Saturday, May 29, 2010

Hydrogen Production By Steam Reforming


Management of the gas is critical for petroleum refiners

Ray Elshout Energy, Systems Engineering

Steam reforming of natural gas at petroleum refining facilities is the 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.

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 ultralow-sulfur 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.

If the hydrogen partial pressure cannot be maintained, the recycle gas should be bled. With pressure swing adsorption (PSA) processes producing hydrogen of purity in the range of four-nines (99.99%), this is not a problem.

Other hydrogen users, like those engaging in milder hydrotreating, can use lower-purity hydrogen at lower pressures (600 psig or lower). One approach that makes sense is recovering hydrogen from the users requiring higher pressure and reusing it at the lower pressure levels.

Minimizing the hydrogen bled into the fuel gas can keep the hydrogen production levels manageable. However, the hydrogen plant feed usually includes some hydrogen that goes through for a “free ride,” except for the cost of heating it up to reformer temperature and ultimately cooling it back down to recovery level.

A recently employed practice in the industry is for the hydrogen to be produced for adjacent producers and sold to the user as “over-the-fence” hydrogen. This keeps the production costs off the books from the adjacent user and has found popularity not only in the U.S. but also in Europe.

Figure 1. Steam-methane reforming is still responsible for the bulk of hydrogen production in petroleum refineries

Steam-Methane 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.

As the molecular weight of the feedstock increases, such as when heavier hydrocarbons (such as ethane, propane or butane) are included in the feed, the reactions are shown by Equations (3) and (4), with the corresponding heat requirements [2].

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.

Hydrogen 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 — usually a mixture of hydrogen, methane and other light hydrocarbons — 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 five-stage 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

The sulfur-free gas is mixed with a fixed amount of superheated steam to maintain the desired steam-to-hydrocarbon ratio. The steam-to-hydrocarbon ratio is kept within a range that is high enough to prevent laydown of coke on the reforming catalyst, but low enough to avoid overloading the reformer duty. Typically for a methane feed, the ratio would be three, whereas the theoretical requirement is somewhat less.

The combination of hydrogen and steam is heated to about 760oC (1,400oF). Since all of the reforming reactions are endothermic, additional heat is required to maintain the reaction temperature as the mixture flows down through catalyst-filled reformer tubes.

A critical factor in the reformer heater design is keeping the tube-wall temperature uniform and hot enough to promote the reforming reaction. Two types of heater designs have been employed for this purpose. Figures 2 and 3 show schematic diagrams of the side-firing reforming furnace, and the roof-fired heater design approach is shown in Figures 2 and 4.

Figure 2. Maintaining a tube-wall temperature
that is hot enough for the reforming reaction
is a critical factor in reformer heater design

Figure 3. A typical reformer furnace could
have over 300 burners

Figure 4. Hydrogen plants with single heaters
and capacities up to 100,000 ft3/d have used
a down-firing approach

Side-fired reforming heaters. 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 to access the burners at each of the four burner levels. A 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 HL-40.

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 of the heater connects another set of pigtails to the outlet transfer line. The pigtails provide for thermal expansion as the heater goes from startup temperature to reaction temperature. The objective is to have an equal pressure drop across each tube, which produces uniform flow to each of the tubes. The convection section includes several different coils. The hottest coil is a steam generation coil that protects the other coils from radiant heat. Usually, there is also a steam superheat coil, a feed preheat coil and another steam generation coil. Above these coils, there may be a boiler feed water (BFW) pre-heater and deaerator preheat coil.

Typically an induced draft fan is used to keep the fire box pressure slightly negative. Some reformers also have an air pre-heater and a forced draft fan.

Top-fired reformer. This 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. All burners are located in the 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 ft3/d have used the vertical, 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.

Transfer-line steam generator

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 opposite ends of the reformer and feeding two steam generators. Figure 3 shows the two transfer line steam generators.

Feed preheat exchanger.Gas is cooled to about 650oF and is moved out of the steam generator. It then enters the tube side of the feed preheat exchanger. Feed gas is preheated to about 600F using heat from the effluent gas. This temperature can be controlled by partial bypass of the effluent side to maintain the desired hot-shift gas reactor temperature.

Hot shift-gas reactor.Effluent gas containing carbon monoxide and steam is passed over the hot gas-shift catalyst, where the water-shift gas reaction shown in Equation (5) occurs. This reaction is slightly exothermic, resulting in a temperature rise across the reactor.

More steam generation.Additional medium-pressure steam is generated, reducing the hot-shift reactor effluent to a temperature of about 500oF, which shifts the reaction equilibrium toward more hydrogen production.

Cold shift-gas reaction.Additional hydrogen is produced by the gas-shift reaction at the lower temperature. The shift reaction is exothermic, which results in a temperature rise across the reactor.

Condensate removal. Cold gas-shift effluent is cooled by heat exchange with BFW, deaerator feedwater, and cooling water to about 34oC (100oF). Condensate is separated from the gas in a vertical knockout drum.

Hydrogen purification

Hydrogen purification is generally carried out using one of two approaches — solvent-based systems or pressure-swing adsorption (PSA) processes.

Solvent systems 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).

Figure 5. Most older units remove carbon
dioxide from the hydrogen-rich gas with
a solvent

Remaining carbon oxides (primarily carbon monoxide) are reacted with hydrogen in a methanator reactor to convert them to methane. Methane is an undesirable component in the makeup gas to a hydrocracker because it builds up in the recycle gas, requiring bleeding of the recycle gas to maintain the 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 heat-exchanged 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 the adsorbers are rotated through a higher-pressure adsorption portion, followed by a pressure reduction, which allows the contaminants to be released from the adsorber. The hydrogen gas passes through the adsorber as almost-pure hydrogen. The contaminants flow into a fuelgas surge drum.

Figure 6 shows a schematic diagram of such a system. The valve openings and closings are all controlled by the central processing unit.

Figure 6. A PSA unit separates carbon monoxide, carbon dioxide and unconverted hydrocarbons from hydrogen. Adsorbers operate in a high-pressure 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.

Pre- and post-reforming

These are two techniques used to expand the capacity of exisiting plants where the reformer furnace is heat-transfer-limiting.

Pre-reforming 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.

Post-reforming. Post-reforming is an attempt to provide additional reforming catalyst outside the reformer heater. A down-flow reactor is added between the outlet transfer line and the waste heat steam generator. This can present a space and piping problem. The additional post-reformer catalyst reduces the overall total space velocity of the combined reformer and post reformer, thus achieving additional reaction. This reduces the downstream shift-reaction requirements. Edited by Scott Jenkins

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5 komentar:

HYDROGEN PRODUCTION BY STEAM. REFORMING OF HYDROCARBONS. Niels R. Udengaard. Haldor Topsoe Inc. 17629 El Camino Real, Suite 300 ...

I am an engineering student, I would like to ask how to calculate the heat energy generated that is given on the catalyst ?

Hi. May I know where did you source this from?

Source:
http://accessintelligence.imirus.com/Mpowered/book/vche1/i2/p44

Hiya! Do you somehow make sure that the content of your blog is exclusive in the web and no other blogger is using it without making sure you are aware of it?

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