Identification and Characterization of Nonflammable Azeotropic Mixtures

IDENTIFICATION AND CHARACTERIZATION OF NONFLAMMABLE AZEOTROPIC MIXTURES FOR PRECISION CLEANING CHAPTER I Introduction & Theory The purpose of this research project was to provide methods that could be used to search for azeotropic solvent blends with targeted properties as well as to characterize these blends. This study focused on non-flammable, non-aqueous, binary blends for cleaning applications. However, these methods should be applicable to all solvent blends used in any application where a blend with targeted properties is wanted. Another goal was to explore a way to predict azeotrope formation mathematically, without any vaporliquid equilibrium (VLE) data. Azeotropy An azeotrope is a mixture of two or more components that maintains the same composition in both the liquid and gas phases. An azeotrope has properties that reflect its components’ properties. For example, one solvent may have extremely good solvency but an extremely low flash point, while another solvent has a poor solvency but no flash point. If the two solvents form an azeotrope, the component with no flash point may quench the flammability of the other component. The solvency of the mixture will suffer due to the poor solvent, but sometimes the level of solvency required to do the job is actually much lower than that of the good solvent anyway. There are other important considerations besides physical properties, which can be improved using azeotropes. An example of one such consideration is an occupational exposure limit. An occupational exposure limit is a maximum that an employee can be subjected to a particular chemical substance or other work-related health hazard such as extremely loud noise. Some of these limits are law and are enforced by the Occupational Safety and Health Administration (OSHA) in the United States. There are several types of exposure limits including permissible exposure limits (PELs), threshold limit values (TLVs), time weighted average PELs (TWA-PELs), and short term exposure limits (STELs). A PEL is a purely legal limit that states that a worker cannot be exposed to more than a certain amount of a health hazard on any given day.6 A TLV is a more scientific limit that defines a maximum amount that it is believed a worker can be exposed to a health hazard during every day of the person’s working lifetime without having adverse effects.7 A TWA-PEL is applicable to health hazards that are only noticeably harmful after accumulation, and allows workers to be subjected to these health hazards in large amounts for short periods of time without breaking the PEL.6 STELs are applicable to health hazards that are immediately harmful to the human body, this type of exposure limit does not allow the exposure to be averaged over the entire work day.6 These exposure limits for chemical substances are measured in parts per million (ppm) by volume. Considering all of these types of limits, some substances have no exposure limit, while others have exposure limits at very low concentrations. If a 1 to 1 mixture between one substance with no exposure limit, and one with a low exposure limit forms an azeotrope, then the exposure to each substance is halved. This means that a worker could be exposed to the azeotrope twice as long as the pure substance (with the low exposure limit) before reaching the limit. By definition, an azeotrope cannot be separated using distillation, which can be a good or bad quality depending on the point of view. It is a good quality if the goal is to make a flammable solvent non-flammable by adding a flame retardant as was done in this study. This is because a certain amount of the flame retardant is required to quench the flammability, and if the composition were to shift, the mixture may become flammable again later. However, if there is a mixture of miscible liquids that needs to be separated, and they form an azeotrope, the fact that it cannot be separated by distillation can be very troublesome. There are ways to “break” an azeotrope, but these methods are beyond the scope of this thesis. There are several types of binary azeotropes that have been reported. The categories are as follows: homogeneous azeotropes in completely miscible systems, homogeneous azeotropes in partially miscible systems, heterogeneous azeotropes, lowboiling or pressure maximum azeotropes, high-boiling or pressure minimum azeotropes, and double azeotropes.8 A completely miscible system means that the mixture will always be one phase when temperature or pressure is changed. However, a partially miscible system will be one phase in a certain range of temperatures or pressures, but multiple phases in other ranges. A low-boiling azeotrope is an azeotrope that has a lower boiling point than the lowest boiling component of the mixture. Analogously, a highboiling azeotrope is an azeotrope that has a higher boiling point than the highest boiling component of the mixture. A double azeotrope is a mixture that forms two different azeotropes at two different compositions, a low-boiling azeotrope at one composition, and a high-boiling azeotrope at a different composition. A binary azeotropic mixture is always defined as being in two of the categories given above, one of the first three categories and one of the last three categories. For example, a low-boiling azeotrope in a completely miscible system, uses two of the descriptive categories to form an enhanced description of a binary azeotrope. Some mixtures have also been reported as “nearazeotropic”, meaning that they are not actually azeotropic mixtures, but are treated similarly because they are extremely difficult to separate. The different types of binary azeotropic systems are represented by the phase diagrams in Figure 1. Figure 1. Different types of binary azeotropic systems.8 In Figure 1, the variables x1 and y1 are the mole fractions of the liquid and gaseous phases respectively in terms of one of the components. It is irrelevant which component is taken to be component one because these phase diagrams are generic examples that do not represent any particular azeotropes, and if the other component was taken as component one, the pattern would be a mirror image of the pattern shown. Column A shows plots of y1 versus x1. Column B shows plots of temperature versus x1, and temperature versus y1 on the same plot. In column B, continuous lines represent x1, and dashed lines represent y1. Row I shows a homogeneous low-boiling azeotrope in a completely miscible system. Row II shows a heterogeneous low-boiling azeotrope. Row III shows a homogeneous high-boiling azeotrope in a completely miscible system. Row IV shows a homogeneous low-boiling azeotrope in a partially miscible system. Row V shows a double azeotrope, which is a mixture that forms a high-boiling azeotrope and a low-boiling azeotrope at different compositions. Row VI shows a homogeneous high-boiling azeotrope in a partially miscible system. Mixture Thermodynamics When preparing mixtures of solvents for dissolving a substance, the mixture needs to be one continuous phase so that the properties are the same throughout the mixture. So, the components need to like each other enough to spontaneously mix. Spontaneous mixing is not to be confused with azeotropy. Azeotropes do not have to be homogeneous. This spontaneous mixing can be predicted using the Gibbs free energy of mixing (ΔGmix). A negative value for ΔGmix means that the substances will spontaneously mix, and a positive value predicts that they will not spontaneously mix. ΔGmix can be expressed in terms of enthalpy (ΔHmix), entropy (ΔSmix), and temperature (T) as seen in eq 1. Equations 1, 2, and 3 were found in several different texts.9–13 ∆ ∆ ∆ (1) This equation shows that higher temperatures increase the likelihood of spontaneous mixing if ΔSmix is positive. Luckily, entropy always favors mixing due to the universe’s tendency toward disorder. This can be seen in eq 2. ∆ In eq 2, (2) is the number of moles of molecules, R is the gas constant and is equal to 8.3145 J mol-1 K-1, and are the mole fractions of components 1 and 2 respectively. Because the mole fractions are always less than one, the natural logarithms are always negative which causes ΔSmix to always be positive. This makes ΔSmix always favor mixing. From this, it can be deduced that ΔHmix is the sole reason for two substances to not spontaneously mix. ΔHmix can be calculated using eq 3. ∆ (3) Equation 3 uses the volume fractions ( ) in terms of the total volume (VT), and the Hildebrand parameter ( ), which is described in the HSP section of this chapter. The 1’s and 2’s represent component 1 and 2 of the mixture respectively. Hansen Solubility Parameters (HSPs) Hansen solubility parameters are a quantitative way of predicting if one substance will dissolve in, or interact with, another. Hansen solubility parameters were produced by Charles M. Hansen in 1967 as part of his Ph.D. thesis. They were developed through the idea that if a molecule interacts with its own kind in a similar way that another molecule interacts with its own kind, then the two different molecules are energetically alike, and like dissolves like. This thinking was started by Joel H. Hildebrand with the Hildebrand solubility parameter.12 Equations 4, 5, 6, and 9 in this section were taken from the HSP user handbook.11 The Hildebrand solubility parameter is defined as the square root of the cohesive energy density as shown in eq 4. (4) In eq 4, represents the total solubility parameter or Hildebrand solubility parameter. The cohesive energy, E, is the sum of all the cohesive bonds in the substance. Thus, E can be measured by breaking the cohesive bonds, or simply evaporating the liquid substance. So, E is equal to the change in enthalpy for vaporization (ΔHvap). represents the molar volume of the substance in mL / mol, and can be calculated by dividing the molar mass by the density of the substance. Hansen reasoned that the total cohesive energy could be split into three parts as seen in eq 5. (5) In eq 5, ED is the dispersion cohesive energy, EP is the polar cohesive energy, and EH is the hydrogen bonding cohesive energy (also called electron exchange cohesive energy). These cohesive energy terms can each be divided by the molar volume, and using the relationship in eq 4, eq 6 can be obtained. (6) Solubility relies on the square root of these three different terms when using the HSP model. These three HSPs are the square root of the cohesive energy density in (J/mL)1/2 from dispersion forces (δD or D), dipolar intermolecular forces (δP or P), and hydrogen bonding forces (δH or H). Large differences in the D, P, and H values can cause the ΔHmix to become large enough to make ΔGmix positive, so that the substances will not spontaneously mix. The HSPs can be estimated for any substance whose refractive index and dipole moment are known. According to the Hansen Solubility Parameters in Practice (HSPiP) e-book, the dispersion term can be estimated by eq 7 using the refractive index (R).10 . . (7) This equation represents a correlation and is by no means perfect. An equation was also given to estimate the polar term from the dipole moment (μ) of the molecule in units of Debyes. This equation can be seen here as eq 8.10 . / (8) The hydrogen bonding term can be calculated by subtracting the polar and dispersion cohesive energies from the total cohesive energy, then dividing by the molar volume and taking the square root as in eq 4. The three parameters can be calculated in other ways such as using group contributions, but these methods are beyond the scope of this thesis. The HSPiP software has an extensive database of HSP values and other properties for many compounds. The HSPiP software also includes a tool that will estimate the HSPs of a molecule whose structure is submitted to the program. The usefulness of the HSPs is easily seen when the parameters are plotted in a 3dimensional plot known as the Hansen space. Each set of HSP values are a set of coordinates in three dimensions. However, this 3-dimensional space has been deemed the Hansen space instead of Cartesian space, because the D parameter is multiplied by two in order to make interaction zone spherical with radius R0. The R0 defines a sphere for the solute and any solvent inside of the sphere is predicted to interact strongly with the solute. R0 is an experimentally determined value. This value is determined by testing the solubility of the solute in many different solvents and finding the distance in Hansen space that is larger than the distance to the solvents that will dissolve the solute, but smaller than the distance of those that will not. The Hansen space can be seen in Figure 2.14 Figure 2. The Hansen space showing the solvents used in this study.14 In Figure 2, the substances are abbreviated according to Table 1, which is located in Chapter II. The colors are weighted combinations of the three colored axes. The closer two substances are in Hansen space, the stronger their interaction is predicted to be. Therefore, the points in Figure 2 with similar colors are predicted to interact strongly. The distance between two points in Hansen space is termed the Hansen distance (Ra). The Hansen distance can be calculated using eq 9. 4 In eq 9, the 1’s and 2’s signify that these are the values that correspond to either substance 1 or substance 2. (9) The Hansen space is very useful for finding suitable solvents for a particular solute or for finding a replacement for a commonly used solvent. Solvent blends are also very useful, and easily made with this tool. For example, if the solvents that are available have HSPs that are outside the interaction sphere of a particular solute, then two solvents that are on opposite sides of the solute in Hansen space could be blended to bring the blend inside the interaction sphere. The HSPs of the blend are just the volume-fraction weighted average of the HSPs of the components. This idea was used in this study to predict possible solvent blends to use for dissolving hydrocarbon grease, however, this method cannot predict if the blend will form an azeotrope. Using the Hansen space, the HSPs of a solid or semi-solid substance can also be easily determined experimentally. Using many different solvents whose HSPs are known and thoroughly cover Hansen space, one could attempt to dissolve the substance. If each solvent is scored, the HSPs of the solute would be somewhere in the middle of the cluster of good solvents in Hansen space. This is not an exact measure, but it is a good estimate. The HSPiP software can help with this endeavor as well, because the program allows the user to select the solvents that were used in the experiment and input the scores that were given to those solvents. Then, the program will pinpoint the likely position of the solute in Hansen space based on the scores that were provided for each solvent, and it will give the R0 of the interaction sphere that contains the good solvents and excludes the bad solvents. Structural & Operational Information about Vapor Degreasers The purpose of this project was to find a solvent replacement for use in vapor degreasers, so it is important to provide some information about what a vapor degreaser is and how it works. A vapor degreaser is a piece of machinery used in industrial settings for precision cleaning. A diagram of a top-down view of the inside of a vapor degreaser is seen in Figure 3 to help describe the parts of a vapor degreaser. Figure 3. Diagram of the inside of a vapor degreaser (top view). In Figure 3, it can be seen that there are two major tanks in the bottom of a vapor degreaser. These two tanks are the boiling sump and the ultrasonic tank. The other important parts are the coils along the walls of the degreaser. When a degreaser is operating, solvent is boiled in the boiling sump. This creates vaporized solvent. The bottom set of cooling coils cool the vaporized solvent, allowing it to condense and drip down into the water separator tank. The top set of cooling coils are referred to as the subzero coils, because they are super-cooled in an attempt to condense all of the vaporized solvent reaching that height in the degreaser. This helps to contain the solvent and prevent it from boiling away. This is very important for many reasons. Any solvent could be harmful if enough of it is inhaled, the solvent could be harmful to the environment, and boiling away all of the solvent is wasteful and defeats the purpose of the machine. These sub-zero coils also trap moisture to keep the solvent dry by condensing and freezing water vapor in the air that gets close to the coils. The part of the vapor degreaser that makes the machine work, is also the part that causes concern. When the vapor degreaser is operating, the heater of the boiling sump is creating as much vaporized solvent as possible and the cooling coils are condensing the solvent when it reaches a certain height. This creates a concentrated vapor zone between the surface of the solvent in the boiling sump, and the first set of cooling coils as seen in Figure 4. The first set of cooling coils cannot contain 100 percent of the vapor, and this creates a vapor concentration gradient. Figure 4.The vapor concentration gradient and explosive environment in a vapor degreaser. The cleaning takes place in the concentrated vapor zone between the boiling solvent and the first set of cooling coils by allowing the solvent vapor to condense on a colder object and drag off any grease. The continuous vaporization and condensation is analogous to distillation because the cooling coils are not perfect at containing the solvent, and the solvent is condensing and running into a water separator before reentering the ultrasonic and boiling tanks. This provides the reasoning for needing a low-boiling azeotrope if a mixture is used. An azeotropic mixture keeps the vapor and liquid the same composition. More cleaning takes place in the ultrasonic tank when the greasy object is submerged in solvent and ultrasonic waves collapse solvent cavities. The collapse of solvent cavities accelerates the solvent molecules, some of which impact the surface of the object and help to remove grease and other particulates. The vapor concentration gradient, seen in Figure 4, from the surface of the liquid to the outside of the degreaser will create an explosive atmosphere if flammable solvent is used. This occurs, because every flammable substance has an Upper and Lower Explosive Limit (UEL & LEL) when vaporized in air. These limits define the concentrations of flammable vapor in air at which there is either too much vapor in too little air to combust (UEL), or there is too little vapor in too much air to combust (LEL). The concentrations in the gradient that are between these two limits will combust if an ignition source is applied. The ignition source could be anything from a small spark caused by plugging something into an electrical outlet to an ignited Bunsen burner across the room, because the cooling coils are not perfect at containing the vapor and the amount of vapor that escapes depends on the sub-zero coils ability to condense the vapor. This is why solvents used in a vapor degreaser should be non-flammable, and this also justifies the need for flash point measurements in this study to determine if a solvent is safe for use in a vapor degreaser.