Steam dilution

Thermal isomerization of a-pinene, usually at about 450°C, gives a mixture of equal amounts of dipentene (15) and aHoocimene (16) (49,50). Ocimene (17) is produced initially but is unstable and rearranges to aHoocimene, which is subject to cyclization at higher temperatures to produce a- and P-pyronenes (18 and 19). The pyrolysis conditions are usually optimized to give the maximum amount of aHoocimene. Ocimenes can be produced by a technique using shorter contact time and rapid quenching or steam dilution (51). 

For recycling to improve the performance of an MCFC network, it must provide benefits that outweigh its inherent disadvantages. If carbon dioxide is not separated from the anode-anode recycle, the concentration of carbon dioxide in the anode is increased. This reduces the Nemst potential. The Nemst potential is similarly reduced by the anode-cathode recycle if steam is not condensed out, since recycled steam dilutes reactant concentrations in the oxidant. In addition, part of the power generated by the network is consumed by the equipment necessary to circulate the recycle streams. Such circulation equipment, along with the additional ducting required by recycling, also increases the capital cost of the MCFC network. 

For a given raw material, the composition of the reaction effluents is obviously related to the variables of temperature, residence time, pressure, and steam dilution rate. At the industrial level, the individual optimization of these parameters normally leads to contradictory requirements hence the solution adopted is generally the result of a compromise in the choice of furnace design on the one hand, and operating conditions on the other. However, an attempt is made to express the overall influence of these factors on the performance of the reaction section by means of a representative value that can indicate the degree of severity of the treatment. 

Experiments were carried out between 720° and 985°C with 1-butene and between 760°and 925°C with the mixed isomers of 2-butene, using steam dilution corresponding to a steam hydrocarbon weight ratio of between 0.17 and 1.2 g/g. All runs were isobaric at total pressure of 1.0 psig. Material balances generally fell within dz 2% for all of the experiments. Tables I and II summarize the individual run conditions, the observed yields and conversions, and the calculated rate constants for pyrolysis of 1-butene and the 2-butenes, respectively. 

In general, with decreasing hydrocarbon partial pressure, unsaturated components such as acetylene, ethylene, propylene, and butadiene increase whereas BTX, pyrolysis fuel oil, and saturated components such as methane, ethane, and propane decrease. Low hydrocarbon partial pressure can be attained either by high steam dilution or by low absolute pressure in the cracking coil, which is determined by furnace outlet pressure and pressure drop in the cracking coil. For each specific case there is an optimum steam dilution. Reduction of steam dilution influences yields, utilities, running times and, in the case of a new ethylene plant, of course, investment costs—but in different ways, either positive or negative. Thus, an optimization has to be carried out to identify the most economic steam dilution. 

The location of the minimum indicates the most economic steam dilution. Higher cracking severity and heavier feed stock shifts the optimum steam dilution to higher values. Existing naphtha furnaces operate mostly with steam dilution between 0.5 and 0.6. An example of the reduction of steam dilution for existing furnaces is discussed at the end of this chapter. 

Figure 2. Optimization of steam dilution for naphtha, k = f (NFC, utility costs, investment costs and...

Fired duty can be reduced by either reduction of dilution steam or combustion air preheating or to a lesser degree by reduction of excess air to the burners. The selection of the optimum steam dilution was discussed previously. 

Great attention should be paid to the convection section. For each feed stock pair, different steam dilution, different initial cracking point, different physical properties, and different hp-steam production all result in different quantities of heat which have to be transferred in the convection section bundles. Table II shows typical heat transfer rates for a... 

Since investment costs had to be kept low, the convection section, the fire box, and the burners were left unchanged. To fulfill these requirements, the new LSCC design was calculated for the same naphtha, P/E, hc-throughput, steam dilution, and crossover temperature. The following improvements in cracking parameters were calculated for the LSCC design pressure drop = 31% less, residence time = 28% less, and MCP = 43% less. 

Figure 9. MCP for various steam dilutions. P/E = const hc-through-...

The dehydrogenation reaction proceeds over an iron or an iron-chromium catalyst that usually also contains potassium in the form of potassium carbonate, so that at elevated temperatures various complex mixed carbonates and oxides are formed, e.g., KFe02. Temperatures are elevated, in the order of 630° C, and pressures are usually subatmospheric for improved per-pass conversions. Steam dilution is performed to further lower the partial pressure of the reactants. Because the reaction is strongly endothermic, various reaction stages with interheat (and interstage addition of superheated steam) are normally employed. Fig. 18 illustrates a typical process scheme for the dehydrogenation of ethylbenzene to styrene. 

This is the same principle as IFGR—the steam dilutes the fuel gas thus reducing both the flame temperature and the localized hot spots. Systems have used up to 8% steam injection (the steam injection rate is 8% of the total steam production from the boiler). 

A typical steam cracker consists of several identical pyrolysis furnaces in which the feed is cracked in the presence of steam as a diluent.The cracked gases are quenched and then sent to the demethanizer to remove hydrogen and methane. The effluent is then treated to remove acetylene, and ethylene is separated in the ethylene fractionator. The bottom fraction is separated in the de-ethanizer into ethane and C3, which is sent for further treatment to recover propylene and other olefins. Typical operating conditions of ethane steam cracker are 750-800°C, 1-1.2 atm, and steam/ethane ratio of 0.5. Liquid feeds are usually cracked at lower residence time and higher steam dilution ratios compared to gaseous feeds. Typical conditions for naphtha cracking are 800° C, 1 atm, steam/hydrocarbon ratio of 0.6-0.8, and a residence time of 0.35 sec. Liquid feedstocks produce a wide spectrum of coproducts including BTX aromatics that can be used in the production of variety of chemical derivatives. 

For an ethylene plant, each reactor has three key operating variables outlet temperature, steam dilution, and feed flow rate. Plants have on the order of 24 parallel reactors (with several within each heater). In addition, the separation section has many variables that can be adjusted for optimization, including recycle compositions, distillation pressures, and refrigeration temperatures. Thus, a plant often has 100 variables or more for optimization. 

High temperature, steam dilution, and low system pressure produce an equilibrium more favorable to styrene. For endothermic vapor-phase reactions, the equilibrium constant increases with temperature and... 

Another method to create a positive shift in equilibrium is the use of steam dilution to reduce the partial pressures of EB, styrene, and hydrogen. Steam dilution provides the same effect as a reduction in total pressure. 

Steam dilution has several other important benefits. First, steam supplies heat to the reacting mixture. Consequently, the drop in temperature for a given EB conversion is lower, allowing greater EB conversions to be obtained with the same inlet temperature. Second, a minimum amount of steam appears to keep the catalyst in the required oxidation state for high activity. The actual quantity of steam varies with the type of catalyst used. Third, steam is believed to suppress the deposition of carbonaceous material on the catalyst. If the carbonaceous material is allowed to accumulate, the catalyst will become fouled and its activity will decline to unacceptable levels. 

The next question is whether this reaction proceeds in a retort. Since the gas compositions change with location in the retort, this question is not answered easily. If we use a typical offgas composition with steam dilution of 40% steam and 5% hydrogen, we find, using the expression for K4, that PH2S must be less than 500 ppm. This indicates that the reaction would be severely limited thermodynamically. At 727°C, AG for Reaction (4) is 11.71 kcal/mol, which results in K4 =... 

Chemical composition (% voL) = paraffins SO. naphthenes 15/aromaucs 5. Steam dilution ratio = 0.60. fields of various pvrolysii products are expressed in % Wt milling to feed. 

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