Ethane binaries

A number of tests were also carried out with liquefied ethane containing various amounts of heavier hydrocarbons. The compositions where RPTs were reported are given in Table VI. Also shown is the ratio TJT which, from previous tests, would be expected to be slightly greater than unity for an RPT to occur. The agreement is excellent. Note also that, for pure ethane, TJT i 1.09, yet no RPT results if this liquefied gas is simply poured into water. The value TJT 1.09 is close to the upper cutoff of the ethane binaries in Table VI. While theory would still indicate that RPTs are possible, they do not often occur when the water temperature is much above the superheat limit temperature of the liquefied gas. 

The phase boundary lines for supercritical ethane at 250 and 350 bar are shown in Figure 2. The surfactant was found to be only slightly soluble in ethane below 200 bar at 37 C, so that the ternary phase behavior was studied at higher pressures where the AOT/ethane binary system is a single phase. As pressure is increased, more water is solubilized in the micelle core and larger micelles can exist in the supercritical fluid continuous phase. The maximum amount of water solubilized in the supercritical ethane-reverse micelle phase is relatively low, reaching a W value of 4 at 350 bar. 

In this article we present an experimental method to measure the selectivity in binary gas adsorption under infinite dilution conditions over a range of pressures and thereby characterize its behavior with respect to both pressure and composition. The method is hist, efficient and robust. We use the methane-ethane binary gas mixture on silicalite as a demonstration. The experimental results obtained using this method are compared with the predictions from two models to check their validity. 

Figure 5.1 A comparison of calculated (lines) and experimental (symbols) data for the methane-ethane binary system. For these calculations k) is 0.02 (Igel, 1985). (14.504 psi = 1 bar)...

This form ensures that ktj — 0 when ncj = nCi it is implied that ncj > nCi.) For the ethylene and ethane binaries, the value of m is 0.0202. A substantially better fit of the data is possible if ky is assumed to be linearly dependent on carbon number ... 

Table II. Optimum Values for Ethylene and Ethane Binaries...

The similarity between ethylene and ethane or even benzene and n-hexane no longer holds when some specific chemical interaction takes place between i and /. A good example of this is provided by the acetylene binaries, which have substantially higher fcy s than the ethylene or ethane binaries. Furthermore, the kij for acetylene-ethylene is 0.02 units lower than that for acetylene-ethane (4). 

The first ionization potential may prove useful in sorting out and ordering the data for binaries of inorganic compounds, but it is certainly of little use in the prediction of ky s for hydrocarbon-hydrocarbon binaries. For example, the I of n-pentane (10.55 eV) is very similar to that of ethylene (10.51 eV), but the kys for the n-pentane binaries are markedly lower than those for the corresponding ethylene binaries (4). On the other hand, the ethylene and ethane binaries have similar kys (Figure 2), even though the I of ethane is 1.25 eV higher than for ethylene (20). 

Figure 12 shows how the ethylene/ethane separation factor decreases with increasing ethylene partial pressure. These data were obtained by performing experiments where the composition of the ethylene/ethane binary fe was varied at four feed and sweep gas total pressures. The decrease of the separation factor is a consequence of the carrier saturation phenomena commonly seen in flicilitated transport which causes the ethylene permeability to decrease with increasing ethylene partial pressure. 

To illustrate calculations for a binary system containing a supercritical, condensable component. Figure 12 shows isobaric equilibria for ethane-n-heptane. Using the virial equation for vapor-phase fugacity coefficients, and the UNIQUAC equation for liquid-phase activity coefficients, calculated results give an excellent representation of the data of Kay (1938). In this case,the total pressure is not large and therefore, the mixture is at all times remote from critical conditions. For this binary system, the particular method of calculation used here would not be successful at appreciably higher pressures. 

So far we have considered only a single component. However, reservoir fluids contain a mixture of hundreds of components, which adds to the complexity of the phase behaviour. Now consider the impact of adding one component to the ethane, say n-heptane (C7H.,g). We are now discussing a binary (two component) mixture, and will concentrate on the pressure-temperature phase diagram. 

Figure A2.5.31. Calculated TIT, 0 2 phase diagram in the vicmity of the tricritical point for binary mixtures of ethane n = 2) witii a higher hydrocarbon of contmuous n. The system is in a sealed tube at fixed tricritical density and composition. The tricritical point is at the confluence of the four lines. Because of the fixing of the density and the composition, the system does not pass tiirough critical end points if the critical end-point lines were shown, the three-phase region would be larger. An experiment increasing the temperature in a closed tube would be represented by a vertical line on this diagram. Reproduced from [40], figure 8, by pennission of the American Institute of Physics.

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (soHd line) for each pure component, ending at the pure component critical point. The loci of critical points for the binary mixtures (shown by the dashed curve) are continuous from the critical point of component one, C , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are CO2—/ -hexane and CO2—benzene. More compHcated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (Hquid—Hquid) immiscihility lines, and even three-phase (Hquid—Hquid—gas) immiscihility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include CO2—hexadecane and CO2—H2O Class IV, CO2—nitrobenzene Class V, ethane—/ -propanol and Class VI, H2O—/ -butanol. 

Adsorption of hard sphere fluid mixtures in disordered hard sphere matrices has not been studied profoundly and the accuracy of the ROZ-type theory in the description of the structure and thermodynamics of simple mixtures is difficult to discuss. Adsorption of mixtures consisting of argon with ethane and methane in a matrix mimicking silica xerogel has been simulated by Kaminsky and Monson [42,43] in the framework of the Lennard-Jones model. A comparison with experimentally measured properties has also been performed. However, we are not aware of similar studies for simpler hard sphere mixtures, but the work from our laboratory has focused on a two-dimensional partly quenched model of hard discs [44]. That makes it impossible to judge the accuracy of theoretical approaches even for simple binary mixtures in disordered microporous media. 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

Carbon forms a huge number of binary compounds with hydrogen. Three major categories of these compounds are alkanes, alkenes, and alkynes. An alkane has only single bonds between carbon atoms. The four simplest alkanes, which are shown in Figure 3-7. are methane, ethane, propane, and butane. An alkene, on the other hand, contains one or more double bonds between carbons, and an alkyne has one or more triple bonds between carbon atoms. Figure shows the structures of ethylene, the simplest alkene, and acetylene, the simplest alkyne. 

Data at two temperatures were obtained from Zeck and Knapp (1986) for the nitrogen-ethane system. The implicit LS estimates of the binary interaction parameters are ka=0, kb=0, kc=0 and kd=0.0460. The standard deviation of kd was found to be equai to 0.0040. The vapor liquid phase equilibrium was computed and the fit was found to be excellent (Englezos et al. 1993). Subsequently, implicit ML calculations were performed and a parameter value of kd=0.0493 with a standard deviation equal to 0.0070 was computed. Figure 14.2 shows the experimental phase diagram as well as the calculated one using the implicit ML parameter estimate. 

Zeck S., and H. Knapp, "Vapor-Liquid and Vapor-Liquid-Liquid Phase Equilibria for Binary and Ternary Systems of Nitrogen, Ethane and Methanol Experiments and Data Reduction", Fluid Phase Equilibria, 25,303-322 (1986). 

Temperature Two modes of temperature parametric-pumping cycles have been defined—direct and recuperative. In direct mode, an adsorbent column is heated and cooled while the fluid feed is pumped forward and backward through the bed from reservoirs at each end. When the feed is a binary fluid, one component will concentrate in one reservoir and one in the other. In recuperative mode, the heating and cooling takes place outside the adsorbent column. Parametric pumping, thermal and pH modes, have been widely studied for separation of liquid mixtures. However, the primary success for separating gas mixtures in thermal mode has been the separation of propane/ethane on activated carbon [Jencziewski and Myers, Ind. Eng. Chem. Fundam., 9, 216-221 (1970)] and of air/S02 on silica gel... 

The interaction parameters for binary systems containing water with methane, ethane, propane, n-butane, n-pentane, n-hexane, n-octane, and benzene have been determined using data from the literature. The phase behavior of the paraffin - water systems can be represented very well using the modified procedure. However, the aromatic - water system can not be correlated satisfactorily. Possibly a differetn type of mixing rule will be required for the aromatic - water systems, although this has not as yet been explored. 

Ethane - Hater System. The data used for the determination of the interaction parameters for the ethane - water binary are those of Culberson and McKetta (21), Culberson et al. (22)... 

Propane - Water System. The interaction parameters for the propane - water system were obtained over a temperature range from 42°F to 310°F using exclusively the data of Kobayashi and Katz (24). This is because among the available literature on the phase behavior of this binary system, their data appear to give the most extensive information. A constant interaction parameter was obtained for the propane-rich phases at all temperatures. The magnitude of the temperature - dependent interaction parameter for this binary was less than that for the ethane - water binary at the same temperature. Azarnoosh and McKetta (25) also presented experimental data for the solubility of propane in water over about the same temperature range as that of Kobayashi and Katz but at pressures up to 500 psia only. However, a different set of temperature - dependent parameters... 

Three-Phase Loci. Figure 11 shows the three-phase loci for the alkane - water systems. No experimental three-phase data were available in the literature for the ethane - water binary. 

Five binary-hydrocarbon mixtures of ethane or ethylene with heavier hydrocarbons were studied (Table III). The only substrate used in these studies was water. If an RPT did not occur, ice always formed rapidly. When n-butane or n-pentane was the heavier component, RPTs were 100% reproducible over a particular composition range. This was not, however, true if the heavier component were propane. 

In addition to binary mixtures, the ethane-propane- -butane ternary system was studied (see Fig. 1). Spills were also made with mixtures containing methane. The addition of as little as 10 mole % methane inhibited RPTs and none were ever obtained with methane concentrations in excess of 19 mole %. 

Binary Mixtures Containing Ethane Spills on 297 12 K Water"... 

Use values of 0.02 for binary interaction coefficient between methane and n-butane, 0.01 between ethane and n-butane, and 0.0 between methane and ethane. 

Figure 15.5. Adsorption of binary mixtures (1) ethane + ethylene. Type 4A MS 25°C, 250Torr (2) ethane + ethylene. Type 4A MS, 25°C, 730Torr (3) ethane + ethylene. Type 4A MS, 75°C, 730Torr (4) carbon dioxide + hydrogen sulfide. Type 5A MS, 27°C, 760Torr (5) n-pentane + n-hexane, type 5A MS, 100°C, 760 Torr (6) ethane + ethylene, silica gel, 25°C, 760 Tort (7) ethane + ethylene, Columbia G carbon, 25°C, 760 Torr (8) acetylene + ethylene. Type 4A MS, 31°C, 740 Torr. (Data from Union Carbide Corp.)...

Of the natural gas components that form simple hydrates, nitrogen, propane, and iso-butane are known to form structure II. Methane, ethane, carbon dioxide, and hydrogen sulfide all form si as simple hydrates. Yet, because the larger molecules of propane and iso-butane only fit into the large cavity of structure II, natural gas mixtures containing propane and iso-butane usually form structure II hydrate (see Section 2.1.3.3 in the subsection on structural changes in binary hydrate structure). 

Similarly, over a wide range of composition for methane and ethane, AZZd values are similar (74 kJ/mol) for components entering both cavities of si. Identical arguments may be used to explain similar AZZd values of 79.5 7 kJ/mol (Mehta and Sloan, 1996) for sH binary mixtures with methane, since all three cavities are occupied. 

While a first approach to phase diagrams is given here, Section 5.2 extends the phase diagrams in this portion of Chapter 4 to single, binary, and ternary mixtures of methane, ethane, and propane. The reader may wish to consult Section 5.2 for a more enlightening discussion that applies the van der Waals and Platteeuw method to the most common components of natural gases. 

In the discussion appendix of the original paper by Carson and Katz (1942), Hammerschmidt indicated that, while the method was acceptable for gases of normal natural gas composition, an unacceptable deviation was obtained for a gas rich in ethane, propane, and the butanes. More work is also required to revise the Kvs -value charts for two components, namely, carbon dioxide and nitrogen. In three-phase hydrate data for binary mixtures of carbon dioxide and propane, Robinson and Mehta (1971) determined that the Kvs method for carbon dioxide gave unsatisfactory results. The API Data Book shows the Kvs values for nitrogen to be only a function of pressure, without regard for temperature Daubert (Personal... 

Although a typical natural gas is mainly comprised of the first three normal paraffins, the phase equilibria of each component with water will differ from that of a natural gas with water. However, a comparison of predictions with data for methane, ethane, and propane simple gas hydrates is given as a basis for understanding the phase equilibria of water with binary and ternary mixtures of those gases. 

To evaluate the phase equilibria of binary gas mixtures in contact with water, consider phase diagrams showing pressure versus pseudo-binary hydrocarbon composition. Water is present in excess throughout the phase diagrams and so the compositions of each phase is relative only to the hydrocarbon content. This type of analysis is particularly useful for hydrate phase equilibria since the distribution of the guests is of most importance. This section will discuss one diagram of each binary hydrate mixture of methane, ethane, and propane at a temperature of 277.6 K. 

Of the possible binary combinations of methane, ethane, and propane, the methane + propane + water system (Figure 5.15) is the simplest. 

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