Separation and Purification by Crystallization - American Chemical

Chapter 19

Crystallization of Sodium Chloride with Amines as Antisolvents

Downloaded by UNIV OF ALBERTA on April 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0667.ch019

T. G. Zijlema, H. Oosterhof, G. J. Witkamp, and G. M . van Rosmalen Faculty of Chemical Technology and Materials Science, Laboratory for Process Equipment, Delft University of Technology, Leeghwaterstraat 44, 2628 CA, Delft, Netherlands

The suitability of the amines diisopropylamine (DiPA) and dimethylisopropylamine (DMiPA) as antisolvents for the crystallization of sodium chloride from its aqueous solution has been demonstrated. Both amines decreased the sodium chloride solubility substantially. The presence of a two liquid phase area offered the opportunity to separate the amines from the mother liquor after crystallization by a temperature increase. In the two liquid phase area the mutual solubilities of the water and the amines were low, so the separability was good. Continuous crystallization experiments were carried out at temperatures below the liquid-liquid equilibrium line in the single liquid phase area. The product consisted of cubic agglomerated NaCl crystals with maximum primary particle sizes of 10-70 μm.

In industry highly soluble inorganic salts with a small solubility temperature dependency like NaCl, are often separated from water by evaporative crystallization. The evaporation of water in these processes is rather energy consuming. To reduce energy costs in the production of such inorganic salts, crystallization with an organic antisolvent could be an interesting alternative, because the energy intensive water evaporation step is substituted by the addition of an antisolvent. The application of antisolvent crystallization however, introduces an additional separation step, in which the antisolvent has to be recovered from the remaining mother liquor after crystallization. The antisolvent recovery must be carried out with little energy consumption in order to make the crystallization process economically feasible. With respect to this recovery diisopropylamine (DiPA) and dimethylisopropylamine (DMiPA) could serve as suitable antisolvents for the crystallization of NaCl, because they can be separated from the water phase by a temperature induced liquid-liquid phase split (7). 230

© 1997 American Chemical Society

In Separation and Purification by Crystallization; Botsaris, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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Crystallization of NaCl with Amines as Antisolvents

231

In Figure 1 a simplified process scheme of the antisolvent crystallization of sodium chloride is displayed. The process is divided into three steps: the crystallization, the solid-liquid separation and the antisolvent recovery or liquid-liquid separation. In the first step sodium chloride is crystallized by mixing the feed brine with an antisolvent. The crystallization is carried out at temperatures below the liquidliquid equilibrium line in the single liquid phase area (see Figure 2). In the second step the crystals are separated from their mother liquor, e.g. by filtration or in a centrifuge. In the third and final step the antisolvent is separated from the water phase at a temperature above the liquid-liquid equilibrium line in the two liquid phase area, in which the ternary amine-water-salt system splits up into an amine and an aqueous phase. The recovered antisolvent is recycled within the process and most ideally the water phase is reused for the dissolution of crude sodium chloride. In this paper the crystallization and the liquid-liquid separation steps will be treated. The goal of this study is to investigate whether DiPA and D M i P A are suitable antisolvents for the crystallization of sodium chloride. Key issues are the phase behaviour of the ternary amine-water-salt system (liquid-liquid and solid-liquid equilibria) and the size, shape and purity of the NaCl product formed by the antisolvent crystallization with amines. To be able to select a crystallization temperature in the single liquid phase region at a given amine fraction, the liquid-liquid equilibrium lines of the amine-waterNaClsat systems were determined. To establish to what extent an antisolvent reduces the sodium chloride solubility and to calculate the maximum obtainable magma densities during crystallization, sodium chloride solubilities in the amine-water mixtures were measured. Finally continuous crystallization experiments were carried out and the feasibility of an antisolvent recovery by a temperature increase was investigated. Experimental Liquid-liquid Equilibrium Lines of the Amine-Water Systems Saturated with NaCl. The liquid-liquid equilibria experiments were carried out in a jacketed 125 ml glass flask. A magnetic stirrer in combination with a stir bar provided agitation and a Lauda RK-8-KP thermostat was used to control temperature. The temperature of the vessel content was measured with an A S L precision thermometer (0.01 °C accuracy, 0.002 °C repeatability) and a small hole in the vessel cover ensured atmospheric pressure. Figure 3 shows a schematic drawing of the set-up. The experiments were carried out by filling the vessel with known amounts of antisolvent and water, and subsequently by saturating the mixtures with excess sodium chloride at room temperature. The systems were saturated with sodium chloride during the entire experiments. Points on the liquid-liquid equilibrium line were determined in two different ways: either by visual observation of the mixing/demixing temperatures at a given overall amine to water ratio or by determining the compositions of both the organic and the inorganic phases at a fixed temperature in the two liquid phase area. The amine concentrations in the liquid samples were determined with a Chrompack


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Crystallization

S-L separation


NaCl crystals^

mm A

t26%NaCl

Â

L - L separation

liquid anti (as-H 0-NaCl) solvent 2

water

Raw Material Storage

x%NaCl

AS

Figure 1. Simplified process scheme of the antisolvent crystallization of NaCl.

2-liquid phases

crystallization

Fraction Antisolvent Figure 2. Crystallization and antisolvent recovery conditions.


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CP 9002 gas chromatograph equipped with a CP-Sil-5 CB column, the sodium chloride concentrations were determined gravimetrically and the water concentrations were calculated from the determined amine and sodium chloride concentrations. When the compositions of both the organic phase and the water phase change strongly with temperature (the middle part of the phase line), points on the liquidliquid phase line are most reliably determined by using the first method (accuracy 0.05 °C). However, when the mixing/demixing temperature is a very strong function of the amine to water ratio in the mixture (the sides of the phase line), the second method is favoured for maximum reliability. Sodium Chloride Solubilities in Amine-Water Mixtures. The sodium chloride solubilities were determined as a function of the amine concentration at the crystallization temperature in the single liquid phase region. The experiments were carried out in the equilibrium vessel as described in the previous section. The vessel was filled with a saturated sodium chloride solution. At the crystallization temperature a known amount of amine was added, resulting in the crystallization of sodium chloride. After a period of at least 60 minutes the stirring was stopped and liquid samples of the mixture were taken. Conductivity measurements showed that this period of time was sufficient to reach chemical equilibrium. The salt concentrations in the liquid samples were determined gravimetrically. Continuous Crystallization Experiments. Continuous crystallization experiments were carried out at temperatures in the single liquid phase region at several brine to amine feed ratios. The residence time was 30 minutes and each experiment lasted at least 8 residence times. The experiments were conducted in a continuous 1 liter crystallizer (Figure 4). The brine and the antisolvent were stored in two 10 1 vessels positioned on two separate balances. From these tanks both antisolvent and brine were transported towards the crystallizer by two membrane pumps. To ensure constant feed mass flowrates the pump frequencies were controlled by dosage controllers. The feedstreams were continuously introduced into the thermostrated baffled crystallizer (4 baffles) directly above a pitched blade stirrer (N = 750 rpm). With 30 minutes intervals solid samples were taken from the crystallizer outlet, which were washed with ethanol after filtration. The amine uptake of the sodium chloride product was determined with TOC (total organic carbon) and G C , and S E M pictures (scanning electron microscopy) of the crystals were taken. The remaining part of the crystallizer outlet stream was discarded into a waste vessel. During the experiments the crystallizer contents were observed continuously with a CCD-camera, which was mounted on an Optem Zoom 70 microscope. The microscope was positioned in front of a window in the crystallizer, which was lit by a lamp from the inside of the crystallizer. The experimental set-up has been fully automated (Intellution FDÉ/MMI) and can be operated at temperatures of -10 to 200 °C and at pressures up to 20 bars. To prevent corrosion hastelloy-C has been used as the main construction material for the crystallizer and most of the piping. The antisolvent recovery step has not been integrated in this experimental set-up, but was investigated separately as will be described in the next section.



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SEPARATION AND PURIFICATION BY C R Y S T A L L I Z A T I O N

Figure 3. Experimental set-up used for determining the liquid-liquid equilibria.

Figure 4. Schematic overview of the experimental set-up used for the continuous crystallization experiments.


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ZLJLEMA E T A L .


Amine Recovery by a Temperature Induced Liquid-Liquid Phase Split The antisolvent recovery experiments were conducted in the experimental set-up as described in the section 'Liquid-liquid Equilibrium Lines of the Amine-Water Systems Saturated with NaCl', and the same analytical techniques were used for sample composition determination. The experiments were carried out by filling the equilibrium vessel with amine-water mixtures of known compositions. The amine water mixtures were saturated with sodium chloride at the crystallization temperature, so that the mixture compositions equaled the mother liquor compositions in an antisolvent crystallization experiment. Subsequently at several temperatures in the two liquid phase region, samples of both phases were taken in order to determine to what extent the amine separated from the aqueous phase. Results and Discussion Liquid-liquid Equilibrium Lines of the Amirie-Water Systems Saturated with NaCl. The liquid-liquid equilibrium lines of the binary D i P A - H 0 and D M i P A - H 0 systems are reported in the literature (2) (Figure 5). The lower critical solution temperatures (LCST) of the D i P A - H 0 and the D M i P A - H 0 systems were estimated to be 27 °C and 65 °C respectively. Below these temperatures there is one single liquid phase present regardless of the concentration of the amine. The determined two-phase envelopes of the D i P A - H 0 - N a C l and the D M i P A H 0 - N a C l systems (saturated with sodium chloride) are also displayed in Figure 5. The LCST's of the DiPA-H 0-NaCl t and the D M i P A - H 0 - N a C l t systems were estimated to be -11.7'°C and 6.6 °C. The LCST of the D i P A - H 0 - N a C l system deviates from the one presented by Weingàrtner et al. who estimated the LCST at -7.5 °C. The LCST's of the amine-water systems as well as the mutual solubilities of the water and the amines in the two liquid phase area decreased substantially as a result of the presence of sodium chloride. The dissolved sodium chloride decreases the miscibility of the amine-water mixtures, due to the fact that it prefers to be surrounded by water molecules and not by amine molecules. At lower temperatures however, the amines are sufficiently hydrophilic to be fully miscible with water saturated with sodium chloride. With the amine-H 0-NaCl at liquid-liquid equilibrium lines the conditions were selected for the continuous crystallization experiments in the single liquid phase area. The experiments with D M i P A were carried out at XDMÏPA = 0.1, 0.3, 0.6 and 0.9 at a temperature of 5 °C. With DiPA sodium chloride was crystallized at XDJPA = 0.9 and Τ = 1 °C (all the amine concentrations are expressed as salt free weight fractions). 2

2

2

2

2

2

2

sa

2

sa

2

2

S

Sodium Chloride Solubilities in Amine-Water Mixtures. To determine to what extent the amines reduce the sodium chloride solubility and to be able to calculate the maximum obtainable magma densities during crystallization, sodium chloride solubilities in the amine-water mixtures were measured. The sodium chloride solubilities were determined at the crystallization conditions in the single liquid phase area. The solubility diagram of sodium chloride in D M i P A - H 0 at Τ = 5 °C is displayed in Figure 6. At amine fractions above 0.8 the sodium chloride solubility approaches zero for both DiPA and D M i P A . 2


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-20

J

0.0

1

0.2

0.4

0.6

amine fraction (M

aminc

0.8

/(M

aminc

+M

1.0 watcr

))

Figure 5. The two phase envelopes of the binary D1PA-H2O and DM1PA-H2O systems and of the ternary DiPA-H 0-NaCl t and D M i P A - H 0 - N a C l t systems. Below the phase lines there is one single liquid phase and above there are two liquid phases present. 2

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Separation and Purification by Crystallization - American Chemical

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