Short-contact-time processing

The significance of these calculations is that lower rank coals will require 5% lower conversion than higher rank coals for a given end product. Also, the more severe a coal is to be upgraded, the lower its conversion has to be in the initial phases of liquefaction. One very pertinent question to be addressed is whether or not coals can be converted to the levels shown in Figure 5 in a short contact time process. This paper will deal with that question as well as what compositional features of the coal and the solvent influence short contact time conversions. 

Similarly, a subbituminous coal (Wyodak-Anderson) which gave only 60% conversion at 427°C in 2 minutes could be converted to >70% at 460°C with no ill effects (see Figure 17). At 470°C there was an indication that conversion had begun to decline at 2 minutes however, this data is extremely limited. The implications of these results with a western subbituminous coal is that a low sulfur boiler fuel may potentially be produced in a single-stage short contact time process. 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

Applications of Structured Catalysts in Short Contact Time Processes... 

The so-called short contact time processes are those processes wherein extremely high throughputs are realized in small reactor volumes, with contact times ranging from milliseconds down to microseconds. These include the partial and selective oxidations of hydrocarbons to produce synthesis gas, olefins and oxygenates. 

It is beyond the scope of the present contribution to provide a detailed report on the complexity of the mechanism and kinetics of short contact time processes. Instead, an effort is made to summarize the advancement of the research in this field, trying to focus on the specific characteristics of monolithic structures that are requested and exploited in the short contact time production of chemicals. The focus of this review is on the selective oxidation (or oxidative dehydrogenation) of small alkanes to olefins. Mention is also made of other short contact time oxidation processes, such as the ammoxidation of methane to HCN. 

The bottoms from the solvent recovery (or a2eotropic dehydration column) are fed to the foremns column where acetic acid, some acryflc acid, and final traces of water are removed overhead. The overhead mixture is sent to an acetic acid purification column where a technical grade of acetic acid suitable for ester manufacture is recovered as a by-product. The bottoms from the acetic acid recovery column are recycled to the reflux to the foremns column. The bottoms from the foremns column are fed to the product column where the glacial acryflc acid of commerce is taken overhead. Bottoms from the product column are stripped to recover acryflc acid values and the high boilers are burned. The principal losses of acryflc acid in this process are to the aqueous raffinate and to the aqueous layer from the dehydration column and to dimeri2ation of acryflc acid to 3-acryloxypropionic acid. If necessary, the product column bottoms stripper may include provision for a short-contact-time cracker to crack this dimer back to acryflc acid (60). 

Today, the air oxidation of toluene is the source of most of the world s synthetic benzaldehyde. Both vapor- and Hquid-phase air oxidation processes have been used. In the vapor-phase process, a mixture of air and toluene vapor is passed over a catalyst consisting of the oxides of uranium, molybdenum, or related metals. High temperatures and short contact times are essential to maximize yields. Small amounts of copper oxide maybe added to the catalyst mixture to reduce formation of by-product maleic anhydride. 

Outside the realm of typical hydrocarbon pyrolysis is the high temperature pyrolysis of methane. In one variant of this process, which has only been commercialized to produce acetylene (with some BTX), methane reacts in an electric arc at about 1500°C (17) with very short contact times. At higher temperatures or with a catalyst and added hydrogen, BTX is produced with fairly high selectivity (18). 

In delayed coking, the reactor system consists of a short contact-time heater coupled to a large drum in which the preheated feed soaks on a hatch basis. Coke gradually forms in the drum. A delayed coking unit has at least a pair of drums. When the coke reaches a predetermined level in one drum, flow is diverted to the other so that the process is continuous. 

The second section of this volume describes several potentially new liquefaction processes which may have higher efficiencies than today s developing technologies. The theme of the Storch Award Symposium, featured throughout these six chapters, was new process potentials through the use of short-contact-time thermal processes followed by catalytic upgrading. 

These observations suggest that new coal liquefaction technology may be possible based on short contact time reactions. The purpose of this and the related papers in this volume by R.H. Heck and W.C. Rovesti is to show some potential advantages for optimized or integrated short contact time liquefaction processes over conventional technology. 

This paper will concentrate on factors which lead to high conversion at short time. R.H. Heck, T.O. Mitchell, T.R. Stein and M.J. Dabkowski discuss the relative ease of conversion of short and long contact time SRCs to higher quality products. C.J. Kulik, W.C. Rovesti and H.E. Liebowitz discuss some new leads presently being explored at the Wilsonville PDU in which short contact time liquefaction is being coupled with rapid product isolation via the Kerr-McGee Critial Solvent Deashing Process. 

In order to understand the potential advantages for short contact time liquefaction processes, let us first consider some of the disadvantages for presently developing long contact time processes. These are enumerated below. 

A combination of this and the related papers will show the following potential advantages for short contact time optimized or integrated processes. 

Effect of Process Parameters on Short Contact Time Conversions... 

Hydrogen donors are, however, not the only important components of solvents in short contact time reactions. We have shown (4,7,16) that condensed aromatic hydrocarbons also promote coal conversion. Figure 18 shows the results of a series of conversions of West Kentucky 9,14 coal in a variety of process-derived solvents, all of which contained only small amounts of hydroaromatic hydrocarbons. The concentration of di- and polyaromatic ring structures were obtained by a liquid chromatographic technique (4c). It is interesting to note that a number of these process-derived solvents were as effective or were more effective than a synthetic solvent which contained 40% tetralin. The balance between the concentration of H-donors and condensed aromatic hydrocarbons may be an important criterion in adjusting solvent effectiveness at short times. 

Although the comparisons are by no means exact, a process based on short-contact time dissolution and catalytic upgrading would appear to have potential for significantly higher yields of high quality liquids from coal. 

Processing Short-Contact-Time Coal Liquefaction Products... 

D [Discriminatory destructive distillation] A thermal deasphalting process which uses the same short contact time concept as the MSCC process and a circulating solid for heat transfer between reactor and generator. It is claimed to be most effective on heavy contaminated whole crude oils or residues. Developed by Bar-Co and now offered by UOP. 

MSCC [Millisecond catalytic cracking] A fluid catalytic cracking process which uses an ultra-short contact time reaction system. It is claimed that less capital investment and higher liquid yields can be achieved using this process, compared with conventional FCC units. Developed by Bar-Co and now offered by UOP it has been operating since 1994. 

A simplified diagram of a typical FCC unit is shown in Figure 16.9. The reaction chemistry described above is carried out in this process at temperatures in the range of 500-540 °C by contachng the fluidized catalyst in the form of particles in the range of 30-120 xm in diameter with the hot feed injected near the top of a riser reactor followed by rapid disengagement after a short contact time (on the... 

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