Definition:Miscibilityis the property of substances to mix in all proportions, forming ahomogeneoussolution. The term is most often applied toliquids, but applies also to solids and gases.Waterandethanol, for example, are miscible because they mix in all proportions.By contrast, substances are said to be immiscible if a significant proportion does not form asolution. Otherwise, the substances are considered miscible. For example,butanoneis significantly soluble in water, but these two solvents are not miscible because they are not soluble in all proportions.Organic compound:Inorganic compounds, theweight percentofhydrocarbonchain often determines the compound's miscibility with water. For example, among thealcohols,ethanolhas twocarbonatomsand is miscible with water, whereas1-octanolwith eight carbons is not. Octanol's immiscibility leads it to be used as a standard forpartition equilibria. This is also the case withlipids; the very long carbon chains of lipids cause them almost always to be immiscible with water. Analogous situations occur for otherfunctional groups.Acetic acid(CH3COOH) is miscible with water, whereasvaleric acid(C4H9COOH) is not. Simplealdehydesandketonestend to be miscible with water, because ahydrogen bondcan form between the hydrogen atom of awater moleculeand theunbonded(lone) pair of electrons on thecarbonyloxygenatom.MiscibilityMiscibility is the ability of two liquids to mix with each to form a homogeneous solution. Water and ethanol, for example, are miscible. They can be mixed in any proportion, and the resulting solution will be clear and show only one phase. Oil and water, on the other hand, are immiscible. A mixture of vegetable oil and water will always separate into two layers or phases, and won't dissolve in each other.Miscibility is often expressed as a wt/wt%, or weight of one solvent in 100 g of final solution. If two solvents are totally miscible in all proportions, their miscibility is 100%. Other solvents are only partially miscible, meaning that only some portion will dissolve in water.Diethyl ether, for example, is partially miscible with water. Up to 7 grams of diethyl ether will dissolve in 93 g of water to give a 7% (wt/wt%) solution. If more diethyl ether is added, a separate diethyl ether layer will appear floating above the water. Most solvents show some miscibility in one another, although it might be very low.Most of the liquids encountered in everyday life are either water-based, called aqueous, or organic, which in the chemical sense means they contain carbon atoms. These can generally be divided into two broad classes. They're either hydrophilic, "water-loving," or lipophilic, "fat loving." Lipophilic solvents are miscible with hydrocarbon solvents, that is, solvents containing only carbon and hydrogen, like fats and oils. Hydrophilic solvents are miscible with water.

How the biodiesel miscible with diesel:Producer:The term "miscible" means (for liquids) two liquids will mix with each other in any proportion. For instance, methanol is miscible in water.The term "soluble" means two liquids will mix with each other. For instance methanol is soluble in biodiesel. There is limit to how much of liquid A will dissolve in liquid B.What does this really mean?Here is an example that helps me to understand the difference.Gasoline containing 10% ethanol is sold in Illinois. Since there is a tax break involved, both the departments of revenue and agriculture want to be sure there is a minimum of 10% ethanol in gasoline advertised as "gasohol". The state uses a simple test.A graduated cylinder with 110 equal marks is filled up to the 90th mark with gasohol. Then water is added to bring the total volume up to the 110th mark. The mixture is then gently agitated and allowed to settle/separate. After just a minute or so, two separate liquids become visible in the cylinder. The lower mixture comes up to about the 17th or 18th mark in the cylinder, while the second fluid extends from there to the top.The lower mixture is alcohol and water, the upper mixture is gasoline.Ethanol is soluble in gasoline, so once mixed to form gasohol they do not separate as long as they are the only two ingredients in the mix.Water has a very low solubility in gasoline, but is MISCIBLE with the ethanol. Given the choice between gasoline and water, the ethanol chooses the water over the gasoline. The combined mixture is heavier than gasoline, so settles rapidly.You would think the ethanol and water would register on the 20th mark on the cylinder. They don't because, at least in part, the ethanol/water mixture is somewhat soluble in gasoline.How does this phenomenon affect methanol recovery from biodiesel?Methanol is soluble in biodiesel, but it is miscible in water. When wash water is added to finished biodiesel, the excess methanol chooses the water over the biodiesel and drops out with the water.Additional washes remove additional soap and glycerol. But, for the most part, the first wash removes the methanol.However, as in the gasohol example, a small portion of the water/methanol mixture is soluble in biodiesel. We dry biodiesel to remove the dissolved water. In so doing a small amount of residual methanol is also removed.Equations of state for miscible processesIn practice, vapor/liquid reservoir phase behavior is calculated by anequation of state(EOS). The two most common EOSs that have been used for oil-recovery solvent-injection processes are the Peng-Robinson EOS[1]and the Soave-Redlick-Kwong EOS.[2]Of the two, the Peng-Robinson EOS seems to be the one most often cited in the literature and is the one discussed in some detail. The Soave-Redlick-Kwong EOS is used in a similar manner to predict solvent/oil phase behavior.Calculating phase behavior with equations of state (EOS)Peng and Robinson originally proposed the two-parameter EOS shown next for a pure component:....................(1)....................(2)....................(3)and....................(4)where= the component acentric factorTc= component critical temperatureandpc= component critical pressure.For heavier components, where> 0.49, the following equation is recommended:....................(5)The constants inEqs. 2and4are often designated aand b.Eq. 1represents continuous fluid behavior from the solvent to liquid state, and it can be rewritten as....................(6)where....................(7)and....................(8)Jhaveri and Youngren[3]adapted a procedure used by Penelouxet al.[4]and modified the originalEq. 1to include a third parameter to allow more-accurate volumetric predictions, which is recommended for solvent/oil simulations. The third parameter does not change the vapor/liquid equilibrium conditions determined by the unmodified, two-parameter equation. Instead, it modifies the phase volumes by making a translation along the volume axis.Eqs. 9and10give the modified three-parameter equation:....................(9)....................(10)wheresis the volumetric shift parameter.For mixtures:....................(11)....................(12)....................(13)and....................(14)InEq. 12,ijis the binary interaction coefficient that characterizes the binary formed by componentsiandj.Eqs. 10through13apply both to pure components and to lumped pseudocomponents that represent two or more pure components in complex mixtures.The following expression derived from thermodynamic relationships and the EOS allows calculation of the fugacity,fj, of componentjin a mixture:....................(15)Thus, by satisfying the equilibrium condition, vapor/liquid equilibrium ratios can be calculated, and flash calculations can be made to calculate the compositions of vapor and liquid in equilibrium, molar splits, and volumes.Solution of the EOS does not calculate phase viscosities directly. This is done from some external calculation once the phase compositions and densities are known. A commonly used calculation for liquid-mixture viscosity is the Lohrenz-Bray-Clark method, which requires the critical volumes of each component or pseudo component in the mixture.[5]Refer toOil viscosityandGas viscosityfor more information on calculating viscosities.Characterizing the fluid systemTo useEqs. 1through15for calculating the phase behavior and properties of solvent compositional processes in oil recovery, the following steps must be taken to "characterize" the fluid system in question:Analyze theoil composition. This can be done by distillation or chromatographic methods. An extended analysis through at least C25+is preferred. The advantage of distillation is that molecular weight, boiling point, and density can be measured on the distillation cuts.Represent the multi-component reservoir fluid by an appropriate division into pure components and pseudo components. Pure components through C 5 plus three to five pseudo components usually will suffice. It may be possible to reduce the number of pure components and pseudo components further by combining similar components.Make an initial assignment of critical pressure and temperature, a centric factor, critical volume (or critical compressibility), volumetric shift parameter, and interaction parameters for each component and pseudo component.Tune the above properties for the pseudo components by comparing predictedphase behaviorand properties with suitable experimental data.Methods for dividing into pseudo components and estimating critical properties, shift parameters, and binary interaction coefficients are described in detail in Whitson and Brule.[6]Because of the approximations inherent in an EOS as well as the approximations required to represent a multi component reservoir fluid in a tractable form, it should be expected that phase-behavior properties and equilibrium compositions predicted with an EOS will depart from measured values over the range of composition and pressure conditions anticipated in areservoir simulation. For this reason, additional adjustment of EOS parameters will be required for predictions to represent experimental measurements adequately. These adjustments usually are made by regression.Reservoir oils usually are subjected to routine pressure/volume/temperature (PVT) experiments that give the volumetric and phase-behavior information necessary for predicting conventional recovery methods such as solution solvent drive orwater flooding. Experiments such as constant-composition expansion, differential liberation, constant-volume depletion, and separator tests provide black-oil properties. Other PVT experiments are more specific for solvent injection. These include swelling tests and multiple-contact experiments.The swelling experiment is sometimes called a pressure-composition diagram determination. Injection solvent is added to reservoir oil in increments to give mixtures that contain increasing amounts of injection solvent. After each addition of solvent, the saturation pressure is measured at reservoir temperature. Overall composition of these mixtures ranges from that of black oil to compositions up to and beyond near-critical conditions (i.e., overall compositions that traverse a range from bubblepoint to dewpoint mixtures at reservoir temperature). Thus, the swelling experiment provides some PVT and phase-equilibrium information on mixture ranges that might reflect compositions as solvent displaces oil through the reservoir. It provides information on the saturation pressure of injection-solvent/oil mixtures, the swelling or increase in oil formation volume factor as solvent is added, the composition of the critical mixture, and the liquid saturation vs. pressure in the two-phase region of the diagram.Multiple-contact tests seek to simulate the solvent/oil multiple contacting that occurs in a reservoir. A forward multicontact experiment tries to simulate multicontacting in a vaporizing-solvent drive. A reverse multicontact experiment tries to simulate the multicontacting that occurs in a purely condensing-solvent drive. The experiments give information concerning equilibrium-phase volumes and compositions.In a reverse-contact experiment, the PVT cell is charged with the reservoir fluid at the desired pressure and temperature, and an increment of injection solvent is added sufficient to form a two-phase mixture (or a three-phase mixture in some tests). The phases are allowed to equilibrate, and phase volumes are measured. The solvent phase is then displaced from the cell, and oil and solvent compositions are measured. The procedure is repeated, with injection of a new increment of injection solvent introduced into the cell to contact equilibrium oil left after the first contact.In the forward-contact experiment, the oil phase is displaced after the first contact, and the remaining equilibrium-solvent phase in the cell is contacted with a fresh increment of reservoir oil.The objective of tuning is to ensure that the EOS predicts fluid properties and phase equilibrium compositions accurately over the range of pressure, temperature (if this varies), and composition that one expects to encounter in a simulation. If the simulation is for a solvent compositional process, then at a minimum the EOS should predict properties and phase equilibrium for the range of injection-solvent/oil mixtures and pressures encountered in the simulation study. It also should predict adequately for any black-oil conditions expected in the simulation (e.g., water flooding or pressure depletion before solvent injection) and for the separator conditions expected.Pedersenet al.[7]observed that an EOS tuned to match a specific set of data may not give reliable predictions for other data not included in the tuning process. However, when both sets of data are included in the tuning process, the prediction for either one may not be quite as good as for tuning against these data individually.It seems prudent that at a minimum, there should be differential depletion data, separator tests, and swell data to tune an EOS against for making solvent-compositional simulations. Swelling tests are necessary when near-critical compositions are expected in the simulation, and it is necessary for the swell tests to explore this composition region. Swell tests with several different injected-solvent compositions might be warranted if optimization of the solvent composition is an objective of the simulation study.The value added by multiple-contact tests is unclear. These are the most difficult and expensive of the experiments discussed earlier, yet they provide direct measurements of vapor/liquid equilibrium compositions and molar splits for a composition path that at least crudely mimics the development of compositions at the leading or trailing edges of the solvent/oil transition zone, which is, of course, what the simulator is trying to calculate. However, for the condensing/vaporizing process, multiple-contact experiments do not give compositions that are very near the critical point.Although they are difficult and expensive to run, slimtube tests give a direct verification of the ability of the EOS to predict minimum miscibility pressure (MMP) or minimum miscibility enrichment (MME). If the EOS after regression of parameters does not predict slimtube MMP or MME, further adjustment of parameters is required.Nomenclaturea=Constant

b=Constant

i=componenti

j=componentj

k=permeability, md

=component acentric factor, dimensionless

Z=fluid compressibility factor, dimensionless

n=number of components

Nca=capillary number, dimensionless

pg=pressure gradient through the displacing phase, psi

R=universal gas constant, units consistent with other equation parameters

s=volumetric shift parameter, dimensionless

T=temperature, R

Tc=critical temperature, R

Tr=reduced temperature,T/Tc

v=volume, cubic ft

vxs=fluid velocity along a streamline, ft/sec

oi=oil viscosity, cp