Borik asitin kristal büyüme kinetiğinin tek kristal hücresinde incelenmesi
Crystal growth kinetics of boric acid in a single crystal cell
- Tez No: 19315
- Danışmanlar: PROF.DR. A. OĞUZ RECEPOĞLU
- Tez Türü: Yüksek Lisans
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 218
Özet
ÖZET Bu çalışmada borik asitin kristalizasyonunun detayına inilmek için büyüme kinetiği incelenmiştir. Bu inceleme tek kristal hücresinde yapılmıştır. Düzgün şekilli ve yüzeyli borik asit kristali ince bir paslanmaz çelik çubuk ucuna yapıştırılarak monte edilmiş daha sonra hücre içine vidalanıp yerleştirilerek asın doygun çözelti ile temasta bırakılmıştır. Mikroskobik incele meye uygun olan hücreden borik asitin büyümesi incelenmiş ve ortalama lineer büyüme hızları hesaplanmıştır. Her deneyde borik asit kristalleri değiştirilerek her bir kristalin büyüme sekli incelenmiştir. önce çeşitli aşırı doygunlukta ve sabit akım hızında deneyler yapılmış daha sonra beş ayrı akım hızında deneyler tekrarlanmıştır. Borik asitin büyüme hızı, deney süresince belirli aralıklarla çekilen fotoğraflarla hesaplanmış tır. Burada, projeksiyon alanı hesaplanarak eşdeğer projeksiyon alanı çapına geçilmiştir: Projeksiyon alanı ölçümü polar planimetre cihazı ile yapılmıştır. Bu ölçüme göre direkt olarak fotoğraftan ölçüm yapılarak ortalama lineer büyüme hızına geçilmiştir. Deneyler 0.0054, 0.0127, 0.0199. 0.0272, 0.0345 m/s lik akım hızlarında yapılmış olup her akım hızında değişik konsantrasyonlarda çalışılmış ayrıca aynı konsantrasyonda birden fazla deney yapılmıştır. Burada amaç borik asit kristalinde büyüme hızı dağılımının (growth rate dispersion) olup olmadığını görmektir. Deneylerden elde edilen büyüme hızlarının aşırı doygunluğa göre değişimi incelenmiştir. Ayrıca bazı kristaller büyütül dükten sonra çözülüp tekrar büyütülerek sekil ve büyüme hızı tekrarlılığı incelenmiştir. Bütün bu deneyler sonunda, literatürlerde de belirtildiği üzere borik asitin her türlü aşırı doygunlukta dentritik büyüme gösterdiği gözlenmiştir. Sabit akım hızında ve aynı aşırı doygunlukta yapılan bir kaç de neylerden elde edilen sonuca göre -425+500 um. elek aralığındaki borik asit kristallerinin büyüme hızı dağılımı gösterdiği ortaya çıkmıştır. Aynı kristalin büyütüldük- den sonra çözülüp tekrar büyütülmesi deneylerinde, ikinci büyümede kristal büyüme hızının düştüğü, büyümenin sekil yönünden de birinci büyümeye göre farklı olduğu gözlenmiştir. -VIII-
Özet (Çeviri)
CRYSTAL GROWTH KINETICS OF BORIC ACID IN A SINGLE CRYSTAL CELL SUMMARY Boric acid produced in industrial crystal lizers show very broad range of crystal size distribution (CSD), very poor washing characteristics, effective dust problem in drier, caking tendency and high level of impurities than normally expected from CSD and mother liquor content of centrifuged product. In order to understand these problems crystallisation kinetics, especially growth rate kinetics, should be well known. Many workers studied this system by using fluidized bed crystal growth cell, mixed suspension mixed product removal ( MSMPR ) type continuous laboratory crystal lizer and programed cooling batch crystal lizer. But crystal growth rate values obtained from different system are very different and even they are not at the same order of magnitude. In order to understand the reason for different results and the real phenomena in growing boric acid crystal, crystal growth rate and shape changing are studied by using single crystal growth cell. In this technique, there is no chance for breakage of growing crystal and therefore will give more real crystal growth rate values than given by previous works. In this study flow-type single crystal growth cell is used. Figure 4.1 shows the experimental set-up and figure 4.2 shows the detail of growth cell. As it can be seen from figure 4.1 double jacketed glass reactor in 1 liter capacity is used as a stock tank for under- saturated solution and this solution is pumped by a magnetic drive centrifugal pump to a cooler. A part of solution is sent back by a by-pass flow and other part is cooled. Second by-pass after cooler gives very high thermal stability during the experiments. Keeping the temperature of thermostat 3“C higher than the saturation point gives permanent undersaturation in stock tank and desired supercooling obtained in growth cell by changing the temperature of crystal and flow ratio of by-pass lines. A flow-meter is used to measure the flow rate in growth cell. A microscope coupled with -IX-a camera permits the visual observation during the experiment and growing crystal in cell is photographed periodically. Crystal holder in growth cell is a stainless steel rod and selected crystal is glued carefully to the holder by cyonoacrylate type adhesive. Holder with crystal is screwed to the cell just before desired experimental condition is obtained. Experiments are conducted by changing supersatura- tion level and flow rate. Boric acid crystals used are carefully selected from high purity boric acid in the particle range of -425+500 am. Boric acid grows dentritical ly and therefore it is difficult to measure growth in any direction. For this reason, growth rate is evaluated from photographs by using planimetric area measurements. Linear growth rate is calculated from the radius of sphere having the same projected area. Therefore measured growth rate values are area projected average growth rates. Experimental results show that this type of measurement gives very consistent and reliable data. Growth rate is calculated from the following expression. dlR İr - 1b G= = dt where ; 1r = Equivalent sphere diameter having the same projection area with crystal at time t [m] 1b = Same diameter at the beginning of experiment [m] t = Time [s] In the experimental part of study, two types of experiments are conducted: a- Crystal growth rate measurements at constant solution velocity ( constant slip velocity -X-between crystal and solution ) but at different supersaturation levels. This type of experiments are conducted at five different solution velocity. Repeated experiments at the same velocity and supersaturation level are done in order to decide if there is any growth rate dispersion at the particle size region of -425+500 urn. In the second group of experiments same crystal is grown two times at the same hydrodynamic and supersaturation condition in order to see if there is any shape and growth rate changing for each case. To the this, after first growth, crystal is dissolved to any desired particle size and than is regrown. Figure 5.1 shows the crystal growth rates measured at different supersaturation levels but at 0.0054 m/s. Increase in average particle diameter with time is shown for each experiment in Al group figures at appendix. Bl group figures at appendix show the photographs of growing crystals at fixed time period. These pictures show clearly how crystals grow and crystal shapes change with time. At Al and Bl group figures, the same run number is used for any particular experiment. Figure 5.2, 5.3, 5.4 and 5.5 show the crystal growth rates as the function of supersaturation (expressed as g/100g of solution) at 0.0127, 0.0199, 0.0272 and 0.0345 m/s solution velocities respectively. At appendix, figures in A2, A3, A4 and A5 groups and in B2, B3, B4 and B5 groups have the same meaning given above respectively. As it can be seen from figures Al to A5 increase in average particle diameter with time is the linear function of time. Therefore, average linear growth rate which is derivative of the particle size function should be constant. All the results show that average growth rate calculated from projection area of particle gives very consistent data, even particles grows dentriticaly. And also they show that each particle has a constant growth rate not changing by time. Common feature of growing crystal shown in figures Bl through B5 is the dentritical growth of crystal at -XI-All supersaturation levels, as it is claimed by previous workers [11,34], As it is clear from these pictures dentritical growth increase with increasing supersaturation but decrease with increasing solution velocity. Growth rate values given in figures from 5.1 to 5.5 are almost 10-15 times higher than from the results of previous workers [11,34], using the well known fluidized bed crystal growth celi. The same values are almost 100 times higher than results given by Matuchova et.al. [35] using bateh type well mixed crystall izer working at constant cooling rate. The main reason för these differences is the breakage of dentrites which is not encountered at our case. it is very surprising that secondary nucleation is very effective in fluidized bed crystal growth celi which has very mi id operation condition. it is concluded that data for industrial crystallisation of boric acid can only produced from the laboratory model vrorking at the same hydrodynamic conditions. Other data has no physical meaning. it is also concluded that kinetic order of crystallisation, i.e. ratio of order of nucleation rate to growth rate, should be around ör below l, because of effective secondary nucleation. This conclusion is consistent with data given by Karakaya [11] and Teodossiev and Kirkova [36]. it is clear from figüre 5.1 that boric acid crystal show broad range of growth rate dispersion. These type of dispersion are given for many substances as it is shovm in table 5.4, but this phenomena is observed at the partide size range up to 50 ym for other substances. For boric acid, growth rate dispersion can easily be detected at the partide size range of -425+500 ym. Accepting the power law: G= ko- AÇ”is valid for this system, Loğ G-LogAC graphs is drawn as it is shovm if figüre 5.6 through 5.10. Table 5.5 shows the calculated orders of average grovrth rates (n), growth rate constant (kG) and correlation coefficients of the results. it is very interesting to see n values -XII-lower than 1 at low solution velocities. At this velocity level, crystal growth rate should be governed by diffusion in that case n values goes to 1. Since n values is lower than 1, growth mechanism should be effected other than diffusion. But this phenomena is still not known. Figure 5.11 shows the crystal growth rate of boric acid when the same crystal is grown two times at the same conditions. After the first experiment, crystal is dissolved to the same original size and than is grown again. In each case, crystal observed to grew dentriticaly. In second growth, growth rate is lower than the first growth. Figure 5.12 show the photographs for each growth. In other experiments after first growth, crystal is dissolved to a lower particle size and growing experiments conducted at the same solution velocity and almost at the same supersaturation. Results are shown in figures 5.13, 5.15, 5.17. particle size after dissolving is clear from this figures. Experiments are conducted in such a way that particle size in second growth decrease to a lower and lower level comparing to original state. Photographs in figure 5.14 and 5.16 is the picture taken during the experiment their results shown in figure 5.13 and 5.15 respectively, figure 5.18 and 5.19 show the picture for first and second growth their growth rates are given in figure 5.17. It is obvious that when the particle is dissolved to the lower size, crystal can grow non-dentritical state and shows very different crystal growth rate. It is concluded that the reason for dentritical growth is the imperfection on crystal surface. When these imperfections are removed by dissolving, boric acid grows regularly. These conclusion show that Burton-Cabrera-Frank (BCF) theory is effectively valid for the growth of boric acid. -XIII-
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