Conversion level

Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

More recendy, advanced generation gasifiers have been under development, and commercialisation of some of the systems has become a reaUty (36,41). In these newer developments, the emphasis has shifted to a greater throughput, relevant to the older gasifiers, and also to high carbon conversion levels and, thus, higher efficiency units. 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

Equation 1 is referred to as the selective reaction, equation 2 is called the nonselective reaction, and equation 3 is termed the consecutive reaction and is considered to proceed via isomerization of ethylene oxide to acetaldehyde, which undergoes rapid total combustion under the conditions present in the reactor. Only silver has been found to effect the selective partial oxidation of ethylene to ethylene oxide. The maximum selectivity for this reaction is considered to be 85.7%, based on mechanistic considerations. The best catalysts used in ethylene oxide production achieve 80—84% selectivity at commercially useful ethylene—oxygen conversion levels (68,69). 

Silver-containing catalysts are used exclusively in all commercial ethylene oxide units, although the catalyst composition may vary considerably (129). Nonsdver-based catalysts such as platinum, palladium, chromium, nickel, cobalt, copper ketenide, gold, thorium, and antimony have been investigated, but are only of academic interest (98,130—135). Catalysts using any of the above metals either have very poor selectivities for ethylene oxide production at the conversion levels required for commercial operation, or combust ethylene completely at useful operating temperatures. 

FIG. 14-12 Effects of reagent-concentration and reagent-conversion level upon the relative values Kca in the C02-Na0H-H 0 system. [Adapted from Eckeii et at, Ind. Eng. Chem., 59f2h 41 (1967).]... 

Table 14-2 illustrates the observed variations in values for different packing types and sizes for the COg-NaOH system at a 25 percent reactant-conversion level for two different liquid flow rates. The lower rate of 2.7 kg/(s-m ) or 2000 lb/(h-ft ) is equivalent to 4 (U.S. gal/min)/ft and is typical of the liquid rates employed in fume scrubbers. The higher rate of 13.6 kg/(s-m ) or 10,000 lb/(h-fU) is equivalent to 20 (U.S. gal/min)/ft and is more typical of absorption towers such as are used in CO9 removal systems. For example. We note also that two different gas velocities are represented in the table, corresponding to superficial velocities of 0.59 and 1.05 m/s (1.94 and 3.44 ft/s). 

One quantitative measure of reactor efficiency at a conversion level x is the ratio of the mean residence time or the reactor volume in a plug flow reactor to that of the reactor in question,... 

The conversion level sometimes is taken at 95 percent of equihbrium, but there is no universal standard. 

This is an obviously difficult task, and it is rarely possible to satisfy both requirements reasonably and simultaneously. This difficulty is compounded by the need to use a preconverter to achieve the various conversion levels where the additional incremental increase in conversion can be measured. The alternative way to a preconverter is to feed the reactor various amounts of products in addition to the starting material. This does not ease the analysis difficulties. 

Row and Column Names F2C1 Feed 2 at conversion level 1... 

F2CAP The availability of feed 2 F4C2 Feed 4 at conversion level 2... 

As seen in Figure 1, the example plant has three major processing steps Reaction, Compression, and Fractionation. There are four available feeds to the plant (FI, F2, F3, and F4). The desire is to use all of the available Feed 1. The model is inhibited from leaving any Feed 1 capacity unused by a large negative SPRICE as seen in the matrix. Feeds 2, 3, and 4 are given a choice of how much of the available material to use. However, Feeds 3 and 4 must be utilized in a 2 1 ratio as dictated by row MIX in the matrix. Feeds 1, 2, and 3 have a choice of 2 conversion levels, while Feed 4 has a choice of 3. 

For items in the solution basis, such as feeds or conversion levels that were selected by the model for the run in question, range information is given. This shows what would happen if the basis item were removed from the basis. 

There is a wide range of conversion levels. The term maximum conversion type has no precise definition but is often used to describe a level of conversion, where there is no net fuel oil manufactured. A fuel products refinery with specialities may manufacture lubricating oils, asphalts, greases, solvents, waxes and chemical feed stocks in addition to the primary fuel products. The number and diversity of products will naturally vary from one refinery to another. Refineries produce chemical feed stocks for sale to the chemical affiliates and do not have responsibility for the manufacture of chemical products directly. Both operations may be carried out at the same physical location but the corporate product responsibilities are usually separate. 

Seasonal chances in gasoline sales and heating oil sales compel some modifications to be made in conversion level. Therefore, the conversion pattern of a given catalytic cracking unit can vary from season to season. In summer operations, for instance, higher yields of motor gasoline are desired, both from direct production of 5/430° FVT catalytic naphtha and also from conversion of butylenes and isobutane to alkylate. 

Volume of CFSTR and plug flow reactors with respect to conversion level for a first order reaction... 

The MTBE reaction is equilihrium limited. Higher temperatures increase the reaction rate, hut the conversion level is lower. Lower temperatures shift the equilihrium toward ether production, hut more catalyst... 

ZSM-5 s effectiveness depends on several variables. The cat crackers that process highly paraffinic feedstock and have lower base octane will receive the greatest benefits of using ZSM-5. ZSM-5 will have little effect on improving gasoline octane in units that process naphthenic feedstock or operate at a high conversion level. 

DO is the lowest priced product and the goal is to reduce its yield. The DO s yield largely depends on the quality of the feedstock and the conversion level. Naphthenic and aromatic feedstocks tend to yield more bottoms than paraffinic feedstocks. If the conversion is m the low- to mid-70 s, increasing catalyst-to-oil ratio or using a catalyst with an active matrix can reduce slurry yield. Raising conversion reduces bottoms yield. If the conversion is in the 80 s, little more can be done to reduce bottoms yields. 

During the copolymerization 4 is, therefore, consumed faster, and at the initial stage the copolymers are enriched with 4. At high conversion levels its relative portion in the macromolecules of the copolyner decreases, resulting in the formation of copolymers with non-uniform composition. 

In thioamidation the nitrile groups of PAN have a much higher reactivity than those of the corresponding model compounds. This fact is explained by the specific character of the polymeric nature of PAN and by the mutual influence of adjacent groups. As it is seen from the data presented in Fig. 4, the highest reaction rate and conversion level, as compared with low-molecular nitriles, is observed in the thioamidation of PAN. 

Fig. 4. Dependence of the conversion level on the time of thioamidation of PAN and low-molecular nitriles at 20 °C in DMFA. (1) PAN (2) 1,35-pentanetricarbonitrile. (3) glutaro-dinitrile. (4) acetonitrile...

These two parameters describe the change in fraction unconverted with a percentage change in kt or in c0. The first sensitivity is also the slope of the curves in Fig. 28. The values of these sensitivities are given in Table IX. In a piston flow reactor where the conversion level is c/c0 = 0.1, the value of Stt is —0.23 for the first-order kinetics, —0.90 for the zero-order kinetics, and —4.95 for the negative first-order kinetics. In the stirred tank reactor, the value of the sensitivities Skt is —0.09 for the first-order kinetics, — 0.90 for the zero-order kinetics, and +0.11 for the negative first-order kinetics. A positive sensitivity means that as kt is increased, the fraction unconverted also increases, clearly an unstable situation. 

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