Contact process


Contact process

The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Platinum was formerly employed as a catalyst for the reaction, but as it is susceptible to poisoning by arsenic impurities in the sulfur feedstock, vanadium(V) oxide (V2O5) is now preferred.[1]

This process was patented in 1831[citation needed] by the British vinegar merchant Peregrine Phillips.[2] In addition to being a far more economical process for producing concentrated sulfuric acid than the previous lead chamber process, the contact process also produces sulfur trioxide and oleum.

Contents

Process

The process can be divided into four stages:

  1. combining of sulphur and oxygen;
  2. adding excess of oxygen to sulphur dioxide in presence of catalyst vanadium oxide;
  3. sulphur trioxide formed is added to sulphuric acid which gives rise to oleum (disulphuric acid);
  4. the oleum then is added to water to form sulphuric acid which is very concentrated.

Purification of air and SO2 is necessary to avoid catalyst poisoning (i.e. removing catalytic activities). The gas is then washed with water and dried by sulphuric acid.

To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.

Sulphur dioxide and oxygen then react as follows:

2 SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = −197 kJ mol−1

According to the Le Chatlier's principle , a lower temperature should be used to shift the chemical equilibrium towards the right, hence increasing the percentage yield. However too low of a temperature will lower the formation rate to an uneconomical level. Hence to increase the reaction rate, high temperatures (450 °C), medium pressures (1-2 atm), and vanadium(V) oxide (V2O5) are used to ensure a 96% conversion. Platinum would be a more effective catalyst, but it is very costly and easily poisoned.[citation needed] The catalyst only serves to increase the rate of reaction as it does not change the position of the thermodynamic equilibrium. The mechanism for the action of the catalyst comprises two steps:

1. Oxidation of SO2 into SO3 by V5+:

2 SO2 + 4V5+ + 2 O2- → 2 SO3 + 4V4+

2. Oxidation of V4+ back into V5+ by oxygen (catalyst regeneration):

4 V4+ + O2 → 4 V5+ + 2 O2-

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated H2SO4 in the absorption tower to form oleum:

H2SO4(l) + SO3(g) → H2S2O7(l)

Note that directly dissolving SO3 in water is impractical due to the highly exothermic nature of the reaction. Acidic vapor or mists are formed instead of a liquid.

Oleum is reacted with water to form concentrated H2SO4.

The average percentage yield of this reaction is around 30%.

H2S2O7(l) + H2O(l) → 2 H2SO4(l)

DCDA

The next step to the Contact Process is DCDA or Double Contact Double Absorption. In this process the product gases (SO2) and (SO3) are passed through absorption towers twice to achieve further absorption and conversion of SO2 to SO3 and production of higher grade sulfuric acid.

SO2 rich gases enter the catalytic converter, usually a tower with multiple catalyst beds, and get converted to SO3, achieving the first stage of conversion. The exit gases from this stage contain both SO2 and SO3 which are passed through intermediate absorption towers where sulfuric acid is trickled down packed columns and SO3 reacts with water increasing the sulfuric acid concentration. Though SO2 too passes through the tower it is unreactive and comes out of the absorption tower.

This stream of gas containing SO2, after necessary cooling is passed through the catalytic converter bed column again achieving up to 99.8% conversion of SO2 to SO3 and the gases are again passed through the final absorption column thus resulting not only achieving high conversion efficiency for SO2 but also enabling production of higher concentration of sulfuric acid.

The industrial production of sulfuric acid involves proper control of temperatures and flow rates of the gases as both the conversion efficiency and absorption are dependent on these.

References

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • contact process — Chem. a catalytic method for producing sulfuric acid from sulfur dioxide and oxygen. [1900 05] * * * ▪ chemistry       modern industrial method of producing sulfuric acid; (sulfuric acid) it has largely replaced the chamber, or lead chamber,… …   Universalium

  • contact process — Chem. a catalytic method for producing sulfuric acid from sulfur dioxide and oxygen. [1900 05] …   Useful english dictionary

  • Contact process (mathematics) — The Contact Process (on a 1 D lattice): Active sites are indicated by grey circles and inactive sites by dotted circles. Active sites can activate inactive sites to either side of them at a rate r/2 or become inactive at rate 1. The contact… …   Wikipedia

  • Anaerobic contact process — The anaerobic contact process is a type of anaerobic digester. Here a set of reactors are created in series, often with recycling. This recycled material is pumped up into the bottom of the first reactor, an upflow reactor. The upflow anaerobic… …   Wikipedia

  • Contact print — For the photolithography technique, see contact lithography. An example of a contact print from small format film strips intended for image review. A contact print is a photographic image produced from film; sometimes from a film negative, and… …   Wikipedia

  • Contact — For information on contacting Wikipedia, see Wikipedia:Contact us. Contact may refer to: Contents 1 Social interaction 2 Mathematics and engineering 3 …   Wikipedia

  • Contact electrification — is an obsolete scientific theory from the Enlightenment that attempted to account for all the sources of electric charge known at the time.[1] In the late 18th century, scientists developed sensitive instruments for detecting electrification ,… …   Wikipedia

  • Contact Dance — Contact Dance, or Contact Improvisation ( Contact Improv ), is an emerging dance art that can be done solo (in contact with the self, the ground, an object, etc...), yet usually is improvised with others generally one partner, sometimes with many …   Wikipedia

  • Contact improvisation — “jam” Contact improvisation (CI) is a dance technique in which points of physical contact provide the starting point for exploration through movement improvisation. Contact Improvisation is a form of dance improvisation and is one of the best… …   Wikipedia

  • Contact Sign — is a variety or style of language that arises from contact between a Deaf sign language and a spoken language (or the written or manually coded form of the spoken language). Contact languages also arise between different sign languages, although… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.