REFINERY GASES—1: Hydrogen, nitrogen assist compliance with new, tougher environmental regs

Nov. 23, 2009
Refineries primarily process crude oil into motor and heating fuels, although other products can be made.

Refineries primarily process crude oil into motor and heating fuels, although other products can be made. These include gaseous fuels, feed gases for petrochemical processes, raw material for lubricant production, and asphalt.

The widespread use of fossil fuels has greatly affected the environment. Specifically the increase in automotive and air traffic has caused substantial pollution. One important result has been increasingly more stringent rules with respect to the environmental acceptability of automotive and heating fuels. In the European Union, for instance, new regulations for automotive fuels have been in force since 2000 that required substantial reductions in harmful components in gasoline and diesel. Table 1 summarizes the new guidelines.1 2

Use of industrial gases can play a major role in a refinery's compliance with these and other new requirements. For instance, desulfurization of products can be increased—as required by the regulations—by increased use of hydrogen. The reduced capacity of Claus plants due to increased desulfurization can be compensated for by use of oxygen.

The three-part series, which this article inaugurates, will discuss the effects of developments on refineries and their utilization of gases. At the same time, it will become clear that use of industrial gases offers possibilities for meeting the new requirements while avoiding major investments in new plants.

This first article will focus on uses of hydrogen and nitrogen. Part 2 (OGJ, Dec. 7, 2009) will discuss uses of oxygen. And the concluding article (OGJ, Dec. 14, 2009) will discuss uses of specialty gases and gas production plants.

Background

In a refinery, the new requirements have major effects on many plants. Fig. 1 shows their positions in a refinery.

Use of industrial gases in refineries has largely been limited to, for example, gassing reactors with nitrogen for inerting during catalyst changouts or use of analytical gases for the laboratories.

To the extent that hydrogen was used for hydrogenations, it was produced essentially from refinery gases, such as the exhaust from the reformer or the fluid catalytic cracker (FCC). Gases were products of the refineries and, in essence, were exported, not imported.

The new developments alter this situation thoroughly.

• More severe environmental protection laws require cleaner products and fewer emissions both in production and in use of the products. Among the emissions, not only must toxic gases be considered, such as NOx and SO2, but increasingly the greenhouse gas CO2 as well.

• Long-term market trends are shifting the range of products. Here are some examples:

—Heavy heating oil with high sulfur content is increasingly harder to sell.

—Increasing air traffic requires more kerosine.

—The rising proportion of diesel vehicles requires more diesel fuel, compared with gasoline. That means that more middle distillate must be produced in the refinery at the expense of the very light and very heavy products.3

• Globalization increases the pressure for operating refineries at optimum efficiency. That has already resulted in reduction of the number of refineries in Europe and the US and a slight decrease in capacity. One can see a trend to balance lack of capacity by importing refined products rather than by maintaining surplus capacity.4

Currently Europe exports gasoline mainly to the US and must import diesel, mainly from Russia. Mexico, which imported about 30% of its gasoline from the US, must now upgrade its refineries to produce the gasoline it cannot buy from the US anymore. There are many such chain reactions worldwide in order to adapt to new developments.

Hydrocracking, hydrotreating

There are many hydrocracking processes because of different feedstocks and objectives of the various refineries. For instance, atmospheric gas oil and vacuum gas oil are used in this process. The longer-chain hydrocarbons are broken down to lighter products and hydrogenated, as well as being partially desulfurized and freed of other heteroatoms.

The operating conditions are between 400° C. and 450° C. and between 70 and 210 bar (1,015 and 3.47 psi). Some hydrocracking processes have several stages in the reaction, with one stage for hydrotreating with an amorphous catalyst and one stage for hydrocracking with a zeolite catalyst. The chemical requirement for hydrogen in hydrocracking depends on the feedstock and is, for example, 2.75-2.90 wt % of the feed.

Hydrocracking is a major process for producing diesel fuel. Massive increases in demand for diesel fuel over the last few years and greater processing of heavy residues will result in expansion of hydrocracking capacities.

There are also various processes for hydrotreating, with the choice of process depending primarily on the feedstock and on the product grade desired. Sulfur, nitrogen, and metals in particular are removed from the various feedstocks, such as middle distillates and atmospheric or vacuum gas oils. That is intended to prevent catalyst poisoning to improve the product quality and of course to meet requirements of ultralow sulfur fuels. The chemical hydrogen requirement varies between 0.40 wt % and 1.35 wt % of the feed.

There are also special processes for hydrodesulfurization in which primarily organically bound sulfur is hydrogenated to hydrogen sulfide (H2S). The hydrogen sulfide can be separated from the gas phase in a scrubber. Then it is available as feed for a Claus plant.

Table 2 shows that the hydrogen requirement for desulfurization increases more than proportionately with the degree of desulfurization.5 One reason is that hydrogenation of organic nitrogen compounds to ammonia also occurs along with desulfurization. The more severe the desulfurization has to be in order to make clean fuels the more ammonia is formed as a by-product.

Demand for products with lower contents of sulfur, nitrogen, and metals is increasing in connection with more stringent environmental laws.

Isomerization

Isomerization is a reaction process being used increasingly to improve the properties of refinery products. Normal paraffins are converted to isoparaffins. Conversion of C5 and C6 paraffins from atmospheric distillation increases the research octane number (RON) and motor octane number (MON).

In hydroisomerization, for instance, the fraction rich in C5-C6 hydrocarbons is mixed with hydrogen and converted to the corresponding isomerizates in a reactor at about 150° C. to 200° C. with a zeolite, metal oxide, or chlorinated aluminum catalyst.

Isomerization can be linked with reduction of the benzene content. In the process, the double bonds of the benzene are saturated with hydrogen, producing cyclohexane so that an aromatic becomes a cycloparaffin. As the aromatic content will be even more strictly limited in the future, isomerization can provide reduction and rational use of aromatics. Isomerization, too, will increase in importance.

The ratio H2/hydrocarbon is usually 0.1-4 mole/mole. Hydrogen is often recycled to ensure optimal utilization.

Equalizing hydrogen balance

As explained previously, the need for hydrogen is increasing today because of the stricter environmental legislation, greater utilization of residues, and higher consumption of diesel fuel compared with gasoline. Table 3 shows a survey of the specific hydrogen consumption of the various hydrogenation processes, depending on the feedstock used.6

Catalytic reforming or platforming is the principal producer of hydrogen in a refinery. From 125 to 315 std. cu m/ton of feed can be produced in the process. Hydrogen production is much smaller in the fluid catalytic cracking process and in thermal processes such as visbreaking, in which hydrogen can amount to 2-16 std. cu m/ton of feedstock.

We can estimate, on a crude oil basis, that a crude oil with about 15 wt % naphtha will yield hydrogen in the reformer to the extent of about 0.4 to 0.6 wt % of the crude oil.

To make up the rest of the hydrogen requirement resulting from the developments described, it will be necessary to produce more and more hydrogen. The following processes are the main ones currently in use:

• Generation by steam reforming of methane.

• Gasification of residues from petroleum processing, obtaining hydrogen from the synthesis gas.

• Recovery from refinery gases, such as wastes from reformers, cokers, and FCC gas.

These processes for recovering and producing hydrogen will be discussed in Part 3 of this series.

Inerting, cryocondensation

Displacement of atmospheric oxygen and flammable gases with inert gases is proven to prevent oxidation, fire, and explosion. Inerting with nitrogen is one measure for primary explosion prevention.

There are four methods of inerting:

• Maintenance of overpressure.

• Permanent flushing.

• Controlled addition of inert gas.

• Pulsed injection of inert gas for stirring plus inerting the surface (Pulsair process).

Inerting is used particularly often in refineries for liquid storage tanks in which the liquids are covered with inert gases. Where it is favorable to stir the liquids, for example, to avoid settling of solids, the Pulsair process often is applied. This is especially true for tanks with biofuel. The process both protects the oil against degradation by oxidation and stirs the vessel. Fig. 2 shows the process principle; Fig. 3 shows a tank farm equipped with Pulsair.

This tank farm is equipped with Pulsair system for inerting plus stirring. The four tanks in the lower left have a content of 14,700 cu m each, filled with biofuel (Fig. 3).

If nitrogen inerting is planned and if the nitrogen is provided in liquid form, the cooling power of the nitrogen can be used for exhaust air cleanup, before serving for inerting. For that purpose, the major gas companies offer standardized units. Fig. 4 depicts the process.

While some units are tailored to the specific demands of the customer, others are standardized to a high degree. Tailored units are more flexible with respect, for example, to the pressure of the treated exhaust gas. Standardized units on the other hand can be kept in stock and may therefore be more quickly available on site.

In both types of units, the treated exhaust gas can be released to the environment as a purified gas. The dual use of the nitrogen in this variation of the process is particularly economical.

In processes with cryocondensation, a gas stream containing pollutants or valuable materials is chilled so strongly in heat exchangers that pollutants or valuable materials in the gas stream condense or freeze out because the temperature is below their dew points. The process uses liquid nitrogen as the chilling agent and is particularly easy to adapt to the present cooling demand, even if this demand fluctuates rapidly.

The required condensation temperature is controlled by the purity required in the exhaust gas. Temperatures below –150° C. are attained in certain cases.

Cryogenic condensation with liquid nitrogen (–196° C. at 1 bar) makes it possible to:

• Reach condensation temperatures far below those of conventional refrigeration machines.

• Adapt to the required condensation power, within wide limits, by variable addition of the chilling agent, nitrogen.

• Treat waste-gas streams heavily loaded with hydrocarbons.

• Achieve minimum residual loadings down into the ppm region.

• Combine this with other processes such as adsorption without problems.

Industrial service

Industrial service includes an extensive range of services based on long experience and tested proprietary processes. Some of the services are carried out in cooperation with service companies. The processes aim at substantially shortening maintenance or shutdown and start-up.

This custom-made cryocondensation unit has a gas capacity throughput about 100 std. m cu/hr and cooling power of about 100 kw (Fig. 5).

An example is cooling of a hydrodesulfurization reactor. These reactors must be operated under increasingly severe conditions in order to achieve very low residual sulfur levels in the treated crude. Accordingly, the service life of the catalysts drops so that exchange must occur more frequently than the turnaround cycles of the refinery. Then any time saving by enforced cooling becomes economic.

Cooling such reactors from the operating temperature to less than 200° C. usually can be executed by internal means, especially stopping to heat the process gas. But reducing it to ambient temperature is then very time consuming. Here enforced cooling by cold gaseous nitrogen is advantageous (Fig. 6).

While cooling this reactor without enforced cooling typically required 4 days, it could be carried out in only 11⁄2 days with cold nitrogen and required about 300,000 std. cu m of N2. The longer operational time gained from the time saved in cooling easily paid for this service.

Similarly, Claus plants are increasingly cooled with cold nitrogen for repair or catalyst exchange between turnarounds. Roughly 1 tonne of nitrogen is required for each daily tonne of sulfur capacity, e.g., 50 tonnes of nitrogen for a 50-tonne/day Claus plant. This allows for cooling a Claus plant within only 3-4 days to ambient temperature; without enforced cooling, usually 8-10 days are required.

In many cases, it is even possible to avoid the time-consuming disassembly of parts of units. In that way, maintenance costs are saved and production can be restarted sooner.

The increasing requirements for environmental protection are also met by the service processes because the gases used, usually nitrogen and/or helium, are nonflammable, nontoxic, and noncorrosive. They place no stress at all on the environment. Fewer materials that do contaminate the environment are produced in treatment and cleanup work. Fire and explosion hazards are also reduced, making safer start-up and shutdown of plants possible.

Major gas companies usually are able to offer the following services, sometimes in cooperation with refinery service companies:

• Tube and pipeline cleaning with-without addition of abrasives.

• Leak tests with nitrogen-helium mixtures

• Nitrogen flushing, drying, and pressure testing of equipment, tubing systems, and pipelines.

• Reactor and reformer service.

• Pigging and pipeline cleaning, including camera inspection by intelligent PIGs.

• CryoClean process for cleaning surfaces of equipment with dry ice.

CO2 for pipe cleaning

The CryoClean process is of particular interest for turnarounds. As it is finding rapidly growing interest in refineries, it will be discussed in more detail.

In any refinery process, there occurs buildup of unwanted deposits in pipes, valves, and on equipment surfaces in general. Because of the process technology, these contaminants must be removed at certain intervals. The materials used in the standard cleaning processes (blast-cleaning agents or solvents) are often questionable because of the costs of disposal of contaminated blasting agents, abrasive effects of many of them, undesired moisture content, contamination of products by residues of the blasting abrasives and environmental problems.

A pig trap and nitrogen injection point was installed for this pipeline restoration at Rongellen, Switzerland (Fig. 7)

CryoClean is a surface treatment process similar to sandblasting. In this case, however, the blasting is done by granules of dry ice (solid CO2) in rice-grain size. The dry ice pellets are metered from a supply hopper into a stream of compressed air or nitrogen, and accelerated to speeds of 180 to 330 m/sec in the blasting nozzle. The thermal and kinetic forces effect the cleaning.

Cleaning pipes in a heat exchanger in the chemical industry uses the CryoClean process (Fig. 8).

The local cooling of the deposit when contacted by dry ice sheers off the deposit from the material below and forms cracks in the deposit's surface. Dry ice particles hitting the surface at high speed partially liquefy similar to ice under the skates of an ice skater. This liquid CO2 diffuses into the cracks formed in the deposits and is vaporized there. It expands eventually and thus helps to remove the deposit and push it into the blasting gas stream, which takes it out of the equipment cleaned.

As the process leaves no residue of blasting agent, it is often possible to clean components in place. Therefore, the time for dismantling and reassembly is reduced and often even the heating phase of items can be omitted. The nonabrasive action of the process also increases the lifetimes of the components. Because the process is easily controlled, even complex units can be cleaned easily.

There have been very good results for the following substances, among others: oils, fats, waxy substances, salt deposits, anticorrosion paints, adhesives, and resins.

References

1. Aitani, A.M., and Ali, S.A., "Hydrogen management in modern refineries," Erdöl und Kohle, Vol. 48 (1995), pp. 19-24.

2. Farshid, D., et al., "Hydroprocessing solutions to Euro Diesel specifications," Petroleum Technology Quarterly, Winter 1999-2000, pp. 29 ff.

3. "European Refining, the Quality Challenge," Purvin & Gertz, The European Refining Technology Conference, Paris, Nov. 22-24, 1999.

4. 1998 Annual Report of the Mineralolwirtschaftsverband (Petroleum Economic Union), Hamburg, 1999, pp. 51 ff.

5. Shorey, S.W., Lomas, D.A., and Keesom, W.H., "Use FCC feed pretreating methods to remove sulfur," Hydrocarbon Processing, Vol. 78 (1999), No. 11, pp. 43 ff.

6. Heisel, M.P., Kummann, P., and Tsujino, T., "Cleaning up on Economics," Power Engineering International, December 1999, pp. 15 ff.

The authors

Michael Heisel ([email protected]) is a project manager with Linde Gas and Engineering based in Germany. He is responsible for gas applications in refineries, especially novel applications for process intensification. Previously he worked in Linde's engineering and contracting division. Heisel holds a PhD from the Technical University of Munich.
Bernhard Schreiner ([email protected])has been with the central application department of Linde Gas since 1999, active in gas uses in the chemical process industry. He started work at Linde AG, first in the research and development department of the engineering group specializing on gas processing plants. Schreiner obtained a PhD in chemistry at the Ludwig-Maximilian-University in Munich in 1991.
Wilhelm Bayerl ([email protected]) manages the specialty gas application department at Linde AG, which he joined in 1992. He started as lab manager, changed to specialty gas production, cylinder logistics, and finally became head of the department. Previously, he worked for BMW as a research and development engineer for engine development. Bayerl studied physical chemistry at Fachhochschule Munich, receiving his diploma in 1983.

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