Surface exposure time

As an alternative to investigating the kinetics of a gas-liquid reaction on a laboratory scale, the mass transfer resistance may be minimised or eliminated so that the measured rate corresponds to the rate of the homogeneous liquid-phase reaction. This method of approach will be considered after first describing those reactors giving rise to controlled surface exposure times. 

Integrating Equation 1 over the total surface exposure time 0 (= distance ahc/irDN) gives the effective film coefficient, for that particular film ... 

The same treatment applies to the regenerated nip surface film, dc. The velocity gradient would be zero at the surface, except perhaps when elastic forces predominate. The surface exposure time may be approximated by 0 = distance dc/irDN sin(f)y where screw helix angle measured from the cross-sectional plane. 

The operation of bubbling sulphonators with surface exposure times of 10 milliseconds for typical bubble size ranges do not appgar to be prone to inter facial temperatures in the region of 100 u above datum. However, the industrial sulphonation of linear alkyl benzenes presents an interesting problem of associated severe discolouration and the formation of malodourous compounds when sulphonation is carried out at high gas composition of SO. The productivity of sulphonators is limited by these factors and the role of localised high temperatures in the absorption process has yet to be fully appreciated. 

In general, bond quality diminished with increased surface exposure time (SET). The best overall adhesive evaluated was epoxy film adhesive, which was found to be the least sensitive to the method of surface preparation used. In general, the best result was obtained when GRP laminates were bonded within 4 h after sanding. If absolutely necessary, bonding can be carried out after periods of time of up to 14 days SET, wifli only moderate strength loss. 

Fig. 11. TEM images of Co-SiO catalyst surface after different exposure times to acetylene at 700 C (a) 1 minute (b) 5 minutes and (c) 20 minutes.

Flame treatment is predominantly used with articles of relatively thick section, such as blow moulded bottles, although it has been applied to polyolefin films as well. The most important variables in the process are the air-gas ratio and their rate of flow, the nature of the gas, the separation between burner and surface, and the exposure time. 

This PDE is subject to the initial condition that a = / at t = 0 and boundary conditions that a = a at a = 0 and a = ai at x = oo. The solution is differentiated to calculate the flux as in Equation (11.35). Unlike that result, however, the flux into the surface varies with the exposure time t, being high at first but gradually declining as the concentration gradient at a = 0 decreases. For short exposure times,... 

This result gives the flux for a small portion of the surface that has been exposed for exactly t seconds. Other portions of the surface will have been exposed for different times and thus will have different instantaneous fluxes. To And the average flux, we need the differential distribution of exposure times,/(f). Danckwerts assumed an exponential distribution ... 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

Exposure time of the wall layer Exposure time of the pool surface Volume of the compound... 

An increase in reducible surface-bound material during ennoblement was demonstrated using galvanostatic reduction" techniques to monitor potential as a stainless steel coupon was cathodically polarized. Coulombs of reducible material were calculated from the duration of regions of polarization rate lag that indicated reduction of surface-bound material. Longer exposure times and thicker biofouling were not sufficient to increase the abundance of reducible surface-bound material. The increase seemed to be associated with increased... 

Fig. 4.30. Variation rate of the electric conductivity of a ZnO film as a function of the intensity of the electron beam bombarding the film, for different times of preliminary exposure of the film to molecular hydrogen at room temperature (/) - ZnO surface free of hydrogen, (2) - the exposure time is 5 min, (i) - the exposure time is 30 min, (4) - the exposure time is 120 min.

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