Short-contact-time production

Catalytic upgrading of short contact time products allows catalysts to be used in more optimal conditions selectivities are improved and aging rates decreased. 

The data above show that the structure of short-contact-time products of coal liquefaction is indicative of the main characteristics of the carbon skeleton in coal itself, the distribution and abundance of the majority of heteroatomic functions, and the statistical distribution of the weak bonds in the parent coal. 

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). 

A major step in the production of nitric acid [7697-37-2] (qv) is the catalytic oxidation of ammonia to nitric acid and water. Very short contact times on a platinum—rhodium catalyst at temperatures above 650°C are required. 

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. 

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. 

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. 

ANALYSES OF PRODUCT FRACTIONS FROM HYDROPROCESSING OF 33% BLEND OF SHORT-CONTACT TIME SRC... 

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

PRODUCT RECOVERY OF SHORT CONTACT TIME FEEDS... 

The catalysts exhibiting the highest activity are the high-surface sodium on activated-alumina catalysts. They were used by O Grady et al. (12), to obtain equilibrium distributions of olefin isomers in short contact times and at relatively low temperatures, as shown in Table I. Since the same composition was reached with different starting materials and different reaction times, equilibrium distributions of products are easily obtained. The preparation of these high-surface sodium catalysts has been described (15). 

Rich catalytic combustion will offer wide opportunities with respect to most of the above issues, including flexible integration in different machines, low-temperature ignition ability, tolerance to fuel concentration and temperature non-uniformities and fuel flexibility. Further, the production of syngas in short contact time catalytic reactors could be exploited in several energy-related applications such as fuel cell and oxy-fuel combustion. 

A feasible procedure for the recovery of oil from the residual solids in the first stage of coal hydrogenation consists in treating the heavy oil slurry from the hot catchpot with superheated steam (25). At short contact times of a spray of heavy oil slurry with superheated steam, a high recovery of oil, with little or no coking or secondary asphaltene production, was achieved. 

Complete exclusion of moisture, short contact time of the reactants, and distillation of the reaction product in vacuo arc required for successful fluorination of (dichloromethyl)ben-zene and its 3- and 4-substituted fluoro and nitro derivatives. The hydrogen fluoride formed in side reactions is immediately removed to prevent polymerization of the (difluorometh-yl)benzenes.14... 

In the ethane-ethylene reaction in a flow system with short contact time, exclusive formation of n-butane takes place (longer exposure to the acid could result in isomerization). This indicates that a mechanism involving a trivalent butyl cation depicted in Eqs. (5.1)—(5.5) for conventional acid-catalyzed alkylations cannot be operative here. If a trivalent butyl cation were involved, the product would have included, if not exclusively, isobutane, since the 1- and 2-butyl cations would preferentially isomerize to the rm-butyl cation and thus yield isobutane [Eq. (5.9)]. It also follows that the mechanism cannot involve addition of ethyl cation to ethylene. Ethylene gives the ethyl cation on protonation, but because it is depleted in the excess superacid, no excess ethylene is available and the ethyl cation will consequently attack ethane via a pentacoordinated (three-center, two-electron) carbocation [Eq. (5.10)] ... 

---