Cubic Plus Association Equation Of State Research Articles

Association parameters for Cubic-Plus-Association equation of state determined by molecular dynamics and Petroleomics: Prediction of asphaltene precipitation

In this study, petroleomics characterization, molecular dynamics (MD) simulations, and a cubic-plus-association (CPA) equation of state (EoS) were conducted to understand how the structural characteristics affect the phase behavior of a fluid. To do this, the asphaltene onset pressure (AOP) was estimated by CPA EoS using association parameters determined from MD simulations. Petroleomics characterization and gas chromatography were conducted to identify appropriate representative molecular structures for describing a live crude oil. The American Petroleum Institute (API) gravity, dead and live crude oil densities, and viscosities at reservoir conditions were calculated from the MD simulation results and were found to differ from the experimental measurements by less than 5%, indicating accurate characterizations for both real dead and live crude oil. A formation damage diagnosis from a compositional perspective was conducted because oxygenated aromatic compounds, which are highly active in aggregation terms, were identified in the aromatic fraction by Fourier-transform ion cyclotron resonance mass spectrometry (FT–ICR MS). The association parameters for the self-associating organics (So: asphaltenes and oxygenated aromatics) and the cross-interaction between these Sos and the maltenes were calculated from the MD simulation results. To the best of our knowledge, this is the first time that association parameters have been obtained from an MD model and successfully implemented in a CPA EoS, allowing the prediction of AOP with an accuracy close to 1% compared to the experimentally determined value for live crude oil. This study provides a promising framework to develop an EoS for crude oils based on their molecular characteristics, reducing the need to perform parametric tuning with experimental data. In addition, this structural characterization proposed could be important to other key topics such as enhanced oil recovery (EOR) and carbon capture use and storage (CCUS), relating the structural composition to phase behavior.

Dew point of the methane-ethanol system: Data via acoustic technique and modeling via CPA EoS

The use of gas-lift systems for chemical injection presents a smart solution in numerous cases, particularly when production wells are already equipped with such systems. However, ensuring optimal operating conditions demands a thorough understanding of the phase equilibria curves of natural gas - inhibitor mixtures. The primary objective of this study is to experimentally determine the dew point curve of the methane - ethanol mixture, with methane being a significant constituent of natural gases, and ethanol being one of the most widely utilized gas hydrate inhibitors. The experiments covered temperatures within the range of 313.15–353.15 K and pressures ranging from 1.2 MPa to 8.5 MPa. Both visual and acoustic techniques were applied as synthetic methods to obtain accurate data. Additionally, a thermodynamic model employing the Cubic-Plus-Association (CPA) equation of state, fine-tuned with the experimental bubble and dew point data, was utilized to establish the dew point curve under optimized conditions. An average absolute relative deviation of 4.4% was obtained between the experimental dew points obtained in this work and the thermodynamic model. Moreover, this study fills a noticeable gap in the literature by providing reliable experimental data, which had been scarce. Furthermore, the acoustic experimental technique showcased in this work presents a feasible option for determining dew points. By gaining deeper insights into the behavior of the methane - ethanol mixture, this research contributes to the enhancement of gas-lift systems' efficiency and reliability in chemical injection applications. The findings are expected to have a positive impact on flow assurance strategies and ensure safer and more efficient operations in oil and gas production.

Flash point prediction of binary partially and totally miscible alcohol-water mixtures by various equation of states and experimental validation

In the present article, the experimental flash points (FPs) of partially miscible and non-ideal mixtures including 3-pentanol + water, 2-pentanol + water, and 1-hexanol + water are reported for the first time. The measured FPs were predicted based on the vapor pressure concept and ‎thermodynamic equilibrium. The cubic-plus-association (CPA) equation of state (EoS) along with the CR-1 combining rule and Elliot combining rule (ECR) were used to investigate the non-ideality of investigated systems. In the following, the same method was used with the Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR) EoSs to evaluate whether simpler EoSs can accurately predict the FPs of non-ideal mixtures. Therefore, in addition to 3-pentanol + water, 2-pentanol + water, and 1-hexanol + water, numerous totally and partially miscible aqueous-organic systems containing 1-pentanol, isobutanol, 2-butanol, 1-‎butanol, isopropanol, propanol, ethanol, and methanol were studied in terms ‎of FP prediction. The findings indicated that SRK is marginally more effective than PR. Moreover, ‎SRK EoS was almost as effective as the CPA EoS with the CR-1 combining rule. The CPA EoS ‎that used ECR, on the other hand, was far superior to the SRK and PR EoSs. To the best of our knowledge, CPA parameters for 2-pentanol, 3-pentanol, and 1-‎hexanol were calculated and reported for the first time.

Asphaltene Precipitation Investigation Using a Screening Techniques for Crude Oil Sample from the Nahr-Umr Formation/Halfaya Oil Field

Many oil and gas processes, including oil recovery, oil transportation, and petroleum processing, are negatively impacted by the precipitation and deposition of asphaltene. Screening methods for determining the stability of asphaltenes in crude oil have been developed due to the high cost of remediating asphaltene deposition in crude oil production and processing. The colloidal instability index, the Asphaltene-resin ratio, the De Boer plot, and the modified colloidal instability index were used to predict the stability of asphaltene in crude oil in this study. The screening approaches were investigated in detail, as done for the experimental results obtained from them. The factors regulating the asphaltene precipitation are different from one well to another, from the high-pressure-temperature reservoir to surface conditions. All these factors must be investigated on a case-by-case basis. Because the Halfaya oil field is still developing its petroleum sector, modelling, and forecasting the phase behavior and asphaltene precipitation is crucial. This work used crude oil bottom hole samples with an API of equal to 27 from a well in the Halfaya oil field/Nahr-Umr formation to create a thermodynamic model using Multiflash software. The data included the compositional analysis, the PVT data, and reservoir conditions. The thermodynamic model of asphaltene phase behavior was proposed using the Cubic-Plus association equation of state. All the screening techniques' results revealed the presence of an asphaltene precipitation issue (asphaltene unstable), which was confirmed by a thermodynamic fluid model. The aim of this paper is to predict the problem of asphaltene precipitation so that future proactive remedial methods can be developed to decrease the time and expense associated with it.

A cage-specific hydrate equilibrium model for robust predictions of industrially-relevant mixtures.

Understanding the thermophysical properties and phase behaviour of gas hydrates is essential for industrial applications ranging from energy transport and storage, CO2 capture and sequestration, to gas production from hydrates found on the seabed. Current tools for predicting hydrate equilibrium boundaries typically use van der Waals-Platteeuw-type models which are over-parameterised containing terms with limited physical basis. Here we present a new model for hydrate equilibrium calculations with 40% fewer parameters than existing tools but with equivalent accuracy, including for multicomponent gas mixtures and/or systems with thermodynamic inhibitors. By eliminating multi-layered shells from the model's conceptual basis and focusing on Kihara potential parameters for guest-water interactions specific to each hydrate cavity type, this new model provides insight into the physical chemistry governing hydrate thermodynamics. The model retains the improved description of the empty lattice developed recently by Hielscher et al. but couples the hydrate model with a Cubic-Plus-Association Equation of State (CPA-EOS) to describe fluid mixtures with many more components including inhibitors such as methanol and mono-ethylene glycol used by industry. An extensive database of over 4000 data points was used to train and evaluate the new model and compare its performance against existing tools. The absolute average deviation in temperature (AADT) achieved with the new model is 0.92 K for multicomponent gas mixtures, compared with 1.00 K for the widely-known model of Ballard and Sloan, and 0.86 K for the CPA-hydrates model implemented in the MultiFlash 7.0 software package. With fewer, more physically justified parameters, this new cage-specific model provides a robust basis for improved hydrate equilibrium predictions particularly for industrially-important, multi-component mixtures containing thermodynamic inhibitors.

Multicomponent vapor-liquid equilibrium measurements and modeling of triethylene glycol, water, and natural gas mixtures at 6.0, 9.0 and 12.5 MPa

Triethylene glycol (TEG) is commonly used for natural gas dehydration, and with subsea processing facilities on the rise, an increasing interest is being shown in complex phase equilibria involving petroleum fluids and polar compounds. In this study, examples of such phase equilibria have been studied. New measurements for multicomponent vapor-liquid equilibrium (VLE) of TEG/Water/Natural Gas systems have been performed at pressures ranging from 6.0 MPa to 12.5 MPa and temperatures in the range 15 °C to 40 °C. Two types of natural gas, a Synthetic Gas (xCH4=0.62) and a Real Gas (xCH4=0.92), were mixed with an aqueous TEG solution, with TEG weight percentages ranging from 95 wt.% to 99 wt.%. Glycol in gas (y1), water in gas (y2), and gas solubility in liquid (xNG) were measured with relative experimental uncertainties below 32%. Since the systems are highly associating, the Cubic-Plus-Association (CPA) equation of state (EoS) was selected to model their phase behavior . Different association schemes (4C, 4F, 5F, 6F and 5C) were tested for TEG, and the classical 4C scheme is shown to perform best. The CPA EoS performed in general terms well, considering the number of components in the systems, the associated complexities, and high measurement uncertainty for such low concentrations (<1 ppm) in the gas phase. Furthermore, the content of N2, CO2, CH4 and ethane were modelled in the liquid phase at various conditions.

Thermodynamic properties of water from SAFT and CPA equations of state: A comprehensive assessment

The performance of eight versions of the representative and most commonly used Statistical-Association-Fluid-Theory (SAFT) equations of state for water along with the Cubic-Plus-Association (CPA) equation are examined in detail both throughout the entire liquid phase region and at supercritical conditions. In addition to the temperature–pressure dependence of density, the fundamental response functions, namely, isothermal compressibility, isobaric expansivity, and isobaric heat capacity, have been evaluated and compared with experimental data along five isobars from P=0.1 MPa up to 1000 MPa, and along four isotherms within the range from 300 K to 750 K. It turns out that to draw a general conclusion on the quality and accuracy of these equations (for pure water) is practically impossible with different equations reproducing reasonably well different properties and at different thermodynamic conditions but failing in other instances. In general, the equations seem to be able to capture simple pressure dependence at isothermal conditions but fail primarily in estimating isobaric properties. All the equations (i) tend to perform reasonably well at pressures above the critical one where the excluded volume begins to predominate and the effect of hydrogen bonding becomes insignificant, (ii) but fail again at very high pressures, and (iii) have the most serious problems with predicting the residual isobaric heat capacity. The most successful results are obtained from the recently developed Association Dependent PC-SAFT equation, which can be mainly attributed to its fitting using a broader set of experimental data compared to other equations; nonetheless, its performance still cannot be considered as overall reliable.

Density, speed of sound, refractive index of binary mixtures containing 2-ethoxyethanol and some alcohols: Measurement and correlation

The aim of this research to investigate thermophysical properties of alkoxyethanols with alcohols, including densities, speeds of sound, and refractive index of five binary mixtures constituted by 2-ethoxyethanol (2EE) + methanol, or ethanol, or propan-1-ol, or propan-2-ol, or butan-2-ol, that have been measured at the temperatures: (293.15, 303.15, 313.15 and 323.15) K, over the entire composition range and at pressure of p = 0.1 MPa. The Cubic Plus Association equation of state (CPA-EOS) has been used to correlate the measured properties. From the measured data, excess/derived properties: excess molar volume, isentropic compressibility, deviation in isentropic compressibility and deviation in refractive index were calculated and correlated using the Redlich − Kister equation. The results obtained from the work have been discussed in terms of the nature of molecular interactions between the components of the mixture.

Vapour-liquid equilibria (VLE) and gas hydrate phase equilibria predictions using the cubic-plus association equation of state: CSMGem extension to association EoS model

Inaccurate predictions of the vapour-liquid equilibria of polar gas hydrate formers and organic inhibitors can lead to erroneous hydrate phase equilibrium predictions for these associating components. In this work, parameters of the cubic plus association equation of state (CPA-EoS) were optimized for associating components by simultaneous minimization of absolute errors of saturated liquid densities and vapour pressures compared to experimental data. Model prediction accuracies were compared with experimental data after optimizing binary interaction parameters. The proposed association equation of state (based on CPA-EoS) was employed to predict vapour-liquid equilibria of water/N2/H2+non-associating hydrate formers. Water was modelled using a 4C association scheme, i.e., comprising two proton donor and acceptor sites on each water molecule, which has been used previously (Chapman, 1989; Hammerschmidt, 1934; Kontogeorgis, 2006a; Tsivintzelis, 2010; Tsivintzelis et al., 2014; Wichterle and Kobayashi, 1972; Williams and Katz, 1954). For model development and validation, phase envelopes were predicted and validated with experimental solubility data for several systems. Furthermore, the CPA-EoS was also coupled with the Colorado School of Mines Gibbs Energy Minimization (CSMGem) algorithm for hydrate phase equilibria predictions. The predictions were in close agreement with experimental hydrate phase equilibria measurements.

Experimental investigation of carbon dioxide solubility in the deep eutectic solvent (1 ChCl + 3 triethylene glycol) and modeling by the CPA EoS

CO2 solubilities in the deep eutectic solvent (1 choline chloride +3 triethylene glycol) were measured at the four temperatures of 303.15, 313.15, 323.15 and 333.15 K and pressures up to 30 bars. Measurements were carried out in a newly designed, built and validated high pressure solubility apparatus. The measured data were modeled by the cubic plus association (CPA) and the Soave-Redlich-Kwong (SRK) equations of state, with AARD% values of 5.52% and 7.30% for the CPA and SRK EoSs, respectively, showing the good correlative ability of both models. Furthermore, by calculating the standard Gibbs free energies of dissolution, standard enthalpies of dissolution and standard entropies of dissolution at infinite dilution, we found that the dissolution process is nonspontaneous and exothermic, with the CO2 - DES interactions being the stronger interactions. The solution becomes less chaotic and reaches a higher degree of order in the liquid phase after dissolution of CO2.

Modeling of the Interfacial Behavior of Carbon Dioxide + Methyl Myristate, Carbon Dioxide + Palmitate, and Carbon Dioxide + Methyl Myristate + Methyl Palmitate Mixtures Using CPA-EOS and Gradient Theory

This work presents the vapor–liquid equilibrium and the interfacial behavior modeling of binary and ternary CO $$_2$$ + ester mixtures, with the cubic plus association equation of state (CPA-EOS) combined with the density gradient theory. The binary mixtures, CO $$_2$$ + methyl myristate and CO $$_2$$ + methyl palmitate, were studied at temperatures from 313.15 K to 333.15 K, considering the ester molecules as non-associating and the carbon dioxide as non-associating or self-associating (2B scheme) molecule. Phase behavior was predicted with binary interaction parameters adjusted from binary mixture phase equilibrium experimental data. Results show than there are some points better represented by the associating scheme, but the overall trend shows that the CPA-EOS with the non-associating scheme for all the molecules is the best option for describing the phase equilibrium and calculating the interfacial tension for the CO $$_2$$ + ester binary mixtures. For this reason, the CO $$_2$$ + methyl myristate + methyl palmitate mixture at 313.15 K was modeled only with non-associating fluids. The AAD% for the calculated interfacial tensions, considering non-associating molecules in the mixture is 9.7 % for the CO $$_2$$ + methyl myristate mixture, 15.1 % for the CO $$_2$$ + methyl palmitate mixture, and 7.9 % for the CO $$_2$$ + methyl myristate + methyl palmitate mixture.

Predictions of the Joule-Thomson coefficients and inversion curves from the CPA equation of state

Several approaches involving some particular cubic equation of state (EoS) have been tested over the last 50 years as attempts to assess the Joule-Thomson coefficient (JTC) and inversion curves, though most of the cubic EoS are limited with regard to delivering good results for associating substances. The present work applies the Cubic Plus Association (CPA) EoS to predict the JTC for several pure substances (methane, ethane, propane, ethylene, benzene, toluene, nitrogen, carbon dioxide, sulfur hexafluoride, water, hydrogen sulfide and ethanol) and mixtures, focusing on systems with at least one associating substance. The results were compared with experimental and correlated data from the literature, as well as with the SRK EoS and molecular simulation. CPA EoS was capable of predicting the general trend for the Joule-Thomson coefficient and inversion curves even at extreme conditions, with reduced pressure values up to 20 and reduced temperatures up to 5.

Accounting for quadrupolar interactions in the CPA equation of state: A modelling approach for binary mixtures of CO2 or benzene with n-alkanes and 1-alkanols

The Cubic Plus Association (CPA) equation of state (EoS) is extended with the theory for quadrupolar molecules of Gross, yielding the CPA-G model. CPA-G is compared to the recently developed qCPA, which uses the polar theory of Larsen and coworkers. Each model requires an additional adjustable parameter for quadrupolar molecules.The models are evaluated by means of their ability to model the phase equilibria of binary mixtures of CO2 or benzene with n-alkanes and 1-alkanols, as well as binary mixtures of quadrupolar molecules. A refined technique of accounting for cross-association in the CO2 + 1-alkanol systems is proposed, allowing an improved description of these systems without the use of a binary interaction parameter. Accounting for cross-association in the benzene + 1-alkanol systems investigated in this work did not lead to improved results. Both CPA-G and qCPA provide significant improvements over nonpolar CPA in both n-alkane and 1-alkanol mixtures, with excellent results obtained in the subcritical region without the use of binary interaction parameters. qCPA continues to provide excellent predictions in the supercritical region, while CPA-G overestimates the solubility pressures in the supercritical region. Both quadrupolar models can qualitatively predict the existence of three-phase equilibrium in the CO2 + n-alkane and CO2 + 1-alkanol systems. The simple combining rules used for cross-quadrupolar interactions in both CPA-G and qCPA do not take molecular orientation into account and are unable to truly capture the phase behaviour in the CO2 + benzene system.

Experimental determination and thermodynamic modeling of clathrate hydrate stability conditions in methane + hydrogen sulfide + water system

In the petroleum industry, it is essential to comprehend the hydrate equilibrium qualifications in order to prevent or defer this phenomenon. However, the thermodynamic equilibrium information on hydrogen sulfide containing systems reported in the literature is limited and discrepant in comparison with other gases. In this communication, new experimental investigations of hydrate dissociation conditions of methane (0.995 mole fraction) + hydrogen sulfide (0.005 mole fraction) + water system which forms stable structure I were carried out in temperature and pressure ranges from 282.1 to 288.3 K and 5.63 to 12.19 MPa, respectively. The experiments were carried out by using the stepwise heating method with an isochoric pressure search procedure in a 100 cm3 autoclave. The apparatus and method validity were confirmed by comparison of the methane hydrate equilibrium data from the current study with some selected data from the literature. In this study, a thermodynamic model was developed to estimate the dissociation conditions of the binary CH4+H2S hydrate system. For this purpose, a systematic comparison is made between results from PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory) and CPA (Cubic Plus Association) equations of state (EoS). In the present work, PC-SAFT and CPA equations of state are utilized to model the gas/vapor and water phases and van der Waals Platteeuw (vdW-P) is used for hydrate phase modeling. In this regard, the binary interaction parameters in PC-SAFT/CPA EoS for the aforementioned system were tuned using literature data on H2S/CH4 solubility in water. Model outcomes show that both models are entirely trustworthy over an extensive range of temperatures (273 to 350 K), pressure (2 to 22 MPa) and all H2S concentrations. In addition, a comparison between experimental data, the models results and HWHydrate (version 1.1) predictions ascertain that the PC-SAFT is relatively precise than CPA in the binary hydrate system of CH4-H2S.

Description of phase equilibrium and volumetric properties for CO2+water and CO2+ethanol using the CPA equation of state

Properties of fluids under high pressure is of interest for processes design. A literature survey was conducted on experimental vapor-liquid equilibrium (VLE) and density for CO2+water and CO2+ethanol. Phase equilibrium and density of pure water, ethanol and CO2 were evaluated considering 1868 data points. VLE and saturated liquid density data were collected: 544 data points for CO2+water and 859 data points for CO2+ethanol. Soave Redlich-Kwong (SRK) equation of state (EoS) with Cubic Plus Association (CPA) was applied for calculation of these properties. Peng-Robinson EoS was evaluated to demonstrate CPA improvement in properties description. The binary interaction parameters were estimated from VLE. For pure compounds, the results indicated suitable description for both phase behavior and density. For CO2+water, density calculations were accurate (Δρ = 0.5%) and VLE was adequately described (ΔxH2O = 0.49% and ΔyCO2 = 4.63%). CO2+ethanol volumetric behavior and VLE were also satisfactorily described in CO2 concentrations up to 30% (Δρ = 2.92%, ΔxEtOH = 1.37% and ΔyCO2 = 5.86%, respectively).

The cubic-plus-association equation of state for hydrofluorocarbons, hydrofluoroolefins, and their binary mixtures

Hydrofluorocarbons, hydrofluoroolefins, and their mixtures are widely used as working fluids in energy systems. Weak hydrogen bond exists in these fluids and thus contribute to their thermodynamic properties. In this work, we account for the contribution of the weak hydrogen bond by tailoring the cubic-plus-association (CPA) equation of state (EoS) and its parameter fitting procedure. Overfitting to the liquid properties is avoided by introducing the saturated vapor density into the fitting objective function. The EoS is extended to binary mixtures using the van der Waals mixing rules and Elliot combining rule in which process solving the cross-association fraction is reduced from an equation set consisting of 4 nonlinear equations to a single nonlinear equation. The performance of the present model is investigated in terms of the vapor-liquid equilibria (VLE) and volumetric properties of 15 pure fluids and 40 binary mixtures using fitted and zero binary interaction parameter, and is compared with the reference data, a CPA EoS that uses the traditional parameter fitting procedure, the Soave-Redlich-Kwong (SRK) EoS, and a volume-translated SRK EoS. The present model (including the EoS and fitting procedure) accurately reproduces the vapor pressure, density, and second virial coefficient for the pure fluids and binary mixtures, and is the only valid model among the investigated models for both the VLE and the volumetric properties in both liquid and vapor phases. The contribution of the association term to the pressure is also shown in the temperature-density diagram, presenting a similar behavior as that in water in which the effect of the hydrogen bond is comparatively more pronounced.

Solubility of CO2 in 3-amino-1-propanol and in N-methyldiethanolamine aqueous solutions: Experimental investigation and correlation using the CPA equation of state

The solubility of CO2 in 1.3M and 2M aqueous N-methyldiethanolamine (MDEA) and 3-amino-1-propanol (MPA) solutions was experimentally measured at 298, 313, 323 and 333 K, using a pressure decay method. Since only limited experimental data exist in literature for MPA aqueous systems, the results of this study were compared with literature data mainly for 2M MDEA solutions, showing satisfactory agreement. The obtained new experimental data were used to parameterize the Cubic-plus-Association (CPA) equation of state using a pseudo chemical reaction approach, i.e. to account for chemical interactions as very strong specific intermolecular interactions, which is a crude approximation, but is necessary for applying equation of state models. It was found that the model satisfactorily describes the experimental data using six or four binary parameters for the CO2-MPA or CO2-MDEA interactions, respectively, which, however, are less than the number of adjustable parameters in similar literature models.

Prediction of solid solute solubility in supercritical CO2 with cosolvents using the CPA EoS

Supercritical carbon dioxide (scCO2) is widely used in separation processes applied to the food, chemistry, pharmaceutical and material industries. The knowledge of the solubility of solid solutes in scCO2, with or without cosolvents, is essential for the design and optimization of extraction, fractionation and purification processes. This work evaluates the accuracy of Cubic Plus Association equation of state (CPA-EoS) to predict the solubility of solutes in ternary and/or quaternary systems from the binary interaction parameters. The solubility of 12 solid solutes in scCO2 in presence of different organic cosolvents was investigated, totalizing 19 systems at pressures between 8 and 40 MPa, temperatures ranging from 308 K to 353 K and concentrations of cosolvent varying from 0.73 to 10 mol%. The overall average logarithmic deviation (ALD) between experimental and calculated data with CPA-EoS was 0.47, which is better than that obtained with PR + COSMOSAC previously reported in the literature. Furthermore, the effects of temperature, pressure and of cosolvent type and concentration on the solubility were well estimated with the CPA-EoS.

Experiment and prediction of water content of sour natural gas with an modified cubic plus association equation of state

Abstract In this work, new experimental value for water content in sour natural gas were reported. In addition, to predict the water content in sour natural gas, a modified cubic plus association equation of state (CPA-EoS) was also proposed. In this model, a new energy parameter a was proposed to make an accurate description of saturated liquid density. Additionally, a temperature dependent binary interaction parameter kij for six binary systems was also obtained. Lastly, a comparison between the prediction results of the modified CPA-EoS and the experimental data was presented, and the results showed that the modified CPA-EoS could predict the water content in sour natural gas with high accuracy, which has an AAD of 3.6722% with experimental data in literatures and an AAD of 1.946% for experimental data reported in this work.

Recent advances with association models for practical applications

ABSTRACTAssociation models like Cubic Plus Association (CPA) equation of state and other Statistical Associating Fluid Theory variants have found widespread use, especially over the recent 30 or so years, and this is not limited anymore to universities and researchers. Industry is beginning slowly to adopt such models for some applications and a few of the association models are now provided by commercial simulators. Association models account explicitly for hydrogen bonding (and other complex) phenomena, and for this reason, they are potentially more useful and more successful than traditional models like cubic equations of state and activity coefficient models. Still, for practical applications, all models are judged by their results and these depend on the availability of experimental data, the number and type of adjustable parameters and the performance of the models for phase equilibria and occasionally also for other properties. We will consider four case studies in this work which, will illustrate some of the capabilities and limitations of these association models in different applications. We will offer a ‘model developer’s’ point of view showing in several cases all stages of model development in order to illustrate what worked, what did not and how it was corrected (when possible). The ‘physics’ and ‘application’ aspects of the models will be in all cases discussed. All results will be shown with the CPA equation of state, although we expect that the overall conclusions will be the same for a wide range of association models.