Reformate pressure

The primary reformer is essentially a process furnace in which fuel is burned with air to indirectiy provide the heat of reaction to the catalyst contained within tubes. This area of the furnace is usually referred to as the radiant section, so named because this is the primary mechanism for heat transfer at the high (750—850°C) temperatures required by the process. Reforming pressures in the range 3 —4 MPa (30,000—40,000 atm) represent a reasonable compromise between cost and downstream compression requirements. 

The reformer pressure drop depends on the number of tubes, tube diameter and catalyst selection. The typical pressure drop ranges from 40 to 60 psi. The reforming catalysts are made in a ring or modified ring form. Nickel is the chief catalytic agent. Heavier feedstocks use an alkali promoter is to suppress carbon formation. 

Hydraulic capacity (i.e., pressure drop) shows up in each of the functional areas. For example, reformer pressure drop often becomes limiting at high flow rates. This requires a change in catalyst or provides a reason to install catalyst tubes with a larger diameter86. 

Thermodynamically, the reforming reaction. Equation 3.5.1, shows that the reformer should be operated at die lowest pressure and highest temperature possible. The reforming reaction occurs on a nickel-oxide catalyst at 880 C (1620 "F) and 20 bar, which results in a 25 "C approach to the equihbrium temperature [25,29]. Methane conversion increases by reducing the pressure, but natural gas is available at a high pressure. It would be costly to reduce the reformer pressure and then recompress the synthesis gas later to 100 bar (98.7 atm) for the converter. The steam to carbon monoxide ratio is normally in the range of 2.5 to 3.0 [30]. The ratio favors both the conversion of methane to carbon monoxide and the carbon monoxide to carbon dioxide as indicated by Equations 3.5.1 and 3.5.3. If the ratio is decreased, the methane concentration increases in the reformed gas, but if the ratio is set at three, the unreacted methane is small. The methane is a diluent in the synthesis reaction given by Equation 3.5.2. 

The steam reforming reaction of methane is endothermic and proceeds with an increase in volume. In Figures 30 and 31 [1487] the relationship between equilibrium methane concentration (a measure for the theoretical possible conversion) and temperature, steam-to-carbon ratio S/C, and reforming pressure are plotted for the range relevant for the reaction in the primary reformer. 

The energy saving measures described in Section 5.1.2 have considerably reduced the demand side (e.g., C02 removal, higher reforming pressure, lower steam to carbon ratio, etc.). On the supply side, the available energy has been increased by greater heat... 

Key features are the high reforming pressure (up to 41 bar) to save compression energy, use of Uhde s proprietary reformer design [1084] with rigid connection of the reformer tubes to the outlet header, also well proven in many installations for hydrogen and methanol service. Steam to carbon ratio is around 3 and methane slip from the secondary reformer is about 0.6 mol % (dry basis). The temperature of the mixed feed was raised to 580 °C and that of the process air to 600 °C. Shift conversion and methanation have a standard configuration, and for C02 removal BASF s aMDEA process is preferred, with the possibility of other process options, too. Synthesis is performed at about 180 bar in Uhde s proprietary converter concept with two catalyst beds in the first pressure vessel and the third catalyst bed in the second vessel. 

One of the first success of zeolites as catalysts, and the first commercial molecular shape selective catalytic process, was the use of erionite in a post-reforming process named selectoforming (39). Ihis 8 MR zeolite was able, based on the principle of size exclusion, to selectively crack the short chain n-parafiins to produce LPG. To avoid the deactivation by coke NiS was deposited on the zeolite. The erionite based catalyst is generally located at the bottom of the last reactor of the reformer unit and operates then at the reformer pressure, and at the temperature of the last reformer reactor. When more flexibility was to be achieved from the selectoforming, the catalyst is introduced... 

After the initial purification, natural gas is compressed to reformer pressure, if not already at that pressure, and preheated. Then, any remaining sulfur is removed to avoid poisoning of catalysts. The sulfur may be removed by adsorption on activated carbon at ambient temperature or by absorption by hot zinc oxide (290 -400 C) after the gas has been preheated. 

A typical secondary reformer is a cylindrical, refractory-lined, insulated vessel. The upper part is empty and serves as a combustion chamber in which the gas from the primary reformer is partially oxidized by preheated air. The lower part is filled with a catalyst similar to that in the primary reformer. The air should be free fronrdusT that might clog the catalyst bed arid from catalyst poisons (S, Cl, and As). The air is filtered, compressed to reformer pressure, and mixed with the gas in a burner at the top of the vessel. The combustion causes the temperature to rise to about 1200°C in the combustion chamber. As the hot gas descends through the catalyst bed, it is cooled by the endothermic reformir reactions and leaves the reformer at a temperature of about 950 -1000 C. The gas at this point contains, on a dry basis, about 56% Hg, 12% CO, 8% CO2, 23% Ng, plus argon, and usually less than 0.5% CH4. It also contains excess steam ranging from one-third to one-half of the total gas volume. 

The potassium carbonate system operates mainly isothermal-COg absorption at high pressure and COg release at low pressure. In the absorptim step the pressure is typically about 3.0 MPa (reformer pressure minus pressure losses), and the temperature may be 100°C. The COg is absorbed chemically by the conversion of potassium carbonate to bicarbonate. When the solution pressure is reduced to about atmospheric pressure, part of the COg and water vapor escape. COg release is assisted by steam stripping. The steam is raised in the regenerator reboiler heated by the gas from the LTS shift converter thus, some or most of the heat required by the COg removal process is derived from the heat in the incoming gas. The regenerated solution is returned to the absorber. 

As a result, the amount of available waste heat will be reduced. This will reduce the amount of export steam. The heat lost in the primary reformer will be minimized by improved reformer designs or by eliminating the direct fired primary reformer. The reforming pressures may rise and the ammonia synthesis pressure may decrease, thus the synthesis gas compressor may poten-... 

Thermodynamics. The dehydrogenation of naphthenes to aromatics at typical reforming reactions conditions, such as 500°C, has an equilibrium conversion of 100% as long as the hydrogen concentration is not too high. Figure 3 (9) shows the cyclohexane conversion to benzene as a function of temperature, at different pressures, when the H2/CH initial ratio is 10. At 500°C, only at pressures above 3 MPa, the conversion is not 100%. Reforming pressures in modern reactors is well below this value therefore, the conversion of CH to B will be complete. The equilibrium constant for this reaction is... 

Figures 7A and 7B show the amoimt of aromatics (mol%) in equilibrium as a function of temperature, pressure, and H2/paraffin initial ratio for the dehydrocyclization of re-hexane and re-heptane, respectively. The longer the paraffin, the higher the equilibrium conversion to aromatic. The pressure has a strong influence. At the typical reforming pressures in modem reformers (below 15 atm), the equilibrium conversion of the paraffins to aromatics approach 100%.

For the power consumed by the compressor, we assume polytropic behavior and 100% efficiency in this example. MW gas is the average molar weight, T is the operating tanperature, ti<. the isentropic efficiency, P fonn the reforming pressure, the initial pressure, F the total flow in (kg/s), and W the power consumed by the compressor. 

Figure 3.19 Effect of S/C ratio and O/C ratio [here expressed as the air ratio 7.= (0/C)/4) on carbon formation for methane steam reforming pressure, 1 bar feed pre-heating temperature 400°C[66].

Figure 5.34 Hydrogen partial pressure along the length axis of a membrane tubular reactor operated in parallel (a) and counter-flow (b) arrangements solid lines, retenate partial pressure dashed lines, permeate partial pressure reformate flow rate, 162cm min reformate pressure, 1.36 bar sweep gas flow rate, 40cm min permeate pressure, 1.01 bar left, reaction temperature 300°C right, reaction temperature 500°C [411].

It is known from the above reactions that low-pressures and high-temperatures are beneficial to methane steam reforming. Pressurized reactions are adopted in industry for the sake of economy. 

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