Compact contact time

The most basic analysis is the presentation of force versus time or displacement versus time. These curves are different for eccentric and rotary tableting machines. The data given in Figures 12 and 13 are valid for the contact time of the compaction cycle of one single tablet. Due to the eccentric-driven movement of the punches, the force-time curve can be described by a sharp peak at the maximum force evolving at the punches and the displacement-time curve can be described with a sharp peak at the minimum height of the powder bed. For curves of eccentric tableting... 

There are, however, some potential disadvantages due to the inherent compactness of membrane reactors. First, a membrane reactor typically provides limited contact time between reactants in the reaction zone. This may require in some cases longer membrane elements. Second, the membrane has relatively small area available for catalyst... 

Force measurements made without displacement values are still useful in identifying the dependency of tablet hardness (and other associated characteristics) on compaction force, and also the effect of the tablet press compaction speed on tablet strength (influence of dwell time/effective contact time). Dwell time dependency is an important scale-up factor for the tableting process, and evaluation of the sensitivity of a formulation to dwell time at small scale is useful, although the actual commercial dwell time is not always achievable on instrumented, small-scale tablet presses. 

In addition, it was shown that the lower and upper punches may not move synchronously. Moreover, maximum force does not coincide in time with the minimum punch gap. These and other considerations (press deformation, contact time, etc.) make the effort of simulating a production press on a hydraulic compaction simulator rather impractical. That is why, to quote from a paper by Muller and Augsburger, " Although compaction simulator have been designed to mimic the displacement time behavior of any tablet press, they rarely have been used in that fashion. ... 

Precompression is often used to tamp or apply a small compression force prior to the main compact compression cycle. Rotary tablet presses are often equipped with a separate precompression station, which is positioned between the die-filling feed frame and the main compression station. Typically, precompression is used to improve the quality of tableted products, where it increases the strength of the compact and/or decreases the incidences of capping and lamination. The compact strength is enhanced through the increase in the effective contact time in which the powder particles are in contact under an applied force. During this extended contact time, stronger interparticulate bonds form and stress relaxation occurs. 

Castaldi, M.J., Boorse, R.S., Roychoudhury, S., Menacherry, P., and Pfefferle, W.C. Lightweight, Compact, Ultra-fast Short Contact Time Preferential Oxidation Reactor for Automotive PEM Fuel Cell Applications, NSF National Meeting (ed. National Science Foundation). San Juan, Puerto Rico National Science Foundation, January 2002. 

It appears that for the capillary filtration process the thickness of the compact is proportional to the square root of the contact time substrate/suspension. As shown, the constant C depends on the properties of the support and compact ... 

Resorcinol paste is compact and dilEcult to apply. It is easily softened by putting it in a bain-marie at 45° for around 10 minutes. If that is not possible, small blocks of paste should be applied to the patient s skin. Heat from the skin will slowly melt the paste and make it easier to spread, although the resorcinol may penetrate too deeply where contact time has been longer. 

Third application (third day) contact time 30-35 minutes. The third application must be carried out with extreme caution if the resorcinol membrane that is forming has modified the permeability of the skin, epidermolysis is present and the skin has been badly injured rough handling (or paste that is too compact) could pull away the skin and sharply increase the risk of post-peel complications in the form of erythema and pigmentary changes. 

Pebble bed heat transfer coefficient, = 280 kW/m K volume of pebbles contact time, fractions of seconds. Surface compactness up to 15,000 vrPIvrP. 

Tab. 8.10 summarizes some technical information for high pressure ram extrusion presses. Tab. 8.10a presents machine details and Tab. 8.10b indicates the approximate briquette output per channel of the shapes shown at the top. In Tab. 8.10b impact area means the face area of the ram that is contacting (impacting) the material to be briquetted it is approximately equivalent to the face area of the briquettes as represented by the shapes specified in the first three lines of Tab. 8.10b. As mentioned before, at typical ram speeds the contact time is so short for each cycle (0.04 s was mentioned, see above) that compacting is often referred to as being carried out by a blow.

Catalytic partial oxidation (CPO) is a novel technology based on the same principles as ATR, except that all reactions take place catalytically, without upstream combustion after careful mixing, the reactants are sent directly on to the catalyst. CPO catalysts are usually based on noble metals (Pt, Pd, Rh, Ir) whose high reactivity permits very short contact times (0.1-10 ms) and thus high space velocities, which enables the design of compact reactors. 

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