Ind. Eng. Chem. Res. 1993,32, 989-994
989
Mass Spectrometric Studies of Vaporization of Phosphoric Acids A. Kasem Chowdhury,t Ming-Biann Liu,'J and Aylin Gulbenkian Western Research and Development, Dow Chemical Company, P.O. Box 1398, Pittsburg, California 94565
Vaporization of phosphoric acids was investigated using a mass spectrometer. In the vaporization, water, H3P04, and P4O10 were the vapor species. The mass spectrometric data combining with other factors are used to discuss the literature data of the vapor-liquid equilibrium (VLE) of phosphoric acids and to recommend VLE data for process design of recycling phosphoric acid to make polyphosphoric acid by a vaporization process.
Introduction Polyphosphoric acid (PPA)has been used as solvent for organic reactions and polymer syntheses (Rowland, 1985; Krongauz et al., 1970;Wolf, 1988). Especially, PPA is the solvent for synthesizing polybenzothiazoles (PBTs) and polybenzoxazoles (PBOs) (Wolf, 1988). The material science and engineering of these polymers is given in a book (Adams et al., 1989). PBTs and PBOs are technologically important materials of the future. They are much lighter than steel, and yet their tensile strengths and modules are far superior to those of steel. In addition, they possess chemical resistance, thermal stability up to 500 "C, flame retardance, low dielectric constant, and low moisture pickup. These properties warrant application of PBTs and PBOs in electric/electronic areas and as materials for aircraft/aerospace strucure and for chemical/ hostile environments. In the process of isolating PBTs and PBOs, the PPA in the product mixtures is removed by contacting with water and thereby becomes phosphoric acid. For a commercial production of these polymers, the current supply of PPA is limited and phosphoric acid is the main waste stream. Thus, recycling of phosphoric acid to make PPA is economically and environmentally important. A straightforward way for recycling phosphoric acid is to reconcentrate the acid to make PPA by a vaporization process. Vapor-liquid equilibrium (VLE)data of the phosphoric acid-water system are needed for process design of the vaporization process. Although three seta of VLE data are available in literature (Fontana, 1951; Brown and Whitt, 1952;Mikhailin et al., 1968),they are inconsistent. The inconsistency of the three seta of data is in the vapor composition. Fontana (1951) reported that water was the vapor species and there were no appreciable amounts of acid present in the vapor phase. Brown and Whitt (1952) and Mikhailin et al. (1968) reported water vapor and an increase of phosphorus-oxygen vapor species with increasing composition of P205. All of the reported vapor compositionswere derived from analysesof the condensate. Such derivations are prone to give erratic conclusionssince, upon condensation, any phosphorus-oxygen vapor species readily react with the water vapor to form acid species. The purpose of our mass spectrometric work is to identify the vapor species directly to resolve the inconsistency and to select VLE data for process design for recycling phosphoric acid by a vaporization process. Experimental Section Phosphoric acid (crystals, 99%) was purchased from Aldrich Chemical Co., Milwaukee, WI. Reagent grade + Current address: Analytical Sciences Lab, Texas Operation, Dow Chemical Company, Freeport, TX. * Current address: 386 Mt. SequoiaPlace, Clayton, CA 94517.
polyphosphoric acid of 83 f 2 wt % P205 was obtained from Eastman Kodak Co., Rochester, NY. In each of the mass spectrometric experimenta, a sample of less than 1 mg was loaded into a glass vial of 1.9 mm in diameter and 9.5 mm in length. The vial was fitted in a stainless steel probe. The probe was then mounted horizontally in the ionization source chamber of a Finnigan quadrupole mass spectrometer. The mass spectrometer was operated at a pressure of le7 mmHg. After outgassing, the probe was electrically heated to vaporize the sample directly into an ionizing electron beam of 20 eV. The use of a low-energy electron beam is to minimize the fragmentation of parent ions since appearance potentials of parent ions are typically about 10 eV (Kiser, 1965) and the appearance potential of P4010+is 13.5 eV (Muenow et al., 1970). The vaporization experimenta were carried out either at constant temperature or with a ballistic heating from 50 to 400 "C. A Finnigan MATT SuperIncos data acquisition system was used to periodically record mass scans of positive ions. It took 2 s to obtain a mass scan ranging from 18 to 650 amu (atomic mass units).
Results It should be noted that the results of this study are qualitative, not quantitative. The reasons are the following: 1. The sample size was too small (
