Similarities residence times

This is consistent with the conversion values obtained earlier under steady operation with a platinum catalyst, ranging from 18 to 80%, depending on the pretreatment of the catalyst under similar residence times and temperatures. 

Two major studies of the water chemistry of the northern Great Bahama Bank were done by Broecker and Takahashi (1966) and Morse et al. (1984), and our discussion will focus on their results. The variation in salinity observed by Morse et al. (1984), shown in Figure 5.21, is remarkably similar to that observed by Broecker and Takahashi (1966). Major increases in salinity, to values exceeding 44, were observed as Williams Island was approached. The 14C distribution was also modeled in both studies, but by different methods, to obtain the residence time of the waters. Similar residence time values were obtained Morse et al. (1984) estimated the highest salinity waters had been on the bank for... 

The mean in vitro dissolution time is compared to either the mean residence time or the mean in vivo dissolution time. Level B correlation, like Level A correlation, uses all of the in vitro and in vivo data but is not considered to be a point-to-point correlation and does not uniquely reflect the actual in vivo plasma level curve, since several different in vivo plasma level-time curves will produce similar residence times. A Level C correlation is the weakest IVIVC and establishes a single point relationship between a dissolution parameter (e.g., time for 50% of drug to dissolve, or percent drug dissolved in two hours, etc.) and a pharmacokinetic parameter (e.g., AUC, Cmax, Tmax). Level C correlation does not reflect the complete shape of the plasma drug concentration-time curve of dissolution profile. 

Since there are about 100 mol of CO2 above each square meter (1 atm = KXX) g air cm and the average molecular weight of air is 29 g thus there are 100 g/29 g = 34 mol air molecules above 1 cm of which 0.03% are CO2), a residence time of tqoi = years is obtained. Similar residence times have been established on die basis of carbon-14 data. Thus, under these circumstances, the chemical enhancement of the exchange rate does not appear to be very significant. Thus the diflusion of CO2 is more important than the chemical cycle of HCO and CO3 for the ocean-atmosphere CO2 exchange. 

Thus, on its way out of the reactor, in the volume element, there are species with variable residence times. The species in the volume element that have similar residence times, however, must have been introduced into the reactor at the same time. This is why we can even derive Equations 4.4 and 4.5 on the basis of a volume element entering the reactor. In this case, function E(t) dt describes that portion of the elements in the volume element that attains residence times between t and t + dt. If the distribution pattern of the species is known in the volume element leaving the reactor (at the outlet), it is possible to predict the RTD in an incoming volume element. A precondition for this, however, is that the system should have reached its steady state and that the flow conditions should remain constant during the time the volume element passes the system. 

Photometric detection has been used for the determination of PG and other antioxidants such as BHA [53,54] and BHT [55] using flow injection systems and a diode array detector (DAD), which allows the measurement of the intrinsic absorbance of each analyte at its individual wavelength. Thus, a flow sensor for the determination of PG and BHA has been described using the differential transient retention of the analytes in a solid phase packed in a flow-through cell [53]. The use of hydrophilic adsorbents, such as Sephadex G-15 and G-25, resulted in low retention of the antioxidants, and similar residence times. In contrast, the use of Cig-bonded silica as adsorbent with average particle sizes of 55-105 pm produced good retention and elution of both antioxidants. PG experienced a faster transient retention than BHA, which allowed the separation and determination of both analytes. PG was determined at 40 s from injection, measuring at 272 nm, and BHA... 

As was discussed above, it is difficult to compare rapid expansion in the orifice and the capillary on the basis of similar residence times. Thus, we consider an alternative analysis by calculating the residence time in each section... 

One of the most important characteristics of micelles is their ability to take up all kinds of substances. Binding of these compounds to micelles is generally driven by hydrophobic and electrostatic interactions. The dynamics of solubilisation into micelles are similar to those observed for entrance and exit of individual surfactant molecules. Their uptake into micelles is close to diffusion controlled, whereas the residence time depends on the sttucture of the molecule and the solubilisate, and is usually in the order of 10 to 10" seconds . Hence, these processes are fast on the NMR time scale. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Acetaldehyde can be used as an oxidation-promoter in place of bromine. The absence of bromine means that titanium metallurgy is not required. Eastman Chemical Co. has used such a process, with cobalt as the only catalyst metal. In that process, acetaldehyde is converted to acetic acid at the rate of 0.55—1.1 kg/kg of terephthahc acid produced. The acetic acid is recycled as the solvent and can be isolated as a by-product. Reaction temperatures can be low, 120—140°C, and residence times tend to be high, with values of two hours or more (55). Recovery of dry terephthahc acid follows steps similar to those in the Amoco process. Eastman has abandoned this process in favor of a bromine promoter (56). Another oxidation promoter which has been used is paraldehyde (57), employed by Toray Industries. This leads to the coproduction of acetic acid. 2-Butanone has been used by Mobil Chemical Co. (58). 

HammerMills. One of the most versatile, economical, and widely used impact mills is the hammer mill (Fig. 12). Many variations are produced, with special types available for specialized appHcations, eg, quick screen change for animal feed, heavy duty for minerals, and light constmctions for woodchip. The principle employed is similar to that of the impact cmsher however, the rotation speed can vary from 20 up to 100 m/s with high speed fine-grinding versions. The oudet screen is used to vary the residence time, which in turn affects final particle size. The size of the end product is an order of magnitude finer than the size of the perforations in the outlet screen. 

Having estabhshed the residence time and power input, the scale-up can Be now done using the principle of geometric similarity together with equal power per unit volume discussed earlier. 

To measure a residence-time distribution, a pulse of tagged feed is inserted into a continuous mill and the effluent is sampled on a schedule. If it is a dry miU, a soluble tracer such as salt or dye may be used and the samples analyzed conductimetricaUy or colorimetricaUy. If it is a wet mill, the tracer must be a solid of similar density to the ore. Materials hke copper concentrate, chrome brick, or barites have been used as tracers and analyzed by X-ray fluorescence. To plot results in log-normal coordinates, the concentration data must first be normalized from the form of Fig. 20-15 to the form of cumulative percent discharged, as in Fig. 20-16. For this, one must either know the total amount of pulse fed or determine it by a simple numerical integration... 

A distinc tion is to be drawn between situations in which (1) the flow pattern is known in detail, and (2) only the residence time distribution is known or can be calculated from tracer response data. Different networks of reactor elements can have similar RTDs, but fixing the network also fixes the RTD. Accordingly, reaction conversions in a known network will be unique for any form of rate equation, whereas conversions figured when only the RTD is known proceed uniquely only for hnear kinetics, although they can be bracketed in the general case. 

The optimal network increases total residence time by 48 per cent when compared with an equivalent MSMPR of the same volume and throughput. This increase would translate into a similar increase in mean crystal size and a 78 per cent increase in yield. Exactly the same residence time as for the single crystallizer have been reported from simple cascade configurations previously designed for stage-wise crystallization processes for slight improvements in... 

NMe is now commercially available and is prepd by the vapor phase nitration of methane at a ratio of 9 moles of methane to I mole of nitric acid at 475° and a residence time of 0.18sec (Ref 12) or by the similar nitration of aliphatic hydrocarbons (Ref 8). Other prepns are from Me sulfate and Na nitrite (Ref 26) by the oxidn of Me amine with dinitrogen trioxide in the gas phase or in methylene chloride, yield 27%... 

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