Optimising hydrogen production and use
Knowledge of hydrogen producing and consuming process technologies, systems analyses and process controls can be leveraged to optimise hydrogen use
Ronald Long, Kathy Picioccio and Alan Zagoria
UOP LLC, A Honeywell Company
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Hydrogen plays a critical role in the production of clean fuels, and its use has increased with the introduction of low-sulphur gasoline and diesel fuels. The reduction of benzene in gasoline via benzene saturation will also increase hydrogen consumption, as will the trend toward diesel cetane improvement and aromatics reduction.
Changes in marine fuel oil specifications are also expected to increase hydrogen demand. In 2008, MARPOL Annex VI regulations were passed, setting a framework for regional and global specifications on marine fuel oil quality. These regulations are expected to further increase the demand for hydrogen for desulphurising residual fuel oils and, through the increase in distillate fuel demand, to replace residual fuel oil in marine fuels (see Figure 1).
The overall reduction in demand for heavy fuel oils has encouraged many refiners to install bottoms upgrading capacity such as delayed coking units. The streams produced by coking typically contain higher contaminant levels (sulphur and nitrogen) than the equivalent straight-run streams. Hydrotreating these coker products has also increased hydrogen consumption.
Hydrocracking has become increasingly important for converting heavier crude fractions into high-quality clean fuels. Increased reliance on hydrocracking for clean fuels production has also led to a rise in hydrogen consumption. A hydrocracking unit is typically the largest hydrogen consumer in the refinery, and hydrogen can account for more than 80% of the unit’s operating cost (see Figure 2).
The quality of crude oil is gradually declining. Globally, crude API gravity is declining and the sulphur content is gradually increasing (see Figure 3). Both of these trends in crude quality will contribute to increased hydrogen consumption during refining. The use of synthetic crudes derived from oil sands and other unconventional sources is expected to increase to 2 million b/d by 2020. These synthetic crudes will require additional hydrogen to be refined into usable products.
One of the major sources of hydrogen and gasoline pool octane is the catalytic reformer. The blending of ethanol has reduced the octane requirements from other refinery product streams to maintain the gasoline pool octane; often, refiners respond to this situation by reducing the catalytic reformer’s severity to produce a lower-octane reformate product. However, a lower catalytic reformer severity typically produces less hydrogen. Lower hydrogen production from a lower severity operation is in opposition to the increased demand for hydrogen in the rest of the refinery and compels the refiner to obtain hydrogen from other sources. Some refiners have decided to operate their catalytic reformer for hydrogen production and tolerate some octane giveaway in the gasoline pool.
Many refiners produce or purchase hydrogen to have a sufficient supply available for their refinery. The steam reforming (SR) process is used to produce most of the additional hydrogen required by refiners. The cost of the hydrogen produced is directly proportional to the feed costs. In the US, most of the hydrogen is produced in steam methane reforming (SMR) units and the cost is typically tied to the price of natural gas.
The production of hydrogen by a steam reformer requires significant energy; one tonne of hydrogen produced requires 3.5 to 4 tonnes of hydrocarbons as feed and fuel. Hydrogen production can account for up to 20% of refinery energy consumption. Additionally, the production of hydrogen generates significant amounts of carbon dioxide (CO2); the production of 1 tonne of hydrogen generates 8–12 tonnes of CO2. Future environmental legislation may regulate the amount of CO2 that can be generated and may increase the costs of hydrogen production.
Availability of hydrogen is a requirement for the production of clean fuels, and demand for hydrogen is at an all time high. Anticipated future trends and regulations are expected to further increase hydrogen consumption. At the same time, the production of additional hydrogen is expected to become more expensive.
While it is well understood that the ability of a refiner to produce clean fuels depends on having sufficient hydrogen, what many refiners recognise is that optimum use of hydrogen will maximise refinery profits.
Hydrogen network analysis and improvements
Refinery hydrogen networks typically interconnect many producers, consumers and purification units with different pressures, purities and operating objectives. The network grows with each subsequent refinery project, modified in ways that minimise complexity and the interruption of existing units rather than for refinery-wide optimisation. Hydrogen production costs and constraints on availability are typically much greater than when the network was first envisioned. All of these factors lead to the conclusion that most operating hydrogen networks are not optimised for today’s environment — not for the minimal cost of hydrogen production, nor for maximised refinery margins.
Optimising the hydrogen network
In many hydrogen optimisation schemes, it often occurs that the greater the number of degrees of freedom, the larger the improvement that is possible. The most successful programmes for improving the hydrogen network draw the largest possible envelope and take advantage of all the “knobs” that are available to turn, including network connectivity, increased hydrogen production capacity, target hydrogen partial pressures, process changes in producers and consumers, catalysts, operating procedures, revamped and new purification capacity, pressure swing absorption (PSA) unit feed to product bypass, feed to hydrogen plant, compressor modifications, ability of LP models to accurately represent hydrogen availability constraints, and header pressure control system improvements.
For the optimisation of hydrogen use, the benefits are driven by identifying and alleviating critical constraints in the refinery-wide hydrogen network. Every refinery is different and, from time to time, the active constraints in a refinery can change with different crudes or operating objectives. A refinery network may be constrained by total moles of hydrogen available, hydrogen purity, hydraulics, purifier capacity, compression, H2S scrubbing, fuel system constraints or other refinery-specific issues.
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