10 SundayMonday, March 2930, 2015 American Fuel & Petrochemical Manufacturers | International Petrochemical Conference

Pressure swing adsorption (PSA) technology is well known for hydro-gen (H2) purification of syngaswhich is produced by steam methane reforming (SMR), partial oxidation (POX) or gasificationand refinery offgases. PSA technology can also be used for other gas separation tasks, such as separating valuable C2+ frac-tions from refinery offgases, the puri-fication of methane (CH4), capturing carbon dioxide (CO2), and generating oxygen (O2) or nitrogen (N2).

The selection of the best technol-ogy for a given gas separation re-quires a thorough understanding of

the available production technolo-gies (SMR, POX and gasification) and separation technologies (mem-brane, cryogenics, absorption and adsorption). Recent developments have made the recovery of CO2 or C2+ technically feasible and economi-cally viable with PSA technology.

The main application for PSA in a refinery is the recovery and purifica-tion of H2 from gas streams. The hy-drogen product can be obtained at high purity up to 99.9999% and high recov-ery rates up to 92%. PSA systems and vacuum (regenerated) pressure swing adsorption (VPSA) systems can be applied for the bulk removal of CO2 and the recovery and purification of CO2 for liquefaction. Refinery offgas (ROG) streams containing H2, CH4 and C2+ hydrocarbons are usually avail-able in refineries and are often used as fuel. It is possible to process these gas streams and recover C2+ by PSA or VPSA as a way to capture greater value of C2+, which can then be used as a chemical feedstock. In this case, the C2+ fraction is recovered on low pres-sure, while H2 and CH4 will stay on the high pressure side. Several units have been placed into commercial opera-tion with very attractive payback.

In a refinery, low-purity O2 can be used to enrich the combustion air in

fluid catalytic cracking (FCC) and sulphur recovery unit (SRU) opera-tions. The production of gaseous O2 at capacities of up to 10,000 Nm3/h and purity of 90%94% can be most ef-fectively achieved by VPSA process, which offers low specific energy con-sumptions and operational simplicity, including simple startup and turn-down capability.

N2 is used in a refinery for inert-ing and blanketing, and PSA systems can also be used to generate N2 for capacities up to 5,000 Nm3/h and pu-rities of 2% to 0.1% (and lower) re-sidual O2. Unlike H2 PSA, CO2 PSA and oxygen VPSA systems, the N2 PSA is based on the difference in ki-netics of adsorption of O2 on carbon molecular sieve.

PSA technology is based on a physical binding of gas molecules to a solid adsorbent material, which can be a combination of activated carbon, silica gel, carbon molecular sieve or zeolite. The attractive forces between the gas molecules and the adsorbent material depend on the gas compo-nent, type of adsorbent material, par-tial pressure of the gas component and operating temperature. Highly vola-tile compounds with low polarity such as H2 or helium (He), are essentially not adsorbed compared to molecules such as CO2, CO, N2 and hydrocar-bons. The relative attractive force of various gas molecules with a typical adsorbent material is shown in FIG. 1.

The PSA process works at con-stant temperature and uses the effect of alternating pressure and partial pressure to perform adsorption and desorption. Adsorption of impurities is carried out at high pressure to in-crease the partial pressure and, there-fore, the loading of the impurities on the adsorbent material. Desorption or regeneration takes place at low pres-sure to reduce the residual loading of the impurities, as much as possible, to achieve a high-purity product and high differential loading between ad-sorption and desorption, thereby pro-viding high recovery.

Because heating or cooling is not required, cycle time can be short. Changes in temperature are caused only by heat of adsorption and desorp-

tion and depressurization, resulting in long adsorbent material lifetime.

Most PSA systems are based on equilibrium considerations (FIG. 2). Adsorption isotherms show the rela-tion between partial pressure of a gas molecule and its equilibrium loading on the adsorbent material at a given temperature. Adsorption is carried out at high pressure, typically in the range of 1040 bar, until equilibrium load-ing is reached, at which time the ad-sorbent material must be regenerated to avoid impurity breakthrough to the product. This regeneration is done by lowering the pressure to slightly above atmospheric pressure, resulting in a corresponding decrease in equi-librium loading. Thus, the impurities on the adsorbent material are desorbed and the adsorbent material is regener-ated. The amount of impurities re-moved from a gas stream in one cycle corresponds to the difference between adsorption and desorption loading.

PSA System. A simplified schematic of an H2 PSA system is shown in FIG. 3. The main process steps are:

Adsorption. The feed gas is fed upwards through the adsorber vessels. The impuritieswater, heavy hydrocarbons, light hydrocarbons, CO and N2 are selectively adsorbed in the vessel from the bottom to the top. High-purity H2 flows to the product line. The adsorber vessels on adsorption are placed on staggered cycles, resulting in a highly flexible purification unit that is not influenced by fluctuations of the composition, temperature or pressure of the feed gas stream. The PSA system allows high performance by the maximum utilization of the H2 stored in an adsorber at the end of adsorption for pressure equalization, re-pressurization and purging of other adsorbers.

Pressure Equalization. To recover most of the H2 stored in an adsorber vessel at the end of the adsorption step, several pressure equalization steps are

PSA TechnologyMore than a hydrogen purifierTOBIAS KELLER and GOUTAM SHAHANI, Linde Engineering

FIG. 3. A simplified schematic of a PSA (H2) system.

Adsorption Pressure equalization and regeneration steps

Repressurization

Pressure equalization

Purging

Product

Feed Ogas

FIG. 4. Modern PSA plant and CO cold box associated with a steam methane reformer.

FIG. 1. Attractive forces for different gas molecules.

Hydrogen/Helium

OxygenArgon

Nitrogen

Carbon monoxideMethane

Strong

Weak

Carbon dioxide

EthaneEthylenePropaneButanePropyleneAmmoniaHydrogen sulfideMercaptansBTX

Water

FIG. 2. Amount of adsorbed impurity per amount of adsorbent.

Adsorptionloading

Desorptionloading

Dierentialloading

PAPA Partial pressureDesorption pressure Adsorption pressure

Adsorptionisotherms

0 C

30 C

50 C

200 C

See PSA TECHNOLOGY, page 11

International Petrochemical Conference | American Fuel & Petrochemical Manufacturers SundayMonday, March 2930, 2015 11

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performed. After termination of regeneration of the adsorbant material, the pressure is increased back to adsorption pressure level by such pressure equalization steps and the process starts again from the beginning.

Regeneration. Once the pressure equalization adsorption steps are completed, the adsorber material is regenerated by depressurization in the counter-current direction to

tail gas pressure (blow-down step) to remove the impurities from the adsorbent; and a purge step carried out at tail gas pressure level with pure H2 from other adsorber vessels to desorb the residual impurities from the adsorbent.

PSA Equipment. The typical scope of supply of PSA units includes prefabricated valve skid, adsorber vessels, specially selected adsorbant material, tail gas drum and process control

system. The scope of the PSA system can be altered to suit specific needs: for example, a feed gas compressor or tail gas compressor can be included as an integrated solution. Key benefits of the process control system include high-purity H2 at constant flow and pressure; steady tail gas flow and pressure; high H2 recovery by optimizing equalization and purge steps; and low sound emissions, among others.

Selecting the most appropriate PSA technology can improve the overall profitability of a refinery by reduc-ing cost and providing improved reli-ability, flexibility and environmental performance. For more details, attend Tobias Kellers (Linde Engineering) session today at 4:30 p.m.

LITERATURE CITED1 Shahani, G. H., and C. Kandziora, CO2 Emissions

from Hydrogen Plants: Understanding the Options, 2014 AFPM Annual Meeting, Orlando, Florida, 2014.

2 www.linde.com.

PSA TECHNOLOGY, continued from page 10

IPC celebrates 40 years of supporting American manufacturing

2015 marks the 40th anniversary of the AFPM International Petrochemical Conference (IPC), which has grown from a breakfast meeting for 125 to a global conference of over 3,000 attendees.

From our beginnings as the National Petro-leum Association (NPA) in 1902, the association has successfully worked on behalf of American manufacturers. NPA was founded with the goal of securing good to the independent oil refiners of Pennsylvania and Ohio. The Western Petroleum Refiners Association (WPA) was created in 1912, merging with the Texas Petroleum Refiners Asso-ciation in 1920 and with the Arkansas-Louisiana Refiners Association in 1936.

The petrochemical industrys technological an-tecedents were in the European coal chemicals in-dustry, largely developed in Germany in the late

1800s through to World War II. Improvements in this technology, married with feedstocks from oil and natural gas by US companies, created the pet-rochemical industry as we know it today.

NPA and WPA then merged in 1961 to become the National Petroleum Refiners Association (NPRA). In 1975, the Petrochemical Committee proposed to the NPRA Executive Committee that a separate conference be organized a week following the Annual Meeting. There were about 1,400 regis-trants for 1976, and the program of that first meet-ing included the Monday morning breakfast with a presentation on the newly passed Toxic Substances Control Act. The topics of discussion included feedstocks from non-oil sources, government over-regulation, financial capital access, and problems of the European petrochemical industry.

1976 was not a happy one for the US: the bi-centennial year came hard on the heels of the Viet-nam War and the civil strife that it engendered; President Richard Nixon resigned; and the first en-ergy crisis of 19731974 hit. The following years brought challenges for the global petrochemical in-dustry: crude oil prices ran up to $40/bbl or more in 19791980 and fell to $10/bbl or less in 1986; tax structures were significantly altered; the Berlin Wall came down and with it the Soviet Union and the Eastern Bloc; and technological advancements and globalization significantly affected the way petrochemical businesses were organized.

After 1976, the IPC settled into a period of steady growth in attendance, from 1,400 in 1976 to almost

See 40TH ANNIVERSARY, page 12

Articles 10.pdfArticles 11.pdf