Steam-to-feed ratio

The objective of this study was to determine the total gas yield and gas composition from the various feedstocks as a function of reactor temperature, air-to-feed ratio, and steam-to-feed ratio. The gas components of greatest interest are hydrogen, carbon monoxide, methane, and ethylene. These components contribute not only to the gas heating value, but also to the value of the gas as chemical synthesis feedstock. 

In the last three chapters, we have developed a number of conventional control structures dual-composition, single-end with RR, single-end with rellux-to-feed, tray temperature control, and so on. Structures with steam-to-feed ratios have also been demonstrated to reduce transient disturbances. Four out of the six control degrees of freedom (six available valves) are used to control the four variables of throughput, pressure, reflux-drum level, and base level. Throughput is normally controlled by the feed valve. In on-demand control structures, throughput is set by the flow rate of one of the product streams. Pressure is typically controlled by condenser heat removal. Base liquid level is normally controlled by bottoms flow rate. 

The use of a steam-to-feed ratio greatly improves this response, as shown in Figure 10.13b. The base-level controller output signal resets the reboiler heat-input-to-feed ratio. The steady-state ratio is 0.038689 (in the required Aspen Dynamic units of GJ/h per kmol/h ). The output range of the level controller is changed from 0 to 0.1, and the TC50 controller is retuned Kc = 3.2 and Ti = 29 min). The deviation in bottoms purity is greatly reduced, and saturation of the bottoms valve is also avoided. 

Flow controllers are installed on the steam to the base of the two columns. These flow rates are ratioed to the feed flows to the respective column by using multipliers. The molar steam-to-feed ratio in the preflash column is 125.9/2722 = 0.04625. The total crude feed is used (after the summer). The molar steam-to-feed ratio in the pipestiU is 302.1/1654 = 0.1827. 

Figure 16.4 shows the Aspen Dynamics flowsheet with controllers installed. A steam-to-feed ratio is used with the ratio changed by the temperature controller. The need for this ratio to improve load performance is illustrated in Figure 16.5. A 50% increase in feed flow rate is the disturbance. The solid lines show responses without the QR/F ratio. There are very large drops in Stage 55 temperature that result in large transient increases in the C4 impurity in the bottoms (xB). The units of the multiplier must be metric in the Aspen Dynamics simulation (GJ/kmol).

The control structure for the chlorobenzene system is given in Figure 11.26. Two features are different from the other control structures. They involve the use of steam-to-feed ratios. In both columns, the reboiler heat input is ratioed to the feed to the column. These are added to improve the load response of the system that was found to be inferior to those found in the other solvent systems. The feed flow is measured and sent to a multiplier block. The other input to the multiplier is the output signal from the temperature controller. The output of the multiplier sets the reboiler heat input. So the temperature controller is looking at temperature and outputting a ratio signal. Controller parameters are given in Table 11.6. Notice that the Qr/F ratios must be in units of GJ per kmol in the Aspen Dynamic convention. 

Figure 12.112 shows the application of full feed rate feedforward on a column with the energy balance scheme. Either or both of the reflux-to-feed and the steam-to-feed ratios can remain with operator entered SPs or their SP can be adjusted by a higher level control (such as tray temperature, inferential or on-stream analyser). 

At this point with flows established, smoothed, and in some cases limited, it will probably be possible to see some improvement in composition control, at least for part of the time. For further improvement provide steam-to-feed ratio control, internal reflux-to-feed or distillate-to-feed ratio control, high AP override on steam to protect against flooding, and a minimum steam flow limiter to protect against dumping. 

We therefore may prepare the signal flow diagram of Figure 18.6. Li and Xi are the liquid flow and its composition from the lowest tray. A composition control system is shown in which V s)/ i s) is the transfer function relating vapor flow to composition controller output. It is assumed that steam flow is controlled by a steam flow controller, but for the moment there are no implications as to whether the steam flow controller set-point signal comes dire y from the composition controller or from a steam-to-feed ratio controller that is reset by the composition controller. 

Naphtha desulfurization is conducted in the vapor phase as described for natural gas. Raw naphtha is preheated and vaporized in a separate furnace. If the sulfur content of the naphtha is very high, after Co—Mo hydrotreating, the naphtha is condensed, H2S is stripped out, and the residual H2S is adsorbed on ZnO. The primary reformer operates at conditions similar to those used with natural gas feed. The nickel catalyst, however, requires a promoter such as potassium in order to avoid carbon deposition at the practical levels of steam-to-carbon ratios of 3.5—5.0. Deposition of carbon from hydrocarbons cracking on the particles of the catalyst reduces the activity of the catalyst for the reforming and results in local uneven heating of the reformer tubes because the firing heat is not removed by the reforming reaction. 

The carbon monoxide concentration of gas streams is a function of many parameters. In general, increased carbon monoxide concentration is found with an increase in the carbon-to-hydrogen ratio in the feed hydrocarbon a decrease in the steam-to-feed-carbon ratio increase in the synthesis gas exit temperature and avoidance of reequiUbration of the gas stream at a temperature lower than the synthesis temperature. Specific improvement in carbon monoxide production by steam reformers is made by recycling by-product carbon dioxide to the process feed inlet of the reformer (83,84). This increases the relative carbon-to-hydrogen ratio of the feed and raises the equiUbrium carbon monoxide concentration of the effluent. 

Product Distribution. In addition to ethylene, many by-products are also formed. Typical product distributions for various feeds from a typical short residence time furnace are shown in Table 5. The product distribution is strongly influenced by residence time, hydrocarbon partial pressure, steam-to-od ratio, and coil outlet pressure. 

Table 6 shows the effect of varying coil oudet pressure and steam-to-oil ratio for a typical naphtha feed on the product distribution. Although in these tables, the severity is defined as maximum, in a reaUstic sense they are not maximum. It is theoretically possible that one can further increase the severity and thus increase the ethylene yield. Based on experience, however, increasing the severity above these practical values produces significantly more fuel oil and methane with a severe reduction in propylene yield. The mn length of the heater is also significantly reduced. Therefore, this is an arbitrary maximum, and if economic conditions justify, one can operate the commercial coils above the so-called maximum severity. However, after a certain severity level, the ethylene yield drops further, and it is not advisable to operate near or beyond this point because of extremely severe coking. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

Table 95 Properties and CO conversion during low temperature shift using a feed containing 4.03%CO in Ar (dry basis) with a steam to gas ratio of 0.7, SV = 4000 h-1 with 0.5 cm3 catalyst465...

Table 128 Screening of modifiers to Au/Fe203-M0x, using a feed containing 10%CO in N2 and a SV of 10 000 ml/h per g cat. Steam to gas ratio of 1 1. Unp means unpromoted 525 ...

The hydrocarbon feed must contain sufficient steam to avoid carbon formation on the catalyst. The steam-to-carbon ratio is defined as moles of steam per mole of carbon in the hydrocarbon. The steam-to-carbon ratios are about 3.0 for hydrocarbon feedstocks but lower values can be used for some feedstocks. Carbon formation is more likely with heavier feedstocks. An alkali-based catalyst can be used to repress carbon formation. 

Other important parameters in the steam reforming process are temperature, which depends on the type of oxygenate, the steam-to-carbon ratio and the catalyst-to-feed ratio. For instance, methanol and acetic acid, which are simple oxygenated organic compounds, can be reformed at temperatures lower than 800 °C. On the other hand, more complex biomass-derived liquids may need higher temperatures and a large amount of steam to gasify efficiently the carbonaceous deposits formed by thermal decomposition. 

Although the stoichiometry for reaction (9.1) suggests that one only needs 1 mol of water per mole of methane, excess steam must be used to favor the chemical equilibrium and reduce the formation of coke. Steam-to-carbon ratios of 2.5-3 are typical for natural gas feed. Carbon and soot formation in the combustion zone is an undesired reaction which leads to coke deposition on downstream tubes, causing equipment damage, pressure losses and heat transfer problems [21]. 

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. 

The thermodynamics of the above reactions are illustrated in Figures 5.6 and 5.7174. Both figures assume a steam-to-methane ratio of 1.0. Figure 5.6 illustrates how the feed and product gases interact when the product gas has a hydrogen-to-carbon monoxide ratio of 3.0. Figure 5.7 illustrates the effects of temperature and pressure on the reactions. As pressure increases, lower conversion can be expected and more methane will not be converted and will be found in the reformer discharge stream. 

Intermediate Duty catalysts are for feeds with a significant content of components from ethanes up to liquid petroleum gas (LPG). The heavier feedstock increases the tendency for catalyst deactivation through carbon laydown and requires a special catalyst in the top 30% to 50% of the reformer tubes. This tendency also occurs when light feeds are run at low steam-to-carbon ratios and/or at a high heat flux. 

The natural gas feed is depressurised again from 100 to 30 bar. The heated gas stream is saturated in a column by counter-current scrubbing with hot water. The saturated gas stream is heated further, before being mixed with additional steam to obtain the required steam to carbon ratio of 3.0. 

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