CATACARBCATACARBCHEMISTRYCHEMISTRY

ByByNOUMAN KHALEEQ BUTTNOUMAN KHALEEQ BUTT

A PEP SESSION ONA PEP SESSION ON

PRODUCTION OF AMMONIA:

• Feed Gas Purification

• Synthesis Gas Production

• Synthesis Gas Purification

• Synthesis of Ammonia

REFORMING REACTIONS:

CH4 + H2O CO + 3H2

CH4 + 2H2O CO2 + 4H2O

SHIFT CONVERSION:

CO + HCO + H22OO COCO22 + H + H22

SYNTHESIS GAS PURIFICATIONSYNTHESIS GAS PURIFICATION

• REMOVAL OF CO2

Synthesis gas contains appreciable amount ofCO2 (15-20%) in it. It is removed at this stageand sent to Urea Production Facilities.

• HOW CO2 CAN BE REMOVED?

AlkanolaminesPotassium CarbonateAmmonia SolutionsOrdinary Water

MP-2503

Semi-Lean Solution

Lean SolutionMP-2504

CO2 ABSORBERC-2519

Feed Gas

Scrubbed Gas

EM-2511

Rich Solution

CATACARBREGENERATOR

C-2515

40 psigsteam

wet CO2

E-2508198 oF, 416 psig

190 oF, 412 psig

250 oF

260 oF

270 oF

240 oF

187 oF

230 oF, 15 psig

SCHEMATIC VIEW OF CATACARBABSORPTION SYSTEM

THE CATACARBTHE CATACARBIt is a solution of Potassium Carbonate K2CO3 in water.K2CO3 acts as main reagent for removal of CO2 from thesystem.

Many other chemicals are also added in to the CATACARBsystem for enhancing the absorption process and maintainproper conditions in the system.

Catacarb works in TWO ways:• Physical Absorption• Chemical Reaction

K2CO3

DEABORICACID

V2O5ANTI-FOAM,DEFOAMER etc

AS SOLUTION INWATER

ROLE OF K2CO3:

To facilitate the presence of additional hydroxyl ion fordirect reaction with CO2. The rate of absorption of CO2 inwater is accelerated.

The faster the rate of absorption, greater will be theabsorption required, the less amount will be needed

K2CO3 itself does not reacts with CO2 directly.

Absorption reaction is characterized in two reactions:

CO2 + H2O HCO3- + H+

CO2 + OH- HCO3-

Concentration of hydroxyl ion is proportional to theconcentration of the carbonate ion. So K2CO3 facilitatesthe hydrolysis of water and rate of CO2 absorption isincreased.These reactions are favored by high pressure and lowtemperatures.

DEA:

It promotes CO2 absorption byforming intermediate salts of aminecarbamate.

CO2 + R2NH R2NCOO- + H+ R2NH + HCO3-

R2NH + HCO3-

H2OAmine Carbamate

DEA degrades with temperature to produceorganic salts of complicated nature,degradation is excessive at 275 F.

BORIC ACID (H3BO3):

It acts a buffer (i.e. a chemical which minimizeschange in pH of a solution when an acid or baseis added to a system.) preventing fluctuation ofpH during CO2 absorption.

It also acts as a promoter for the absorptionreaction. Reaction of carbon dioxide is furthercatalyzed in the presence of borates.

VANADIUM:It is not involved in absorption process. It is

added as a corrosion inhibitor compound toprotect the internals of the catacarb absorber andregenerator. (EXPLAINED LATER)

Vanadium exists in five differentforms

VANADIUM

V2O5acidic

V2O3

V2O4amphoteric

V2Obasic

V2O2basic basic

Vanadium pentaoxide V+5 is the mostimportant from the corrosion inhibitionpoint of view.

V2O5 is anhydride of vanadic acid whichexists in different forms.

Vanadicadic

Meta

HVO3

PYRO

H4V2O7

Ortho

H3VO4

Per

HVO4

pH 9-12

pH > 12

pH 7-10

pH 2-2.7

Prevention of Foaming:

Foaming is caused by the presence of suspended particlesand organic compounds. Little foaming is good for gasliquid contact but excessive foaming decreases absorberefficiency.

To prevent foaming, antifoam (a chemical added beforefoaming to prevent foam formation) and Defoamer (achemical added after foaming to destroy the foam) areadded to the system.

WBU is an organic antifoam. However, it acts as a gooddefoamer as well.

Formation of Potassium Formate:

Potassium Formate (KCOOK) is an undesirable by-product.

K+ + CO + OH- HCOOK

This reaction is very sensitive to temperature. Rate offormation is high above 300oF

Its presence reduces the effective potassium carbonatesolution strength.

DEA DEGRADATION:During CO2 absorption, DEA absorbs other compunds whichdegrade the amine into complex organic compounds.

This degradation:– Wastes valuable DEA– Fouls equipment– Produces foaming.– Increases strongly with increasing temperature.– increases with pressure up to about 600 psig then

remains constant.

Catacarb Regeneration:Regeneration of catacarb is achieved by Stripping (amass transfer operation where a liquid is contactedwith a gas for the purpose of selectively removing acomponent of the liquid by transferring it to the gas).

For this purpose 40 psig steam is injected into the systemwhich has following benefits:– Reduction of CO2 partial pressure by reducing the mole

fraction of CO2.– Better tray efficiency– Some heat is added to the stripper to regenerate the solution.

CORROSION PHENOMENA:

Catacarb solutions are very corrosive to carbonsteel; corrosion is primarily due to local formationand ionization of carbonic acid.

Corrosion products are mixture of insoluble ironcarbonate and iron oxides. Corrosivity of catacarbsolutions increases with the concentration ofcarbon dioxide.

Fundamental chemical reactions takingplace during corrosion are:

Fe + 2H2CO3 Fe(HCO3) 2 + 2H+

Fe(HCO3)2 FeCO3 + CO2 + H2OFe(HCO3)2 FeO + 2CO2 + H2O

The V5+ is very effective inhibitor. Theaction of inhibitor is to passivate thecarbon steel by means of red-ox couplingreaction with the corroding metal.

This passive film protects the metal bypreventing any further corrosion sincecorrosion rate in passivation zone ispractically zero.

PASSIVATION:“To treat of coat a metal in order to reduce the

chemical reactivity of its surface.”

Different metals have different tendency to forma protective layer (usually by oxidation) of themetal present on the surface.

• FeO is unstable and it reacts with CO2 in the solutionto form respective carbonates as secondary productsof corrosion.

• Fe3O4 has the ability to form the passive layer andprotect carbon steel from further corrosion.

• Fe2O3 becomes transpassive and hence can not stopcorrosion.

During corrosion, iron exists in followingdifferent forms.

O2O2O2

Fe FeO Fe3O4 Fe2O3

ROLE OF V2O5:

By means of Reduction-Oxidation (Red-Ox) reactions,Vanadium Penta oxide, oxidizes FeO to Fe3O4 and duringthis reaction it is reduced to V2O4. Hence a passive layerof Fe3O4 is formed and any further corrosion is inhibted.

FeO Fe3O4

O

+ OV2O5 V2O4

(OXIDATION)

(REDUCTION)

ROLE OF AIR INJECTION:

•The predominant role of air injection in thecatacarb process is reactivation of thecorrosion inhibitor.

•It oxidizes the inactive V4+ to active V5+

form.

•The effect of air injection in the passivationof carbon steel is negligible.

KEY PARAMETERS UNDER OBSERVATION DURINGOPERATION OF CATACARB SYSTEM

EFFECT OF VARIABLES ON CO2 ABSORPTION:

DecreasesIncrease Feed Gas Rate

IncreasesIncrease Solution Strength

DecreasesIncrease Solution content

DecreasesIncrease Temperature

IncreasesIncrease pressure Drop

IncreasesIncrease Circulation Rate