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A Review of the Equations of State and Their Applicability in Phase Equilibrium Modeling

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A Review of the Equations of State and Their Applicability in Phase Equilibrium Modeling International Conference on Chemical and Environmental Engineering (ICCEE'2013) April 15-16, 2013 Johannesburg (South Africa) A Review of the Equations of State and their Applicability in Phase Equilibrium Modeling Sashay Ramdharee, Edison Muzenda and Mohamed Belaid the non-sphericity of molecules and related it to vapor Abstract—This short paper reviews the equations of state and pressure data. In 1972, Soave modified the Redlich & Kwong their application to phase equilibrium modeling. The strengths, EOS by introducing Pitzer’s acentric factor. Peng and weaknesses and applicability of these equations will be assessed. Robinson (1976) proposed their EOS applicable to natural gas Our intention is to test the applicability of the Peng Robinson systems [1]. equation of state in the computation of thermodynamic interactions of volatile organic compounds and biodiesel. Thermodynamic B. Definition of Equation State models are very useful in phase equilibrium prediction as measurements are costly and time consuming. An Equation of State (EOS) is a semi-empirical functional relationship between pressure, volume and temperature of a Keywords—Activity coefficients, equations of state, modeling, pure substance. It is a thermodynamic equation describing the phase equilibrium. state of matter under a given set of physical conditions. Most EOSs are semi-empirical and are generally developed for pure I. INTRODUCTION substances. Their application to mixtures requires an additional variable (composition) and hence an appropriate A. Development of Equation of States mixing rule. The functional form of an EOS can be expressed N 1662, Robert Boyle found that the volume of a gas was as in (3). I inversely proportional to its pressure as shown in (1). =pV constant f(P, V, T, k n1,=k,a p 0=) (1) (3) Charles (1787) postulated that the volume of a gas was Ak and np are the parameters and categories of the proportional to its temperature at isobaric conditions. In 1801, equations of state. np indicates the complexity of the EOS and Dalton (1801) introduced the concept of partial pressures. the more complex is the equation, the more accurate it is. Gay-Lussac (1802) defined the universal gas constant “R”. Currently, there is no single equation of state that accurately Van der Waals (1873)’s quantitative approach proposed the predicts the properties of all substances under all conditions continuity of gases and liquids and provided the most [2]. Many cubic EOSs contain the original van der Waals important contribution to EOS development. In 1875 Gibbs RT made an important contribution to thermodynamics. In 1927, b)-(V 2 Ursell (1927) proposed a series solution (polynomial functional form) for the EOS known as the virial EOS (2). repulsive term cb d The Peng Robinson and Soave-Redlich-Kwong equations +1=P + + ...+ V VV 32 of state are the most applicable in the petroleum industry [3]. The EOSs were developed to predict volumetric, (2) thermophysical and vapour-liquid equilibrium data [3]. In 1949, Redlich & Kwong introduced a temperature The main purpose for the development of the EOS dependency to the attraction parameter “a” of the vdW EOS. approach was for generating of data which included Pitzer (1955) introduced the “acentric factor” to quantify for volumetric, thermo physical data, and to help perform vapor/liquid equilibrium calculations. [3] S. Ramdharee is with the Department of Chemical Engineering, Faculty of Equations of state can be applied over wide range of Engineering and the Built Environment, University of Johannesburg, temperature and pressure as well as to mixtures of diverse Doornfontein, Johannesburg 2028 (email; [email protected]) components, from light gases to heavy liquids. They can also E. Muzenda is with the Department of Chemical Engineering, Faculty of be applied to supercritical phases without encountering any Engineering and the Built Environment, University of Johannesburg, Doornfontein, Johannesburg 2028; phone: 0027-11-5596817; fax: 0027-11- conceptual difficulties. 5596430; e-mail: [email protected]). M. Belaid is with the Department of Chemical Engineering, Faculty of Engineering and the Built Environment, University of Johannesburg, Doornfontein, Johannesburg 2028 (e-mail: [email protected]). 84 International Conference on Chemical and Environmental Engineering (ICCEE'2013) April 15-16, 2013 Johannesburg (South Africa) II. EQUATIONS OF STATE b = 0.08664 *RT /P c c (8) A. Van der Waals Equation of State The Redlich-Kwong equation is expressed as in (9). The van der Waals EOS (vdWEOS) is the basis of various NRT 2 EOSs. The contributions of the vdWEOS are that it laid P = a- N T1/2 (V + bN) foundations for modern cubic EOS, radically improved (V - bN) (9) prediction potential, formulated the principle of corresponding C. Soave-Redlich Kwong Equation states and was the first to predict continuity of matter between gas and liquid. Van der Waals (1973) accounted for the non- In 1972, Soave proposed an important modification to the zero molecular volume and non-zero force of attraction for a Redlich-Kwong Equation of State (EOS). The modification real substance. Van der Waals established that molecules fitted experimental vapor-liquid data well and could predict should have an infinite volume and modified the pressure term phase behavior of mixtures in the critical region. Until the of the Ideal gas equation to account for molecular interaction work of soave (1972), modifications to the vdW EOS focused [4]. The prediction of liquid behavior became more accurate on temperature dependency of the attractive parameter. Soave as the volume approaches a more limiting value, b at high proposed a two-variable dependency for ‘a’ as in (10) accounting for the shape of the molecules through the Pitzer’s pressures as in (4). ω lim V(p) = b acentric factor ( ) [7]. p →∞ a = a(T,ω) (10) (4) The Soave-Redlich-Kwong equation of state can accurately The vdW EOS gives a simple relationship between describe both liquid and vapor phase bevahiour. Various pressure, temperature and molar volume. Considering the modifications have been proposed to the Soave-Redlich- corrections to the ideal gas law, the Van der Waals equation of Kwong equation to improve its accuracy. state is written as in (5). RT aα P = - (V - b) v(v + b) a (11) P = RT/(v - b) - 2 2 α V = [1+ S(1- (Tr )] (12) (5) In (12), Tr is the reduced temperature and S can be In (5) P, T, a, R and b are absolute pressure, molar volume, calculated from (13). 2 absolute temperature, attraction parameter, universal gas S = 0.48 +1.574ω - 0.176 ω (13) constant and the repulsion parameter. The van der Waals The major drawback in the SRK EOS is that the critical equation is not capable of predicting phase equilibrium compressibility factor takes on the unrealistic universal critical accurately. compressibility of 0.333 for all substances. [7] Consequently, the molar volumes are typically overestimated, i.e., densities B. Redlich Kwong Equation are underestimated. Another major drawback of the SRK The Redlich Kwong is a cubic equation of state based on EOS was the poor liquid density prediction. the van der Waals equation [5]. It can be applied to mixtures by using mixing rules to the equation of state parameters. It D. Peng Robinson Equation improved the Van der Waals equation with a better The Peng-Robinson equation of state (1976) was developed description of the attractive term. Redlich and Kwong to handle both vapor and liquid properties near equilibrium introduced an adjustment to the van der Waal’s attractive conditions. The development of this equation was focused on a natural gas systems [9]. It is generally superior to the Soave 2 Redlich Kwong equation in the prediction of liquid densities. pressure term V which improved the prediction the RT aα P = - (14) prediction vapour phase physical properties. The Redlich- (v - b) (v(v + b)+ b (b - v)) Kwong equation is relatively simple and can predict component or nixture behavior based on little data. Its While they kept the temperature dependency of the limitation is that it cannot be used liquid-liquid and vapour- attractive term and the acentric factor as introduced by Soave, liquid equilibrium prediction but gases only [6]. Peng and Robinson introduced various fitting parameters to RT a describe this dependency. P = - (v - b) 2 2 (15) ( T v(v + b)) (6) a = 0.45724 (R * Tc )/Pc T The constants a and b can be calculated from (7) and (8). α = [1+ S(1- ( )] 2 (16) Tc 2 2.5 2 a = 0.42748 (R *Tc )/Pc S = 0.37464 +1.54266ω - 0.26992ω (17) (7) b = 0.07780 *RTc/P c (18) 85 International Conference on Chemical and Environmental Engineering (ICCEE'2013) April 15-16, 2013 Johannesburg (South Africa) ∂z = (30) Peng and Robinson (1978) suggested (19) for ω > 0.49. M ∂x y S = 0.3796 +1.4850ω - 0.1644 ω 2 + 0.01666ω 3 (19) ∂z N = (31) ∂ Equation (20) predicted the vapour pressure of heavy y X hydrocarbons over a wide temperature range better. Consistency requires that: 2 ρNRT aρ ∂M ∂N P = - (20) = (32) (1- bρ ) 1+ 2bρ - b2 ρ 2 ∂ ∂ y X x Y Then the equations of state must obey (33). a and b can be calculated from (21) to (26) ∂C p ∂ ∂v = − (33) a= a cω (21) v T ∂p∂ T ∂T 2 2 p ac = 0.45724 (R * Tc )/Pc (22) 2 F. The Modern Cubic Equation of state α= [1 + κ (1− (Tr )] (23) T Equation (34) is an example of a cubic equation of state = (24) Tr [10] derived from van der Waals equation. Tc b = 0.07780 * RTc/P c (25) 3 RT 2 a ab (34) v −b + v + v - = 0 κ = 0.37464 +1.54226ω - 0.26992ω 2 ρ ρ ρ (26) PV Introducing the compressibility factor Z ( Z = ) into The Peng-Robinson and Soave-Redlick-Kwong equations RT are widely used in industry.
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