2/6/13  

1  

LECTURE 6 INDUSTRIAL GASES Chapter 7 in Shreve’s Chemical Industries

CARBON DIOXIDE

USES: -  Carbonated beverages -  Refrigerating and freezing (food, ice cream) -  Fire extinguishers -  pH control of waste water -  Production of urea

CARBON DIOXIDE - MANUFACTURE q  from synthesis gas in NH3 production q  by-product in the production of substitute natural

gases q  from fermentation q  from natural wells

OBTAINING CO2 FROM FERMENTATION PROCESS

¨  Another source of CO2 is fermentation industry ¨  If yeast is used, alcohol and CO2 are produced

¨  Yield of CO2 varies with mode of fermentation

¨  Recovery and purification of CO2 (from fermentation) requires no cooling (temp nearly 40°C )

¨  So, No special cooling is necessary and CO2 content starts above 99.5%.

2/6/13  

2  

FERMENTATION CO2 PURIFICATION METHOD

¨  3 scrubbers containing stoneware spiral packing; ¨  Weak alcohol solution removes most of the alcohol carried by gas; ¨  next 2 scrubbers use deaerated water (removes water soluble impurities);

¨  Potassium di chromate oxidisex the alcohol and aldehyde in the gas and cools ¨  H2SO4 acts as dehydrating agent. ¨  Sodium carbonate removes entrained acid in gas; when acid is neutralised, CO2 is

released ¨  Oil scurbber contains glycerin; absorbs the oxidsex products and send odorless

gas to compressor ¨  H2SO4 after use is send to distillery for pH control

HYDROGEN ¨  Important gaseous raw

material for chemical and petroleum industries

¨  Sold as gas or liquid ¨  Used in making Ammonia,

methanol, etc. ¨  Envisioned as fuel for

future Renewable fuel (Green)

MANUFACTURING OF HYDROGEN

¨  Derived from carbonaceous materials (primarily hydrocarbons) and/or water

¨  Carbonaceous materials or water is decomposed by application of energy which may be electrical, chemical or thermal

¨  Other methods also exist

2/6/13  

3  

ELECTROLYTIC METHOD (WATER/BRINE)

¨  Produces high purity water (>99.7 % pure) ¨  Passing direct current through an aqueous solution of alkali and

decomposing the water i.e.

¨  Electrolysis cell electrolyzes 15% NaOH solution and uses Iron cathode and Nickel plated iron anode, has asbestos diaphragm

¨  Operates around 60 – 70 °C. ¨  Produces around 56 L of hydrogen ; 28 L Oxygen ; per Mega Joule ¨  Pure H2 is suitable for hydrogenating edible oils

HYDROGEN PRODUCTION IN MICROBIAL ELECTROLYSIS CELL

STEAM-HYDROCARBON REFORMING PROCESS

¨  Catalytically reacting a mixture of steam and hydrocarbons at elevated temperatures

¨  Forms a mixture of H2 and oxides of C ¨  Light hydrocarbons are used e.g. CH4

REFORMING REACTION

¨  First reaction is Reforming

¨  Highly endothermic è high T & low P ¨  Excess steam is used

2/6/13  

4  

SHIFT REACTION ¨  Second reaction is water-gas-shift reaction

¨  Mildly endothermic è Low T ¨  Excess steam used to force reaction to completion ¨  Catalyst is used (Fe2O3)

STEAM REFORMING

¨  Both reactions occur in Steam Reforming Furnace at Temp 760 – 980 °C.

¨  Composition of product stream depends upon process conditions, including T, P and excess steam, which determine equilibrium and the velocity through the catalyst bed (approach to equilibrium)

¨  Product contains app 75% H2, 8%CO, 15% CO2. Remainder N2 and unconverted CH4

PRODUCING ADDITIONAL HYDROGEN

¨  Water gas shift conversion ¨  Additional steam is used ¨  Temp is reduced to 315 °C – 370 °C ¨  Single stage converts 80 to 95% of residual CO to

CO2 and H2

¨  Reaction is exothermic, reaction T rises; enhances the reaction rate but adverse effect on equilibrium

SHIFT CONVERSION

¨  When high conc of CO exist in feed, shift conversion is conducted in 2 or more stages

¨  Interstage cooling to prevent excessive temp rise ¨  First stage at High T, to obtain high reaction rate ¨  Second stage at low T, to obtain good conversion

2/6/13  

5  

HYDROGEN MANUFACTURE – PARTIAL OXIDATION PROCESS

¨  Rank next to steam-hydrocarbon process in the amount of Hydrogen made

¨  Use natural gas, refinery gas or other hydrocarbon gas mixtures as feedstock

¨  Benefit– also accept liquid hydrocarbon feedstocks such as gas oil, diesel oil and heavy fuel oil

PARTIAL OXIDATION PROCESS

¨  Non catalytic partial combustion of the hydrocarbon feed with oxygen in the presence of steam

¨  Combustion chamber temp 1300 and 1500 °C ¨  When methane is used

¨  First reaction is highly exothermic and produces enough heat to sustain the other two reactions

Overall Reaction

PARTIAL OXIDATION PROCESS

¨  For efficient operation, heat recovery using Waste Heat Boilers is important

¨  Product gas composition depends upon the carbon/hydrogen ratio in feed and amount of steam added

¨  Pressure does not have a significant effect and conducted at 2 – 4 MPa.

¨  This permits the use of more compact equipment and reducing compression costs

2/6/13  

6  

COMPOSITION OF MIXTURE

¨  Process has higher carbon oxides/hydrogen ratio than steam-reformer gas

REMAINING CONVERSION

¨  Same as for Steam-hydrocarbon reforming process ¤ Water gas shift conversion ¤ CO2 removal via mono/di ethanol amine scrubbing ¤ Methanation

COAL GASIFICATION PROCESS

¨  More emphasis on Coal as feedstock for hydrogen due to diminishing oil and gas resources

¨  Will be discussed later in Coal Gasification ¨  Gases produced require the water-gas shift

conversion and subsequent purification to produce high-purity hydrogen.

COMPARISON FOR 4 MAIN PROCESS FOR H2 MANUFACTURE

2/6/13  

7  

HYDROGEN PURIFICATION

CO, CO2 & H2S removal ¨  CO Removal èWater gas shift reaction ¨  CO2 & H2S è MEA/DEA (mono/di ethanolamime).

Chemical Reactions

¨  Stripping with steam at 90-120°C ¨  Capable of reducing CO2 conc to < 0.01% by volume

DISADVANTAGE OF ETHANOLAMINES ¨  Corrosive nature of ethanolamines ¨  Corrosion most severe at elevated temps and high conc

of acid gas in solution ¨  Use of stainless steel on vulnerable areas ¨  Limiting the conc of ethanolamines in aq solution to limit

CO2 in solution, removing O2 from system and degradation products

¨  Use of corrosion inhibitor

HOT POTASSIUM CARBONATE PROCESS

¨  Similar to Amine treatment ¨  Less purity than amine treatment (CO2 conc down to

0.1% volume); though more economical for conc down to 1% or greater

¨  Hot/Boiling solution absorbs CO2 under pressure ¨  Steam consumption is reduced and Heat Exchangers

eliminated. ¨  Catacarb process mainly important à (catalyst)

2/6/13  

8  

ADSORPTION PURIFICATION ¨  Fixed bed adsorption can remove CO2, H2O, CH4, C2H6, CO, Ar and N2 impurities ¨  Thermal and Pressure Swing Adsorption ¨  Thermalà impurity adsorbed at Low T and desorbed

thermally by raising Temp ¨  Pressure Swing Adsorption (PSA) à Impurities are

adsorbed by molecular sieve under pressure and desorbed at same T but low Pressure

¨  Purge gas may be used to aid desorption ¨  For continuous operation 2 beds are normally employed.

ADVANTAGE OF PSA OVER THERMAL ADSORPTION

¨  Operate at shorter cycle ¨  Thereby reduces vessel sizes and adsorbent

requirements ¨  Capable of purifying typical hydrogen stream to

less than 1 to 2 ppm total impurities (high purity)

CRYOGENIC LIQUID PURIFICATION ¨  Highly purity >99.99% obtained when hydrogen

separated from liquid impurities (N2 and CO, CH4) ¨  Employed at -180°C; 2.1 MPa ¨  Final purification with activated Carbon, silica gel or

molecular sieves OXYGEN

2/6/13  

9  

Manufacturing ¨  Air separation methods: a)  Cryogenic process b)  Pressure swing adsorption process c)  Electrolysis of water d)  By chemical reaction in which oxygen is freed

from a chemical compound

PROCESS FLOW SHEET FOR OXYGEN & NITROGEN PRODUCTION

¨  The air is compressed to about 94 psi (650 kPa or 6.5 atm) in a multi-stage compressor. It then passes through a water-cooled cooler to condense any water vapor, and the condensed water is removed in a water separator.

¨  The air passes through a molecular sieve adsorber. The adsorber contains zeolite and silica gel-type adsorbents, which trap the carbon dioxide, heavier hydrocarbons, and any remaining traces of water vapor. Periodically the adsorber is cleaned to remove the trapped impurities. This usually requires two adsorbers operating in parallel, so that one can continue to process the air-flow while the other one is flushed

¨  The pretreated air stream is split. A small portion of the air is diverted through a compressor, where its pressure is boosted. It is then cooled and allowed to expand to nearly atmospheric pressure. This expansion rapidly cools the air, which is injected into the cryogenic section to provide the required cold temperatures for operation.

¨  The main stream of air passes through one side of a pair of plate fin heat exchangers operating in series, while very cold oxygen and nitrogen from the cryogenic section pass through the other side. The incoming air stream is cooled, while the oxygen and nitrogen are warmed. In some operations, the air may be cooled by passing it through an expansion valve instead of the second heat exchanger. In either case, the temperature of the air is lowered to the point where the oxygen, which has the highest boiling point, starts to liquefy.

¨  The air stream—now part liquid and part gas—enters the base of the high-pressure fractionating column. As the air works its way up the column, it loses additional heat. The oxygen continues to liquefy, forming an oxygen-rich mixture in the bottom of the column, while most of the nitrogen and argon flow to the top as a vapor.

¨  The liquid oxygen mixture, called crude liquid oxygen, is drawn out of the bottom of the lower fractionating column and is cooled further in the subcooler. Part of this stream is allowed to expand to nearly atmospheric pressure and is fed into the low-pressure fractionating column. As the crude liquid oxygen works its way down the column, most of the remaining nitrogen and argon separate, leaving 99.5% pure oxygen at the bottom of the column.

¨  Meanwhile, the nitrogen/argon vapor from the top of the high-pressure column is cooled further in the subcooler. The mixed vapor is allowed to expand to nearly atmospheric pressure and is fed into the top of the low-pressure fractionating column. The nitrogen, which has the lowest boiling point, turns to gas first and flows out the top of the column as 99.995% pure nitrogen.

¨  The argon, which has a boiling point between the oxygen and the nitrogen, remains a vapor and begins to sink as the nitrogen boils off. As the argon vapor reaches a point about two-thirds the way down the column, the argon concentration reaches its maximum of about 7-12% and is drawn off into a third fractionating column, where it is further recirculated and refined. The final product is a stream of crude argon containing 93-96% argon, 2-5% oxygen, and the balance nitrogen with traces of other gases.

2/6/13  

10  

The air is compressed to about 94 psi (650 kPa or 6.5 atm) in a multi-stage compressor. It then passes through a water-cooled cooler to condense any water vapor, and the condensed water is removed in a water separator. The air passes through a molecular sieve adsorber. The adsorber contains zeolite and silica gel-type adsorbents, which trap the carbon dioxide, heavier hydrocarbons, and any remaining traces of water vapor. Periodically the adsorber is cleaned to remove the trapped impurities. This usually requires two adsorbers operating in parallel, so that one can continue to process the air-flow while the other one is flushed The pretreated air stream is split. A small portion of the air is diverted through a compressor, where its pressure is boosted. It is then cooled and allowed to expand to nearly atmospheric pressure. This expansion rapidly cools the air, which is injected into the cryogenic section to provide the required cold temperatures for operation. The main stream of air passes through one side of a pair of plate fin heat exchangers operating in series, while very cold oxygen and nitrogen from the cryogenic section pass through the other side. The incoming air stream is cooled, while the oxygen and nitrogen are warmed. In some operations, the air may be cooled by passing it through an expansion valve instead of the second heat exchanger. In either case, the temperature of the air is lowered to the point where the oxygen, which has the highest boiling point, starts to liquefy.

The air stream—now part liquid and part gas—enters the base of the high-pressure fractionating column. As the air works its way up the column, it loses additional heat. The oxygen continues to liquefy, forming an oxygen-rich mixture in the bottom of the column, while most of the nitrogen and argon flow to the top as a vapor. The liquid oxygen mixture, called crude liquid oxygen, is drawn out of the bottom of the lower fractionating column and is cooled further in the subcooler. Part of this stream is allowed to expand to nearly atmospheric pressure and is fed into the low-pressure fractionating column. As the crude liquid oxygen works its way down the column, most of the remaining nitrogen and argon separate, leaving 99.5% pure oxygen at the bottom of the column. Meanwhile, the nitrogen/argon vapor from the top of the high-pressure column is cooled further in the subcooler. The mixed vapor is allowed to expand to nearly atmospheric pressure and is fed into the top of the low-pressure fractionating column. The nitrogen, which has the lowest boiling point, turns to gas first and flows out the top of the column as 99.995% pure nitrogen. The argon, which has a boiling point between the oxygen and the nitrogen, remains a vapor and begins to sink as the nitrogen boils off. As the argon vapor reaches a point about two-thirds the way down the column, the argon concentration reaches its maximum of about 7-12% and is drawn off into a third fractionating column, where it is further recirculated and refined. The final product is a stream of crude argon containing 93-96% argon, 2-5% oxygen, and the balance nitrogen with traces of other gases.

HIGHER OXYGEN PURITY If higher purity is needed, one or more additional fractionating columns may be added in conjunction with the low-pressure column to further refine the oxygen product. In some cases, the oxygen may also be passed over a catalyst to oxidize any hydrocarbons. This process produces carbon dioxide and water vapor, which are then captured and removed. If the oxygen is to be liquefied, this process is usually done within the low-pressure fractionating column of the air separation plant. Nitrogen from the top of the low-pressure column is compressed, cooled, and expanded to liquefy the nitrogen. This liquid nitrogen stream is then fed back into the low-pressure column to provide the additional cooling required to liquefy the oxygen as it sinks to the bottom of the column.

2/6/13  

11  

USES n It is one of the life-sustaining elements on Earth and is needed by all animals.

n Oxygen and acetylene are combusted together to provide the very high temperatures needed for welding and metal cutting

n When oxygen is cooled below -297° F (-183° C), it becomes a pale blue liquid that is used as a rocket fuel.

n  It is used in blast furnaces to make steel, and is an important component in the production of many synthetic chemicals, including ammonia, alcohols, and various plastics.