Carbon efficiency

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

Coal is the main energy reserve for many countries. In order to ensure energy security, many of these countries are looking at implementing large-scale coal-to-liquid (CTL) plants. Current CTL Fischer-Tropsch (FT)-based processes at best have a carbon efficiency near 45% that is, less than half the carbon that is fed to the process ends up as hydrocarbon fuels. This has large implications for the environment, in that more than half of the carbon in the feed to the CTL ends up... 

It is clear from the mass balance that C02 is inevitably produced and that for every mole of product, half a mole of C02 must be produced. In terms of a carbon efficiency (defined as the amount of carbon that ends up in product), this means a 67% carbon efficiency. This is simply the result of the wrong ratio of the atoms (C and H) in the feedstock. 

Taking the energy requirements into account results in a reduction of the carbon efficiency for the CTL process from 67% to 60%. 

This process is the idealized process mass balance, representing the process with the highest carbon efficiency and lowest possible C02 emissions. The process may consist of various subsystems (reactions, phase change, etc.). Achieving this... 

FIGURE 17.6 Comparison of two CTL routes in terms of (a) carbon efficiency and (b) lost work. 

Figure 17.6 shows a comparison of the two CTL processes in terms of carbon efficiency and lost work. It is clear the newly developed process has the advantage of being more efficient, producing less carbon dioxide, and also less lost work. 

Some of the oxygenates in the Fischer-Tropsch aqueous product could be recovered by distillation to improve the overall carbon efficiency of the Fischer-Tropsch refinery. However, it was pointed out that complete separation of the short-chain oxygenates was difficult due to the formation of azeotropic mixtures.23... 

J. Schwender, F. Goffman, J. B. Ohlrogge, and Y. Shachar Hill, Rubisco without the Calvin cycle improves the carbon efficiency of developing green seeds. Nature 432, 779 782 (2004). 

Acetic acid has been generated directly from synthesis gas (CO/H2) in up to 95 wt % selectivity and 97% carbon efficiency using a Ru-Co-I/Bu4PBr "melt" catalyst combination. The critical roles of each of the ruthenium, cobalt and iodide catalyst components in achieving maximum selectivity to HOAc have been identified. Ci Oxygenate formation is observed only in the presence of ruthenium carbonyls [Ru(C0)3l3] is here the dominant species. Controlled quantities of iodide ensure that initially formed MeOH is rapidly converted to the more reactive methyl iodide. Subsequent cobalt-catalyzed carbonylation to acetic acid may be preparatively attractive (>80% selectivity) relative to competing syntheses where the [00(00)4] concentration is optimized that is, where the Co/Ru ratio is >1, the syngas feedstock is rich in 00 and the initial iodide/cobalt ratios are close to unity. 

Figure 2 illustrates the effect of incremental changes in ruthenium catalyst content upon the production of acetic acid and its C1--C2 alkyl acetate esters. Acetic acid production is maximized at Ru/Co ratios of ca. 1.0 1.5 however, the data in Figure 2 do show an approximate first order dependence of lOAc (acetic acid plus acetate esters) upon initial ruthenium content—at least up to the 2/1, Ru/Co stoichiometry under the chosen conditions. Selectivity to acetic acid in the liquid product peaks at 92 wt % (carbon efficiency 95 mol %) for a catalyst combination with initially low Ru/Co ratios (e.g. 1 4). The formation of C1-C2 alkanols and their acetate esters rapidly exceeds acetic acid productivity when the Ru/Co atomic ratio is raised above 1.5, although two-carbon oxygenates continue to be the predominant fraction. Smaller quantities of glycol may also be in evidence. 

Assuming reported typical methane levels are formed in this experiment ethanol carbon efficiency among liquid products is 63%. 

Calcium oxide dates from prehistoric times. It is produced by heating limestone to drive off carbon dioxide in a process called calcination CaCO , —-—> CaO, + CO ,At tem-peratures of several hundred degrees Celsius, the reaction is reversible and calcium oxide will react with atmospheric carbon dioxide to produce calcium carbonate. Efficient calcium oxide production is favored at temperatures in excess of 1,000°C. In prehistoric times limestone was heated in open fires to produce lime. Over time, lined pits and kilns were used to produce lime. Brick lime kilns were extensively built starting in the 17th century and the technology to produce lime has remained relatively constant since then. 

Carbon efficiency determines greenness of the loss of carbon in a reaction (Equation 13.8).34... 

Solvents are not included in the calculation. Like effective mass yield, carbon efficiency strives to make a value judgment on the relative importance of various wastes. Elimination of water from a molecule would not count against the carbon efficiency of a reaction, but it would give a decreased atom economy. By not including all wastes, carbon efficiency is not as strict as most green metrics. Of course, all metrics have their shortcomings and carry their own assumptions. 

The emissions reduction effects are caused in part by a reduction in UK output - a combination of a change in the share of the UK market held by UK producers and of weaker demand from consumers as a consequence of higher prices. They are also caused by investment in carbon-efficient technologies. Most of the emissions reduction is attributable to greater carbon efficiency, driven by the carbon abatement curves prepared for The Carbon Trust by Ecofys. Only in the steel and cement sectors does output reduction contribute to a significant carbon reduction. 

We stress that not only is our trade representation (i.e. no product differentiation, focus on transport, no inertia in trade) responsible for this important leakage around 2010, but so also is the technical inertia leakage decreases as time goes by with the introduction of more carbon-efficient techniques. Furthermore, the reader should bear in mind that we assume no climate policy outside the EU which explains a part of this high leakage rate. 

Figure 2.4.3 Comparison of different renewable energy pathways and carbon transfer schemes from carbohydrates, as well as their typical conditions. Here, preservation of combustion energy and the carbon efficiency (CE) of the transformation are compared. The combustion energy always concerns the complete side of the reaction equation. The sum formula of the coalified plant material is a schematic simplification.

Highly Selective Synthesis Catalyst. Very low byproduct formation, and very high loop carbon efficiency is achieved by the use of ICI s 51-2 methanol catalyst, which is highly selective. 

Palladium on activated carbon has turned out to be a highly versatile, simple heterogeneous catalyst for one-pot multistep syntheses. Recently, Djakovitch and coworkers [42] have demonstrated that low catalyst loadings of Pd on activated carbon efficiently catalyze the Heck reaction of bromo benzene and styrene giving rise to T-stilbene (1) (92%), Z-slilbcnc (1%), and 1,1-diphenylethene (7%). If the Heck products are not isolated but an atmosphere of 20 bar of hydrogen is imposed onto the reaction vessel the sequence furnishes 1,2-diphenylethane (2) in 93% yield (Scheme 1). 

Practically speaking, in order to achieve the simultaneous conversion of CO and COj to metbanoL one can also introduce the concept of carbon efficiency, defined as follows ... 

In a particular experiment, p-cresol (23.54 g, 0.22 mol), methyl-f-butyl ether (19.36 g, 0.22 mol) and a silica/zirconia catalyst (3.5 wt%) were heated at 100°C for 3 hours. After cooling, 13.0 g of the product, 2-t-butyl-p-cresol was obtained, and 10.78 g of p-c resol was left unreacted. The FW of the product = 164 g/mol. Calculate the atom economy, yield, selectivity, carbon efficiency, and environmental factor. 

Cobalt- rather than iron-based FT catalysts have been examined, in order to minimize the competing water-gas shift reaction, which would result in a lowered carbon efficiency. Most cobalt FT catalysts have been prepared by coprecipitation of Co salts with various promoters onto a slurried oxide support to afford mixed phase systems (J ). Reduction to the active catalyst was controlled by addition of various promoters (e.g. MgO, Th02, AI2O3) (2). In part, these promoters are necessary to maintain good metal dispersion in the catalyst and resistance to sintering. Dispersion... 

The UT-Austin experimental tests have shown that high (98+ %) carbon combustion efficiency is possible in a fluidized bed combustor burning Texas lignite and operating on a once-through basis. Future tests incorporating recycle of elutriated material will be performed to determine the extent to which this increases carbon efficiency. 

Fig. 25. Carbon efficiencies of oxygenated products in the CO + H2 reaction and ethanol selectivities on bimetallic RhCo cluster-derived catalysts impregnated on Zr02 (0.2 wt% metal) (CO/H2 ratio 20/45 cmHg, 200°C). O.C. represents oxygenated products on a carbon basis.

Rh precursor Turnover frequency Selectivity in carbon efficiency % Rh mean particle size s... 

Madhavaram H, Idriss H (2002) Carbon efficiency and the surface chemistry of the actinides. Direct formation of furan from acetylene over P-UO. J Catal 206 155... 

A melt RuCoI/Bu4PBr catalyst system yields acetic acid from syn gas at 95 % selectivity and 97 % carbon efficiency [130],... 

Here the carbon efficiency nc is defined by nc - carbon in permanent gases -v carbon in solid feed. 

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