Simulation on thermodynamic state of ammonia carbonation at low temperature and low pressure

doi:10.1007/s11705-013-1370-1

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Jingcai ZHAO, Xingfu SONG, Ze SUN, Jianguo YU. Simulation on thermodynamic state of ammonia carbonation at low temperature and low pressure. Frontiers of Chemical Science and Engineering, 2013, 7(4): 447-455 Copy to clipboard

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Simulation on thermodynamic state of ammonia carbonation at low temperature and low pressure

Jingcai ZHAO, Xingfu SONG, Ze SUN, Jianguo YU

National Engineering Research Center for Integrated Utilization of Salt Lake Resource, East China University of Science and Technology, Shanghai 200237, China

xfsong@ecust.edu.cn

Abstract

This study on thermodynamic property of NH₃-CO₂-H₂O system provided the basic data for ammonia carbonation. Simulations on vapor-liquid equilibrium (VLE) of ammonia carbonation with different physical properties were discussed in NH₃-H₂O and NH₃-CO₂-H₂O systems, respectively. The results indicated that at low temperature (303.15 K–363.15 K) and pressure (0.1–0.4 MPa), the PR (Peng-Robinson) equation was suitable for the description of the thermodynamic state in NH₃-H₂O system. NRTL (Non-Random-Two-Liquid) series models were selected for NH₃-CO₂-H₂O mixed electrolyte solution system. VLE data regression results showed that NRTL series models were suitable for describing thermodynamic properties of NH₃-CO₂-H₂O system, because average relative error fitting with each model was about 1%. As an asymmetric electrolytes model in NRTL model, E–NRTLRK (Electrolyte NRTL Redlich Kwong) could most accurately fit VLE data of NH₃-CO₂-H₂O system, with fitting error less than 1%. In the extent temperature range of 273.15 K–363.15 K, the prediction of product component using E-NRTLRK model for ammonia carbonation agreed well with the data reported in literature.

Keyword: vapor-liquid equilibrium; activity coefficient; carbon dioxide; ammonia; NRTL

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Introduction

A new energy-saving process, in which waste calcium sulfate reacted with ammonia carbonized product at low temperature of 273.15-363.15 K and low pressure of 0.1-0.4 MPa, has been proposed for comprehensive utilization of sulfur resources, energy conservation and low-carbon environment [ 1, 2]. Therefore, ammonia carbonation process plays an important role in the conversion of calcium sulfate, and investigation on thermodynamic properties of ammonia carbonation process would provide basic data for further study of the conversion of calcium sulfate.

Ammonia carbonation process can be divided into two steps: (1) the absorption of ammonia into ammonia solution and (2) carbon dioxide absorption by ammonia solution. Over the past sixty years, vapor-liquid equilibrium (VLE) experimental data of NH₃-H₂O [ 3] and NH₃-CO₂-H₂O have been reported, and these data were measured in urea, ammonium bicarbonate, soda ash production and flue gas absorption process [ 4, 5, 6]. However, ammonia carbonation is quite sensitive to process condition. Early data were measured using several points at a narrow scope, and there is not enough data of VLE in ammonia carbonation. A number of effort have focused on applying thermodynamic model to describe and predict [ 7, 8] the thermodynamic state of ammonia solution system by simulation [ 9, 10, 11, 12].

Several simulation VLE data about NH₃-H₂O and NH₃-CO₂-H₂O at low temperature and low pressure have been reported since the last century. Recently ZENG et al. [ 13] have reported that in the strong polar solvent system of NH₃-H₂O, with 16 thermodynamics models in common use, fitting error are all more obvious in vapor phase than in liquid phase. Smolen et al [ 14] have used a modified cubic equation of state and different mixture parameters in vapor and liquid phase in the NH₃-H₂O system, and the experimental VLE data have been converted to T-P-X data with calculation deviation less than 8%. In NH₃-CO₂-H₂O system, molecule-molecule, ion-molecule and ion-ion interactions have been incorporated into early models to describe the VLE state of the multi-step chemical equilibriums system, with numerous complex parameters in the model [ 15, 16, 17, 18]. A state equation based on perturbation theory [ 19] has been used to simplify the model parameters with similar related results. Generalization relational models based on various modifications of the Pitzer equation [ 20] have reduced the number of parameters in the new equation for (exceed Gibbs free energy) electrolyte solutions. Artificial Neural Network (ANN) model [ 21] has been applied to estimate VLE data for ternary system of NH₃-CO₂-H₂O. The comparison of prediction with the experimental data indicated that Multilayer Perception (MLP) and Radial Basis Function (RBF) models predict the system better than the thermodynamic models.

UNIQUAC (Universal Quasi Chemical Activity Coefficient) series models initially proposed by Thomsen, K and Rasmussen predicted VLE data in NH₃-CO₂-H₂O system withfitting accuracy in vapor phase much lower than in liquid [ 22]. Total average deviation with original model in NH₃-CO₂-H₂O system in vapor phase was around 6% compared with original experimental data [ 23], whereas average deviation of ammonia in vapor phase was 4.72% [ 24]. Zhang et al [ 25] has regressed the VLE data of NH₃-CO₂-H₂O at 303.15 K-363.15 K and 0.1-0.4 MPa with adjusted UNIQUAC model, and found that average relative fitting errors in vapor and liquid were about 1%. Electrolyte activity coefficient series model has been reported to be suitable for mixed electrolyte solution system [ 26, 27]. Que et al. [ 28] has regressed VLE data reported in ammonia carbonation process with E-NRTL model, with 1%-10% fitting error of ammonia in vapor phase.

Although thermodynamics of NH₃-CO₂-H₂O system at high temperature and high pressure has been numerously reported, little VLE data at low temperature and pressure has been reported. Several models have been proposed to describe the thermodynamics state of NH₃-CO₂-H₂O system but simulation accuracy still needs to be improved. Moreover, few reports are available about the prediction of ammonia carbonation product related to the conversion of calcium sulfate at low temperature and pressure. The aim of this paper is to provide a suitable thermodynamics model with basic parameters in detail to obtain a high degree of accuracy for the description of ammonia carbonation at low temperature and low pressure, and further extend temperature scope to 273.15 K-363.15 K to apply it in the ammonia carbonation process for product prediction.

Calculation

Model description

VLE model theory includes state equation, activity coefficient, law of corresponding state, perturbation theory and polynomial function [ 12]. Thermodynamics physic methods in common use are state equation and activity coefficient models.

A state equation describes the pressure, volume and temperature (p, V, T) behavior of a pure component or a mixture, assuming that the difference between a liquid and an ideal gas is much larger than that between a real gas and an ideal gas. The assumptions in RKS (Redlich-Kwong-Soave), PR (PENG-ROB), and LKP(Lee-Kesler-Plöcker) usually do not apply to highly non-ideal chemical systems. Activity coefficient models includes molecular models such as NRTL (non random two liquid), group contribution models such as UNIFAC (universal functional activity coefficient) and UNIQUAC (universal quasi-chemical), and electrolyte activity coefficient models such as E-NRTL (electrolyte NRTL) series extended models.

The activity coefficient method is the best way to represent a highly non-ideal liquid mixture at low pressure (below 10 atm). Binary parameters can be estimated from experimental data, such as phase equilibrium data, and are valid only in the temperature and pressure ranges of the data.

For activity coefficient models, the NRTL model can not describe liquid-liquid separation at all, whereas UNIQUAC, UNIFAC and E-NRTL can suitably consider interaction and binary interaction parameters. NRTL series equations can be applied to polar and non-polar compounds, strong non-ideal mixture and partially miscible system. Without special instructions, NRTL can not be applied in electrolyte system, except for E-NRTL series models proposed for electrolyte solution.

Expression of E-NRTL equation includes two contributions: one is Pitzer-Debye-Hückel activity coefficient terms of long-range electrostatic interactions, and another is the NRTL local composition model contributed by short range interaction of ionic bonds [ 29, 30]. Pitzer-Debye-Hückel activity coefficient term can only be decided by the system temperature and concentration that mainly expressed by asymmetric activity coefficient of ions, and short-range local contribution are expressed by characteristic parameters of the solution [ 31]. E-NRTLRK (electrolyte non random two liquid Redlich Kwong) is an extended asymmetric E-NRTL model that is suitable for mixed multivariate electrolyte solution system. The E-NRTLRK method is identical to ELECNRTL (electrolytic NRTL model) for systems containing a single electrolyte. However, for mixed electrolyte systems, the E-NRTLRK method uses the mixing rules only to calculate interaction parameters.

Binary parameters calculation

NH₃-H₂O is a strong polar system. To determine the proper thermodynamics model in low temperature range of 303.15 K-363.15 K and low pressure range of 0-0.4 MPa, the obtained binary parameters in the model is a main objective in the research. Here Peng-Robinson equation (PR equation) is introduced in detail as an example.

The PR equation is a mathematical expression for the relation of temperature T, pressure P, and molar volume V of a pure fluid, as shown in Eq. (1):

P = \frac{R T}{V - b} - \frac{a}{V (V + b)+ b (V - b)} .

(1)

In the PR equation, mixed equation coefficients are important parameters, as showed in Eqs. (2)-(4):

a_{m} = \sum_{i = 1}^{2} \sum_{j = 1}^{2} x_{i} x_{j} a_{i j},

(2)

a_{i j} = (1 - k_{i j}) a_{i}^{1 / 2} a_{j}^{1 / 2},

(3)

b_{m} = \sum_{i = 1}^{2} x_{i} b_{i},

(4)

where a_m is a temperature-dependent energy parameter and b_m is a co-volume parameter [ 32]. Volume parameter b_m can be estimated from the mole fraction weighted average of pure component contributions. Taking the interaction between molecules and non-ideal characteristics into account, binary interaction coefficients k_ij, a_ij were introduced as cross terms and key parameters in the PR equation [ 33].

On the basic simulation in NH₃-H₂O binary system, NRTL series model is considered for NH₃-CO₂-H₂O multicomponent system, in which there are molecular parts including NH₃, CO₂ and H₂O, and ion parts including $N H_{4}^{+}$ , $C O_{3}^{2 -}$ , $H C O_{3}^{-}$ , etc.

Activity coefficient in E-NRTL equation can be expressed in the following equation:

\ln γ_{i}^{*} = \ln γ_{i}^{* P D H} + \ln γ_{i}^{* N R T L}

(5)

Where

γ_{i}^{*}

is asymmetric activity coefficient of ion i. Pitzer-Debye-Hückel activity coefficient term can only be decided by the system temperature and concentration.

Each type of electrolyte NRTL parameter consists of both the nonrandomness factor, a, and energy parameters, t. Activity coefficient in NRTL term can be expressed in Eq. (6):

\ln γ_{a}^{* N R T L} = \frac{τ_{c a, m} x_{m}^{2} G_{c a, m}}{{(x_{a} G_{c a, m} + x_{c} G_{c a, m} + x_{m})}^{2}} + \frac{τ_{m, c a} Z_{a} x_{m} G_{m, c a}}{x_{c} + x_{m} G_{m, c a}} - \frac{τ_{m, c a} Z_{c} x_{c} x_{m}^{2} G_{m, c a}}{{(x_{m} G_{m, c a} + x_{a})}^{2}} - τ_{c a, m} G_{c a, m} - τ_{m, c a} Z_{a},

(6)

G_{a m} = G_{c m} \exp (- α τ_{c a, m}),

(7)

G_{m, a c} = G_{m a, c a} = \exp (- α τ_{m, c a}),

(8)

where i, j, k represent different species, c is short for cation, a for anion, m for solvent, and G_ji, G_ji,ki are energy interaction parameters of solution.

τ_{c a, m} = τ_{c m} = \frac{g_{c m} - g_{m m}}{R T} = τ_{a m} = \frac{g_{a m} - g_{m m}}{R T},

(9)

τ_{m, c a} = τ_{m a, c a} = \frac{g_{m a} - g_{m m}}{R T} = τ_{m c, a c} = \frac{g_{m c} - g_{a c}}{R T},

(10)

where t_ij is equation parameter in NRTL term, representing interaction characteristic of different species, and a_ij, g_ij are characteristic parameters of the solution in short-range local contribution of NRTL activity coefficients that need to be known.

Results and discussion

Model determination in NH₃-H₂O binary system

Temperature scope in this study is in the range of 303.15 K to 363.15 K for NH₃-H₂O binary system. VLE data published [ 34] at 305 K and 340 K has been selected for model validation. Evaluation criteria have been defined and objective functions are in Eqs. (11) - (13). RSS_l and RSS_v are absolute sum of squared residuals in liquid and vapor phases, respectively. s is relative squared residuals sum of gas plus liquid phases, x_i, y_i represent calculation data in liquid and vapor phases, respectively; while x_c, y_c represent experimental data in liquid and vapor phases, respectively.

R S S_{l} = \sum_{i} {(x_{i} - x_{c})}^{2},

(11)

R S S_{v} = \sum_{i} {(y_{i} - y_{c})}^{2},

(12)

s = {\sum_{i} [(x_{i} - x_{c}) / x_{c}]}^{2} + {\sum_{i} [(y_{i} - y_{c}) / y_{c}]}^{2} .

(13)

Commonly used models at low pressure are state equation models such as LK-PLOCK, PR, RKS and PSRK, and activity coefficient models such as NRTL, NRTL-RK, UNIQUAC, UNIFAC, E-NTRL, they all have been selected for the fitting of VLE data.

Figure 1 shows P-X-Y VLE image of ammonia solution regressing with PR, PSRK, LKP, SRK and RKS, respectively. At constant temperature and pressure, molar fraction of ammonia is higher in liquid phase than in vapor phase. When ammonia molar fraction is lower than 50%, simulation result by PR equation agrees best with experimental data. However, when ammonia molar fraction is higher than 50%, model simulation errors are obvious. Bubble point line calculation mainly contributes to the calculation deviation.

Table 1 is the accurate evaluation value of the fitting result. As shown in Table 1, fitting errors with selected state equations are all lower than those reported in literature [ 34]. Moreover, fitting accuracy in vapor phase is higher than that in liquid phase with any of models in this study, because ammonia system is a highly non-ideal system, and interaction of association and polarization between ammonia molecular and water leads to a serious deviation of ammonia solution from ideal state.

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	Fig.1 NH₃-H₂O VLE data regression with different models

Tab.1 Regression result in NH₃-H₂O

Among the methods of LKP, PR, RKS, SRK and PSRK for state equations, properties decreases in the order PR>LK-PLOCK>PSRK>SRK>RKS. Of all the models, PSRK and LK-PLOCK models apply to light gas system with higher temperature and pressure. Predictive equations of state of PSRK are mainly used at high pressure, and it is quite suitable at low temperature and low pressure. The assumptions in the simpler equations of state (RKS and LKP) do not apply to highly non-ideal chemical systems. Ammonia solution system is a strong polar system, so obvious simulation deviations exist in ammonia solution system at 303.15 K-363.15 K and 0.1-0.4 mol∙L^-1.

The PR equation has a wide range for nonpolar or light polar system in all range of temperature and pressure, and also has a good practicability in some strongly polar system such as NH₃-H₂O system. Table 1 shows that the PR equation has the smallest fitting error in the models selected, and also has a much higher fitting accuracy than that reported in literatures. PR equation parameters for NH₃-H₂O are listed in Table 2.

Tab.2 PR equation parameters for NH₃-H₂O system

In summary, the PR equation is better for describing the thermodynamic properties of NH₃-H₂O system due to its high fitting accuracy.

Model determination in NH₃-CO₂-H₂O multicomponent system

On the basis of NH₃-H₂O binary system, phase equilibrium data have been fitted in NH₃-CO₂-H₂O ternary system [ 25]. As a mixed multicomponent system, NH₃-CO₂-H₂O system contains a variety of anions and cations, polar molecules (NH₃, H₂O) and non-polar molecules (CO₂) that are far from ideal state. To predict the nature of multicomponent system solution, VLE data of NH₃-CO₂-H₂O system at temperature range of 303.15-363.15 K has been regressed by NRTL series equations. Evaluation criteria in gas and liquid phases have been defined in formula (14) and (15). s_l and s_v are absolute sum of relative error in liquid and vapor phases, respectively. s is relative squared residuals sum of gas plus liquid phases, x_i, y_i represent calculation data in liquid and vapor phases, respectively, and x_c, y_c represent experimental data in liquid and vapor phases, respectively.

s_{l} = \sum_{i} ((a b s (x_{i} - x_{c}) / x_{i}) / n),

(14)

s_{v} = \sum_{i} ((a b s (y_{i} - y_{c}) / y_{i}) / n) .

(15)

At 303.15 K-363.15 K and 0.1-0.4 MPa, fitting results of NH₃-CO₂-H₂O system by NRTL series models are shown in Table 3. It can be seen that fitting results by NRTL, NRTL-RK and E-NRTL models are almost the same. Ammonia is the main solute in NH₃-CO₂-H₂O system, and thus ammonia calculation errors are paid more attention in the study. Ammonia calculation errors using electrolyte activity models are obviously smaller than those using ordinary NRTL models. E-NRTLRK even has the smallest fitting error among the models mentioned above as well as models reported in literatures, especially for vapor-phase system. s_l calculated by E-NRTRK is 0.79% in this study while it is 0.84% in the literature. s_v calculated by E-NRTRK is 0.72% while it is 1.03% in the literature. Therefore, asymmetric electrolyte activity model has a higher prediction accuracy in the description of NH₃-CO₂-H₂O system.

Tab.3 NH₃-CO₂-H₂O VLE regression by NRTL series models

Regression results of the experimental data by E-NRTRK model are shown in Table 4. From Table 4, it can be seen that when the temperature increases from 303.15 K to 363.15 K and the pressure increases from 0.101 MPa to 0.39 MPa, calculation results are closer to experimental data [ 25] in liquid phase for NH₃, CO₂ and H₂O. Ammonia is the main solute and main component in liquid and vapor phase, respectively. With the temperature and pressure increasing, relative calculation deviations are obvious except for NH₃. With the ascending of temperature and pressure, interaction between molecules is rising because of the increasing of molecular movement and collision, and this interaction is more obvious in the vapor phase than that in liquid. Therefore, calculation errors are larger in vapor phase than in liquid phase in the whole.

Tab.4 NH₃-CO₂-H₂O VLE regression by E-NRTLRK

From Table 5, binary interaction parameter of E-NRTLRK can be seen as AIJ, AJI, BIJ, BJI and CIJ. AIJ, AJI, BIJ and BJI are temperature-dependent coefficient that is related to energy parameter. AIJ and AJI are constant item, whereas BIJ and BJI are coefficient for the reciprocal of temperature. From Table 5, it can be seen that binary interaction between NH₃ and CO₂ is easier affected by temperature in the NH₃-CO₂-H₂O system than in the NH₃-H₂O system. CIJ is a nonrandomness factor, and usually considered as temperature-independent. In a non-polar or weak polar system, CIJ is commonly defined as 0.2 while in polar ion system, CIJ is defined as 0.3. Repeated verification indicates that in the NH₃-CO₂-H₂O system, the nonrandomness factor of molecular interaction should be 0.2, and the polarity of molecule-molecule is ignored while the polarity of ion-molecule is obvious and should be considered in the calculation.

Tab.5 Binary interaction parameters of E-NRTLRK in the NH₃-CO₂-H₂O system

Component i	NH₃	NH₃	CO₂	H₂O	H₂O	H₂O	H₂O
Component j	CO₂	H₂O	H₂O	$N H_{4}^{+}$	$N H_{2} C O O^{-}$	$H C O_{3}^{-}$	$C O_{3}^{2 -}$
Temperature units	°C	°C	°C	°C	°C	°C	°C
AIJ	33.216	11.480	-15.720	0	0	0	0
AJI	22.393	-11.739	37.7014	0	0	0	0
BIJ	-14913.300	-4198.140	4945.892	0	0	0	0
BJI	-6475.170	4354.897	-11840.100	0	0	0	0
CIJ	0.2	0.2	0.2	0.3	0.3	0.3	0.3

Tab.5 Binary interaction parameters of E-NRTLRK in the NH₃-CO₂-H₂O system

Binary interaction parameters and interaction energy parameters are the main parameters in E-NRTLRK model. Interaction energy parameters (GMELCC) have been listed in Table 6. For the electrolyte-electrolyte pair parameters, the two electrolytes must share either one cation or one anion that can be seen in Table 6. Interaction energy parameters can be calculated with Eq. (9) and Eq. (10).

Tab.6 Parameters of electrolyte pairs (GMELCC)

Molecule i or Electrolyte i	Molecule j or Electrolyte j	Value
H₂O		$H_{3} O^{+}$	$N H_{2} C O O^{-}$	8.045
H₂O		$H_{3} O^{+}$	OH^-	8.045
H₂O			$H C O_{3}^{-}$	8.045
H₂O			$C O_{3}^{2 -}$	8.045
H₂O		$N H_{4}^{+}$	$N H_{2} C O O^{-}$	4.669
H₂O		$N H_{4}^{+}$	OH^-	8.045
H₂O		$N H_{4}^{+}$	$H C O_{3}^{-}$	4.654
H₂O		$N H_{4}^{+}$	$C O_{3}^{2 -}$	8.045
$H_{3} O^{+}$	$N H_{2} C O O^{-}$	H₂O		-4.072
$H_{3} O^{+}$	OH^-	H₂O		-4.072
$H_{3} O^{+}$	$H C O_{3}^{-}$	H₂O		-4.072
$H_{3} O^{+}$	$C O_{3}^{2 -}$	H₂O		-4.072
$N H_{4}^{+}$	$N H_{2} C O O^{-}$	H₂O		-2.929
$N H_{4}^{+}$	OH^-	H₂O		-4.072
$N H_{4}^{+}$	$H C O_{3}^{-}$	H₂O		-1.761
$N H_{4}^{+}$	$C O_{3}^{2 -}$	H₂O		-4.072
NH₃		$N H_{4}^{+}$	$N H_{2} C O O^{-}$	10
$N H_{4}^{+}$	$N H_{2} C O O^{-}$	NH₃		-2
NH₃		$N H_{4}^{+}$	$H C O_{3}^{-}$	10
$N H_{4}^{+}$	$H C O_{3}^{-}$	NH₃		-2
CO₂		$H_{3} O^{+}$	OH^-	15
$H_{3} O^{+}$	OH^-	CO₂		-8
CO₂		$H_{3} O^{+}$	$H C O_{3}^{-}$	15
$H_{3} O^{+}$	$H C O_{3}^{-}$	CO₂		-8
CO₂		$H_{3} O^{+}$	$C O_{3}^{2 -}$	15
$H_{3} O^{+}$	$C O_{3}^{2 -}$	CO₂		-8

Tab.6 Parameters of electrolyte pairs (GMELCC)

Application of thermodynamics models in product prediction for ammonia carbonation

The models determined above were applied in ammonia carbonation process simulation. At a lower temperature of 298.15 K and 1 atm [ 35], and with mole ratio of carbon dioxide to ammonia as 0.5, carbonate component predicted by E-NRTLRK model also matches well with experimental data measured by Raman spectra [ 35] with an average relative deviation of 1.5%, as seen in Table 7.

Tab.7 Comparison of simulation result on ammonia carbonation product with experimental data [ 35]

$C_{{NH}_{3}}$ /mol^-1	$C_{{CO}_{3}^{2-}}$ /mol^-1 (cal.)	$C_{{CO}_{3}^{2-}}$ /mol^-1 (exp.)	Average relative deviation
1.31	0.1	0.1	0.015
1.99	0.15	0.15
2.7	0.21	0.21
3.43	0.26	0.26
4.18	0.31	0.32
4.98	0.36	0.38

Tab.7 Comparison of simulation result on ammonia carbonation product with experimental data [ 35]

Process of product control in ammonia carbonation is a key objective in the conversion of calcium sulfate. Simulations were also carried out at temperature 273.15 K-363.15 K and 0.1-0.4 MPa. Initial conditions were set as ammonia flow rate of 10 mol/L and CO₂ flow rate of 4 mol/L.

Figure 2 is the process simulation result for the preparation of (NH₄)₂CO₃. As shown in Fig. 2, to produce carbonate with composition content larger than 80%, ammonia mole fraction should be at range of 0.12-0.35, and the ratio of NH₃/CO₂ at range of 1.5-4.5; with the ammonia mole fraction larger than 0.20, carbonate composition content is larger than 90%.

Phase diagram [ 36] reported in literature indicated the following conditions for preparation of specified (NH₄)₂CO₃: temperature was no higher than 298.15 K, and mole concentration of ammonia was around 0.20-0.35. Our simulation results agree well with information in reported phase diagram, indicating that E-NRTLRK model can be widely used in the ammonia carbonation process at low temperature of 273.15 K-363.15 K and low pressure of 0.1-0.4 MPa.

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	Fig.2 Process conditions of (NH₄)₂CO₃ production simulation

Conclusions

Simulation methods can completely express the thermodynamics state of a system under the condition of limited practical data, whereas the determination of a proper physical method is the core of the calculation.

Ammonia carbonation is a widely used process while few existing reports are on products at low temperature and low pressure. Our simulation results indicated that at 303.15 K-363.15 K and 0-0.4 MPa, vapor phase of ammonia in NH₃-H₂O system deviated obviously from its ideal state, and the PR equation was suitable for the NH₃-H₂O thermodynamics state description for its high prediction accuracy.

NRTL series activity coefficient equations proved suitable for NH₃-CO₂-H₂O multicomponent mixed solution system. As an asymmetric electrolyte activity coefficient equation, E-NRTLRK had a smaller relative average deviation than models reported in literatures in fitting VLE data of NH₃-CO₂-H₂O, so it could be used for the description of NH₃-CO₂-H₂O mixed solution thermodynamic properties.

E-NRTLRK model was further applied in the process of ammonia carbonation calculation to predict process condition of ammonia carbonation product. To prepare carbonate with composition content larger than 80%, ammonia mole fraction should be larger than 0.20, and the ratio of NH₃/CO₂ should be in the range of 1.5-4.5.

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