Hydrogen Production by Steam Reforming Chemical Engineering Processing
Hydrogen Production by Steam Reforming Chemical Engineering Processing
Hydrogen Production by Steam Reforming Chemical Engineering Processing
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5/29/2010 01:10:00 PM
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Managemen of he ga i c i ical fo pe ole m efine Ray Elshout Energy, Systems Engineering Steam reforming of natural gas at petroleum refining facilities is the
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2011 (18) 2010 (6) May (2) Burner Operating Characteristics Hydrogen Production By Steam Reforming April (4) 2009 (58)
predominant means of producing hydrogen in the chemical process industries (CPI). Areas where hydrogen is heavily consumed include ammonia production, the cryogenics industry and methanol production (Table 1). Because hydrogen needs within various sectors of the CPI are at their highest levels in history, and are continuing to grow, an understanding of this method of hydrogen production and purification can be useful. Search
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A major percentage of hydrogen used in the CPI goes toward production of ammonia, which continues find greater demand in the chemical fertilizer industry. On the other hand, methanol usage is declining in connection with its use as a feedstock for making methyl tert-butyl ether (MTBE; by reaction of methanol with tertiary butylene). In the U.S., MTBE had been used as a gasoline blend stock until recently, when use of the chemical as a gasoline oxygenate was phased out in favor of ethanol. In addition to being producers of hydrogen, largely through steam reforming, petroleum refineries are also large consumers of the gas. Consumption of hydrogen by petroleum refineries has increased recently due to clean-fuels programs, which require refiners to produce low-sulfur gasoline and ultralowsulfur diesel fuel. Management of hydrogen is a critical concern for refiners because various processes require different hydrogen pressure levels and purity. Hydrogen-using processes that require high pressures and high purity, including hydrocracking, use hydrogen above the 100 kg/cm2 (1,500 psig) level. When a recycle gas system is used, the higher pressures are needed to maintain hydrogen partial pressure at the desired level as methane concentration in the hydrogen feed to a hydrocracker increases. Sufficient hydrogen partial pressure promotes the intended reactions without producing undesirable coke.
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Figure 1. Steam-methane reforming is still responsible for the bulk of hydrogen production in petroleum refineries S eam-Me hane Reforming Refinery hydrogen comes primarily from two sources catalytic reforming of byproduct gas from the dehydrogenation of naphthenes into aromatics and high-octane gasoline blend stocks, as well as from direct hydrogen manufacture. The bulk of direct hydrogen manufacturing in a petroleum refinery is still accomplished via either steam-methane reforming (Figure 1) or steam-naphtha reforming. Partial oxidation of heavier hydrocarbons is also used to a limited extent. In the overall steam methane reforming (SMR) reaction, methane reacts with steam at high temperatures and moderate pressures in catalyst-filled tubes to generate synthesis gas, a mixture of hydrogen, carbon monoxide and some carbon dioxide. The reactions for the two simultaneous SMR mechanisms are shown as Equations (1) and (2). Both are endothermic, as shown by the positive heat of reaction. The reaction requires heat transfer to maintain temperatures favorable to the equilibrium reactions.
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Product gas from the steam reforming of the methane and naphtha contains equilibrium amounts of hydrogen, carbon dioxide, carbon monoxide and excess steam. The calculated effluent composition of a reformer always needs to be checked against the equilibrium constant equations to ensure that simulations agree with known values. Excess steam above the theoretical requirements is maintained to prevent the reforming catalyst from coking. The temperature exiting the reformer furnace tubes is usually about 760oC (1,400oF), a level that provides maximum hydrogen production within the temperature limitation of the reformer tube metallurgy (discussed later).
Water-shift gas reactions Additional hydrogen can be generated from the carbon monoxide byproduct following the reforming reaction. First, the reformer effluent gas is cooled in two steps to favor the equilibrium toward the right side of the reaction. The first cooling step is followed by the high-temperature shift reactor, and the second cooling step is followed by a low-temperature shift reactor. Shift reactions are promoted as effluent gas flows down through the fixed catalyst reactor containing a ferric oxide catalyst in accordance with the reaction in Equation (5). Note the water-shift reaction is exothermic, which results in a temperature increase across the reactors as water reacts with CO to form CO2 and more H2.
Water shift gas equilibrium is not affected by pressure, since there is no volume change. Reduced temperatures favor the conversion of CO to H2, as might be expected by its exothermic nature. A variety of catalysts are available for the service.
H drogen Plant Process Figure 1 shows a schematic of a conventional steam-reforming hydrogen plant [4]. The plant is based on a feed gas with high sulfur content, requiring plant operators to hydrotreat the feed before the zinc oxide removes the sulfur compounds. The H2 purification at the end of the process is based on the removal of CO2 with a pressure swing adsorber (PSA) system shown as the H2 purification block. The reformer is shown as a vertical furnace type with side firing. The reformer furnace design alternatives will be discussed below. Feed gas hydrocarbons usually a mixture of hydrogen, methane and other light is first compressed to about 300 psig. The initial
compression has been found to provide product hydrogen at a pressure that can easily reach the desired hydro-processing pressure with a four- or fivestage reciprocating compressor. This equipment is not part of the hydrogen plant. The feed gas is preheated with reformer effluent gas and hydrotreated to convert the various sulfur compounds (such as mercaptans, carbonyl sulfide and carbon disulfide) to hydrogen sulfide. The gas is then passed through desulfurization reactors, usually containing a zinc oxide catalyst, which adsorbs the hydrogen sulfide. Low-sulfur feeds may not require the hydrotreating step.
Reforming furnace
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hea e . The coil arrangement in a typical side-fired reformer furnace (Figure 3) consists of two parallel rectangular fire boxes connected at the top with horizontal duct work into the vertical convection stack. Two rows of vertical tubes arranged on a staggered pitch are present in each of the radiant boxes. Several (typically four) rows of burners are used to fire each side of the two radiant sections. This arrangement allows direct radiant fire to reach most of the tube wall. Platforms are provided burner to levels. access A the burners at each of the four typical reformer furnace could have over 300 burners. Reformer tubes typically have diameters of 5 in. (127 mm), walls 0.5-in. (13 mm) thick and about 34 ft (11.5 m) of wall exposed to the burners. The tube metallurgy is usually 25% chrome, 20% nickel or a high-nickel steel such as HL40. The inlet manifold at the top of the heater has pigtails, which uniformly transfer the feed gas to the top of the tubes. Another manifold at the bottom connects of the heater set of another Figure 2. Maintaining a tube-wall temperature that is hot enough for the reforming reaction is a critical factor in reformer heater design
pigtails to the outlet transfer line. The pigtails provide for thermal heater expansion as goes from to the startup reaction
temperature
temperature. The objective is to have an equal pressure drop across each tube, which
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type of reformer heater is usually a rectangular box. The tubes are still vertical, and inlet and outlet pigtails are used to connect the inlet header and the outlet transfer line, respectively. Figure 4 shows a schematic diagram of a down-fired reformer furnace [9]. The tubes are spaced on a pitch, which allows the burners to fire down between the tubes. The burners have a special pencil-shaped flame design. located All in burners the are penthouse
above the inlet manifold. The flame and the flow through the tubes travel in the same direction. Hydrogen plants with single reformer heaters and capacities up to 100 million Figure 4. Hydrogen plants with single heaters 3 ft3/d have used the vertical, and capacities up to 100,000 ft /d have used a down-firing approach down-firing approach. Each burner s radiant flame covers one-quarter of four adjacent vertical tubes (except for the outside burners, which cover half of the two adjacent tubes). The radiant gases exit the box horizontally through a horizontal convection section. The horizontal convection section is located about 3 m above grade to allow enough height for passage. The horizontal convection provides for a simpler support structure than that of the side-fired unit.
T an fe -line
eam gene a o
The outlet transfer line from the reformer is used to generate high-pressure (usually 650 psig) steam. The reformer effluent gas exits through the transfer line at about 1,400oF and enters the tube side of a single-pass steam generator. BFW is fed through the shell side and becomes 650 psig steam. Depending on the size of the reformer, there may be two transfer lines exiting
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H drogen purification Hydrogen purification is generally carried out using one of two approaches solvent-based systems or pressure-swing adsorption (PSA) processes. Solvent s stems Most older units remove carbon dioxide from the hydrogen rich gas using a solvent, such as Catacarb or amines, in a typical acid gas separation unit (Figure 5). Remaining reacted with carbon hydrogen to oxides in a
them to methane. Methane is an component makeup gas to a hydrocracker because it builds up in the recycle gas, requiring bleeding of the gas to maintain the recycle
desired hydrogen partial pressure in the hydrocracker. Most solution-type carbon dioxide removal systems are similar. Gas enters the bottom of the absorber, where it contacts lean solution. The carbon dioxide is absorbed from the gas, leaving the rest of the contaminants and hydrogen relatively untouched. The rich solution is then heatexchanged with lean solution and enters the top of the stripper. The stripper uses a steam reboiler to regenerate the solvent, stripping out the absorbed carbon dioxide. The overhead from the stripper goes through a condenser to condense solvent and then to an overhead drum, where the carbon dioxide is separated from the stripper reflux. PSA unit.The newer PSA process produces a hydrogen stream of four-nines (99.99%) purity. It separates carbon monoxide, carbon dioxide and unconverted hydrocarbons. A bank of adsorbers operates in a cycle where Figure 5. Most older units remove carbon dioxide from the hydrogen-rich gas with a solvent
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Figure 6. A PSA unit separates carbon monoxide, carbon dioxide and unconverted hydrocarbons from hydrogen. Adsorbers operate in a highpressure to low-pressure cycle to adsorb and then release contaminants The fuel gas is relatively low-BTU carbon oxides. It is supplemented with natural gas or other fuels as feed to the reformer furnace burners.
P e- and po - efo ming These are two techniques used to expand the capacity of exisiting plants where the reformer furnace is heat-transfer-limiting. P e- efo ming Pre-reforming is used when spiking the feed with liquified petroleum gas, which is used to increase the capacity of the existing unit. Examining the reforming Equations (1), (2) and (4) reveals the advantage of a heavier feed that yields more hydrogen per feed mole. The pre-reformer reaction breaks down the heavier hydrocarbons (propane and butane) to methane ahead of the heat-intensive reforming reactions, essentially shifting part of the load upstream of the reformer heater as shown in Figure 7 [8].
Figure 7. A pre-reformer breaks down heavier hydrocarbons into methane ahead of the reforming reactions Feed at 950oF passes down through the pre-reformer reactor, where the breakdown reactions occur. Then the pre-reformed feed passes through another convection coil to reheat it to about 1,100oF before entering the reformer. Adding the pre-reformer as a retrofit to an existing facility presents two problems one of space and one of compatibility. Physical space contraints may not allow adding a feed reheat coil within the convection section. Also, the metallurgy of the inlet pigtails may not be able to handle the higher feed temperature. Po - efo ming. Post-reforming is an attempt to provide additional reforming
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HYDROGEN PRODUCTION BY STEAM. REFORMING OF HYDROCARBONS. Niels R. Udengaard. Haldor Topsoe Inc. 17629 El Camino Real, Suite 300 ...
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