Conversion thermodynamic limits

Carbon is deposited on a catalytic reactor bed as a result of the cracking of hydrocarbons. Periodically, hydrogen gas is passed through the reactor in an ef-fon to remove the carbon and to preserve the reduced state of the catalyst. It has been found, by experiment, that the effluent gas contains about 10 mol % methane and 90 mol % hydrogen when the temperature is 1000 K and the pressure is 1 bar. Is this conversion thermodynamically limited Can a higher concentration of methane be produced by re-... 

Dehydrogenation of /i-Butane. Dehydrogenation of / -butane [106-97-8] via the Houdry process is carried out under partial vacuum, 35—75 kPa (5—11 psi), at about 535—650°C with a fixed-bed catalyst. The catalyst consists of aluminum oxide and chromium oxide as the principal components. The reaction is endothermic and the cycle life of the catalyst is about 10 minutes because of coke buildup. Several parallel reactors are needed in the plant to allow for continuous operation with catalyst regeneration. Thermodynamics limits the conversion to about 30—40% and the ultimate yield is 60—65 wt % (233). 

A much more selective reaction is possible by using vapor-phase fluonnahon over a chromia" catalyst at 300 to 400 °C, but conversions are thermodynamically limited to 10-20% under acceptable operating conditions [fO 11 Despite this disadvantage, this process has been selected by ICI and Hoechst for their first plants... 

The series of reactors and exchangers which methanates a raw syngas without pretreatment other than desulfurization is collectively termed bulk methanation. The chemical reactions which occur in bulk methana-tion, including both shift conversion and methanation, are moderated by the addition of steam which establishes the thermodynamic limits for these reactions and thereby controls operating temperatures. The flow sequence through bulk methanation is shown in Figure 1. 

The chemical industry of the 20 century could not have developed to its present status on the basis of non-catalytic, stoichiometric reactions alone. Reactions can in general be controlled on the basis of temperature, concentration, pressure and contact time. Raising the temperature and pressure will enable stoichiometric reactions to proceed at a reasonable rate of production, but the reactors in which such conditions can be safely maintained become progressively more expensive and difficult to make. In addition, there are thermodynamic limitations to the conditions under which products can be formed, e.g. the conversion of N2 and H2 into ammonia is practically impossible above 600 °C. Nevertheless, higher temperatures are needed to break the very strong N=N bond in N2. Without catalysts, many reactions that are common in the chemical industry would not be possible, and many other processes would not be economical. 

In typical diesel exhaust gas, the N02/N0x fraction is only 5-10%. This fraction may be increased by passing the gas over a strong oxidation catalyst containing platinum as the active component. However, it is difficult to obtain useful fractions of N02 at temperatures below 200°C at high space velocities due to the strong temperature dependency of the NO oxidation over platinum [42], As expected the NO conversion rises exponentially with temperature, but declines at higher temperatures due to the thermodynamic limit, of the reaction (Figure 9.13). 

In the above manner, we may proceed downstream in the reactor until we either reach the desired conversion level, run into thermodynamic limitations on the reaction rate, or exceed the effluent temperature constraint (see Table... 

Sometimes, the maximum conversion is limited by thermodynamics. In such cases, different approaches need to be taken to surpass the thermodynamic limitation. Standard methods that are also realized on an industrial scale are ... 

Kragl and coworkers investigated using organic-solvent-free systems to overcome the thermodynamic limitations in the synthesis of optically active ketone cyanohydrins. With organic-solvent-free systems under optimized reaction conditions, conversions up to 78% with > 99.0 enantiomeric excess (ee) (S) were obtained. Finally, 5 mL of (S)-acetophenone cyanohydrin with an ee of 98.5% was synthesized using MeHNL [52]. 

A.W. Adamson, University of Southern California You mentioned a thermodynamic limitation to the efficiency of conversion of solar energy to useful work. This is an interesting point, and I would like to hear more about it. I can see an entropic effect in terms of the following hypothetical cell for the system A = A, where A is the excited state of species A ... 

C. Thermodynamic Limits on the Conversion of Light Energy to Work... 

Many authors have treated the problem of thermodynamic limits on the conversion of light to work (e.g., electricity or chemical free energy) (7-12) however, Ross and Hsiao (13) have recently published a particularly lucid treatment which I will briefly summarize here. 

Curve P in Fig. 2 represents the ideal thermodynamic limit for conversion efficiency and cuarve C sets an approximate limit for the efficiency of conversion to stored chemical energy. There are, however, other loss factors to be considered which vaary according to the device. These loss factors have been considered in considerable detail for photovoltaic devices (15, 19) and estimates for the ultimate achievable conversion efficiency to electricity vaary from 25 - 2 8% however, only recently have efficiencies been considered for conversion to stored chemical energy (5). In this case... 

Within the kinetic and thermodynamic limitations on the conversion of light energy to chemical energy, I have shown that a reasonable efficiency goal would be 25 - 28% for conversion to electricity and 10 - 13% for storage as chemical energy. Five types of photochemical converters have been defined and described with examples where possible. 

Ross RT (1966) Thermodynamic limitations on the conversion of radiant energy into work. J Chem Phys 45 1-7... 

So far this discussion has covered the effect of kinetics on reaction selectivity. However, it is important to realise that selectivity can also be determined by thermodynamics. It is thus necessary to write kinetics expressions that ensure the thermodynamic limitations are fulfilled for reactions where thermodynamic limitation is likely. For example, if the experiment shown in Fig. 12 were continued to higher the temperature, the CO conversion would eventually fall off with increasing temperature due to the water gas shift reaction reaching equilibrium. 

In seeking new and improved ways for achieving the ultralow levels of sulfur in the fuels of the future, it is important to understand the nature of the sulfur compounds that are to be converted (especially PASCs), as described in Section III. It is equally important to understand how these transformations occur through interactions with catalytic surface species, the pathways involved during these transformations, and the associated kinetic and thermodynamic limitations. These considerations dictate the process conditions and reactor process configurations that must be used to promote such transformations. In this section, we describe the reactor configurations and process conditions being used today what is known about the catalyst compositions, structure, and chemistry and what is known about the chemistry and reaction pathways for conversion of PASCs in conventional HDS processes. 

There are no real thermodynamic limits in the removal of sulfur from any organic sulfur compound by reaction with hydrogen (1, 2, 5). There are, however, limits on the overall rates of conversion that may be achieved by increasing the temperature of the reaction. A classic limitation in rates is the result of the inverse relationship between adsorption on a catalytic surface and temperature. This may be a problem with dialkyldibenzothio-phenes, which have steric limitations for adsorption. 

The fast and easy conversion of activated amino acids 18 with a AG0/ above the thermodynamic limit of Fig. 3 into NCA and the similar conversion of N-substituted derivatives 23 involving good leaving groups show that most extremely activated amino acid derivatives must have been converted into NCAs in bicarbonate-buffered aqueous solution at moderate pH (5.5—8.5). 

At times, the reaction may be exothermic with conversion being limited by thermodynamic equilibrium. In such cases, packed beds in series with interstage cooling may be used as well. The performance enhancement associated with this approach is shown for two cases in Table 19-2. Such units can take advantage of initial high rates at high temperatures and higher equilibrium conversions at lower tempera-... 

Fig. 5.6. Above Activity in benzene hydrogenation, as % of conversion at 100°C per m2 total surface area of unsupported alloys, as a function % Cu in Ni. Below Conversion with a standard amount catalyst as a function of temperature. (1) pure Ni (2) 10% Cu-Ni alloy (3) thermodynamic limit at reaction conditions (for other details see [1]).

The main drawbacks of the non-oxidative dehydrogenation reaction can be summarized as, the thermodynamic limitation, the low conversion rate, the need for recovery of unreacted ethylbenzene, the high energy consumption, and deactivation of the catalyst. Thus in recent years several alternatives to overcome those problems have been investigated. 

In membrane reactors, the reaction and separation processes take place simultaneously. This coupling of processes can result in the conversion enhancement of the thermodynamically-limited reactions because one or more of the product species is/are continuously removed. The performance of such reactors depends strongly on the membrane selectivity as well as on the general operahng conditions which influence the membrane permeability. 

Preliminary results obtained in an effort to model the dehydrogenation of ethylbenzene to styrene in a "membrane reactor" are described below. The unique feature of this reactor is that the walls of the reactor are conprised of permselective membranes through which the various reactant and product species diffuse at different rates. This reaction is endothermic and the ultimate extent of conversion is limited by thermodynamic equilibrium constraints. In industrial practice steam is used not only to shift the ec[uilibrium extent of reaction towards the products but also to reduce the magnitude of the ten erature decrease which accon anies the reaction when it is carried our adiabatically. 

If several reactants are charged in non-stoichiometric amounts, their fractional conversions differ. As a convention, reference then is to the limiting reagent, that is, the reactant that could be consumed completely (barring thermodynamic limitations). 

Table 6 shows an example of the cyclic steady state performance of the SERP concept using an admixture of a SMR catalyst (noble metal on alumina) and a CO2 chemisorbent (K2CO3 promoted hydrotalcite) in the reactor [20]. The reactor temperature was 490°C. The feed H20 CH4 ratio was 6 1. The concept can directly produce 95% H2 product (dry basis) with a CH4 to H2 conversion of 73%. The trace impurities in the product gas contained less than 40 ppm COx- The corresponding product gas composition (thermodynamic limit) of a SMR reactor operated without the CO2 chemisorbent will be -67.2% H2, 15.7% CH4, 15.9% CO2, and 1-2% CO (dry basis), and the CH4 to H2 conversion will be only 52%. Thus, the SERP concept may be attractive for direct production of a CO free H2 stream for fuel cell applications. 

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