'ACROSS-THE-FENCE' HYDROGEN PLANT STARTS UP AT CALIFORNIA REFINERY

Nitin M. Patel, Ruth A. Davis, Norm Eaton
Air Products & Chemicals Inc.
Allentown, Pa.

Daniel L. Carlson
Tosco Refining Co.
Martinez, Calif.

Fred Kessler, Vinay Khurana
Kinetics Technology International Corp.
San Dimas, Calif.

An alliance of Air Products & Chemicals Inc and Kinetics Technology International Corp. (KTI) put the alliance's first on-site hydrogen plant on stream this past November at Tosco Refining Co.'s Avon refinery in Martinez, Calif. (Fig. 1).

Plant construction met cost and schedule expectations because minimal design and scope changes were required.

The plant was designed for a future capacity increase of as much as 40%. Based on the performance test results, the new plant qualifies as one of the most energy efficient hydrogen plants in operation.

The plant can convert various gaseous and liquid refinery hydrocarbon streams to high-purity hydrogen (H2). Numerous process optimization and design features were verified satisfactorily during plant performance testing.

Air Products and KTI decided to invest a little extra in the plant to make possible a future capacitor expansion. This preinvestment approach enables the refiner to achieve increased hydrogen capacity at low cost with minimal interruption of hydrogen supply. The additional hydrogen is expected to be available by early 1996.

HYDROGEN DEMAND

As environmental legislation mandating cleaner gasoline and diesel fuels is implemented around the world, demand for new and supplemental refinery hydrogen capacity continues to increase. And complicating this picture are the accompanying changes in processing schemes, which reduce hydrogen supply from the catalytic reformer.

In addition, the world's available crude supply creates a discount for heavier and sour crudes, which further increases hydrogen demand.

Several processes are available to meet the rising world demand for hydrogen. However, steam reforming is the dominant preferred process for converting light hydrocarbons to hydrogen. Demand for diesel desulfurization, resid upgrading, high-sulfur crude processing, and reformulated gasoline production is projected to result in a 11.5 billion scfd increase in on-purpose hydrogen demand in the U.S.

Regional trends influencing hydrogen supply include:

Europe-Significant demand growth caused by: Refinery product slate upgrades, EEC diesel regulations (1996), reformulated gasoline (limited), and heavy feedstock disposal.
Asia/Pacific-Modest demand growth created by: New refinery and chemical centers and diesel regulations in Japan (1996) and Thailand (1997).

PROJECT OVERVIEW

The hydrogen plant project was jointly developed, engineered, and built by Air Products and KTI. The Avon facility is the first of three California hydrogen plants under contract to Air Products and KTI, since their marketing alliance was formalized in August 1992.

The key components of this plant are KTI's reformer design and Air Products' pressure swing adsorption (PSA) system. For the alliance's projects, Air Products provides overall project management and operating experience, while KTI furnishes process design, engineering, procurement, and construction management.

The hydrogen plant is owned, operated, and maintained by Air Products to supply hydrogen and steam to the Avon refinery under a long-term supply agreement.

The hydrogen from this plant enables Tosco Refining Co. to reduce the sulfur and aromatics content of its diesel fuel, as required by California and federal requirements. The plant also enables the refiner to maximize liquid yields without incurring a hydrogen-balance constraint.

With Air Products owning and operating the hydrogen plant, Tosco Refining is able to concentrate more of its financial and operating resources on its core business of producing and marketing transportation fuels.

The base capacity of the plant is 25 MMscfd H2. Working closely with the refinery during the scope-development phase of this project, the following key objectives and considerations were identified:

Minimize use of rotating equipment for improved reliability
Provide feedstock flexibility
Maintain high thermal efficiency
Minimize export-steam generation
Facilitate cost-effective expansion capabilities
Minimize plot space to stay within refinery battery limit.

The hydrogen plant occupies about 1.1 acres and is surrounded by refinery operating units (Fig. 2). To ensure safe operation of the hydrogen plant, Air Products and KTI included in the facility a pressurized control/maintenance building, a pressurized power-distribution center, and a barrier wall with combustible-gas detectors (because of the proximity of the butane tank farm).

In addition, a detailed three-phase process hazards review was performed, including:

Design hazards review
Design verification review
Operational readiness inspection before start-up.

Air Products and KTI worked with the refinery's engineering and operations departments to build an efficient, safe, and reliable hydrogen plant which would meet cost and schedule expectations. The end result was a plant that came on stream 7 weeks ahead of the contracted commitment date (Fig. 3).

DESIGN OVERVIEW

Table 1 lists the conditions and specifications of the H2 and export steam produced by the plant. The available feedstocks and fuels include high or low-nitrogen (N2) natural gas, liquid propane, liquid butane, and refinery fuel gas.

Table 2 lists the composition of these feedstocks and fuels for the plant-design basis.

The reforming section is designed to handle multiple feeds to produce high-purity H2 and a very limited amount of export steam. The primary feedstock for this plant is a blend of high or low-N2 natural gas. During times when excess propane or butane are available, however, either or both of these can be blended into the natural gas feed stream or used separately.

The refinery fuel gas is used only as supplemental reformer furnace fuel.

The process scheme, shown in Fig. 4, is traditional, with the addition of a prereformer integrated with the primary reformer to minimize export steam and maintain high thermal efficiency. The plant is designed to deliver high-purity H2 at 420 psig with feed compression only. This scheme contributes to higher overall plant reliability.

The feed portion of the plant comprises two parallel systems to handle the various feedstocks. For the high or low-N2 natural gas feed, a single-stage reciprocating compressor boosts the feed pressure to about 600 psig.

The second system consists of a pressurized vaporizer for the liquid propane and butane feeds available at 600 psig.

The hydrocarbon feed is preheated to 700 F. using process waste heat, then desulfurized. The desulfurization step includes hydrogenation of organic sulfur to H2S, followed by processing in zinc oxide beds to remove the H2S.

Downstream of the desulfurizers, the superheated process steam is added to the hydrocarbon feed to obtain a steam-to-carbon ratio of 3.5. The steam/hydrocarbon mixture is preheated to 900-950 F. using flue-gas waste heat, then fed to the prereformer.

The prereformer is an adiabatic reactor which converts the desulfurized hydrocarbon-steam mixture to an equilibrium mixture of methane, carbon oxides, and H2 via the reforming and shift reactions.

The prereformer effluent is heated further to 1,150-1,200 F., also with flue gas waste heat, before being fed to the primary reformer. With the heavy hydrocarbons converted to methane and H2, the primary reformer feed can be heated safely to these high temperatures without the typical concerns of cracking.

The primary reformer completes the reforming process, which is highly endothermic. Primary reforming results in a synthesis gas mixture of H2, carbon oxides, and unreacted methane and steam. The heat necessary for the reaction is supplied by externally fired burners.

The reformed gas exits the primary reformer and enters the process-gas boiler. A majority of the steam generation occurs in this boiler. The carbon monoxide in the synthesis gas then is "shifted" (converted to CO2) to increase the hydrogen concentration.

The heat generated by the shift reaction is used to preheat the reformer feed and boiler water for steam generation. After the H2-rich syn-gas is cooled to ambient temperature, it enters the PSA unit.

In the PSA, all of the impurities in the syngas-such as methane, nitrogen, and carbon oxides-are adsorbed to produce high-purity H2. Offgas from the PSA is recycled to the primary reformer for use as fuel. Natural gas or refinery fuel gas is used to supplement the PSA offgas to supply the total primary-reformer furnace duty.

The convection section of the primary-reformer furnace recovers the flue gas waste heat by generating and superheating steam, and by preheating process feeds and combustion air. Low-Nox burners and a selective catalytic reduction (SCR) unit are incorporated into the design to meet California's stringent emission regulations.

REFORMER DESIGN

The primary reformer is a KTI box type with downflow and top-fired furnace. The reformer is fully integrated with the process design of the plant and utilizes the latest catalyst innovations and improvements in tube metallurgy. The operating pressure of the reformer was determined by the available feed pressure and the required product pressure. The most economical plant design was selected based on feed compression alone. This resulted in a reformer outlet pressure much greater than the conventional range of 350-400 psig.

Because reforming is favored by lower pressures and higher temperatures, methane slip in the reformer effluent was minimized by designing for a high outlet temperature of 1,600 F.

The reformer tubes are designed in the creep-rupture range. At elevated temperatures and pressures, creep occurs constantly. Because allowable creep-rupture decreases with increasing temperature, it was imperative that the tubewall temperatures be predicted accurately to design the reformer properly. KTI's reformer program was used to predict the tubewall temperatures, which were verified subsequently by field measurements.

The reformer tube design is based on newly developed alloys which result in thinner tubes. Thinner tubes lower the thermal gradients and are better suited to the somewhat cyclic nature of reformer operation. KTI selected for the tubes microalloys with the nominal composition of chrome/nickel and other proprietary doping materials.

This composition is expected to result in longer reformer tube life in spite of the severe operating conditions in the reformer. These alloys exhibit greater creep-rupture strength and resistance to carburization.

The reformer design for the plant includes four rows of tubes and five rows of burners. Taking advantage of the top-fired configuration, it was possible to install fewer burners relative to the number of tubes, with all burners operating at a single level. Both outer rows of burners are designed to generate 50% of the heat required by the inner row of the burners, to allow for even distribution of that heat.

The top-fired configuration is best suited for this application because it increases the radiant-box efficiency of the reformer. Maximum heat liberated in the top section of the reformer is utilized for the reforming reaction and for heating the inlet process gases. The remainder of the reforming process and the specified conversion are achieved at the reformer outlet by maintaining a high process temperature.

PREREFORMER TECHNOLOGY

The hydrogen plant at Avon has one of the largest prereformer reactors operating on natural gas, LPG, and butane feeds. A prereformer is an adiabatic reactor containing high-activity, nickel-alumina catalyst. It converts the desulfurized hydrocarbon feedstock and steam to an equilibrium mixture of hydrogen, methane, and carbon oxides.

Prereformer operation with heavy feedstocks results in a slightly exothermic reaction, whereas the use of natural gas causes an endothermic reaction. With operation on 100% natural gas feed, a 110-120 F. temperature decrease across the catalyst was observed. The prereformer effluent composition was analyzed, revealing 22-24% hydrogen, 66-68% methane, and less than 10 ppmv ethane.

Similarly, with operation on 100% butane feed, a 20 F. temperature increase across the catalyst bed was observed. The effluent stream analysis showed 30-34% hydrogen, 50-55% methane, and less than 10 ppmv ethane.

Prereformer technology was considered in the process design for this project because it:

Limits export-steam generation by utilizing waste heat for reforming
Provides flexibility to process multiple refinery feedstocks
Reduces reformer furnace radiant/convection-section physical size.

The primary concern with a prereformer reactor is maintaining the catalyst in an active, reduced state. As steam is primarily an oxidant, exposing the catalyst to steam, without the presence of hydrogen or a hydrocarbon, during start-up and shutdown must be minimized, if not eliminated. This is contrary to the normal practices associated with steam reforming technology.

Air Products and KTI pioneered a new procedure for reducing the oxidation of high-nickel prereformer catalyst with steam during start-up and shut down.

Fig. 5 shows the piping and controls associated with the prereformer vessel. With a mixed-feed preheat coil in the convection section and a full process-steam bypass around the reactor, the prereformer vessel can be heated to the desired temperature during start-up using N2 alone. Process steam is injected into the reactor a few seconds before hydrocarbon feed is introduced, thus avoiding an oxidizing atmosphere in the reactor.

During plant shutdown or loss of hydrocarbon feed to the prereformer, the main process-steam control valve shuts to prevent steaming of the prereformer catalyst. The process-steam bypass valve then sends steam to the primary reformer to protect the tubes from overheating.

In addition, as a precaution, a 20-sec delay is incorporated into the main process steam valve closure to help purge the hydrocarbon off the catalyst bed during plant shutdown. The use of the prereformer also is expected to extend the life of the primary reformer catalyst and reformer tubes by acting as a guard bed.

SPECIAL FEATURES

In addition to being safe and highly reliable, the hydrogen plant incorporates the following design features, based on the latest technology developments, to improve efficiency:

High mixed-feed preheat temperature
High combustion-air preheat temperature.

In conjunction with using the prereformer to provide feedstock flexibility and limit export-steam generation, a high mixed-feed preheat temperature was established at proven design limits to minimize firing in the reformer.

The additional mixed-feed preheat was achieved by utilizing the waste heat from the flue gas in the reformer convection section. This helped decrease fuel consumption and achieve the design goal of reduced export-steam generation.

Greater combustion-air preheat was included in the design to maximize recovery of flue-gas waste heat, thus reducing fuel consumption and minimizing export steam. A fluid-modeling program, and other KTI in-house reforming simulators, were used effectively to determine the correct convection layout with the most efficient integration of air preheater and SCR designs.

The combustion air preheater was split into two sections-high and low metallurgy-to reduce capital cost and accommodate the SCR unit installation.

SCR/CEMS

The primary source of air emissions at this facility is the flue gas from the reformer furnace. The flue gas is generated from the combustion of natural gas or refinery offgas, plus the PSA purge gas.

In the flue gas waste-heat recovery section, the flue gas passes through an SCR unit. The SCR unit consists of an aqueous ammonia injection/distribution grid, followed by a flow-through honeycomb catalyst for destruction of NOx before it exits the stack (Fig. 6).

The composition of the SCR catalyst is proprietary to the catalyst supplier. Typical major components, however, include titanium dioxide, tungsten trioxide, vanadium pentoxide, and molybdenum trioxide.

The stack also is equipped with a continuous emission monitoring system (CEMS) to measure, store, and report continuously the following data:

NOx emissions
Carbon monoxide (CO) emissions
Oxygen concentration
Volumetric flowrate
Flue gas temperature.

In early 1994, the hydrogen plant was tested via manual stack sampling to verify that NOx and CO emissions complied with permit limits and to certify the CEMS measurements. The test was successful, and the results verified that the reformer-furnace flue gas was in compliance with all permit limits and that the CEMS met its accuracy requirements.

The data acquisition and reporting system design is based on input from the Bay Area Air Quality Management District to ensure compliance with the permit-to-operate conditions.

FUTURE EXPANSION

To ensure an ease of transition from 25 to 35 MMscfd capacity, all unit operations and detailed design aspects were thoroughly reviewed during the early basic engineering phase. Cost-effective preinvestment was included in the base case 25 MMscfd H2 plant without significantly impacting its thermal efficiency. The additions necessary to increase capacity in the future were minimized to accommodate the entire upgrade within a 2-week turnaround time.

The preinvestment includes the installation of:

Oversized equipment for key components
Larger reactor vessels without the incremental catalyst
Variable-speed motors on the furnace fans
Piping configurations to accommodate future equipment additions
Extra space in the convection section for future convection coils.

This scheme ensures that future expansion cost is minimized, incremental hydrogen cost is low, and, most importantly, that the 40% capacity expansion is feasible.

START-UP

The Air Products/KTI alliance created a team concept, which led to a much smoother and more efficient plant precommissioning and startup compared to the more usual turnkey approach. As a result of proper interfacing between Air Products and KTI, it was possible to conduct certain activities in parallel during precommissioning and start-up. This enabled the two organizations to reduce the overall schedule by several weeks.

Precommissioning activities included:

Loading catalyst
Blowing and flushing lines
Running in all rotating equipment satisfactorily
Completing all instrument loop checks
Verifying shutdown devices.

During the first cold startup of this facility, no major problems were encountered, although a few minor ones were identified and resolved:

Errors in combustion air flow shutdown set points
Low SCR ammonia-vapor temperature
False level indication in the LPG vaporizer.

The plant delivered on-specification hydrogen within 10 days.

PERFORMANCE

After successful start-up of the plant, a detailed performance test was conducted jointly by Air Products and KTI in coordination with the Avon refinery operations department. The stack emissions testing for SCR and CEMS certification also were performed at the same time.

During the test the following data were collected:

Incoming and outgoing plant flow measurements, all compensated for temperature, pressure, and density effects.
Lab composition analysis of feed and fuel gas, hydrogen product, and effluent from the prereformer, primary reformer, and high-temperature shift converter.
Flue-gas temperature profile using a high-velocity thermocouple.
Furnace and convection-section pressure profile using a water manometer.
Overall plant pressure profile using a calibrated Hiese gauge.
Reformer tube temperature using an optical pyrometer.
Transfer-line and process-gas boiler skin temperatures.
SCR ammonia injection rate and NOx/CO emissions.

Before the performance test, all the important process flow elements were calibrated to ensure accuracy of the data. Although this plant is designed to process a variety of hydrocarbon feeds, LPG was not available during the testing period. The test therefore was conducted with high and low-nitrogen natural gas and butane feeds. The plant was operated at a range of throughputs, from 100% capacity down to a turndown rate of 50%, with each of these feeds and with mixed feeds. Table 3 details plant performance at design capacity with the various feeds.

ACKNOWLEDGMENTS

The authors thank Joseph Abrardo, Randy Kessler, and Jennifer Castello for review and comments; Bill Baade, Kevin Murphy, and Doug Lubbers for their advice and support on this article.