Viscosity, critical hydrocarbons

Viscosity of Hydrocarbons. Although Fig. 5-16, taken from Uyehara and Watson, is based primarily upon light hydrocarbons and organic compounds, it is probably the best method available for estimating the viscosities of heavy refinery vapors. Critical viscosities of such vapors can be estimated from... 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

For prediction of the vapor viscosity of pure hydrocarbons at low pressure (below Tr of 0., the method of Stiel and Thodos is the most accurate. Only the molecular weight, the critical temperature, and the critical pressure are required. Equation (2-97) with values of N from Eqs. (2-98) and (2-99) is used. 

If critical pressure and critical temperature are given in Pa and K, respectively, viscosities in centipoise result. The variable Io is either the low pressure pure component or mixture viscosity according to whether a pure component or mixture is being considered. For mixtures, simple molar average pseiidocritical temperature (Kay s rule), pressure, and density, and molar average molecular weight are used. The vapor density can be predicted by the methods previously discussed. Errors of above 5 percent are common for hydrocarbons and their mixtures. Experimental densities will reduce the errors slightly. 

Triacetin resistance is especially critical when filter tips are made in one location, stored, and then shipped to another location. For these operations, polyethylene-based adhesives are used because of their low polarity and therefore excellent resistance to triacetin. Where filter plugs are attached at the same location shortly after production, EVA-based adhesives are suitable and preferred. Both types of adhesives use low odor, clean tackifiers such as hydrogenated hydrocarbons or pure monomer resins (typically a-methylstyrene based). Rosin, rosin esters, and phenol-containing tackifiers are not acceptable. EVA-based adhesives use a higher level of wax (about 1 /3 of the formula) than polyethylene-based adhesives (5-20% wax) due to the lower crystallinity and slower set of EVA vs. PE. Application viscosities are 2000-5000 cP. 

Figure 6-6 gives the viscosity of ethane.2 Note the similarity between this figure and the graph of the densities of pure hydrocarbons given in Chapter 2. The dotted line is the saturation line, and the point of maximum temperature on the dotted line indicates the critical point. 

Supercritical water is neither a liquid nor a gas, but it has properties between the liquid and gas phases (i.e., density approaching its liquid phase and diffusivity and viscosity approaching its gas phase). At the critical point, hydrogen bonds disappear, and water becomes similar to a moderately polar solvent. Oxygen and almost all hydrocarbons become completely miscible... 

To this point we have applied the critical temperature to both viscosity and density calculations. Already this critical property Tc is seen as valued data to have for any hydrocarbon discrete single component or a mixture of components. It is therefore important to secure critical temperature data resources as much as practical. I find that a simple table listing these critical properties of discrete components is a valued data resource and should be made available to all. I therefore include Table 1.3 listing these critical component properties for 21 of our more common components. A good estimate can be made for most other components by relating them to the family types listed in Table 1.3. 

Schick and Fowkes (11) studied the effect of alkyl chain length of surfactants on critical micelle concentration (CMC). The maximum lowering of CMC occurred when both the anionic and nonionic surfactants had the same chain length. It was also reported that the coefficient of friction between polymeric surfaces reaches a minimum as the chain length of paraffinic oils approached that of stearic acid (12). In order to delineate the effect of chain length of fatty acids on lubrication, the scuff load was measured by Cameron and Crouch (13). The maximum scuff load was observed when both hydrocarbon oil and fatty acid had the same chain length. Similar results of the effect of chain length compatibility on dielectric absorption, surface viscosity and rust prevention have been reported in the literature (14-16). 

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