Revision 9670444 of "Pressure swing adsorption" on cawiki

{{ Petició de traducció a|EN|Pressure swing adsorption|Pàgina ER| [[Usuari Discussió:Joan manel]] }}
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[[Image:Adsorbimento - layout rigenerazione.jpg .3. thumb .3. 400px .3. Schematic drawing of Psa process]] ..
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((((( Adsorció per oscil·lació de pressió ))))) ((((((( PSA ))))))) 

L'adsorció per oscil·lació de pressió és una tècnica de separació de components d'una barreja de gasos. Es designa sovint amb l'acrònim PSA, que deriva de les inicials angleses d'aquesta tècnica: "[[:en:Pressure Swing Adsorption|Pressure Swing Adsorption]]".

L'adsorció és el fenomen pel qual la superfície d'una fase sòlida o líquida reté àtoms, ions o molècules i els manté en concentració més elevada que a l'interior d'aquest sòlid o líquid. És, doncs, un fenomen superficial i en això es distingeix de l'absorció, ja que aquesta té lloc quan es dóna una penetració més o menys uniforme en la massa d'absorbent.

La quantitat de matèria adsorbida augmenta amb la pressió. A més és reversible, de manera que, quan es redueix la pressió, les substàncies adsorbides a major pressió tendeixen a desorbir-se. El terme oscil·lació de pressió es refereix a això: el sistema es porta successivament des d'un nivell de pressió alt per promoure l’adsorció fins a un nivell de pressió baix que afavoreix la desorció, repetint aquest cicle indefinidament.

Aquest fenomen es pot aplicar a la separació de components d'una mescla gasosa perquè no tots els gasos tenen la mateixa tendència a ser adsorbits en un determinat adsorbent. Triant convenientment l'adsorbent es pot aconseguir que el procés sigui més selectiu a uns gasos que a altres. Els materials preferits com a adsorbents solen ser materials molt porosos per la gran superfície que presenten: carbó actiu, gel de sílice, alúmina, [[zeolita]], etc. A més les zeolites tenen la capacitat d'actuar com a tamisos moleculars, és a dir, d'impedir el pas al seu interior de molècules més grans que la mida de les cavitats internes, propietat que les fa encara més apropiades per a aquest tipus de separacions. Sovint s'utilitzen diversos adsorbents diferents apilats en un mateix recipient.

S'ha fet esment que l'adsorció és funció de la pressió; també depèn de la temperatura, encara que en sentit contrari: la quantitat de matèria adsorbida disminueix amb la temperatura. Segons això i aplicant el principi de Le Chatelier es dedueix que durant l'adsorció es desprèn calor, és a dir, que és exotèrmica. També amb la temperatura s'observa una reversibilitat semblant a la que es dóna amb la pressió: en augmentar la temperatura es desorbeixen les substàncies adsorbides a temperatures baixes. Per analogia amb la PSA es parla també de TSA, "Temperature Swing Adsorption", és a dir, adsorció per oscil·lació de temperatura.
En separacions especialment difícils es poden simultaniejar ambdós processos en el mateix recipient i combinar així PSA i TSA, és a dir, variar la pressió i al mateix temps també la temperatura.






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(==) Process (==) ..
Pressure swing adsorption processes rely on the fact that under high pressure , gases tend to be attracted to solid surfaces , or "adsorbed". The higher the pressure , the more gas is adsorbed ; when the pressure is reduced , the gas is released , or desorbed. Psa processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly. If a gas mixture such as AIR , for example , is passed under pressure through a vessel containing an adsorbent bed of ZEOLITE that attracts NITROGEN more strongly than it does OXYGEN , part or all of the NITROGEN will stay in the bed , and the gas coming out of the vessel will be enriched in oxygen. When the bed reaches the end of its capacity to adsorb nitrogen , it can be regenerated by reducing the pressure , thereby releasing the adsorbed nitrogen. It is then ready for another cycle of producing oxygen enriched air. ..
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This is exactly the process used in portable oxygen concentrators used by EMPHYSEMA patients and others who require oxygen enriched air to breathe. ..
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Using two adsorbent vessels allows near-continuous production of the target gas.  It also permits so-called pressure equalisation , where the gas leaving the vessel being depressured is used to partially pressurise the second vessel.  This results in significant energy savings , and is common industrial practice. ..
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(==) Adsorbents (==) ..
Aside from their ability to discriminate between different gases , adsorbents for Psa systems are usually very porous materials chosen because of their large surface areas. Typical adsorbents are ACTIVATED CARBON , SILICA GEL , ALUMINA and ZEOLITE. Though the gas adsorbed on these surfaces may consist of a layer only one or at most a few molecules thick , surface areas of several hundred square meters per gram enable the adsorption of a significant portion of the adsorbent's weight in gas. In addition to their selectivity for different gases , zeolites and some types of activated carbon called carbon MOLECULAR SIEVES may utilize their molecular sieve characteristics to exclude some gas molecules from their structure based on the size of the molecules , thereby restricting the ability of the larger molecules to be adsorbed. ..
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(==) Applications (==) ..
One of the primary applications of Psa is in the removal of CARBON DIOXIDE (Co<sub>2</sub>) as the final step in the large-scale commercial synthesis of HYDROGEN (H<sub>2</sub>) for use in OIL REFINERIES and in the PRODUCTION OF AMMONIA (Nh<sub>3</sub>). Refineries often use Psa technology in the removal of HYDROGEN SULFIDE (H<sub>2</sub>S) from hydrogen feed and recycle streams of HYDROTREATING and HYDROCRACKING units. Another application of Psa is the separation of carbon dioxide from BIOGAS to increase the METHANE (Ch<sub>4</sub>) content. Through Psa the biogas can be upgraded to a quality similar to NATURAL GAS. ..
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NITROGEN GENERATOR units employ the Psa technique to produce high purity nitrogen gas (99.5 %  or greater) from a supply of compressed air. ..
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Research is currently underway for Psa to capture Co<sub>2</sub> in large quantities from COAL-FIRED POWER PLANTS prior to GEOSEQUESTRATION , in order to reduce GREENHOUSE GAS production from these plants. <ref>. [http://www.co2crc.com.au/Research/research_c_2_4.html http://www.co2crc.com.au] </ref>.  ..
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Psa is an economic choice for small-scale production of reasonable purity OXYGEN or NITROGEN from AIR. Psa technology has a major use in the medical industry to produce oxygen , particularly in remote or inaccessible parts of the world where bulk cryogenic or compressed cylinder storage is not possible.  ..
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Psa is also used in HYPOXIC AIR FIRE PREVENTION SYSTEMS to produce air with a low oxygen content. ..
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Psa has also been discussed as a future alternative to the non-regenerable sorbent technology used in SPACE SUIT PRIMARY LIFE SUPPORT SYSTEMS , in order to save weight and extend the operating time of the suit. <ref>.  ..
{{ cite web ..
   .3.  last = Alptekin  ..
   .3.  first = Gokhan ..
   .3.  title = An Advanced Rapid Cycling Co2 and H2O Control System for Plss ..
   .3.  publisher = Nasa ..
   .3.  date = 08-01-05 ..
   .3.  url = http://sbir.gsfc.nasa.gov/Sbir/abstracts/04/sbir/phase2/Sbir-04-2-X3.01-8100.html ..
   .3.  accessdate = 2007-02-24 ..
}} </ref>.  ..
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(==) Variations of Psa technology (==) ..
Double Stage Psa (Ds-Psa , sometimes referred to as Dual Step Psa). ..
With this variation of Psa developed for use in Laboratory Nitrogen Generators  <ref>. [http://www.dot-red.com/gas-generators/nitrogen-n2-gas-generators/high-capacity-high-purity-n2-generators-sirocco/ http://www.dot-red.com] </ref>.  generation of nitrogen gas is divided into two steps: in the first step , the compressed air is forced to pass through a carbon molecular sieve to produce nitrogen at a purity of approximately 98 %  ; in the second step this nitrogen is forced to pass into a second carbon molecular sieve and the nitrogen gas reaches a final purity up to 99.999 % . The purge Gas from the second step is recycled and partially used as feed gas in the first step. ..
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In addition , the purge process is supported by active evacuation for better performance in the next cycle. The goals of both of these changes is to improve efficiency over a conventional Psa process. ..
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(==) See also (==) ..
*  ADSORPTION ..
*  GAS SEPARATION ..
*  HYDROGEN PINCH ..
*  HYDROGEN PURIFIER ..
*  OXYGEN CONCENTRATOR ..
*  AIR DRYER ..
*  VACUUM SWING ADSORPTION ..
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(==) References (==) ..
 <references/>.  ..
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[[Category:Chemical engineering]] ..
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{{ Chem-stub}} ..
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paraulesenllacos ..
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ADSORBENT ..
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DISTILLATION#INDUSTRIAL_DISTILLATION .3. CRYOGENIC DISTILLATION TECHNIQUES OF GAS SEPARATION ..
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ZEOLITES ..
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MOLECULAR SIEVE ..
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AIR ..
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ZEOLITE ..
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NITROGEN ..
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OXYGEN ..
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NITROGEN ..
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EMPHYSEMA ..
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ACTIVATED CARBON ..
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SILICA GEL ..
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ALUMINA ..
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ZEOLITE ..
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MOLECULAR SIEVES ..
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CARBON DIOXIDE ..
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HYDROGEN ..
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OIL REFINERIES ..
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PRODUCTION OF AMMONIA ..
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HYDROGEN SULFIDE ..
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HYDROTREATING ..
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HYDROCRACKING ..
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BIOGAS ..
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METHANE ..
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NATURAL GAS ..
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NITROGEN GENERATOR ..
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COAL-FIRED POWER PLANTS ..
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GEOSEQUESTRATION ..
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GREENHOUSE GAS ..
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OXYGEN ..
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NITROGEN ..
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AIR ..
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HYPOXIC AIR FIRE PREVENTION SYSTEMS ..
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SPACE SUIT ..
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PRIMARY LIFE SUPPORT SYSTEMS ..
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ADSORPTION ..
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GAS SEPARATION ..
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HYDROGEN PINCH ..
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HYDROGEN PURIFIER ..
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OXYGEN CONCENTRATOR ..
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AIR DRYER ..
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VACUUM SWING ADSORPTION ..

[[de:Druckwechsel-Adsorption]]
[[en:Pressure swing adsorption]]
[[nl:Pressure swing adsorptie]]
[[pl:Adsorpcja zmiennociśnieniowa]]