. 5-1 shows the experimental system. The nucleation cell has a capacity of 30 ml and is fitted with a water jacket, a magne tic stirrer, a contact thermometer and a thermometer with +0.1 °C sensitivity. Sodium perborate solutions used are prepared by using extra pure sodium perborate tetrahydrate crystals for stoichiometric conditions or by using extra pure reactants to produce a perborate solution contains excess of anyone of reactants. After preparation of perborate solution, crystals are filtered and chemical analysis and physical tests of the mother liquor are performed immediately. The clear perborate solution is placed in nucleation cell and heated to a few degree higher than its dissolution temperature and then nuc leation runs are carried out for three different cooling rates at a constant stirring rate. In order to attain a linear cooling rate during the experiment, a contact thermometer coupled with a control led speed motor, a cryostat and a IR-Iamp are connected to the sys tem. After determination of the nucleation temperature for a cons tant cooling rate, the solution is heated slowly to see its disso lution temperature. The defference between nucleation and dissolu tion temperature is maximum supercooling (ATmaxh After finishing the nucleation runs, physical and chemical analyses of the solution in the nucleation cell are performed to check of any decomposition of sodium perborate during nucleation runs. All of the nucleation runs are carried out without seed. The metastable limit is measured by the visual method. Equations 5-1 and 5-2 shows the relationship between metastable zone width which can be related to maximum supercooling and cooling vi i irate. lo9ATm= A+ (1/Dlogb (5.1) log kM =(1-1) ^-Av'I (5.-2) H dT A plot of logATmax versus logb should yield a straight line if the data represent homogenous nucleation and nucleation order (1) may be calculated from the slope of this line. Intercept of this line is used to calculate the nucleation rate constant according to equation 5.2. If concentration of sodium perborate is plotted versus tempera ture a curve is obtained. dC*/dT in eq. 5.2 may be calculated from slope of this curve for any temperature. In this study, dC*/dT va1~ ues is calculated by using the solubility curve of sodium perborate against sodium metaborate concentration given by Smith [26J and shown in fig. 2-1C. Unit of dC*/dT is kg NaB03.4H20/°C.kg free water. 1C*/dT is calculated 1.5x10~3 for stoichiometric condition. Similar values calculated for perborate solutions containing impuruties are.jvsn Table 6-3, 6-5, 6-7 for NaB02, Na2B4Ü7 and H2O2 + NaOH respec tively. The results obtained for pure sodium perborate solution and solutions containing tetraborate, hydrogen peroxide and sodium hydroxide and magnesium sulphate used as impurity are shown in Table 6-1, 6-2, 6-4, 6-6 and 6-8, respectively. Evaluation of nucleation parameters from these results are shown in figure 6-1 for pure solution, in figure 6-15 for the pre sence of excess hydrogen peroxide, in figure 6-16 for the presen ce of excess sodium hydroxide and in figure 6-17 for the presence of magnesium sulphate as an external impurity. Similarly, evalua tion of the results obtained for the presence of excess sodium me- taborate are shown in figure 6-2, 6-3, 6-4, 6,5, 6-6, 6-7 and 6-8 and for the presence of sodium tetraborate are shown in figure 6-12 and 6-13 and 6-14. Table 6-1 and 6-2 shows that allowable maximum supercooling changes with the change of metaborate concentration for the same cooling rate. This change is shewn in fig. 6-11. It is seen that the change of the maximum supersaturation as a function of metabo rate concentration shows similar tendency at the all cooling rates. Density and refractive index values of sodium perborate solu tions which contain excess of sodium metaborate is given in fig. 6-9 and 6-10 against NaB0o/NaB03 mole ratio. During these expe riments, since density and refractive index magnitudes hasn't been ixdone at the same temperature, these magnitudes can not be taken into consideration as exact values. But these figures show that these. values are good control parameters. From the experimental results, the following conclusions can be obtained. a) In real stoichiometric condition, nucleation order (1.) is about 5 and nucleation rate constant is about 3.5x105. b) Nucleation order is approximately constant with increasing metaborate concentration up to 50 g/1 NaB02 concentration. c) Excess metaborate in the mother liquor up to 50 g/1 NaB02 concentration causes the nucleation rate constant to decrease very deeply. d) Maximum allowable supercooling (ATmax) increases with in creasing sodium metaborate concentration in sodium perborate solu tion. e) No sharp effect is observed with excess sodium tetraborate in solution. f) No detectable effect is observed with excess hydrogen per oxide and sodium hydroxide. g) No definite effect is obtained in nucleation behaviour of sodium perborate. When technical magnesium sulfate equivalent to 100 ppm MgO is added to solution. Increasing technical magnesium sulfate concentration causes to increase the decomposition rate of sodium perborate. h) The most suitable crystallization media is obtained at the presence of 30-50 g/1 NaBÜ2 in sodium perborate solution, from the point of nucleation rate constant and maximum allowable supercooling. i) Excess hydrogen peroxide decomposes quickly, even there is no impurty in perborate solution. j) When excess hydrogen peroxide and sodium hydroxide, are exist together in perborate solution, excess sodium hydroxide enhances the decomposition rates of hydrogen peroxide and sodium perborate.k) Density or refractive index are good control parameters in the crystallization of sodium perborate tetrahydrate. XT

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