Poisoning activity profiles

The effect of sulphur on the activity profile is shown in Figure 2. The sulphur poisoning arises from sulphur impurities found in the synthesis gas, which is derived from coal. Sulphur is a common F-T catalyst poison [2,11] and it is apparent from the results that the top portion of the reactor is acting as a guard bed" to remove this impurity. The finding that sulphur is only found to be present in the top section of the catalyst bed has been observed previously for F-T and related catalysts [12-15]. 

The orthogonal collocation polynomial approximation using a single parameter trial function was employed to solve equations (l)-(3), In addition to the solution for time concentration and activity profiles, effectiveness factors representing the combined effect of mass transfer resistance and poisoning in terms of pellet surface conditions were computed according to... 

It was found that for impurity poisoning concentration of benzene and thiophene decrease from a maximum value at the surface to the centre. This is reflected in the residual activity profile which has a maximum at the centre of the pellet. It was also found that there is a satisfactory degree of correlation between calculated effectiveness factor and obtained with experimental observation. 

First Cycle Poisoning Dynamics. Upon attaining an isothermal temperature profile for an impurity free feed, the next phase of the experiment was begun. Thiophene was introduced into the benzene feed as the feed impurity. The thiophene also caused another problem which manifested itself as a migrating temperature depression, a result of the active zone moving towards the end of the reactor. This movement of the reaction zone decreased the heat generation at the upper section of the reactor and therefore a temperature depression resulted. A comparsion between the simulated activity profile and the temperature migration in Fig.4 illustrates the above phenomena. 

Frequently,for higher values of or RQ,the one-dimensional model can qualitatively predict a rather complex interaction between the temperature and concentration fields. Such a situation is presented in Figures 5-7. For high values of E the activity 0 is very sensitive to temperature fields and the activity calculated from one- and two-dimensional models can be different. For higher values of R (e.g. R lM),the activity profile can be affected by both the temperature and concentration fields. With higher temperatures, the consumption of benzene and poison and the rate of deactivation is higher however, the concentration of poison is lower. This complex interaction may result in radial profiles of activity with minima outside the reactor axis (c.f., Figure 8). Of course, the one-dimensional model cannot correctly describe such a behavior. 

Effect of Tube Diameter. For 1" tubes,the radial profiles of activity are parabola -like functions with minimum value at the center of the tube. For tubes with higher values of the diameter, e.g., 2" tubes, the picture can be rather different. High radial temperature gradients result also in large gradients of benzene and poison. For a deactivation process with high E, a minimum on the activity profile can occur between the reactor axis and wall, see Figure 8. Blaum ( 3) observed radial hot spots of temperature between the reactor axis and wall for a very rapid deactivation. For slow deactivation,these hot spots are not likely. 

OSHA PEL TWA 0.5 mg(As)/m3 ACGIH TLV BEI 35 (As)/L inorganic arsenic and methylated metabolites in urine DOT CLASSIFICATION 6.1 Label Poison SAFETY PROFILE Poison by an unspecified route. Moderately toxic by ingestion and intraperitoneal routes. Experimental teratogenic and reproductive effects. A skin and eye irritant. Questionable carcinogen with experimental tumorigenic data. Mutation data reported. Used as an herbicide, defoliant, and silvicide. Hazardous when water solution is in contact with active metals, e.g., Fe, Al, Zn. When heated to decomposition it emits toxic fumes of As. 

Flammable Liquid, Poison SAFETY PROFILE A poison by ingestion, skin contact, and subcutaneous routes. Very irritating to skin, eyes, and mucous membranes. Human systemic effects by ingestion convulsions, change in motor activity, coma. An agricultural chemical and pesticide. Flammable when exposed to heat or flame can react vigorously with oxidizing materials. When heated to decomposition it emits very toxic fiimes of NO and SOx. See also THIOCYANATES. 

In Figure 1, activity profiles affected by poisoning and activity profiles affected by both sintering and poisoning for three different ki values, T = 503 K and two months of operation are shown. It can be seen that the profiles are sharper when ki increases, consequently the average activity, as defined by eqn. (11), remains high. When both deactivation effects are superimposed, the... 

Figure 1. Activity profiles during poisoning and simultaneous sintering and poisoning.

Pavlou, S., and Costas, G. V, Optimal Catalyst Activity Profile in Pellets with Shell-Progressive Poisoning The Case of Fast Linear Kinetics, Chem. Eng. Sci. 45 (3) (1990) 695-703. 

Comparing eq. (8.183) and (8.117) it can be concluded that the activity profiles are very similar in the cases of coking and poisoning. 

When the poisoning is nonuniform, there exists an activity distribution within the pellet, and the generalized effectiveness factor obtained in Chapter 4 for a nonuniform activity profile can be used to arrive at the following expression ... 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

There seems no reason, therefore, to eschew the description of lithium therapy as the treatment of manic patients by lithium poisoning, in the words of an early critic (Wikler 1957). Anticonvulsants do not share the toxicity profile of lithium, but by virtue of their use to prevent epileptic fits, they all exert depressant effects on the activity of the central nervous system. With this profile of drug-induced effects in mind, let us examine the research on whether lithium or other drugs currently used in manic depression act in a disease-centred way and consider whether they have any real benefits. 

Figures 28 and 29 show the transient methanation activity of a Ni/Al203 flat-plate catalyst and the gas-phase H2S concentration profile, respectively. The presence of just 13-ppb H2S caused about a 200-fold loss in steady-state methanation activity. Increasing the H2S level to 62 ppb resulted in an additional tenfold activity loss an increase to 95 ppb lowered the activity further. However, increasing the H2S level above 95 ppm did not cause a significant additional decrease in activity (Fig. 30) and decreasing the H2S level from 95 to about 15 ppb reversibly restored the activity level originally observed at this latter concentration level, thereby demonstrating that sulfur adsorption and poisoning by sulfur are reversible, and that a truly dynamic...

Industrially, catalyst activity maintenance is often screened via "temperature increase requirement" (TIR) experiments. In these experiments, constant conversion is established and the rate of temperature increase required to do so is used as a measure of the resistance of the catalyst to deactivation. However, this type of operation may mask the effect of particle size, temperature, temperature profile, and heat of reaction on poison coverage, poison profile, and the main reaction rate. This masking may be particularly important in complicated reactions and reactor systems where the TIR experiment may produce positive feedback. 

Temperature Increase Dynamics after the First Cycle. As with the start up of the bed, subsequent temperature cycles resulted in the formation of a mild hot spot. The occurrence of this temperature fluctuation is undesirable since the past history of the catalyst may be altered. The adsorption of thiophene upon the active hydrogenation sites was assumed to be irreversible and therefore unaffected by temperature. However, as will become apparent later, the effect of temperature may have altered the poison coverage/or profile. Lyubarski, et. al [73 determined that, as a result of the hydrogenation of thiophene and subsequent hydrogenolysis to butane, the adsorption capacity of a suported Ni... 

A comparison between axial temperature profiles of temperature, activity, poison,and benzene concentrations calculated from one-and two-dimensional models is presented in Figures 1-4, respectively. The values calculated from a two-dimensional model are drawn in Figures 1-4 at the radial position r=0.707Ifo. The theory reveals (12) that the axial profile taken for a two-dimensional model at r = 0.707Ro should well agree with the one-dimensional approximation. The calculated results prove that there is a very good agreement between the one- and two-dimensional models. 

Figure 8. Radial profiles of (a) temperature (b) activity (c) concentration of poison. E /R 15,000[K],k =1.5XlO ...

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