Phase Behavior Of Hydrates Research Articles

Modeling Phase Behavior of Semi-Clathrate Hydrates of CO2, CH4, and N2 in Aqueous Solution of Tetra-n-butyl Ammonium Fluoride

Abstract Semi-clathrate hydrates are members of the class of clathrate compounds. In comparison with clathrate hydrates, where the networks are formed only by H2O molecules, the networks of semi-clathrate hydrates are formed by mixtures of H2O and quaternary ammonium salts (QASs). The addition of QASs to the solution enables to improve the formation of semi-clathrate hydrates at much milder conditions comparing to clathrate hydrates. In this work, we study the phase equilibria of semi-clathrate hydrates of CH4, CO2, and N2 gas in an aqueous solution of tetra-n-butyl ammonium fluoride (TBAF). An extension of the Chen–Guo model is proposed as a thermodynamic model. The Peng–Robinson equation of state (PREOS) was applied to calculate the fugacity of the gas phase and in order to determine the water activity in the presence of TBAF, a correlation between the system temperature, the TBAF mass fraction, and the nature of the guest molecules has been used. These equations were solved simultaneously and through optimizing tuning parameters via the Nelder–Mead simplex algorithm. The results are compared to experimental data and good agreement is observed.

Revisions to standard theory provide clues to understanding phase behavior of clathrate hydrates

Retooling the van der Waals and Platteeuw theory of clathrate hydrates facilitates evaluating phase boundaries between water, hydrates and guest molecules.

Multiphase flash calculations for gas hydrates systems

In this study, the van der Waals and Platteeuw model was coupled with the Cubic Plus Association (CPA) equation of state (EoS) for equilibrium calculations in systems with gas hydrates. It has been applied to simple and complex multicomponent systems involving methane, ethane, propane, isobutane, carbon dioxide, nitrogen and hydrogen sulfide. Methanol, ethanol, monoethylene glycol, calcium chloride, sodium chloride and potassium chloride were contemplated as thermodynamic hydrate inhibitors. The calculations were performed in the presence of single and mixed inhibitors. The mole fraction of components in all phases were determined using flash algorithm procedures to improve the calculations accuracy. To evaluate the ability of the methodology, the prediction of hydrate phase behavior in the presence and absence of inhibitors was compared with the experimental data. Additionally, the binary interaction coefficients were considered as a linear function of temperature. Furthermore, the Langmuir constants were optimized for each gas hydrate former. The results obtained with the proposed approach agreed well with the experimental data, predominantly for hydrate systems comprising mixture of different inhibitors as salts and alcohols.

Advancements in hydrate phase equilibria and modeling of gas hydrates systems

Reliable hydrate phase behavior predictions are critical to petroleum and natural gas processing and design, and operation of process equipment. Inaccurate predictions of phase equilibria can also lead to erroneous design of process facilities and subsequently may cause safety hazards and flow assurance issues. This work reviews the experimental data on hydrate phase equilibria in the presence of inhibitors (Salts + Organic Inhibitors). The statistical thermodynamic model of van der Waals and Platteeuw for hydrate phase equilibria prediction is also critically reviewed. Recent studies have shown that the basic assumptions of the van der Waals and Platteeuw model, including the spherical symmetry of molecules, no guest–guest interactions, and no lattice distortions (due to guest molecules) can introduce errors in the predicted results. In addition, the limitations of the fluid phase models, which do not account for the effect of hydrogen bonding and electrolyte contributions, can lead to severe prediction errors in hydrate forming systems containing polar hydrate formers, inhibitors, and salts, especially at high concentrations. Thermodynamic predictions of gas hydrate phase equilibria for polar hydrate formers and inhibited systems (e.g., NaCl, KCl, CaCl2, and also methanol, ethanol, Ethane-1,2-diol) are of major concern because of the large errors in fluid phase equilibrium predictions. The unavailability of phase equilibria data, an appropriate electrolyte model, and an associative equation of state as well leads to various problems in predicting the properties of aqueous associating fluids and inhibited systems.

A new predictive thermodynamic framework for phase behavior of gas hydrate

This study presents a thermodynamic framework for predicting hydrate equilibrium pressure of gas species including methane, ethane, propane, nitrogen, carbon dioxide and hydrogen sulfide and their mixtures. For this purpose, Peng–Robinson–Stryjek–Vera equation of state (PRSV-EOS) along with modified Huron-Vidal (MHV1) mixing rule and UNIQUAC model were employed to calculate fugacity and activity coefficient of water in equilibrated fluid phases. To represent phase behavior of solid hydrate, a modified version of van der Waals and Platteeuw (vWd-P) model was utilized which accounts for guest interaction with water molecules beyond first shell of cavity and also asymmetry of encapsulated molecules and cage structure. In addition, dissolution of gas compounds in aqueous phase and subsequent non-ideality of water was taken into account. It was realized that both physical phenomena are of marked influence on the prediction accuracy.In contrast to conventional modelling approaches which scarify some part of equilibrium data to regress some adjustable parameters, advantageously, our model does not require any pre-matching. It was observed that present framework gives accurate prediction for equilibrium pressure of hydrates of CH4, C2H6, C3H8, CO2, N2 and H2S with overall average absolute percent deviation (AAPD) 3.2, 3.4, 3.7, 1.5, 2.8 and 1.3, respectively. Also, incipient pressure of hydrate formed by binary gas mixtures comprised of aforementioned single components could accurately be predicted by proposed approach, which is detailed in the manuscript. Overall, formation pressures predicted by proposed modelling approach are more accurate (or at least comparable) to outstanding models presented in the hydrate literature.

New insight into prediction of phase behavior of natural gas hydrate by different cubic equations of state coupled with various mixing rules

Progress in hydrate thermodynamic study necessitates robust and fast models to be incorporated in reservoir simulation softwares. However, numerous models presented in the literature makes selection of the best, proper predictive model a cumbersome task. It is of industrial interest to make use of cubic equations of state (EOS) for modeling hydrate equilibria. In this regard, this study focuses on evaluation of three common EOSs including Peng–Robinson, Soave–Redlich–Kwong and Valderrama–Patel–Teja coupled with van der Waals and Platteeuw theory to predict hydrate P–T equilibrium of a real natural gas sample. Each EOS was accompanied with three mixing rules, including van der Waals (vdW), Avlonitis non-density dependent (ANDD) and general non-quadratic (GNQ). The prediction of cubic EOSs was in sufficient agreement with experimental data and with overall AARD% of less than unity. In addition, PR plus ANDD proved to be the most accurate model in this study for prediction of hydrate equilibria with AARD% of 0.166. It was observed that the accuracy of cubic EOSs studied in this paper depends on mixing rule coupled with them, especially at high-pressure conditions. Lastly, the present study does not include any adjustable parameter to be correlated with hydrate phase equilibrium data.

Effect of aromatic/aliphatic based ionic liquids on the phase behavior of methane hydrates: Experiments and modeling

Abstract In this study, eight ionic liquids (ILs) from the two varieties of ILs, namely, aromatic and aliphatic ILs, have been considered to carry out experimental studies for their effect on the phase behavior of methane hydrate. We have employed five aromatic based ILs with several cations, such as 1-butyl-3-methyl imidazolium, 1-hexyl-3-methyl imidazolium, 1-octyl-3-methyl imidazolium and various anions, such as [Cl]−, [Br]−, [HSO4]−, and three aliphatic based ILs with various cations, such as di-ethyl-ammonium, tri-propyl-ammonium, tri-butyl-ammonium and [HSO4]− anion. All the experiments were performed in the hydrate equilibrium pressure and temperature ranges of 3.86–7.66 MPa and 276.68–283.18 K, respectively. It has been observed that all the investigated ILs have shown inhibition effect on methane hydrate system. Aromatic ionic liquids have shown their dominance over aliphatic ionic liquids in terms of methane hydrate inhibition. ILs with similar class of cation with varying carbon chain length have not shown significant improvement in hydrate inhibition. However, the replacement of anion by [HSO4]− in imidazolium-based ILs improves methane hydrate inhibition. 1-butyl-3-methyl imidazolium sulphate ([BMIM]+[HSO4]−) found to be the best methane hydrate inhibitor among all investigated ILs. In addition, a phase behavior model incorporating a single tuning parameter has been proposed to predict the phase behavior of methane hydrate in the presence of various ionic liquids and salt solutions. The absolute average deviation in pressure (AARD-P, %) for proposed model with an experimental data generated in this work and for various data sets from the literature has been found to be within ±2.90% of the experimental values. Both cation and anion of the ionic liquids have shown to exhibit inhibition effect on the methane hydrate phase stability. The study indicates that the selection of ionic liquids with tunable cation and anion may provide an opportunity to design the best inhibitor for methane hydrate.

Phase Behavior of Carbon Dioxide Hydrates: A Comparison of Inhibition Between Sodium Chloride and Ethanol

In view of the possibilities for hydrate formation caused by carbon dioxide-rich fluids in the production lines of the Brazilian Pre-Salt fields, this study focus on experimental measurements to obtain fundamental insight into the phase behavior of carbon dioxide hydrate forming systems. This work considers the influence of sodium chloride and ethanol, hydrate inhibitors, on this phase behavior and its implications to practical applications. In addition, the inhibiting effect of ethanol on hydrates was compared with that of sodium chloride at the same mass fractions. The carbon dioxide hydrate phase behavior was measured using a high-pressure equilibrium cell in the temperature range of 272–279 K and pressures up to 3.9 MPa. Experimental measurements using the isothermal method were performed by monitoring the pressure response of the system with volume changes. Enthalpies of dissociation for carbon dioxide hydrates were estimated from the measured three-phase (Lw–H–V) data by applying the Clausius–Clapey...

Inhibition effect of amino acids on carbon dioxide hydrate

The formation of gas hydrates hinders oil & gas transportation in pipelines, however, adequate knowledge and understanding of the gas hydrate phase behaviour is a key to its prevention. Thus, this study reports the inhibition effect of glycine, alanine, proline, serine and arginine on the carbon dioxide hydrate phase equilibrium condition using the isochoric T-cycle method in the pressure range of 2.53–4.0MPa. The studied amino acids concentrations ranged from 5 to 20wt%. The presence of all the studied amino acids significantly shifted the carbon dioxide hydrate phase boundary to higher pressures and/or lower temperatures area, hence indicating inhibition effect. Glycine is found to show the highest inhibition strength with an average depression temperature of 1.83K at 10wt%. In agreement with COSMO-RS sigma profile analysis, the amino acids inhibitions strengths are found to be influenced by their ability to reduce water activity in hydrate formation via hydrogen bonding with water molecules. The calculated carbon dioxide hydrate dissociation enthalpies in the presence of amino acids, suggested that, amino acids do not take part in carbon dioxide hydrate cage occupation and structure during hydrate formation. Furthermore, the measured carbon dioxide hydrate phase conditions in the presence of amino acids are predicted and are found to be in good agreement with the experimental data.

Phase behavior of methane hydrate in the presence of imidazolium ionic liquids and their mixtures

The phase equilibrium conditions of methane hydrate in single ionic liquid aqueous solutions containing low dosage 1-ethyl-3-methylimidazolium nitrate ([Emim][NO3]) at mass fractions of 0.01, 0.025, 0.05 are measured in a pressure range of 3.50–15.0 MPa via an isochoric step-heating pressure search method. The thermodynamic inhibition effect of mixed ionic liquids containing [Emim][NO3] in conjunction with 1-ethyl-3-methylimidazolium chloride ([Emim][Cl]) at low and high concentrations on methane hydrate formation is also investigated for the possible presence of synergistic effects. It is observed that for all the inhibitor systems containing [Emim][NO3] or [Emim][Cl], the inhibition effect increases progressively with the increased concentration and pressure. In addition, the mixture of [Emim][NO3] and [Emim][Cl] does not always exhibit synergetic effect on the thermodynamic inhibition of methane hydrate at higher concentrations up to 0.2 mass fraction. The higher concentrations are, the positive interactions between [Emim][NO3] and [Emim][Cl] possibly happen at higher pressures.

Thermodynamic modeling of gas hydrate formation in presence of thermodynamic inhibitors with a new association equation of state

Formation of hydrate crystal particles has been one of the major difficulties facing gas industry in the generation, transmission, and processing that causes blockage of pipelines and equipment resulting in decreases of level of safety and immunity Which necessitating proper methods to prevent hydrate formation. One of the most common treatments are injection of chemical inhibitors. Keeping excessive cost of hydrate remediation and inhibition in mind, using a reliable predictive model is the key step of designing a natural gas transmission operation.In this paper the van der Waals and Platteeuw (vdW-P) model was coupled with Dehaghani et al.'s association equation of state (DA-EOS) for prediction of equilibrium temperature of methane hydrate in presence of methanol (MeOH), ethanol (EtOH), and mono-ethylene glycol (MEG) as thermodynamic inhibitors. To increase the calculation accuracy, the mole fraction of components in the liquid and gas phases were obtained using the results of flash algorithm calculations. In addition, effect of gas dissolution in aqueous phase was considered by Henry's law and water activity coefficient calculation was performed based on φ- φ approach. Additionally, the binary components interaction coefficients in the system, was considered as a linear function of temperature.To evaluate the capability of DA-EOS, the prediction of methane hydrate phase behavior in presence and absence of inhibitors was compared with the experimental data and also the prediction of Peng-Robinson EOS (PR-EOS). In addition, a comparison was made between DA-EOS and Elliott–Suresh–Donohue uation of State (ESD-EOS) to gain insight of performance of present model. The prediction of DA-EOS and ESD-EOS was close to each other. DA-EOS proved to be effective in representing self-association between water and cross-association between water-inhibitor components in the case of hydrate solution. Also, it was concluded that performance of vdW-P model strongly depends on reliability of EOS used for calculation of fugacity coefficient.

Phase behavior of gas hydrates in nanoporous materials: Review

A precise understanding of phase behavior for a variety of both artificial and natural processes is essential to achieving scientific and technological goals. There has been growing research interest in gas hydrates confined in nanoporous media aiming to simulate and analyze the unique behavior of natural gas hydrates in sediments. Moreover, the appearance of peculiar properties due to the confinement effect stimulates research on gas hydrate technology for gas separation, such as CO2 capture from versatile pre/post combustion emissions. In spite of their importance, reliable phase equilibrium data on gas hydrates confined at a nanoscale are scattered throughout the literature, while those in bulk state are abundant. Accordingly, we surveyed the previous studies on the phase behavior of gas hydrates in various nanoporous materials to include and provide valuable information and knowledge for start-up researchers in various gas hydrate fields.

Bulk phase behavior of tetra-n-butylammonium bromide hydrates formed with carbon dioxide or methane gas

We report the bulk phase behavior of ionic clathrate hydrates of tetra-n-butylammonium bromide (TBAB) formed with a common guest substance: CO2 or CH4. We formed the bulk samples by a classical mixing reactor for gas hydrates, and measured them by the powder X-ray diffraction (PXRD). PXRD patterns of the TBAB+(CO2 or CH4) hydrates formed with 0.32 of TBAB mass fraction in the aqueous phase were obtained. They are consistent with the orthorhombic hydrate (Shimada et al., Acta Crystallogr. 2005; Muromachi et al., Chem. Commun. 2014), but not identical with the other stable phase, i.e., the tetragonal TBAB hydrate (Rodionova et al., J. Phys. Chem. B 2013). When the aqueous solutions are under the substantial pressure of CO2 or CH4 gas, TBAB is likely to form the orthorhombic Pmma and/or Imma phases. A question for the bulk orthorhombic TBAB hydrate phase about the scarce gas incorporation is newly proposed.

Inhibition effect of 1-ethyl-3-methylimidazolium chloride on methane hydrate equilibrium

The dissociation conditions of methane hydrate in the presence of 0.1, 0.2, 0.3 and 0.4 mass fraction of 1-ethyl-3-methylimidazolium chloride (abbreviated by EMIM-Cl hereafter) were experimentally determined. A high pressure micro-differential scanning calorimeter equipped with a motorized pump was applied to measure the dissociation temperature of the (hydrate+liquid water+vapor) three-phase equilibrium under a constant pressure process with a pressure ranging from (5.0 to 35.0)MPa. The addition of EMIM-Cl would inhibit the methane hydrate formation. The most significant inhibition effect was observed at 0.4 mass fraction of EMIM-Cl in aqueous solution to lower the dissociation temperature by 12.82K at 20.00MPa in comparison to that of the (methane+water) system. The Peng–Robinson–Stryjek–Vera equation of state incorporated with COSMO-SAC activity coefficient model and the first order modified Huron–Vidal mixing rule were applied to evaluate the fugacity of vapor and liquid phase. A modified van der Waals and Platteeuw model with an explicit pressure dependence of the Langmuir adsorption constant was applied to determine the fugacity of hydrate phase. The predictive thermodynamic model successfully describes the tendency of phase behavior of methane hydrate in the presence of EMIM-Cl in the range from 0.1 to 0.4 mass fraction with absolute average relative deviation in predicted temperature of 0.70%.

Phase Behavior and Structural Characterization of Ionic Clathrate Hydrate Formed with Tetra-n-butylphosphonium Hydroxide: Discovery of Primitive Crystal Structure

This paper reports phase equilibrium measurements and crystal structure analysis on the ionic clathrate hydrate formed from tetra-n-butylphosphonium hydroxide (TBPOH). Phase equilibrium temperatures were measured in the mole fraction range of TBPOH in aqueous solution from 0.0072 to 0.0416. The highest ionic clathrate hydrate–solution equilibrium temperature was determined to be 290.2 K at a TBPOH mole fraction of 0.0340, which corresponds to the congruent composition. Single-crystal X-ray diffraction measurements were performed on the crystal formed at 288.7 K, and the chemical composition of the TBPOH hydrate crystal was determined to be TBPOH·29.6H2O, which is consistent with the congruent composition obtained by the phase equilibrium measurement. The crystal structure of the TBPOH hydrate has a superstructure identical with Jeffrey’s type I cubic structure, with an I43d space group with a lattice constant of 24.5191(13) A. The TBPOH hydrate structure is compared with the same hydrate structure formed...

Enhanced CH₄ Recovery Induced via Structural Transformation in the CH₄/CO₂ Replacement That Occurs in sH Hydrates.

The CH4/CO2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH4 recovery induced via structural transformation with a CO2 injection. In this study, neohexane (NH) is used as a liquid hydrocarbon guest in the sH hydrates. Direct thermodynamic measurements and spectroscopic identification are investigated to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in the sH (CH4 + NH) hydrate. The hydrate phase behavior and the (13)C NMR and Raman spectroscopy results of the CH4 + CO2 + NH systems demonstrate that CO2 functions as a coguest of sH hydrates in CH4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO2-rich conditions. CO2 molecules are found to preferentially occupy the medium 4(3)5(6)6(3) cages of sH hydrates or the large 5(12)6(2) cages of sI hydrates during the replacement. Due to the favorable structural transition and resulting re-establishment of guest distributions, approximately 88% of the CH4 is recoverable from sH (CH4 + NH) hydrates with a CO2 injection. The hydrate dissociation and subsequent reformation caused by the structural transformation of sH to sI is also confirmed using a high-pressure microdifferential scanning calorimeter through the detection of the significant heat flows generated during the replacement.

Vapor–hydrate two-phase and vapor–liquid–hydrate three-phase equilibrium calculation of THF/CH4/N2 hydrates

Tetrahydrofuran (THF) is a popular thermodynamic promotor for gas hydrates. Nonetheless, the phase equilibrium calculations of THF hydrates are limited given the abundant experimental data. In this work, we successfully applied the Chen–Guo hydrate model to the THF/CH4/N2 system with different compositions of CH4/N2 gas mixtures. In consideration of various reaction conditions, we proposed the vapor–hydrate two-phase and vapor–liquid–hydrate three-phase models for the equilibrium separations of THF/CH4/N2 via hydrate formation. The hydrate formation condition and equilibrium separation of CH4/N2 in 6mol% THF solution were calculated and compared with experimental results. The average absolute relative deviations between the experimental and calculated data for hydrate formation pressure and equilibrium separation compositions are 6.86% and 7.56%, respectively. This demonstrates that the Chen–Guo model has relatively high accuracy for the equilibrium calculation of THF/CH4/N2 hydrates. This work provides an effective approach to predict the thermodynamic phase behavior of hydrates containing promotors or inhibitors.

Structural transition and phase behavior of N2 gas hydrates with pinacolyl alcohol and tert-amyl alcohol

The influences of large molecular alcohols (LMAs) (pinacolyl alcohol (PCA) and tert-amyl alcohol (tAA)) on N2 hydrate were examined with a primary focus on the hydrate phase equilibria and structural transition. The four-phase (H–Lw–LLMA–V) equilibria of the N2+PCA+water and N2+tAA+water systems were experimentally measured in order to determine the thermodynamic stability conditions of the N2 hydrates with LMAs. The H–Lw–LLMA–V curves of both N2+PCA and N2+tAA hydrates were significantly shifted to more thermodynamically stable regions. Powder X-ray diffraction (PXRD) and Raman analyses verified that the structure of N2 hydrates with LMAs was transformed from the original structure of sII to sH (P6/mmm) as a result of the inclusion of LMAs. In addition, the formation of sH hydrate from the N2+PCA+water system was further verified using an endothermic dissociation thermogram from a high pressure micro-differential scanning calorimeter (HP μ-DSC) by confirming the appearance of three distinct peaks that correspond to ice, sII hydrate, and sH hydrate. Therefore, the experimental results that were obtained in this study are expected to be informative for understanding the roles of LMAs in affecting the macroscopic hydrate phase behavior and microscopic hydrate structure.

Hydrate and Phase Behavior Modeling in CO2-Rich Pipelines

Carbon dioxide transport will be a key part of any Carbon Capture and Storage (CCS) system. Generally for CCS, flow assurance modeling of these pipelines has been restricted to pure CO2 systems; however CO2 streams coming from capture processes are not pure and contain various components. These fluids have a certain level of moisture, so dehydration is needed before delivering CO2 rich fluid to the export pipeline, in order to prevent potential hydrate formation, two-phase flow and corrosion in the export line. In this contribution, the hydrate phase equilibria of the binary CO2 + (N2 or CH4 or O2 or Ar or CO) systems in the presence of a free water phase were determined as well as direct hydrate dissociation conditions of liquid CO2 in absence of a free phase. A rigorous and generalized model is presented to predict the phase behavior, hydrate dissociation pressures and the dehydration requirements of CO2 rich gases. A statistical thermodynamic approach, with the Cubic-Plus-Association equation of state, is employed to model the phase equilibria. The hydrate-forming conditions are modeled by the solid solution theory of van der Waals and Platteeuw. Predictions of the developed model are first validated using simple systems and then for more complicated synthetic multicomponent systems. Showing that accurate phase behavior prediction of CO2 rich stream is critical for accurate predictions of the hydrate phase behavior of these fluids.

Modelling of cyclopentane promoted gas hydrate systems for carbon dioxide capture processes

A thermodynamic model based on the Cubic-Plus-Association equation of state and the van der Waals–Platteeuw hydrate model is applied to perform a thermodynamic evaluation of gas hydrate forming systems relevant for post-combustion carbon dioxide capture.A modelling study of both fluid phase behaviour and hydrate phase behaviour is presented. Cycloalkanes ranging from cyclopropane to cyclohexane, represents a challenge for CPA, both in the description of the pure component densities and for liquid–liquid equilibrium (LLE) in the binary systems with water. It is concluded that an insufficient amount of reliable LLE data exist for the binary system of water and cyclopentane. Additional water-in-oil data in particular are desired for this system.An unpromoted hydrate-based capture process, operating isothermally at a temperature of 280K is simulated. The minimum pressure requirement of the first stage is estimated to be 24.9MPa. Applying three consecutive hydrate formation/dissociation stages (three-stage capture process), a carbon dioxide-rich product (97mol%) may be delivered at a temperature of 280K and a pressure of 3.65MPa.A second capture process, where cyclopentane is incorporated as a thermodynamic hydrate promoter is simulated. At the presence of cyclopentane the minimum pressure requirement of the first stage (operating at 285K) is lowered to 1.04MPa. This process needs four consecutive hydrate formation/dissociation stages to produce a 95mol% carbon dioxide-rich product stream. The vapour phases in the cyclopentane promoted process contains several mole percent cyclopentane at hydrate equilibrium conditions. At temperatures below 284K, the entire cyclopentane bulk phase evaporates completely at hydrate forming conditions (pressures below 0.55MPa).The present study suggests the hydrate-based separation technology to be unsuitable for the specific case of post-combustion carbon dioxide capture from power station flue gases, where operating pressures should preferably remain close to atmospheric. Even though the hydrate structure becomes available at low pressure conditions (by use of thermodynamic promoters), carbon dioxide may not necessarily enter the solid phase in significant amounts.