The thermal deconlposition of systems derived from tetrafluoroethylene, occumng in a nickel vessel at temperatures from 660" to 760", is described.The tetrafluoroethylene-perfluorocyclobutane equilibrium mixture decomposes at temperatures above 660" forming perfluoropropene by a first-order mechanism. This in turn decomposes giving perfiuoroisobutene in a reaction with an order of 1.6. A reaction scheme, based on the difluoromethylene radical as intermediate, is shown to be in accord with the experimental results.At temperatures above 700" perfluoroisobutene is shown to disproportionate giving perfluoroethane and various non-volatile products by a first-order mechanism. A reaction scheme, based on the tduoromethyl radical, is suggested. RESULTS reported by Miller and Atkinson andTrenwith show that the thermal decomposition of tetrafluoroethylene can be divided into three phases. At low temperatures perfluorocyclobutane is the main product ; at medium temperatures perfluoropropene and a perfluorobutene are produced ; and at high temperatures perfluoroethane is formed.Atkinson and Trenwith also performed a detailed investigation between 300" and 590", showing that a reversible cyclic dimerisation occurs : BCFCCF, -(CFJ4, k, = 10.3 x lo7 exp (-25,400/RT) 1. mole-l sec.-l . (1) (CF,), -2CF,:CF,, k, = 8.9 x 10ls exp (-74,100/RT) sec.-1 . . (2) 1 Miller, '' Preparation and Technology of Fluorine and Organic Fluorine Compounds," National

Equations are given that relate the entrance length to Reynolds number for pipe and channel geometries with a flat velocity profile as the initial condition. These equations are linear combinations of the creeping flow and boundary layer solutions. The former is obtained by minimization of the viscous dissipation using the finite element method. The equation for the pipe entrance is shown to be in good agreement with experimental data.The entrance region for laminar flow has received considerable attention in recent years. Not only is such study of importance for industrial and viscometric applications, but also as a simple developing flow it provides a useful vehicle for the evolution and evaluation of numerical solution procedures for nonlinear partial differential equations, for example, the Navier-Stokes equations. In consequence, the restrictions imposed on the solution by use of the Prandtl boundary-layer hypothesis have been overcome and, in principle, it is now possible to achieve a solution for all Reynolds numbers and boundary conditions.The early numerical solutions were obtained by Bodoia and Osterle ( 1 ) for parallel plates, and Hornbeck ( 2 ) for pipes, both using a flat velocity profile as an initial condition and applying the boundary-layer hypothesis to yield initial value problems. The velocity and pressure fields were then obtained by finite difference procedures ( 3 ) . These studies have been extended to the lower Reynolds number region where the problem is of the boundary value type, by Wang and Longwell ( 4 ) in the case of parallel plates and for pipes by Vrentas, Duda, and Bargeron ( 5 ) . In the former, solutions are presented at a Reynolds number of 300 using both the flat velocity profile (Figure 1) and the stream-tube (Figure 2) as the initial condition, whereas in the latter, the stream-tube condition was used exclusively at Reynolds numbers of 0, 1, 50, 150, and 250. The significant features of these solutions lie in the transport of vorticity upstream of the tube entrance when using the stream-tube condition (4, S), the way the entrance length varies with Reynolds number ( 5 ) , the finite entrance length at zero Reynolds number ( 5 ) , and the fact that the maxima are predicted in the velocity profiles at locations other than the center line ( 4 ) .The associated experimental investigations, all in pipes, due to Nikuradse ( 6 ) , Reshotko (7), and Pfenninger (8) are discussed by Atkinson, Kemblowski, and Smith (9), who have confirmed beyond reasonable doubt the high Reynolds number asymptote of the Navier-Stokes equations applied to entrance region problems.Experimentally, one of two methods is used in an attempt to produce an acceptable approximation to a flat velocity profile for use as an initial condition. These are the type of device developed by Atkinson, et al. (9) and the conical entrance section (7,8).In view of the computations of Vrentas, et al. ( 5 ) application of the latter method is questionable at low Reynolds numbers because of the upstream axial diffusion of vo...

ABS TRACTThe thermal decomposition of perfluorocyclopropane has been studied in a static reactor. Pressure changes during the reaction were followed with a pressure transducer and products analyzed by gas-solid chromatography.

The thermal decomposition of tetrafluoroethylene at temperatures from 309" to 890" is described. Below 600" dimerization to octafluorocyclobutane, and the reverse reaction, are much faster than any of the other reactions taking place. Hexafluoropropene and octafluorobutenes are formed a t 600-800". A t 800" hexafluoroethane and tars are also obtained. Vapour-pressure measurements for the major products are reported.

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