Reforming convective steam

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

In some cases a plant may have a pre-reformer. A pre-former is an adiabatic, fixed-bed reactor upstream of the primary reformer. It provides an operation with increased flexibility in the choice of feed stock it increases the life of the steam reforming catalyst and tubes it provides the option to increase the overall plant capacity and it allows the reformer to operate at lower steam-to-carbon ratios166. The hot flue gas from the reformer convection section provides the heat required for this endothermic reaction. 

Figure 105. Modern integrated single-train ammonia plant based on steam reforming of natural gas (Clide process) a) Sulfur removal b) Primary reformer c) Steam superheater d) Secondary reformer e) Waste heat boiler f) Convection section g) Forced draft fan h) Induced draft fan i) Stack k) TIT and LT shift converters ...

The feed is sent to a convective steam reformer where it is partially converted into hydrogen then hydrogen is recovered through a Pd alloy membrane separation module, while the retentate is sent to the next step or recycled to the first module. By means of a heat recovery system, the operating temperature can be reduced to about 450°C before the membrane unit and again increased before the second reactor. It is possible to replicate the RMM until the desired natural gas conversion is achieved. 

Figure 3.11 Topsoe convective steam reforming pilot, Houston, Texas. Capacity...

Incorporation of a feed gas saturator cod in the convection section of the primary reformer allows for 100% vaporization of the process condensate. The steam is used as process steam in the reformer. 

Convective Heat Transfer Reformers provide additional reforming capacity by using the heat contained in the primary reformer exit gases. Several designs are available, but not all have been commercialized. These units typically replace a portion or the entire duty of the waste heat boiler. So they significantly reduce the steam capability of the reformer. Potential increases in capacity of between 10% and 30% are possible. The modifications are capital intensive but relatively easy to implement170. 

The Haldor Topsoe Convection Reformer (HTCR) has been developed for the production of hydrogen from hydrocarbons without steam generation. The elementary unit of the reformer consists of two concentric tubes (Fig. 1.21). The annular space... 

One of the most effective reformer modifications is to use heat from the convection section to preheat radiant section feed. This will reduce radiant section heat load, reduce radiant section firing rate, and potentially unload other areas such as steam generation. This option has been used to increase capacity by 10 percent without increasing the arch temperature in the radiant section.86... 

Also important is the effect of the size and shape of the catalysts [428] on heat transfer and consequently performance. Unlike the most processes carried out under substantially adiabatic conditions, the endothermic steam reforming reaction in the tubes of the primary reformer has to be supplied continuously with heat as the gas passes through the catalyst. The strong dependency of the reaction rate on the surface temperature of the catalyst clearly underlines the need for efficient heat transfer over the whole length and crosssection of the catalyst. However, the catalyst material itself is a very poor conductor and does not transfer heat to any significant extent. Therefore, the main mechanism of heat transfer from the inner tube wall to the gas is convection, and its efficiency will depend on how well the gas flow is distributed in the catalyst bed. It is thus evident that the geometry of the catalyst particles is important. 

Other types of more compact steam reformers are also used. In most cases, the heat transfer is then accomplished by convection with the reformed gas itself, a fine gas, or by a combination as illustrated in Fig. 4. In this case, about 80% of the heat is transferred to the process and export of steam from the plant can be reduced or avoided. 

Another type of steam-reforming reactor that is attracting increasing attention is known as gas heated reformers or heat exchange reformers. In such reformers, heat is transferred by convection and the heat source is a hot process gas from another reformer or a partial oxidation reactor. A number of different installations of heat exchange reformers can be envisaged. In Fig. 5, the installation of a heat exchange reformer either in series or in parallel to an auto-thermal reformer (ATR) is illustrated. 

Two concepts of a He - He intermediate heat exchanger for a heat rating of 125 - 170 MW have been selected. For both, a 10 MW test plant has been operated in the KVK loop verifying the operation of reformers with convective helium. A 10 MW decay heat removal system cooler, hot gas ducts including insulation and liner, hot gas valves, and a steam generator were other components of the KVK loop. Furthermore, a helium purification system was operated in a bypass of the main system. Starting in 1982, the KVK facility was operated for 18,400 h with approx. 7000 h above 900 C [28]. Hot gas duct with internal insulation was operated at temperatures up to 950 °C. The KVK experimental loop has demonstrated reliability and availability even of newly developed components. 

The reformer is a direct fired chemical reactor consisting of numerous tubes located in a firebox and filled with catalyst. Conversion of hydrocarbon and steam to an equilibrium mixture of hydrogen, carbon oxides and residual methane takes place inside the catalyst tubes. Heat for the highly endothermic reaction is provided by burners in the firebox. The heat is transferred to the catalyst filled reactor tubes by a combination of radiation and convection. 

It can also be used to expand an existing steam methane reformer. This technology will allow the capacity of an existing unit to be increased by about lO /o. However, it is expensive because of the need to revamp the convection section of the primary reformer in order to add an additional prereforming coil. 

Shell Oil Company has recently patented a process and apparatus for the production of pure hydrogen by steam reforming. This process integrates the steam reforming and shift reaction to produce pure hydrogen with minimal production of CO and virtually no CO in the hydrogen stream, provides for CO2 capture by sequestration, uses a steam reforming MR and is powered by heat from a heater convection section. 

F304 is the common convection bank for two fired heaters in the reforming unit. Currently 1300-1900 t/day of flue gas from F304 leaves the stack at temperatures around 390 °C. This is considerably higher than the typical value of 220 C seen in peer units within the industry. One of the options proposed is to add surface area in the convection section for extra MP steam generation of 85 t/day with a cost savings of 703,800 per year at the expence of 1.9 MM capital cost. 

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