Methane Slip

A promoted nickel type catalyst contained in the reactor tubes is used at temperature and pressure ranges of 700-800°C and 30-50 atmospheres, respectively. The reforming reaction is equilibrium limited. It is favored at high temperatures, low pressures, and a high steam to carbon ratio. These conditions minimize methane slip at the reformer outlet and yield an equilibrium mixture that is rich in hydrogen. ... 

The Methane Slip (or % Methane that is not converted) has a big influence on the selection of reformer operating conditions. As shown in Figure 5.8, the best operating conditions will be within the following ranges ... 

Figure 5.8. Methane Slip at different process conditions 174.

The reforming process is completed in the authothermic secondary reformer, which is a refractory lined vessel containing a fixed-bed catalyst. The remainder of the endothermic heat requirement is provided by the combustion of part of the primary reformer effluent directly with air. This allows much higher process temperatures, of the order of 1000°C, to be attained at the secondary reformer exit and consequently low methane slips in the range of 0.2-... 

A high reformer exit temperature of 1616°F is made possible by high alloy tube materials such as Manaurite 36X and Paralloy. This leads to a reduction in methane slip, and an increase in the CO/CO ratio. Both effects enhance the plant s efficiency and result in a reduction of feedstock natural gas consumption of 3.5% over previously used reforming conditions. 

Increased Process Air Supply to the Secondary Reformer. Decreased heat supply in the primary reformer means that increased internal firing is necessary to achieve approximately the same degree of total reforming. A somewhat higher methane slip (and thus a lower secondary reformer outlet temperature) is acceptable and preferable in this type of process. This is because methane is removed in the final purification.53... 

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. 

The same group [2.354] has also recently reported on the performance of a membrane reactor with separate feed of reactants for the catalytic combustion of methane. In this membrane reactor methane and air streams are fed at opposite sides of a Pt/y-A Os-activated porous membrane, which also acts as catalyst for their reaction. In their study Neomagus et al. [2.354] assessed the effect of a number of operating parameters (temperature, methane feed concentration, pressure difference applied over the membrane, type and amount of catalyst, time of operation) on the attainable conversion and possible slip of unconverted methane to the air-feed side. The maximum specific heat power load, which could be attained with the most active membrane, in the absence of methane slip, was approximately 15 kW m with virtually no NO emissions. These authors report that this performance will likely be exceeded with a properly designed membrane, tailored for the purpose of energy production. 

As the H2/CO ratio is lowered below the stoiciometric ratio of 3 1 there is a progressive increase in the total and imported CO2 and, consequently, an increase in both the total feed rate to the reformer and the overall heat load. Even though the heat of reaction for conversion of carbon dioxide and hydrogen, and carbon dioxide and methane are lower than that of the reforming reaction, the total heat load is nevertheless increased. The methane slip falls only slightly and, as a result, the total syngas (CO + H2) product rate stays about the same. Even though low H2/CO ratios... 

The additional methane formed by methanation is insignificant compared to the residual unreacted methane (or methane slip) in the product gas. Product gas is cooled following methanation and entrained water is removed in a dehydrator. The product gas, which is primarily hydrogen and methane, is then delivered to the plant battery limits for use in downstream applications. 

The CALCOR process is similar to a conventional steam methane reformer with an amine acid gas removal system, except that the CO2 from the amine system is recycled to the reformer furnace. The reformer operates at a very low pressure to reduce reforming severity. The synthesis gas from the CO2 removal system is just above atmospheric pressure. It is saturated with water and residual CO2 and must be compressed before entering downstream separation equipment. The process features a very low methane slip below 500 ppm in the synthesis gas [11]. 

ATR H2/CO ratio favorable Lower operating temperature comparing with POX Low methane slip Oxygen required... 

A eomprehensive set of plant operating eonditions was not available for this case, but the plant indicated a methane slip of about 8.5 % was observed by the plant for diis outlet temperature. The calculated outlet methane dry mole percent of 8.8 % is very close to the 8.5 % that was observed, and since all the independent conditions (feed rate, steam to carbon ratio, etc.) are not precisely known for this case, the discrepancy may be easily attributed to differences in simulated versus actual conditions. 

The unacceptable temperatures can be avoided. Two alternatives of avoiding them are simulated, one resulting in different unacceptable operating conditions (high methane slip), and the other with very acceptable conditions. 

The first alternative focuses on the unacceptable temperatures encountered in the first few meters of the reactor, which may cause coking. This alternative keeps the temperature at the approximate one meter from the inlet at the same temperature as die base, 100% catalyst activity, case. This temperature was 449°C. Table C.2 and the accompan5dng plot shows these results. To keep that temperature at 449°C, and maintain the firing profile, the reactor outlet specification has to be relaxed. The independent operating condition is just moved from the reactor outlet, to the catalyst temperature. As illustrated, under these operating conditions, as the catalyst activity falls from 100% to 90%, and 70%, the methane slip rises dramatically. So does the reactor outlet temperature. As mentioned, this alternative is unacceptable, due to very high methane slip. 

POX plants require no steam. Some soot is formed, but since there is no catalyst, there is obviously no possibility of catalyst bed plugging. With proper design and operation, the soot can be processed and removed in the downstream equipment without affecting the plant on-stream time. METHANE SLIP - The exit gas from each reactor contains some residual methane, which corresponds to unreacted natural gas feed. This is typically referred to as the methane slip. A low methane slip increases the purity of a synthesis gas product. It also reduces the natural gas requirement. 

An SMR typically has a relatively high methane slip (3 to 8 mol pet, dry basis). This is primarily because the SMR operates at a lower outlet temperature. The other technologies operate at higher outlet temperatures and typically have low slips (0.3 to 0.5 mol pet, dry basis). 

If a methane-wash cold box is used, typically a methane slip of at least 1 to 2 mol percent is required to maintain the methane level in the box. This is not a problem for an SMR design because its methane slip is considerably more than this. It is also easily achieved by the SMR/O2R and ATR designs by simply reducing the reactor outlet temperature. However, this methane level is more difficult to achieve with a POX, since in the absence of a catalyst, a high temperature and corresponding low methane slip should be maintained to ensure consistent performance. 

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