Selectivity under periodic operation

Figure 8. Mean selectivity under periodic operation...

The compositional modulation technique has been applied to the Fischer-Tropsch synthesis (FTS) reaction [2-5], It was found that the cyclic feeding of CO/H2 had an influence on the selectivity of the FTS products. Among the conclusions was that for an iron catalyst the selectivity for methane increased under periodic operation compared to the steady state operation [5], In the study [5] it was found that the propane/propene ratio increased under periodic operation and the largest changes were with periods between one and ten minutes. Due to the limitations of the anal5dical technique utilized, they could not separate ethane and ethene so that the selectivity basis was for the C3 hydrocarbons. In this study the analytical procedure permitted analysis of products only to the Cg-compoimds. 

Reliability. The rehabiUty of a system is defined as the probabiUty that the system will perform its intended function satisfactorily for a specified interval of time when operating under stated environmental conditions. It has to be realized that supposedly identical products fail at different times, thus rehabihty can be quantified only as a probabiUty. For any product there is some underlying function that describes this success pattern. Typical reUabiUty functions are shown in Figure 1 for two different products. These products can be compared at the same rehabihty level R or the rehabihty levels can be compared for any selected time period, t. ... 

The significance of the coupling of micro- and macrorelaxations for resonance phenomena observed in catalytic systems under forced periodic operation (cycling) (15) implies that the wave-front analysis of transients of this kind can eventually suggest a more effective strategy in seeking the optimal conversion and selectivity. Finally the existence of certain surface structures and complexes could be established, if the transients of the surface intermediates will be followed e.g. by infrared spectroscopy (see e.g. (16, 17)). 

One instmment capable of measuring the dynamic shear modulus is a dynamic mechanical analyzer (DMA). A DMA measures the viscoelastic properties of a material by measuring the mechanical response that is deformed under periodic stress. Operation of a DMA tool offered by TA Instruments is as follows The sample is clamped between the ends of two parallel arms, which are mounted on low-force flexure pivots, allowing motion only in the horizontal plane. The distance between the two arms is adjustable by means of a precision mechanical slide to accommodate a wide range of sample lengths (from < 1 mm up to 65 mm). An electromagnetic motor attached to one arm drives the arm/sample to a strain (amplitude) selected by the operator. As the arm/sample system is displaced, the sample undergoes a flexural deformation [as depicted schemati-... 

Isothermal temperature control in the fluid-bed reactor was easily maintained under all process conditions investigated. The temperature gradients in the catalyst bed did not exceed 5 C even at mean temperature gradients of 200 to 300 C between the catalyst bed and the heat transfer medium. The plant accumulated 17 months-on-strearn of MTG/MTO operation, including 5 months at MTO conditions. MTO operation started with sensitivity studies to determine the effects of temperature and pressure on selectivity. Deactivation periods to vary catalyst activity and to Drovide a comDarison with the 4 B/D Dilot Diant were Derformed... 

We observed that the reaction using noble metal catalysts, particularly Pt and Pd, under cycling conditions was superior to that under static conditions, and that catalyst perfomance depends on the cycling period and feedstream composition. These phenomena may be used to improve the activity of three-way catalysts by selection of suitable periodic conditions. Further, it is very important to clarify the mechanism of the periodic operation effects over various noble metal catalysts. Our object was to investigate the performance of conversions over noble metal catalysts under periodic conditions. 

Also for CO sensing, the present sensors are available only for the field of security not for environmental use because of the insufficient sensitivity and selectivity to monitor CO in the atmosphere. Examples of CO sensor which have been improved their sensitivity and selectivity are, for example, SnO semiconductor sensors operated under periodic temperature cycle[85-87], a electrochemical sensor using nafion membrane[88], a catalytic combustion sensor composed of catalysts and hydrophobic pol uner[89], a SnOj diode sensor doped with Pd[90] and an optical fiber catalytic sensor with Au/CogO as combustion catalyst[91]. 

It is normal for the CO2 selectivity to decrease when the conversion over an iron catalyst is decreased [8]. The change in selectivity that was observed in figure 13 is also observed under steady state when the conversion falls from 60% to 25% due to an increase in the space velocity. The efficiency (amount of CO converted to FTS products and not CO2, fig 14) is also an indication that the change observbed with an increase in the period is normal for an iron catalyst. The periodic operation therefore only caused the conversion to decrease and a result of that decrease was that the selectivity for hydrocarbon products, relative to CO2, increased. 

The conditions under which different steels can be used in high-temperature hydrogen service are listed in API 941. The principal data are presented in the form of Nelson curves, as shown in Figure 2.11. The curves are based on long-term refinery experience, rather than on laboratory studies and are periodically revised by the API Subcommittee on Materials Engineering and Inspection. The latest edition of API 941 should be consulted to ensure that the proper steel is selected for the operating conditions encountered. 

The most selective methods for cleaving organic molecules at carbon-carbon double bonds involve glycols as intermediates. Oxidations of alkenes to glycols was discussed in Section 12.2.1. Cleavage of alkenes can be carried out in one operation under mild conditions by using a solution containing periodate ion and a catalytic... 

The analytical phase generally involves the use of very dilute solutions and a relatively high ratio of oxidant to substrate. Solutions of a concentration of 0.01 M to 0.001 M (in periodate ion) should be employed in an excess of two to three hundred percent (of oxidant) over the expected consumption, in order to elicit a valid value for the selective oxidation. This value can best be determined by timed measurements of the oxidant consumption, followed by the construction of a rate curve as previously described. If extensive overoxidation occurs, measures should be taken to minimize it, in order that the break in the curve may be recognized, and, thence, the true consumption of oxidant. After the reaction has, as far as possible, been brought under control, the analytical determination of certain simple reaction-products (such as total acid, formaldehyde, carbon dioxide, and ammonia) often aids in revealing what the reacting structures actually were. When possible, these values should be determined at timed intervals and be plotted as a rate curve. A very useful tool in this type of investigation, particularly when applied to carbohydrates, has been the polarimeter. With such preliminary information at hand, a structure can often be proposed, or the best conditions for a synthetic operation can be outlined. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

After a steady catalytic behavior was reached, the catalyst was treated in air at 350°C, in order to reoxidize it. Thereafter, the reaction was run again under isobutane-rich conditions (Figure 14.5), in order to understand the role of the POM reduction level on catalytic performance. The reoxidized catalyst exhibited a selectivity to methacrylic acid that was initially around 20%, and approximately 20-30 hours were necessary to recover the original performance of the equilibrated, reduced catalyst. On the contrary, the activity of the catalyst was almost the same as before the oxidizing treatment. This confirms that a partially reduced POM is intrinsically more selective to methacrylic acid than a fully oxidized one, and that one reason for the progressive increase in selectivity to methacrylic acid that occurs during the equilibration period was the increase in the POM reduction level, as a consequence of the operation under isobutane-rich conditions. 

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