Steam-hydrocarbon ratio

Chemistry of Petrochemical Processes Steam/Hydrocarbon Ratio... 

The process uses two radial reactors in series with one preheater and one interstage heater. Steam is used as an energy carrier (adiabatic reactor) and diluent [43,50,51]. Reactor temperatures and pressures are 570-630°C and 1.5 bar, respectively. Total hydrocarbon mass flow (96 wt% ethylbenzene) is 95,000 kg/h. The steam/hydrocarbon ratio is 2. Typical conversion, selectivity and yield numbers are 71, 92 and 66%, respectively. Definitions are given in the appendix. Reaction equations and kinetics are taken from literature [43,51]. 

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. 

Figure 9. Effect of steam-hydrocarbon ratio on gas composition for reforming hexane (same as predicted from equilibrium calculations)...

Steam-Hydrocarbon Ratio. The next series of tests was made to show the effect of the steam-hydrocarbon ratio on gas composition (Figure 9) and to determine the minimum practical steam-hydrocarbon ratio. The trends shown are approximately the same as predicted by equilibrium calculations. The lack of complete temperature profile data makes it difficult to show how closely the trends agree with equilibrium predictions. Steam-hydrocarbon weight ratios as low as 1.6 (molar ratio of 7.7) were adequate to prevent carbon deposition. The product gas heating value at this low ratio was about 774 B.t.u./std. cu. ft. It could be raised to 957 B.t.u./std. cu. ft. if the exit gas carbon dioxide were reduced to 2 mole % by scrubbing. 

Reforming of Various Feedstocks. When the catalyst developed proved to be capable of reforming pure hexane successfully at low steam-hydrocarbon ratios, we decided to test various feedstocks having a range of molecular weights and various types of hydrocarbons. The first tests were conducted with octane and benzene to show whether... 

Test results for octane and benzene reforming are summarized in Table II. The steam-hydrocarbon ratios were deliberately set high to avoid possible carbon formation with these heavier or aromatic feedstocks. 

Phillips Pure Grade butene-1 and butadiene-1,3 were used separately as reacting gases, and the molar steam/hydrocarbon ratio was normally 15 1 for butere and 20 1 for butadiene. The space velocity of hydrocarbon was 100 v/v/hr. 

This tendency increases with the increase in temperature and the percentage of higher hydrocarbons in the feed, and it decreases with the increase in steam/hydrocarbon ratio. 

Steam injection introduces an additional source of noise. An effective flare tip is one which achieves a good balance of smoke and luminosity reduction without exceeding acceptable noise levels. Low-frequency noise is encountered at relatively high steam to hydrocarbon ratios. 

A higher steam/hydrocarhon ratio favors olefin formation. Steam reduces the partial pressure of the hydrocarbon mixture and increases the yield of olefins. Heavier hydrocarbon feeds require more steam than gaseous feeds to additionally reduce coke deposition in the furnace tubes. Liquid feeds such as gas oils and petroleum residues have complex polynuclear aromatic compounds, which are coke precursors. Steam to hydrocarbon weight ratios range between 0.2-1 for ethane and approximately 1-1.2 for liquid feeds. 

The morphology of the carbon on the surface can assume several forms a two-dimensional film or so-called whisker carbon, which is formed when the carbon dissolves in the supported metal catalyst, diffuses through the metal, and forms a growing filament that lifts the metal from the catalyst surface. Whisker carbon is typically associated with Ni-based catalysts because carbon is soluble in Ni at reforming conditions. Whisker carbon tends to form at higher temperatures, low steam to hydrocarbon ratios and higher aromatic content of the feeds. This type of carbon formation may be minimized by the use of precious metals as catalysts, because these metals do not dissolve carbon. On a nickel surface, the whisker mechanism can be controlled by sulfur passivation. 

Carbon formation is also different for diesel and gasoline. The long chain hydrocarbons present in diesel or kerosene fuel are more difficult to reform than the shorter chain hydrocarbons present in gasoline, while aromatics in gasoline hinder the overall reaction rate. An example is found in the results of Ming et who showed that SR of n-Ci required a higher steam/ carbon ratio to avoid coke formation than i-Cg. The cetane number of the feed had little effect on carbon formation. Carbon formation can often be attributed to fuel pyrolysis that takes place when the diesel fuel is vaporized. This is considerably decreased when the steam content in feed increases. 

In tlie steam-hydrocarbon reforming process, steam at temperatures up to 850°C and pressures up to 30 atmospheres reacts with the desulfurized hydrocarbon feed, in the presence of a nickel catalyst, to produce H2. CO, ( G CH4, and some undecomposed steam. In a second process stage, these product gases are further reformed. Air also is added at this stage to introduce nitrogen into the gas mixture. The exit gases from this stage are further puntied to provide the desired 3 parts H. to 1 part Nj which is the correct empirical ratio for NH3 synthesis. See also Ammonia. 

Steam reforming of hydrocarbons has become the most widely used process for producing hydrogen. One of the chief problems In the process Is the deposition of coke on the catalyst. To control coke deposition, high steam to hydrocarbon ratios, n, are used. However, excess steam must be recycled and It Is desirable to minimize the magnitude of the recycle stream for economy. Most of the research on this reaction has focused mainly on kinetic and mechanistic considerations of the steam-methane reaction at high values of n to avoid carbon deposition ( L 4). Therefore, the primary objective of this studyis to determine experimentally the minimum value of n for the coke-free operation at various temperatures for a commercial catalyst. 

When steam reforming of saturated hydrocarbons was carried out on nickel catalysts at a high steam - carbon ratio (eg >1.5 mol/atom) [1,8], no carbon deposition was observed after several hours of reaction. The same results (on N1/AI2O3) were observed during CO methanation at H2 CO ratios of 1 to 3 [4]... 

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. 

As known from literature, the gas composition depends mainly on the used fuel, on the temperature and on the steam-fuel ratio [4,8,9]. The gas composition depends also on the residence time, but in all experiments the residence time was kept as constant as possible. Therefore in the following diagrams the dependency of the dry product gas composition to these parameters is shown. The nitrogen content for all experiments was below 5 vol% and is not shown in the diagrams. The rest to 100 % is nitrogen and higher hydrocarbons. From the gaschromatographic analysis it can be seen, that the main component of the these hydrocarbons is ethene. 

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

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