Phase Equilibria for the Glycine–Methanol–NH4Cl–H2O System

1 Oct 2014 - Department of Mining and Materials Engineering, McGill University, 3610 University ... Yan Zeng , Zhibao Li , and George P. Demopoulos...
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Phase Equilibria for the Glycine−Methanol−NH4Cl−H2O System Yan Zeng and Zhibao Li* Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

George P. Demopoulos Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, Quebec H3A 2B2, Canada S Supporting Information *

ABSTRACT: An investigation of the phase equilibria of the glycine−methanol−NH4Cl−H2O system was carried out with the objective of optimizing the monochloroacetic acid (MCA) process for the production of glycine. Phase equilibrium of the glycine−NH4Cl−H2O system at temperatures over the range of 283.2−353.2 K was determined for concentrations ranging up to the multiple saturation points. The solubilities of both glycine and NH4Cl were found to increase with increasing temperature, as well as with increasing concentration of other solutes. The Bromley−Zemaitis model for ions and the Pitzer formulation for glycine neutral species implemented in the OLI platform were used in the regression of the experimental solubilities. The average absolute deviations between the regressed solubility values and the experimental data were found to be 1.4% for glycine and 0.93% for NH4Cl. Three binary interaction parameters of the Pitzer formulation were newly obtained and coupled with the Bromley−Zemaitis parameters documented in OLI’s databank to predict the multiple saturation points of the system. Additionally, the solubility of glycine in methanol−H2O mixtures was also measured from 283.2 to 323.2 K, and a sharp decline was observed as a function of the content of methanol. Such thermodynamic information is definitely useful for improving the existing industrial process, as well as providing fundamentals for the development of new glycine production processes. containing glycine. Held et al.9 applied the perturbed-chain statistical association theory (PC-SAFT) to describe the solubility and activity coefficient as well as other properties of glycine in water. Khoshkbarchi and Vera10,11 determined the solubility of glycine in aqueous solutions of NaCl and KCl at 298.2 K and developed a perturbed-hard-sphere model for representing the activity coefficients required in the correlation of the solubility data. Gao and Li12 also reported the solubility of glycine in NaCl and KCl solutions, over a wide range of temperatures from 283.2 to 363.2 K and for a full concentration range up to the multiple saturation point were performed. Ferreira et al.13,14 investigated the effects of KCl and Na2SO4 on the solubility of glycine at 298.2 K. Because a large discrepancy was found between their experimental values and those previously published for the glycine−KCl−H2O system, the Pitzer−Simonson−Clegg equations were applied for the correlation of activity coefficients of this ternary system with a satisfactory accuracy, indicating the reliability of their solubility data. The solubility of glycine in (NH4)2SO4 solution at 298.2 and 323.2 K was also measured by Ferreira et al.,15 in which a salting-in effect was found on the solubility of glycine in the presence of (NH4)2SO4. It should be noted that, although the thermodynamic properties including solubility of glycine have

1. INTRODUCTION As the simplest amino acid, a building block for proteins, glycine plays an important role in the food, medical, and chemical industries. It is most popularly used as a food additive or a starting material for the synthesis of medicines and agricultural chemicals. There are two main chemical processes for the production of glycine. One is the Strecker process, in which glycine is synthesized by treating an aldehyde with hydrogen cyanide and ammonia or amines.1 The other is called the MCA process and involves the amination of monochloroacetic acid (MCA) with ammonia.2,3 Although the Strecker process has become the dominant method in some countries, such as the United States and Japan, the latter method using monochloroacetic acid and ammonia as raw materials is still widely utilized in other countries, such as China, which has an annual output of more than 600000 tons. In the MCA process, methanol is commonly added during the purification stage to separate glycine and ammonium chloride through crystallization.4−7 However, the subsequent regeneration of methanol by distillation is energy-intensive and causes serious pollution. It is generally known that knowledge of phase equilibrium is essential to the design and optimization of crystallization processes. To obtain the optimal conditions for glycine crystallization, systematic investigation of the phase equilibrium of the glycine−methanol−NH4Cl−H2O system is needed. The thermodynamic properties of glycine in pure water and electrolyte solutions, such as NaCl, KCl, Na2SO4, and (NH4)2SO4, have been investigated for decades.8−15 In addition to experimental measurements, various models have been used to correlate the experimental data for electrolyte solutions © 2014 American Chemical Society

Received: Revised: Accepted: Published: 16864

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the vessel, and the addition was continued until the last trace of glycine solid was undissolved. When approaching the solubility where the solid dissolved quite slowly in the solution, roughly 0.050 g of glycine was added each time to ensure that the relative uncertainty was less than 0.002. The total mass of glycine added to the vessel before the last addition was established as its solubility under certain conditions. All chemical reagents were prepared by weighing the analytically pure components with an uncertainty of ±0.001 g. When the solubility determination was completed, excess solid glycine was added to the saturated solution at the same temperature as before. After 6 h of equilibration, the solid was filtered, washed, dried, and subjected to X-ray diffraction (XRD) analysis. It should be mentioned that, during the measurement of the solubility of glycine in the methanol−H2O mixtures, an electronic balance with an uncertainty of ±0.1 mg was used, and approximately 0.001 g of glycine was added each time when the studied system was approaching equilibrium. For the measurement of the multiple saturation points of the glycine−NH4Cl−H2O system, glycine and NH4Cl, both in excess amounts relative to those required for saturation, were added to the deoinized water simultaneously. After 12 h of agitation and 6 h of settling at a constant temperature, the clear supernatant solution was withdrawn with a preheated syringe, whereas the solid was filtered rapidly and then dried thoroughly. The dried sample was weighed with an accuracy of 0.001 g and analyzed by XRD. The concentration of NH4Cl in the solution was determined by titration with a standard AgNO3 solution. The concentration of glycine was calculated from the total mass of added reagents, the weight of the dried solid, and the concentration of NH4Cl. Each experiment was replicated three times, and the data reported are the averages of the replicates. Error analysis for the measurements was performed based on calculations of the standard deviation (SD) for each experimental point as

been studied extensively, no data related to glycine in NH4Cl solution have been reported. For the solubilities of glycine in organic solutions, Gekko’s work,16 in which the solubility at 298.2 K was measured, was the first to investigate the solubility behavior of glycine in methanol−water mixtures. Orella and Kirwan17 employed an excess solubility approach together with the Wilson activity coefficient equation to fit Gekko’s solubility data. Ferreira et al.18 represented the solubility data of glycine in aqueous methanol solution at 298.2 K reported by Gekko using the nonrandom two-liquid (NRTL) model. The NRTL parameters of glycine−water and glycine−methanol were obtained through the correlation, whereas other parameters for water−methanol were taken from the DECHEMA Chemistry Data Series.19 Bouchard et al.20 measured the solubilities of α-, β-, and γglycine in methanol aqueous solutions at 310 K. In fact, the majority of the data available related to the glycine−methanol− H2O system are at ambient temperatures. However, the influence of temperature on the solubility of glycine plays an important role in the crystallization-based separation. In the present study, the phase equilibrium of the glycine− NH4Cl−H2O system in the temperature range from 283.2 to 353.2 K and the solubility of glycine in methanol−H2O mixtures from 283.2 to 323.2 K were measured. The Pitzer formulation for neutral species and the Bromley−Zemaitis model embedded in the OLI system were employed to correlate the solubility data obtained in this study for the glycine−NH4Cl−H2O system. With the newly obtained model parameters, multiple saturation points in the glycine−NH4Cl− H2O system were predicted with the aid of OLI Stream Analyzer 9.1.21 The effects of the temperature, the concentration of NH4Cl, and the pH on the distribution of glycine species in unsaturated solutions were analyzed. Interpretation of the separation process in the production of glycine is presented based on the phase diagram of the glycine−NH4Cl− H2O system.

n

∑i = 1 (Xi − M )2

SD =

2. EXPERIMENTAL SECTION 2.1. Chemical Agents. Glycine (99.8% purity, was supplied by Sinopharm Chemical Reagent Co., Ltd. Ammonium chloride (NH4Cl, 99.5% purity) and anhydrous methanol (99.5% purity) were supplied by Xilong Chemical Plant. All chemical reagents were used without further purification. Deionized water with a conductivity of less than 0.1 μS·cm−1 produced in a local laboratory was used in all experiments. 2.2. Experimental Procedure. Solubilities for the glycine− NH4Cl−H2O and glycine−methanol−H2O systems were determined by the dynamic method described in the literature.12,22 Experiments were carried out in a jacketed quartz vessel with a volume of 250 mL that was capped with a stopper and placed on a magnetic stirrer. A circulating water bath was well-controlled and afforded thermostatic water through the jacket of the vessel. All experiments for solubility determinations were performed by the same approach, described here for the solubility of glycine in NH4Cl solution at 298.2 K as a typical example. The deionized water was weighed and introduced into the vessel. The temperature of the thermostatic bath was kept constant at 298.2 ± 0.1 K. A known mass of NH4Cl solid was then dissolved in the water to prepare the NH4Cl solution with a certain concentration in terms of molality. Glycine was then added to the solution with an initial amount equal to 50% of the expected solubility. After it had dissolved completely, a small amount of glycine was added to

n−1

(1)

where Xi represents the individual data point, M is the average value of replicates, and n is the replicate time.

3. THERMODYNAMIC MODELING 3.1. Chemical Equilibrium Relationships in the Glycine−NH4Cl−H2O System. To establish a chemical model for the glycine−NH4Cl−H2O system, various species and their respective dissociation reactions, such as those listed below, must be considered NH 2CH 2COOH(s) = + NH3CH 2COO−

(2)

+

NH3CH 2COO− = NH 2CH 2COO− + H+

(3)

+

NH3CH 2COOH = + NH3CH 2COO− + H+

(4)

NH4Cl(s) = NH4 + + Cl− +

(5)

−

+

where NH3CH2COO , NH3CH2COOH, and NH2CH2COO− are the zwitterion, cation, and anion species of glycine, respectively. The equilibrium constants, denoted as K1−K4 for eqs 2−5, respectively, are given by K1 = a+NH3CH2COO− = γ+NH CH COO−m+NH3CH 2COO− 3

16865

2

(6)

dx.doi.org/10.1021/ie502846m | Ind. Eng. Chem. Res. 2014, 53, 16864−16872

Industrial & Engineering Chemistry Research K2 = =

a NH 2CH 2COO−a H+

=

S P°r , Tr is the standard-state partial molal entropy for the sum at the reference state (298.15 K, 1 bar); T and Tr are the temperature and reference temperature of 298.2 K, respectively; P and Pr are the pressure and reference pressure of 1 bar, respectively; a1−a4 are pressure-independent parameters for aqueous species; c1 and c2 are temperature-independent parameters for aqueous species; Y is the Born function; ω is a species-dependent equation-of-state parameter at temperature T; Ψ and Θ are solvent-dependent parameters equal to 2600 bar and 228 K, respectively, for water; and ε is the dielectric constant of water at temperature T. In this work, the equilibrium constants of +NH3CH2COO− and + NH3CH2COOH (i.e., K2 and K3 in eqs 7 and 8, respectively) were determined by the HKF equation of state. The HKF parameters (a1−a4, c1, c2, and ω) and the thermodynamic property data required for calculating K2 and K3, together with relevant information for other main species in the glycine− NH4Cl−H2O system were taken from the OLI Databank (version 9.1) and are listed in Tables S1 and S2 in the Supporting Information. The second method makes use of an empirical equation of the form

a+NH3CH 2COO− γNH CH COO−m NH 2CH 2COO−γH+m H+ 2

2

γ+NH CH COO−m+NH3CH 2COO− 3

K3 =

Article

2

(7)

a+NH3CH 2COO−a H+ a+NH3CH 2COOH γ+NH CH COO−m+NH3CH 2COO−γH+m H+ 3

2

γ+NH CH COOHm+NH3CH 2COOH

(8)

K4 = a NH4+aCl− = γNH +m NH4+γCl−mCl−

(9)

3

2

4

where a is the activity, γ is the activity coefficient, and m is the concentration in terms of molality. 3.2. Equilibrium Constants. The equilibrium constant was calculated from the standard-state partial molal Gibbs free energy of reaction, ΔR G°, as ln K = −

ΔR G° RT

ΔR G° =

∑ νiΔ Gf° i

(10)

log K = A + (11)

(13)

where T is the temperature in Kelvin and A−D are empirical parameters obtained by fitting to experimental solubility data for a pure salt in water. Equilibrium constants K1 and K4 for NH2CH2COOH(s) and NH4Cl(s) were determined in this way. Parameters in eq 13 for each solid phase were taken from the OLI Databank (version 9.1) and are listed in Table S3 (Supporting Information). 3.3. Activity Coefficients. It is clearly known from eqs 6−9 that the calculation of the solubility of either glycine or NH4Cl is associated with the activity coefficients of relevant ions and neutral aqueous species. Different types of activity coefficient models are embedded in the OLI system, from which the Pitzer formulation for neutral species and the Bromley−Zemaitis model were selected in this work for the representation of solubility in the glycine−NH4Cl−H2O system. In the case of neutral aqueous species, the formulation proposed by Pitzer26 was applied to obtain the activity coefficients

where νi is the stoichiometric coefficient and Δ Gf° is the standard-state partial molal Gibbs free energy of formation of species i. There are two alternative methods in the OLI system that can be used to calculate the equilibrium constant of a reaction. The first method is the Helgeson−Kirkham−Flowers (HKF) equation of state that was originally proposed by Helgeson and co-workers.23−25 It is used to calculate the standard-state partial thermodynamic properties of aqueous species in terms of electrostatic and nonelectrostatic interactions between aqueous species and the solution. The following equation was proposed to calculate the standard partial molal Gibbs free energy of formation of an aqueous species Δ GP°, T = Δ Gf°Pr , Tr − S P°r , Tr(T − Tr) ⎤ ⎡ ⎛T ⎞ − c1⎢T ln⎜ ⎟ − T + Tr ⎥ ⎥⎦ ⎢⎣ ⎝ Tr ⎠ ⎧⎡ ⎪ ⎛ 1 ⎟⎞ ⎛ 1 ⎞⎤⎜⎛ Θ − T ⎟⎞ ⎨⎢⎜ − c 2⎪ −⎜ ⎟⎥ ⎩⎢⎣⎝ T − Θ ⎠ ⎝ Tr − Θ ⎠⎥⎦⎝ Θ ⎠

ln γaqPitzer = 2β0(m−m)mm + 2β1(m−s)ms

(14)

where β0(m−m) and β1(m−s) represent the adjustable parameters as a function of temperature for molecule−molecule interactions and molecule−ion interactions, respectively, and mm and ms are the molalities of neutral species and ions, respectively. β0(m−m) and β1(m−s) are expressed as

⎫ T ⎡ T (T − Θ) ⎤⎪ ⎬ + a1(P − Pr) − 2 ln⎢ r ⎥⎪ ⎣ T (Tr − Θ) ⎦⎭ Θ ⎛Ψ + P⎞ ⎛ P − Pr ⎞ ⎟ + a 2 ln⎜ ⎟ + a3⎜ ⎝T − Θ⎠ ⎝ Ψ + Pr ⎠ ⎛ Ψ + P ⎞⎛ 1 ⎞ ⎛1 ⎞ ⎟ + ω⎜ + a4 ln⎜ − 1⎟ ⎟⎜ ⎝ ⎠ ⎝ ⎠ ε ⎝ Ψ + Pr ⎠ T − Θ ⎛ 1 ⎞ − ω Pr , Tr⎜⎜ − 1⎟⎟ + ω Pr , TrYPr , Tr(T − Tr) ⎝ εPr , Tr ⎠

B + CT + DT 2 T

β0(m−m) = B01ij + B02ij t + B03ij t 2

(15)

β1(m−s) = B11ij + B12ij t + B13ij t 2

(16)

where t is the temperature in degrees Celsius and B01ij, B02ij, B03ij, and so forth are adjustable parameters between species i and j. For the ionic species, the Bromley−Zemaitis activity coefficient model,27 which represents only ion−ion interactions, was used to calculate the activity coefficients. This

(12)

where Δ Gf°Pr , Tr is the standard-state partial molal Gibbs free energy of formation at the reference state (298.15 K, 1 bar); 16866

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Figure 1. Standard deviation of the experimental data in terms of the solubilities (mol·kg−1) of (a) glycine in NH4Cl−H2O solutions, (b) NH4Cl in glycine−H2O solutions, (c) glycine in methanol−H2O solutions, and (d) NH4Cl in NH4Cl−glycine−H2O multisaturated solutions.

4. RESULTS AND DISCUSSION 4.1. Phase Equilibrium in the Glycine−NH4Cl−H2O System. The dynamic method was applied and verified in previous works22 and was also tested for the present investigated systems by comparing the solubility data in pure water obtained in this work with those reported in the literature.11−14,28−30 Prior to the comparison, standard deviations of all of the experimental points were calculated and are plotted in Figure 1. The maximum standard deviation for the solubility of glycine in NH4Cl aqueous solutions is about 0.10 in terms of the molality of glycine, that for the solubility of NH4Cl in glycine aqueous solutions is 0.12 in terms of the molality of NH4Cl, and that for the multiple saturation points is 0.20 on the basis of the molality of NH4Cl. As shown in Figures 2 and 3, which represent the solubilities of glycine and NH4Cl, respectively, in pure water at various temperatures, good agreement was found between the experimental data and the reported values. The error analysis of the measurements and the comparison with literature data indicate the validity and accuracy of this experimental method. The measured solubility data in the temperature range from 283.2 to 353.2 K for the glycine−NH4Cl−H2O system is compiled in Table S4 (Supporting Information) and depicted in Figure 4. As can be seen in Figure 4, the solubility of glycine increases with temperature and also shows an upward trend, at a constant temperature, with increasing concentration of NH4Cl. Similarly, both the temperature and the glycine concentration have increasing effects on the solubility of

model has been successfully applied for electrolytes with concentrations in the range of 0−30 M in the temperature range from 0 to 473.15 K. For the case of cation i in an electrolyte solution, it is given by ⎡ ⎢ (0.06 + 0.6B )|Z Z | −AZi I ij i j = + ∑⎢ + Bij 2 ⎢ 1+ I ⎛ 1.5I ⎞ j ⎜ ⎟ 1 + |Z Z | ⎢ ⎝ i j ⎠ ⎣ 2

log γi B−Z

⎤ ⎥⎛ | Z | + | Z | ⎞ 2 i j 2⎥ + CijI + DijI ⎜ ⎟ mj ⎥⎝ 2 ⎠ ⎥ ⎦

(17)

where j represents all anions in solution; A is the Debye− Hückel parameter; I indicates the ionic strength of the solution; Bij, Cij, and Dij are temperature-dependent coefficients; and Zi and Zj denote the charge numbers of the cation and anion, respectively. Bij is expressed as Bij = B1ij + B2ij t + B3ij t 2

(18)

where t is the temperature in degrees Celsius and B1ij, B2ij, and B3ij are adjustable parameters between species i and j that can be obtained through the regression of experimental data such as solubility. Cij and Dij have similar forms of temperature dependence. 16867

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various concentrations and temperatures calculated by OLI Stream Analyzer 9.1 are also listed in Table S4 (Supporting Information). 4.2. Experimental Solubility of Glycine in Methanol− H2O Mixtures. The solubility of glycine in methanol−H2O mixtures was determined in the temperature range from 283.2 to 323.2 K. Standard deviations for all of the experimental data are depicted in Figure 1. It can be seen that the standard deviations for most of the points are less than 0.02 and that the maximum is about 0.06 on the basis of the molality of glycine in methanol−H2O mixtures. Compared with the data reported by Gekko16 at 298.2 K and Bouchard et al.20 at 310.0 K, the solubilities obtained in this work are consistent with those in the literature (Figure 5). The solubility data for the system Figure 2. Comparison of the experimental and literature data on the solubility of glycine in pure water at different temperatures. Each experimental point is the average of three replicates, and the error bars represent the standard deviations.

Figure 5. Comparison of the experimental and literature data on the solubility of glycine in methanol−H2O mixtures at 298.2 and 310.0 K. Each experimental point is the average of three replicates, and the error bars represent the standard deviations.

throughout the investigated temperature range are reported in Table S5 (Supporting Information) and illustrated in Figure 6. It can be seen from Figure 6 that the solubility of glycine increases with increasing temperature but drops sharply increasing methanol content. When the mass fraction of methanol exceeds 0.8, the solubility of glycine is minimal, with a value of less than 0.1 mol·kg−1.

Figure 3. Comparison of the experimental and literature data on the solubility of NH4Cl in pure water at different temperatures. Each experimental point is the average of three replicates, and the error bars represent the standard deviations.

Figure 4. Experimental and modeling solubility in the glycine− NH4Cl−H2O system from 283.2 to 353.2 K.

NH4Cl throughout the investigated temperature and concentration ranges. Density values of saturated solutions with

Figure 6. Experimental solubility of glycine in methanol−H2O mixtures from 283.2 to 323.2 K. 16868

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Table 1. Newly Regressed Parameters of the Pitzer Formulation for the Glycine−NH4Cl−H2O System parameter

B01 (×10−1)

B02 (×10−3)

B03 (×10−5)

βglycine−glycine βglycine−NH4+

−0.443239 0.254775

0.800564 4.14732

−0.0146738 1.23336

βglycine−Cl−

−1.06411

−3.80270

−1.67816

4.3. Model Parameterization for the Glycine−NH4Cl− H2O System. The Pitzer formulation and the Bromley− Zemaitis model were used to perform the regression for the phase equilibrium of the glycine−NH4Cl−H2O system. Whereas the ion−ion interaction parameters of the Bromley−Zemaitis model were kept the same as those in the OLI’s Public Databank, the molecule−molecule and molecule−ion interaction parameters, namely, βglycine−glycine, βglycine−NH4+, and βglycine−Cl−, in the Pitzer formulation were determined by regression of the experimental solubilities of glycine and NH4Cl in the glycine−NH4Cl−H2O system from 283.2 to 353.2 K. The subscript glycine herein refers to the neutral species of glycine, which is also known as zwitterionic glycine with the formula +NH3CH2COO−. The newly obtained parameters were reported in Table 1, and a comparison between the regressed values and the experimental solubility in the glycine− NH4Cl−H2O system is presented in Figure 4. It is obvious that all of the regressed solubility data are in excellent agreement with the experimental values. The relative deviation (RD) and the average absolute deviation (AAD) were calculated as

RDi =

B11 0.0125444 2.63823 −2.66204

B12 (×10−2)

B13 (×10−4)

−0.0878693 −4.82245

0.0359397 2.23874

4.59530

−2.62980

absolute deviation of 0.93%, indicating that the newly obtained parameters are capable of accurately representing the experimental data. Multiple saturation points in the glycine−NH4Cl−H2O system in the temperature range from 283.2 to 353.2 K were predicted using the newly regressed Pitzer parameters with the aid of OLI Stream Analyzer 9.1. To carry out the prediction, the scaling tendency or supersaturation was calculated as a function of temperature and solution composition. If the scaling tendency (ST) of a solid is less than 1, then the solid is undersaturated; if its ST exceeds 1, then it is supersaturated; and if its ST is equal to 1, then it is at saturation. For the calculations, excess glycine was added, and the solid phase of glycine was fixed at saturation so that its scaling tendency remained constant at 1. At a certain temperature, the scaling tendency of NH4Cl varies with its concentration. The variations of the scaling tendencies of glycine and NH4Cl at 293.2 K are displayed in Figure 8. It can be seen from Figure 8 that the

Xi ,calc − Xi ,exp Xi ,exp n

AAD =

∑ i=1

(19)

Xi ,calc − Xi ,exp Xi ,exp

(20)

where Xi,calc and Xi,exp refer to the calculated and experimental values, respectively, for individual point i and n denotes the number of data points. The results are plotted in Figure 7, from which it can be observed that the relative deviations for the solubility of glycine are within the range from −4% to 5%, with an average absolute deviation of 1.4%, whereas for the solubility of NH4Cl, the RDs range from −2% to 1%, with an average Figure 8. Scaling tendencies of glycine and NH4Cl as functions of the NH4Cl concentration at 293.2 K.

saturation point of NH4Cl occurs at a concentration of 7.9290 mol·kg−1. Using this method, multiple saturation points can be calculated for the glycine−NH4Cl−H2O system over the investigated temperature range. As displayed in Figure 4, the prediction of multiple saturation points agrees well with the experimental data. The deviation results in Figure 9 show that the relative deviation between the predicted and measured values of the solubility of NH4Cl at the multiple saturation points is greatest at 283.2 K (about 12%) but less than ±4% at other temperatures, for which the average absolute deviation is 2.16%. 4.4. Speciation Calculations. There are three relevant species that might exist in aqueous solutions containing glycine, namely, the neutral zwitterionic species +NH3CH2COO−, the cation +NH3CH2COOH, and the anion NH2CH2COO−.31 The effects of temperature, NH4Cl concentration, and pH on the distribution of glycine species were investigated based on the new chemical model established in this work. Figures 10−12

Figure 7. Relative deviation of the calculated values from the new model for solubilities in the glycine−NH4Cl−H2O system. Horizonal reference lines are the average absolute deviations for the solubilities of glycine and NH4Cl. 16869

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almost no influence on the distribution of glycine species in unsaturated solutions containing 2 mol·kg−1, in which the neutral species +NH3CH2COO− is predominant. According to Figure 12, on the other hand, the pH value does have an impact

Figure 9. Relative deviation of the calculated values from the new model for multiple saturation points in the glycine−NH4Cl−H2O system. The horizonal reference line represents the average absolute deviation. Figure 12. Speciation distribution of glycine species, as a function of pH, for unsaturated aqueous solutions containing 2 mol·kg−1 glycine at 298.2 K.

on the speciation distribution of glycine. It can be observed that + NH3CH2COO− is the major species in solutions with pH values between 3 and 10 and nearly the only species for pH values between 4 and 8, in which the cation +NH3CH2COOH and the anion NH2CH2COO− are negligible. Referring to the industrial MCA process for glycine production, the pH is one of the key factors in the ammonolysis reaction, which is typically controlled within the range of 4−8. This choice of pH range has been previously attributed to the prevention of the formation of byproducts.6,32 The results shown in Figure 12 could further explain this optimum pH range from the point of view of the speciation distribution; that is, within the pH range of 4−8, +NH3CH2COO− is the predominant species with insignificant dissociation, and therefore, a high yield of glycine can be obtained. 4.5. Phase Diagram Application for Glycine Production. The equilateral triangle diagram has the advantage of visually representing the equilibrium relationships in ternary systems and is thus utilized commonly in crystallization-based separation processes. Using the newly established chemical model implemented in OLI Stream Analyzer 9.1, an equilateral triangle diagram for the glycine−NH4Cl−H2O system was constructed as shown in Figure 13 with four isothermal lines for temperatures from 293.2 to 353.2 K. Take the isothermal curve ABC at 353.2 K as an example. Curve AB indicates the compositions of saturated ternary solutions that are in equilibrium with solid glycine; curve BC corresponds to those in equilibrium with solid NH4Cl. Point B, which is called the multiple saturation point, represents a solution that is saturated with respect to both glycine and NH4Cl. The lower left-hand area enclosed by AbCB represents homogeneous unsaturated solutions, and the right-hand area aBc represents systems that are in equilibrium with both solids. In the MCA process for the production of glycine, a reaction mixture containing glycine, ammonium chloride, and water is formed initially as an unsaturated solution. Cooling is normally the first step to perform crystallization separation from the liquor. As the specific composition of the reaction products is in

Figure 10. Speciation distribution of glycine species, as a function of temperature from 283.2 to 353.2 K, for unsaturated aqueous solutions containing 2 mol·kg−1 glycine.

represent the speciation distributions of glycine species as functions of these three factors. As can be seen in Figures 10 and 11, the temperature and the concentration of NH4Cl have

Figure 11. Speciation distribution of glycine species, as a function of NH4Cl concentration, for unsaturated aqueous solutions containing 2 mol·kg−1 glycine at 298.2 K. 16870

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Stream Analyzer 9.1, the prediction of multiple saturation points in the glycine−NH4Cl−H2O system was carried out in good agreement with the experimental values. The speciation distribution of glycine species was also determined considering effects of temperature, NH4Cl concentration, and pH. All of the results generated by solubility determination and modeling indicate that cooling alone cannot provide pure glycine from NH4Cl-containing reaction mixtures. Use of methanol, although a feasible method for achieving the separation target, entails intractable economic and environmental problems, thus making it necessary to optimize the existing process and develop more practical processes as well.

■

ASSOCIATED CONTENT

S Supporting Information *

HKF parameters for the main aqueous species in the glycine− NH4Cl−H2O system (Table S1), thermodynamic data of the related species (Table S2), empirical parameters of eq 13 (Table S3), experimental solubility for the glycine−NH4Cl− H2O system (Table S4), and experimental solubility of glycine in methanol−water mixtures (Table S5). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 13. Phase diagram of the glycine−NH4Cl−H2O system from 293.2 to 353.2 K.

accordance with the conditions under which the reaction has been carried out, a typical case is chosen here to interpret the effect of cooling on such a system. As shown in Figure 13, point x refers to the composition (25% NH4Cl, 33% glycine, 42% H2O) of a certain system that is unsaturated at 353.2 K. Pure solid glycine would crystallize out at about 343.2 K if the temperature were reduced from 353.2 K. Further cooling would cause the deposition of more pure glycine and allow the composition of the solution change, following line ax. When the solution concentration reached point y, which refers to the intersection of line ax with the mixed-solid line BD, solid NH4Cl would start crystallizing out together with glycine. The solution composition would thereafter vary along line BD with any further decrease in temperature. The final result of this cooling step would be a glycine−NH4Cl mixture containing a small amount of water, indicating that only water had been isolated from the ternary system. Therefore, to achieve an efficient separation from which a high yield of pure glycine can be obtained, other measures must be taken. In the existing industrial process, methanol is usually used to separate glycine and NH4Cl. This is because NH4Cl has a much higher solubility than glycine in aqueous methanol solutions. As shown in Figure 6, glycine is nearly insoluble in solutions containing more than 0.8 mass fraction of methanol and can thus be isolated from such mixtures. The solid glycine obtained from filtration is then subjected to recrystallization. The filtrate is distilled for the recovery of methanol, and NH4Cl can be crystallized out as a solid. Although this purification process using methanol has its advantages, it should be noted that distillation is energy-intensive and the large amount of wastewater generated in the overall process will cause severe pollution. In view of such problems, more research on both the optimization of this process and the development of other methods for the separation of glycine should be done as soon as possible.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax.: + 86 10 62551557. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully thank the National Natural Science Foundation of China (Grants 21476235 and 21206165) and the National Basic Research Development Program of China (973 Program with Grant 2013CB632605) for financial support of this work.

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5. CONCLUSIONS The Bromley−Zemaitis model for ions and the Pitzer formulation for neutral species were used and found to be applicable for the representation of the solubility data determined in this work for the glycine−NH4Cl−H2O system. With the newly obtained Pitzer parameters inserted in OLI 16871

NOMENCLATURE a = activity A = Debye−Hückel parameter, empirical parameter of log K a1, a2, a3, a4 = pressure-dependent terms of the HKF equation of state B = empirical parameter of log K, parameter of the Pitzer equation, parameter of the Bromley−Zemaitis equation c1, c2 = temperature-dependent terms of the HKF equation of state C = empirical parameter of log K, parameter of the Bromley−Zamaitis equation D = empirical parameter of log K, parameter of the Bromley−Zamaitis equation Δ Gf° = standard-state partial molal Gibbs free energy of formation, J·mol−1 Δ GP°, T = standard-state partial molal Gibbs free energy of formation, J·mol−1 Δ Gf°Pr , Tr = standard-state partial molal Gibbs free energy of formation at the reference state (298.15 K, 1 bar), J·mol−1 I = ionic strength K = dissociation or dissolution equilibrium constant m = molality of a species, mol·kg−1 M = molarity of a species, mol·L−1 P = pressure, Pa dx.doi.org/10.1021/ie502846m | Ind. Eng. Chem. Res. 2014, 53, 16864−16872

Industrial & Engineering Chemistry Research

Article

(18) Ferreira, L. A.; Macedo, E. A.; Pinho, S. P. Solubility of amino acids and diglycine in aqueous−alkanol solutions. Chem. Eng. Sci. 2004, 59, 3117−3124. (19) Gmehling, J.; Onken, U.; Arlt, W. Vapor−Liquid Equilibrium Data Collection, Aqueous−Organic Systems; Chemistry Data Series; DECHEMA: Frankfurt, Germany, 1981; Supplement 1, Vol. 1, Part 1a. (20) Bouchard, A.; Hofland, G. W.; Witkamp, G. Solubility of glycine polymorphs and recrystallization of β-glycine. J. Chem. Eng. Data 2007, 52, 1626−1629. (21) OLI Analyzer Studio: Stream Analyzer, version 9.1.2014; OLI Systems, Inc.: Cedar Knolls, NJ, 2014; available at http://www. olisystems.com/new-streamanalyzer.shtml (accessed Oct 15, 2014). (22) Zeng, Y.; Zhang, Y.; Li, Z. B.; Demopoulos, G. P. Measurement and chemical modeling of the solubility of Na2SiO3·9H2O and Na2SiO3 in concentrated NaOH solution from 288 to 353 K. Ind. Eng. Chem. Res. 2014, 53, 9949−9958. (23) Helgeson, H. C.; Kirkham, D. H.; Flowers, G. C. Theoretical prediction of the thermodynamic behavior of aqueous electrolytes at high pressures and temperatures. IV. Calculation of activity coefficients, osmotic coefficients, and apparent molar and standard and relative partial molar properties to 600 °C and 5 kb. Am. J. Sci. 1981, 281, 1249−1516. (24) Tanger, J. C.; Helgeson, H. C. Calculation of thermodynamic and transport properties of aqueous species at high pressures and temperatures; Revised equations of state for the standard partial molal properties of ions and electrolytes. Am. J. Sci. 1988, 288, 19−98. (25) Shock, E. L.; Helgeson, H. C. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Correlation algorithms for ionic species and equation of state predictions to 5 kb and 1000 °C. Geochim. Cosmochim. Acta 1988, 52, 2009−2036. (26) Aqueous System Modeling Course and Workshop, OLI’s Manual; OLI Systems, Inc.: Cedar Knolls, NJ, 2002. (27) Zemaitis, J. F.; Clark, D. M.; Rafal, M.; Scrivner, N. C. Handbook of Aqueous Electrolyte Thermodynamics: Theory & Application; AIChE: New York, 1986. (28) Wang, D. G.; Li, Z. B. Modeling solid−liquid equilibrium of NH4Cl−MgCl2−H2O system and its application to recovery of NH4Cl in MgO production. AIChE J. 2011, 57, 1595−1606. (29) Seidell, A. Solubilities of Inorganic and Metal Organic Compounds; A Complication of Quantitative Aolubility Data from the Periodical Literature; Van Nostrand: New York, 1940. (30) Ji, X. Y.; Lu, X. H.; Zhang, L. Z.; Bao, N. Z.; Wang, Y. R.; Shi, J.; Lu, B. C. -Y. A further study of solid−liquid equilibrium for the NaCl− NH4Cl−H2O system. Chem. Eng. Sci. 2000, 55, 4993−5001. (31) Figueiredo, B. R.; Silva, F. A. D.; Silva, C. M. Non-ideality and solubility modeling of amino acids and peptides in aqueous solutions: New physical and chemical approach. Ind. Eng. Chem. Res. 2013, 52, 16044−16056. (32) Couriol, C.; Fauduet, H.; Porte, C.; Delacroix, A. Setup of glycine continuous synthesis by ammonolysis of monochloroacetic acid. Lab. Rob. Autom. 1999, 11, 29−35.

S P°r , Tr = standard-state partial molal entropy for the sum at the reference state (298.15 K, 1 bar), J·mol·K−1 t = absolute temperature, °C T = absolute temperature, K Y = Born function Z = cation or anion charge β = parameter of the Pitzer equation γ = compound activity coefficient ε = dielectric constant of water at temperature T Θ = solvent-dependent parameter equal to 228 K for water ν = stoichiometric coefficient Ψ = solvent-dependent parameter equal to 2600 bar for water ω = species-dependent equation-of-state parameter at temperature T

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REFERENCES

(1) Strecker, A. The artificial synthesis of lactic acid and a new homologue of glycine. Liebigs Ann. Chem. 1850, 75, 27−45. (2) Robertson, G. R. The reaction of chloro-acetic acid with ammonia and the preparation of glycine. J. Am. Chem. Soc. 1927, 49, 2889−2894. (3) Block, R. J. The isolation and synthesis of the naturally occurring α-amino acids. Chem. Rev. 1946, 38, 501−571. (4) Orten, J. M.; Hill, R. M. A simple method for the preparation of glycine. J. Am. Chem. Soc. 1931, 53, 2797−2799. (5) Tobie, W. C.; Ayres, G. B. An improved procedure for the preparation of glycine. J. Am. Chem. Soc. 1942, 64, 725−725. (6) Guinot, H. M. Preparation of amino acids. U.S. Patent 3,215,736, Nov 2, 1965. (7) Colburn, C. S. Method of making glycine. U.S. Patent 3,510,515, May 5, 1970. (8) Dalton, J. B.; Schmidt, C. L. A. The solubilities of certain amino acids in water, the densities of their solutions at twenty-five degrees, and the calculated heats of solution and partial molal volumes. J. Biol. Chem. 1933, 103, 549−578. (9) Held, C.; Cameretti, L. F.; Sadowski, G. Measuring and modeling activity coefficients in aqueous amino-acid solutions. Ind. Eng. Chem. Res. 2011, 50, 131−141. (10) Khoshkbarchi, M. K.; Vera, J. H. Measurement of activity coefficients of amino acids in aqueous electrolyte solutions: Experimental data for the systems H2O + NaCl + glycine and H2O + NaCl + DL-Alanine at 25 °C. Ind. Eng. Chem. Res. 1996, 35, 2735− 2742. (11) Khoshkbarchi, M. K.; Vera, J. H. Effect of NaCl and KCl on the solubility of amino acids in aqueous solutions at 298.2 K: Measurements and modeling. Ind. Eng. Chem. Res. 1997, 36, 2445− 2451. (12) Gao, W. C.; Li, Z. B. Determination and chemical modeling of phase equilibria for the glycine−KCl−NaCl−H2O system and its application to produce crystals with anticaking characteristics. Ind. Eng. Chem. Res. 2012, 51, 8315−8325. (13) Ferreira, L. A.; Macedo, E. A.; Pinho, S. P. Effect of KCl and Na2SO4 on the solubility of glycine and DL-alanine in water at 298.15 K. Ind. Eng. Chem. Res. 2005, 44, 8892−8898. (14) Ferreira, L. A.; Macedo, E. A.; Pinho, S. P. KCl effect on the solubility of five different amino acids in water. Fluid Phase Equilib. 2007, 255, 131−137. (15) Ferreira, L. A.; Macedo, E. A.; Pinho, S. P. The effect of ammonium sulfate on the solubility of amino acids in water at (298.15 and 323.15) K. J. Chem. Thermodyn. 2009, 41, 193−196. (16) Gekko, K. Mechanism of polyol-induced protein stabilization: Solubility of amino acids and diglycine in aqueous polyol solutions. J. Biochem. 1981, 90, 1633−1641. (17) Orella, C. J.; Kirwan, D. J. Correlation of amino acid solubilities in aqueous aliphatic alcohol solutions. Ind. Eng. Chem. Res. 1991, 30, 1040−1045. 16872

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