Hydrogen generation – a brief review
Project manager discusses vital design features
Petroleum refining has seen significant evolution in technologies over the past five decades, thus many process options can improve product selection and quality. Awareness of ecological and environmental issues has advanced secondary processing resulting in hydrogen (H2) being important for product refinement and conversion.
Stringent process requirements and the need for cost-effectiveness requires high-purity H2. Generation processes are standardized for the hydrocarbon processing industries (HPI) and use naphtha/natural gas reforming followed by pressure swing adsorber (PSA) purification. Over time, these technologies have become more sophisticated, and finer details invite the attention of designers, engineers, fabricators and operators.
Evolution
Commercial production was driven by the fertilizer industry, as nitrogenous fertilizers are made from ammonia directly synthesized from H2 and nitrogen. Haber&Bosch were the first to produce viable H2 generation in the early 20th century.
The first 3-5-tpd ammonia plant was built in 1913 in Germany. Since then, technology has evolved, leading to improvements in processes and capacity. H2 generation has followed the same pattern and found various industrial applications. H2 demand is increasing for the HPI as conversion of heavier products into useful ones rises.
The HPI and the fertilizer industry require H2 plants with large capacities, and the HPI requires high-purity H2 (99.96%) for economic reasons. For others, this may not be essential.
Concept and process. H2 is produced commercially by:
- Steam refining of naphtha or natural gas
- Partial oxidation of coal (Koppers-Totzek coal gasification) or heavy oils
- Steam-iron process by decomposition of steam via reaction with iron oxide
- Electro-splitting of water (electrolysis)
- Complex chemical splitting of water.
Of these, steam reforming is accepted by industry due to ease and scale of operation. The first use of steam reforming of hydrocarbons dates back to the 1930s when ICI with I. G. Farben and Standard Oil produced H2 by reforming hydrocarbons.
The first ICI plant operated at atmospheric pressure using hydrocarbons ranging from methane to butane. Later, operating pressures of 40 atm were adopted for better efficiencies and to reduce plant size. In 1959, ICI commissioned the first naphtha-based reforming plant. With the availability of natural gas, naphtha is now being replaced. Availability, cleanliness, ease of operation and environmental requirements favor usage of natural gas.
Steam reforming is based on the reaction between steam and hydrocarbons. It takes place over nickel-based catalyst (in the shape of small hollow cylinders) with methane:
CnH2n+2 + nH2O ® nCO + (2n+1) H2
or
CH4 + H2O ® CO + 3H2
The reaction is endothermic and carried out in a tube-furnace. H2 is formed at pressures of up to 43 bar with temperatures ranging from 740°C to 900°C.
When naphtha is used, mixing with recycled H2 at 42 bar vaporizes it. The mixture is preheated to approximately 380°C and hydrotreated on a Co-Mo catalyst bed in a reactor. Sulfur compounds are converted into hydrogen sulfide (H2S), which is adsorbed on a ZnO bed in another reactor before its entry into the reformer. Otherwise, the sulfur will poison the reforming catalyst.
The pressure drop across the catalyst bed should be kept low, so that the flow rate ensures a complete reaction at process temperatures. The steam-to-carbon ratio is very important and should be monitored. A suitable ratio must be maintained to avoid deposition of carbon on the catalyst or the reaction will not be complete. The ratio should be high (more than 2.5) to avoid deposition. This is achieved by mixing steam with desulfurized naphtha.
Carbon formation on the catalyst blocks will increase the pressure drop over the catalyst bed. Deposits inside the catalyst blocks affect the mechanical strength and activity. The high ratio helps to maintain/reduce the methane-slip decreasing methane in the reformed gas. The steam-to-H2 ratio is important as well. The steam ratio should be around 20 for naphtha and around 70 for natural gas.
Reformer effluent is cooled by a waste-heat-recovery system, which generates reaction steam plus some byproduct steam. This is further passed to the shift-reactor (shift-converter), where carbon monoxide is converted to carbon dioxide and yields more H2:
CO + H2O ® CO2 + H2
H2 produced in the reformer and shift-reactor are mixed with other gases and must be purified. In the case of ammonia synthesis, the H2 produced can be used as it is. Secondary processing/treatment in refineries necessitates further purification. H2-rich gases are fed to the PSA system, impurities are adsorbed on the nickel-based catalyst beds, and purified H2 is collected (Fig.1). For producing 38,000 metric tons per year of H2, around 150,000 metric tons per year of refined naphtha is used. Naphtha is converted to vapor before its entry into the unit.
Design
Steam reformer. The reformer furnace is a critical and major component. It is called the primary reformer and is tubular in type. Cylindrical furnaces were used in earlier ammonia synthesis plants. Recently, the design has been optimized, and the reformer is a rectangular prism. Catalyst tubes are arranged in suspended form in a rectangular box with an outlet manifold system beneath the furnace.
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| Fig. 1. Hydrogen generation flow diagram. |
The feed header is at the top connected by flexible tubes to the catalyst tubes. Burners are either at the bottom, sides or top. Catalyst tubes are connected to the outlet manifold directly to an outlet header or to a hot header and then connected to a cold header. For onstream reliability, design of packed catalyst tubes is critical. The cost of tube material and fabrication are almost half the cost of the steam reformer.
Rectangular reformers are predominant and are of two types. One is the multiple chamber with burners in the sidewalls of the furnace boxes. The other is a single furnace box with burners at the top. The type depends on the licensor and chosen design.
Side-fired furnaces need more space than top-fired furnaces for the same heat duties. The first type is used for small duties/capacities and flue gases exit at the top, making construction simple. The second is used for large duties and capacities. High heat flux design keeps the furnace size small.
Catalyst tubes are supported at the top by springs or by a pulley/counterweight system. At the bottom, they are connected to an outlet manifold which directly connects to a cooler. The manifold consists of a hot header in the chamber connected to a cold collector outside the furnace. In other cases, the catalyst tubes are attached through pigtails to a cold header. Such pigtails can be pinched when required to isolate the defective tubes.
The total vertical expansion of the tubes must be accommodated by flexibility at the top (inlet piping) and outlet end. This is achieved by allowing freedom of linear movement, rotation at the top and linear rotation at the bottom. In alternative designs only rotation at the bottom is allowed, the top remains the same.
The tube size is based on process (flow, temperature and pressure) and economic considerations. Generally, tubes used are 6-in. diameter and heat transfer of the reaction governs the wall thickness. Heat transfer is effected at a metal temperature of 860-950°C. Process temperature may vary from 500°C at the arch level to 800°C at the furnace bottom at 43 bar.
Furnace. Each process licensor has optimized furnace designs based on feed back, experiments, heat flux calculations (modified by experience for fixing process design kinetics and parameters), specifications, degree of reaction, space velocity, catalyst activity, degree of conversion, operating levels, endothermic heat requirements and heat flux etc. Reformer furnace design requires great care. A detailed analysis of furnace type, heat flux variations, temperature gradients, firing type, tube arrangement, etc., are done to achieve optimum design. Once these and the process parameters are established (depending on the reaction kinetics and the mechanization of functioning/operations), the mechanical design is done.
Temperature profile. This governs the reaction completion, effectiveness and maintenance of tube skin temperatures. The temperature profile across the tube portion within the reformer furnace and outside up to the header needs to be monitored (Figs. 2 and 3).
Flexibility analysis. This is governed by the temperature profile. It is done by providing free movement to tubes, containing stress within permissible limits that is caused by furnace heat and giving flexibility/cold pull to take care of expansion during furnace heating.
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Fig. 2. Temperature profile, ºC of tube skin
temperature. |
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Fig. 3. Temperature profile, ºC of reformer tube
length vs. hot header. |
Burners. The fuel chosen-natural gas or naphtha-will govern burner design. This is dependent on proprietary technology and manufacturer design. The size of flame and its envelope need to be defined and chosen based on experience and judgment.
The burners provide heat evenly which by radiation/convection is transferred to the gases inside the tubes. Use of proven burners based on similar services is preferable to avoid future problems. The burner tips must be properly sized for the steam and naphtha to mix inside the venturi and burn outside it.
Waste-heat-recovery system. The furnace is provided with a waste-heat-recovery system to conserve energy. The recovered heat is used to preheat the feed, combustion air and generate steam.
Tubes. Heat transfer in the reformer is about 40,000-50,000 Btu/hr/ft3 at around 1,000°C. Exotic tube materials are required to withstand high temperatures and high pressures in a highly corrosive environment. Tubes are cast out of HK-40 material (25% Cr and 20% Ni). This material has superior resistance to rupture and deterioration at high furnace temperatures.
Based on tube failure feedback, many manufacturers now use refined materials. A material better than HK-40 is 519 (Cr 24%, Ni 24%, Nb 1.5% and Fe). Also, 35 Cr, 25 Ni materials with tungsten (W) are being used, as these withstand high rupture and have better creep properties compared to low alloy materials. Control burner firing, regulate methane and prevent skin temperatures from going beyond allowed limits to help prolong tube life.
Important considerations for startup/commissioning and operations. Steam reformer furnaces need to be operated by experienced personnel. Care in maintaining uniform operating parameters and uniform heat emission from burners is required. Operators should have sufficient experience to trim the burners with valves to attain the designed fuel-to-air supply.
Uniform flowrates and heating must be maintained. Heat transfer is predominantly by radiation, but a uniform vertical gas flow must be established on the flue-gas side by an appropriate hydraulic design. This is achieved by providing ports in the sidewalls of the tunnels.
The number of ports varies over the length of the tunnel. Closing or opening some of the ports allows for final trimming. High skin temperatures also result from high-pressure drops across the catalyst bed. To exercise the required control, skin temperatures are monitored with pyrometers.
Uniform temperature must be achieved in the furnace and tubes, so that permissible skin temperatures are not exceeded. High pressures as per design are to be maintained. At low pressures, the mass/space velocities get reduced-leading to tube "red hotness". This causes high skin temperatures, which shortens tube life.
Important factors to consider are: catalyst tube failure; burner performance; catalyst poisoning/choking; steam ratios; catalyst loading and unloading; frequent shutdowns; assessment of methane threshold limit; penthouse temperature.
Control system. Adaptive controls help achieve complex configurations and are used to control all the plant components, including digital distributed control systems. Controlling feed and fuel systems is important, as the feed system maintains the correct steam-to-carbon ratio. The fuel system ensures that a variety of fuels are burned at desired times with the correct quantity of combustion air and maintaining a constant exit temperature.
Pressure swing adsorption system. PSA comprise of adsorbers (PSA vessels), purge vessels and the valve/control skid. PSA vessels use molecular sieves and Ni-based catalyst. The catalyst adsorbs gases other than H2 in the syngas going to these vessels. Regeneration of adsorbers is done with purge gas that sweeps through the PSA vessels and is collected in a buffer vessel.
PSA operation follows a sequence of cycles consisting of various steps. A pressure/time diagram controls this. Each PSA vessel (absorber) comes in sequence in a set pattern generated/controlled by pressure sensing. Controls provided take care of the depressurization steps, pressurization with the product and a tailgas control.
Construction. The total plant, including the reformer furnaces, can be configured into manageable modules. This is done for transportation and construction to achieve quality and speed. Each module has to be complete in its component form with all mountings and properly sequenced to dovetail into the adjacent one. Implementation schedules should be construction-driven.
All measures to achieve quality need to be fully accepted and followed across the organizational setup. The product handled is hazardous and the flame is invisible. Welding, fabrication, erection tolerances and NDT requirements are to be fully enforced to avoid any mishaps during startup/commissioning and operation.
Stabilized plant. The plant is simple in configuration; however, the steam reformer control and methane threshold limit for the PSA unit are the most important for stabilized plant operation without trouble and tube failure. Care of the critical parameters and design features during implementation will help in mechanical completion of a safe plant. 
Acknowledgments
The author would like to thank Engineers India Ltd. (EIL) for permission to publish, Hydrocarbon Processing for acceptance of the article and fellow professionals of IOCL, NRL, GSFC/GNFC for sharing their experiences. Thanks also to M/S Linde, Uhde, KTI and Haldor Topsøe, whose literature was useful in the compilation of this article.
The Author
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