FREEZE CRYSTALLIZA TION
CHAPTER 6 FREEZE CRYSTAL LI SATION
2. Will the material crystallize?
Bccause rate processes arc critical In freeze crystallization, il is difficult to predict how a mixture will separate without pilot plant results. If a chemical has not been pilot tested, then a silllple freeze test in the laboratory will give some indication of ils behavior. Such a test was conducted in this project and is disclIssed in a laler section. Generally, if the freeze-test analyses show a promising degree of purification, then an industrial separation may bc feasible.
3. Is melting point imponant?
The lower the melting poim of a component, the more expensive the process. This is because refrigeration is required to crystallize low-melting-point components and these costs increase with a decrease in melting point temperatures. Insulation costs also increuse at lower temperatures and at vcry low temperatures the dllctility of materials of constrtlction becomes an Issue.
4. How important is product purity?
In most cases the solid phase contains n pure component. Impurities generally arise during sep:uation of the solid and liquid phase, however implementing a subsequent washing operation can prevent thi!..
5. How important is product recovery?
Recovery in most systems is limited by the existence of a eutectic. In some cases, freeze crystallization produces higher purities and lower product recovery than distillntion. In such a case both techniques can be combined to form a process known as fractional cryslallization.
Lipowicz [1981]. Dye & Ng [1995] and most recently Berry and Ng (1997) discusses these types of hybrid processes together with their applications and tradeoffs. The reader is therefore referred to these reviews for further information on combining. distillation and freeze crystallization processes.
95
CHAPTER (j FREEZE CRYSTALLISATION
6 .3 Solid-liquid equilibr i a
The method of predicting or experimentally determining solid·liquid equilibrium is not important so long as it provides the accuracy for the intended use. Preliminary analysis requires much less precise data than would Lt final design. Experimental methods are cumbersome, time consuming and expensive. Heist [1980] recommends using them only when a decision to lIse u freeze·separation process has already been made und accurate design cbla are required. For preliminary evaluntions, predictive techniques have been developed.
As in the case of vupor·liquid equilibrium (Chapter 3). the cnlcul.::Hion of solid-liquid phase equilibrium enn start from the criterion for equilibrium, which has been developed by Smith & Vnn Nes..; [1987J and is illmaratcd in Appendix 8-1. Therefore. at equilibrium, the respective fugacities
(f.) of the components in both the solid phm:e (S) and liquid phase (L) me' eqllai:
(6·' ) In Equation (6-1) ... 1\ .. denotes Ihe mixture property.
The fugacity of the mixture (f;) is related to the product of the mole fraction (x;). activity coefficient (y,) and the fugacity of the pure species (f..,,,,,,.). Therefore, the following expressions for the solid and liCJl1id phases me obtained:
,.<
r -
xSy'f.'• i - • ; i./""~ (6·2)
, I.
f
I = x, I.y'f.' ; ;./IW,' (6·3)Substituting equations (6-2) .md (6-3) into (6-1) and rearranging gives:
f., , ,
"pu,r x, Yi - , - - =
----ss
f../""r x; Yi
(6-4)
CHAPTER 6 FREEZE CRYSTALLISATION
The ratio of the pure component fugacities (f/"",r /
J/;'''N)
can be calculated via a thermodynamic cycle discussed by Prausnitz. et al [1986]. If the temperature-dependence of the heat capacity difference is neglected, the following equation is obtained:f. ,
611 •' r
1') 6c" ,(1' - 1') 6c" l'1 1 1 - - -i./".rr _ - m.<".' 1 - - - .r "
+ - -
.' 1 n -"r."
RT T R1' R T• 1.1""" "
(6-6)
In the ubove equ;ltioll, 6hm T. ; is the latent heat of fusion at the triple point. T is system
. ".
tCTllperatl1l'e. T" the triple point temperature of component i, and 6cp .; is the difference in liquid and solid molar heat capacities for component i.
To simplify Equation (6-6) further, Jakob et nl [\995] state that conditions nt the triple point are tLSll.dly close to the melting point. Therefore this nllows the substitution of melting point conditions for triple point conditions (i.e. TIr , ; == T"" i and 8h,.,. j == 611",.7".;)' In addition, the last two terms in Equation (6·6) are of opposite sign and tend to cancel Cllt in the vicinity of the melting point.
Substitution of Equation (6-5) into (6-6) with these simplifications gives:
611 '( T )
I nx,"Y,L
=R;'
l -~",.r
(6-7)
Equation (6-7) is used to predict the solid-liquid phase behavior for preliminary process-design ca\clllutions. The data required arc the latent heat of fusion at the melting point temperature. the melting temperatllre nnd the nctivit)' coefficient of the solution. Mehing temperature:s and I"'em heals for organic compounds may be found in books by Weast [1983J or Weast & Grasselli [1989).
Activity coefficients in solid-liquid systems are sparse but they do exist for many binary vapollr- I iquid equilibrium systems. Therefore, ROllsscau & Moyers [19S7J sugg<!st estimating solid-liquid activity coefficients directly from vapor-liquid equilibrium data obtained at higher temperatures,
\Vith this information. the freezing -point temperature of a solution may be calculated as a function of liquid composition.
97
CHAPTER 6 FREEZE CRYSTALLISATION
If a solution is assumed to be ideal, Equation (6-7) further simplifies to the rollowing equation:
::." . ( T )
I .f. - ",.r I
11-1: - -- - - -
, RT T
"'.r
(6-S)
Equation (6-8). also known us the Vun Hoft equation, IS recommended to predict solid-liquid equilibria of binary systems containing isomers (Muir & Howat [1982]). To demonstrate ils lIse as a predictive tool for systems containing isomers. comparison between the predicted and experimental phase curves for the I ,3-xylene+ I ,2-xylene system is presented in Figure 6-2. The prediction is very good with a mean deviation of O.3K over the temperature range and therefore ensures confidence in using the developed equations (0 determine solid-liquid equilibria of binary systems containing isomers. It must be made clear. however, that these techniques become less accurate as the difference in molecular siz.e increase.
250 ·r - - - -- - - -- - - -- -- -- - - -- - - , 245 ·
240 · g 235
i!' ~
~ 230 ·
~
E 225
~ 220 215
• Jacob et al [1995) - - -Calculated
._ - ...
210 L -______ - - - -__ - - - -__ - - - -__ - - - -__ - - - -____________ ~
o
0.1 0.2 0.3 0.4 0.5x,
0.6 0.7 0.8 0.9
CHAPTER 6 FREEZE CRYSTALLISATION
6 .4 Experi mental Set-up and procedure
A simple experiment was set-up (Figure 6-3) 10 demonstrate solidification of butyric acid Ollt of a liquid mixture of butyric acid and isoblltyric acid.
Open-ended test-tube , - - - ,
Cold Finger
st irrer ---j;;;;;;;;;~I~j=========i
Liquid Solution
Cooling ---t~
___ .::.=-____ J
Medium
Temperature Display
Temperature Sensor
Solid
Figure 6-3: Experimental set-up for cryslallization experimenl
A solution cOI1!>isting approximately 87% butyric acid and 13% isobutyric acid was made up to a volume of" 10 ml in a Pyrex glass tube. The butyric acid was purchased from Riedel-de Ilaell. whilst the isobutyric acid was purchased from Fluca Enterprises. The reagents were used without further purification after gas chromatographic analysis showed no significant impurities. The purities of the reagents wert: also checked by their refrnctive indexes and comparisons with literature values are shown in Table 3-4. Chapter 3.
The glass tube containing the acids was then placed into a liquid bath (cooling medium) with polystyrene providing insuLation against the atmosphere. A Eurotherm temperature indicator was used to display the resistance of the PT-lOO temperature sensor placed in the liquid solution. A TECHNE cold finger. with Cl minimum operating temperature of _20uC was used to maintain the temperature of the cooling lllediulll. The cooling medium consisted of a mixture of water and ethylene glycol. Manipulation of the relative concentrations of ethylene glycol and water in the cooling medium prevents the cooling medium from solidifying. This relationship is dctennined from the solid-liquid equilibrium of the binary mixture and has been measured by 011 Cl al (19721 and is shown in Figure 6-4. Using this figure. a mixture of 80-vol% water and 20-vol% ethylene glycol was chosen to make up the cooling medium to prevent it from solidifying.
99
CHAPTER 6 FREEZE CRYSTALLISATION
280
270 -+--Ott et al (1972J
260
?2:
- 250e
o~ 240
~ E 230
~ 220 210 200
o
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9x,
Figure 6-4: Solid- illUld clluilibriulII curve or et lylene g yeo (I) water ( )
6 .5 Results and Discussion
The solid-liquid equilibrium curve for the isobutyric acid/butyric acid system as predicted by Equation (6-8) is shown in Figure 6-5.
1
280
,---,, ---,
270 260
~
•
" 250..
0 "•
240~ 230 E