Mass transfer limit, tests

Table 14-3 presents a typical range of values for chemically reacting systems. The first two entries in the table represent systems that can be designed by the use of purely physical design methods, for they are completely gas-phase mass-transfer limited. To ensure a negligible liquid-phase resistance in these two tests, the HCl was absorbed into a solution maintained at less than 8 percent weight HCl and the NH3 was absorbed into a water solution maintained below pH 7 by the addition of acid. The last two entries in Table 14-3 represent liquid-phase mass-transfer hmited systems. 

The following equation has been derived for testing mass transfer limitation to the gross catalyst particle (19). 

Heat and mass transfer limitations are rarely important in the laboratory but may emerge upon scaleup. Batch reactors with internal variations in temperature or composition are difficult to analyze and remain a challenge to the chemical reaction engineer. Tests for such problems are considered in Section 1.5. For now, assume an ideal batch reactor with the following characteristics ... 

Most of the actual reactions involve a three-phase process gas, liquid, and solid catalysts are present. Internal and external mass transfer limitations in porous catalyst layers play a central role in three-phase processes. The governing phenomena are well known since the days of Thiele [43] and Frank-Kamenetskii [44], but transport phenomena coupled to chemical reactions are not frequently used for complex organic systems, but simple - often too simple - tests based on the use of first-order Thiele modulus and Biot number are used. Instead, complete numerical simulations are preferable to reveal the role of mass and heat transfer at the phase boundaries and inside the porous catalyst particles. 

Owing to its nature as a test reaction, rather the reactor and its operational modes were tested, mainly to determine mass transfer limits (see Section 3.3.11.3). It was also used for kinetic studies on the performance of various catalysts. 

Figure 5 Madon-Boudart test for mass transfer limitation over Pd/C catalysts -a plot of In (activity) versus In (Pd surface concentration).

Tests for external mass transfer limitations on conversion rates, (a) External mass transfer probably does not limit conversion rate. (b) Mass transfer limitations are present, (c) External mass transfer probably does not limit conversion rate, (id) Mass transfer limitations are present at low velocities. Adapted from Chemical Engineers Handbook, Fourth Edition, edited by R. H. Perry, C. H. Chilton, and S. D. Kirkpatrick. Copyright (c) 1969. Used with permission of McGraw-Hill Book Company.)... 

The second approach involves simultaneous variation of the weight of catalyst and the molal flow rate so as to maintain W/F constant. One then plots the conversion achieved versus linear velocity, as shown in Figures 6.4c and 6Ad. If the results are as indicated in Figure 6Ad, mass transfer limitations exist in the low-velocity regime. If the conversion is independent of velocity, there probably are no mass transfer limitations on the conversion rate. However, this test is also subject to the sensitivity limitations noted above. 

Catalytic tests were performed in a glass vessel equipped with a stirrer motor. Two monoliths (diameter 4.3 cm, length 4 cm) were mounted in plane on the stirrer axis. The total reaction volume was 2.5 1. Lipase was assayed in the acylation of vinyl acetate with butanol in toluene. Initial reaction rate was followed by GC analysis. Immobilized trypsin was used in the hydrolysis of N-benzoyl-l-arginine ethyl ester (BAEE) in a 0.01 M phosphate buffer pH 8 at 308 K. The reaction was followed by UV-VIS at 253 nm, and reaction rate was calculated in the mass transfer limited situation. 

Alkene hydrogenation was also suggested to test for mass transfer effects during liquid-phase hydrogenations236,237. The method is based on the linear poisoning of hydrogen addition to alkenes (cyclohexene and apopinene) by CS2. When the active sites of Pd or Pt catalysts are titrated with CS2 the decrease in rate is linear unless mass transfer limitations occur. 

How is a carrier particle with biocatalytic activity tested for potential mass transfer limitations ... 

The basic challenges for parallel test reactor development for high-throughput experimentation are, apart from technological challenges, related to technical demands that arise with the special issues for parallel test reactors, which are identical with the demands for conventional test reactors for gas-phase reactions. The criteria that must be fulfilled to obtain intrinsic catalyst properties from experimental data relate mainly to mass and heat transfer. A sufficient contact between the reactants and the catalyst must be insured to avoid mass transfer limitations inside and outside of the catalyst particles. Isothermal operation under laboratory conditions and avoidance of heat transfer limitations are also crucial. As a general quality check prior to operation intra- and extra-particle limitations should be... 

External mass transfer limitations, which cause a decrease in both the reaction rate and selectivity, have to be avoided. As in the batch reactor, there is a simple experimental test in order to verify the absence of these transport limitations in isothermal operations. The mass transfer coefficient increases with the fluid velocity in the catalyst bed. Therefore, when the flow rate and amount of catalyst are simultaneously changed while keeping their ratio constant (which is proportional to the contact time), identical conversion values should be found for flow rate high enough to avoid external mass transfer limitations.[15]... 

The correct evaluation of catalytic properties demands that heat and mass transfer limitations are eliminated or properly accounted for. It also demands that the catalyst is in the working state, as opposed to the transient state observed at the beginning of most catalytic tests. The absence of gas-phase reactions or reactions catalyzed by the reactor wall should also be verified. This must be kept in mind in the following, in which measurement methods, kinetic analyses including the influence of heat and mass transfer and deactivation or, more generally, time-dependent effects will be examined. Regeneration of catalysts will be examined at the end. 

However, carbon formation and destruction of the cordicntc support were both found to have taken place over the course of vanous test conditions These findings indicated that while the heat transfer into the monolithic catalyst bed improved, (1) the gas-phase hexane cracking reaction that produced carbon precursor species (due to the high void fraction and mass transfer limitation) still existed, and (2) the combination of sustained high temperature and high steam density on cordiente warranted use of only metal monoliths for this application However, the relatively low loading of nickel in the monolith catalyst and the mass transfer limitation still resulted in equivalent conversion under conditions similar to those found in industrial practice... 

Effect of the face gas velocity (mass transfer limitations). It is well known how these monolithic catalysts work in laminar flow because of the small size of their channels. There are transversal gradients of concentration in them, thus. This effect has been quantify in this research by two ways 1st) Some tests were made with the same gas hourly space-velocity [10,000 h (450 C)] varying the face gas velocity (equivalent to Ae superficial gas velocity in fixed beds) from 28 to 83 cm/s, The monoliths were then cut to different lengths (from 10 to 30 cm). Results on oxidation of EC (1,000 ppm at inlet) are shown in Figure 3 for two BASF catalysts, and indicate some external diffusion control at low face gas velocities. To keep it to a minimum extent, most of further tests were made at the maximum (in this facility) face velocity 83 cm/s. 2nd) Mass transfer limitations in these monoliths have also been studied by the well established procedures in chemical engineering, following the detailed... 

Common tests for the determination of mass transfer limitations of heterogeneously catalyzed reactions. 

The question of the effeet of H2 coneentration on enantioseleetivity is partieularly pertinent to asymmetrie hydrogenation by ruthenium eomplexes eontaining BINAP ligands (Seheme 1). With sueh eatalyst preeursors, the enantioseleetivity of hydrogenation in methanol is strongly dependent on the coneentration of H2 in the reaction phase (97,102). In practice, the H2 concentration in the MeOH is a function of the H2 pressure (102) and the stir rate (i.e., mass transfer limitations exist) (97). For some substrates, the enantio-selectivity is greater if there is a greater concentration of H2 in the liquid phase, whereas for others, the opposite is true. Both classes of substrates have been tested in SCFs. 

Before the kinetic experiments some preliminary tests were performed with different hydrogen flows (75 and 100 ml/min) and catalyst particle sizes (<45 pm and 63-90 pm) to ensure the absence of mass transfer limitations in the kinetic experiments. 

Figure 8.7 Comparison of effects of mass transfer limitations in the desorption of alachlor from activated carbon using supercritical carbon dioxide tests with several flow rates and carbon mesh size. SLPM = Standard liters per minute.

After drying and reduction, the Pd-Ag/C catalysts are composed of bimetallic Eilloy nanoparticles ( 3 nm). CO chemisorption coupled to TEM and XRD analysis showed that that, for catalysts 1.5% wt. in each metal, the bulk composition of the alloy is close to 50% in each metal, whereas the surface is 90% in Ag and 10% in Pd [9]. Mass transfer limitations can be detected by testing the same catalyst with various pellet sizes [18]. Indeed, if the reactants diffusion is slow due to small pore sizes, the longer the distance between the pellet surface and the metal particle, the larger the influence of the difiusion rate on the apparent reaction rate. Pd-Ag catalysts with various pellet sizes were thus tested in hydrodechlorination of 1,2-dichloroethane. Results were compared to those obtained with a Pd-Ag/activated charcoal catalyst. Fig. 4 shows the evolution of the effectiveness factor of the catalysts, i.e. the ratio between the apparent reaction rate and the intrinsic reaction rate, as a function of the pellet size. The intrinsic reaction rate was considered equal to the reaction rate obtained with the smallest pellet size. When rf = 1, no diffusional limitations occur, and the catalyst works in chemical regime. When j < 1, the observed reaction rate is lower than the intrinsic reaction rate due to a slow diffusion of the reactants and products and the catalyst works in diffusional regime [18]. 

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