Showing papers in "Industrial & Engineering Chemistry Process Design and Development in 1975"

A Generalized Method for Predicting Second Virial Coefficients

Abstract: Expressions for predicting pure-component and cross second virial coefficients for simple and complex systems have been developed from the bound-pair formalism of Stogryn and Hirschfelder. For pure components, the generalized correlation requires the critical temperature and pressure, Thompson's mean radius of gyration or the parachor, dipole moment, and, if appropriate, a parameter to describe chemical association which depends only in the type of group (hydroxyl, amine, ester, carboxylic acid, etc.). Mixing rules have been developed for predicting cross coefficients and solvation effects can be accounted for in a similar manner to association. Agreement with experimental data on 39 nonpolar and 102 polar and associating compounds, 119 mixed nonpolar systems, and 73 mixed systems involving polar compounds, is comparable to or better than that of several other correlations including those which require data to obtain parameters. The method should be most accurate for systems of complex molecules where no data are available In order to accurately predict phase equilibria involving the vapor phase at pressures above atmospheric, deviations from the perfect-gas law usually need to be taken into account (Prausnitz, 1969; Nagata and Yasuda, 1974). The vinal equation terminated at the second coefficient is a simple but accurate method for conditions up to a density of about one-half the critical and has been employed in completely developed methods for predicting vapor-liquid equilibria such as Prausnitz et al. (1967). Several analytical methods for predicting values for the second virial coefficient have been developed (Black, 1958; O'Connell and Prausnitz, 1967; Kreglewski, 1969; Nothnagel et al., 1973; Tsonopoulos, 1974), but except for the last, all suffer from the disadvantage of often requiring one or more parameters that must be obtained from data, or the results are too inaccurate to be acceptable. This work develops an accurate method for predicting second virial coefficients using only critical properties and molecular parameters. all of which may usually be estimated from molecular structure to the required accuracy. From extensive comparisons with pure component and cross vinal coefficient data, the present method appears to be more consistently accurate than any other purely predictive method. In addition, for strongly associating substances, the method predicts association effects at higher densities in a realistic fashion (Nothnagel et al., 1973) using a parameter which depends only on the group interaction.

Mass Transfer in Packed Beds with Two-Phase Flow

Abstract: Mass transfer coefficients were measured at 25’C and 1 atm for cocurrent liquid (water) and gas flow in beds of small particles (0.054-0.29 cm) of napthalene and CuO-ZnO. The coefficients for transfer between particle and liquid are not greatly different for the three arrangements: upflow, downflow (trickle bed), or liquid full. At high gas rates and low liquid rates upflow gives somewhat higher transport rates, while trickle beds are favored over liquid-full operation at high liquid rates. Desorption of oxygen was measured to obtain liquid-to-gas coefficients kLa when mass transfer in the liquid phase controlled interphase transport. For downflow such coefficients were independent of gas flow rate, but in upflow kLa increased with gas rate. Upflow gave higher coefficients at high gas and liquid rates. Desorption of napthalene from water to air was employed to evaluate the gas-side coefficient in liquid-to-gas transfer for trickle-bed operation. The results showed a high sensitivity to gas flow rate and a modest sensitivity to liquid rate.

Catalyst Deactivation Due to Pore-Plugging by Reaction Products

Abstract: In trickle-bed hydrodesulfurization the gas and liquid phases pass concurrent downflow over a fixed bed of catalyst. The hydrotreating of residuum oils at elevated temperatures and pressures in this way has revealed the presence of (undesirable) demetallation reactions taking place in parallel with the desired desulfurization reactions. Organometallic constituents of the oil, primarily vanadium, nickel and iron, react out of the oil and combine with hydrogen sulfide to produce solid deposits of metal sulfides. The deposition of the reaction products occur both inter- and intra-particle. The intra-particle deposits decrease the effective diffusivity of the catalyst causing deactivation by a pore-plugging mechanism. A simple model is proposed to take into consideration catalyst deactivation in residuum hydrotreating by metal sulfides build-up within the pores of the catalyst. The effect of the metal sulfides are considered the dominant feature of the deactivation process based on recent evidence from the literature. The demetallation rates used in the model are typical of current commercial practice. Predictions for catalyst life are compared with commercial data.