basic purification

INTRODUCTION TO PURIFICATION

• FEEDSTREAM OVERVIEW

• PURIFICATION REQUIREMENTS OVERVIEW

• IMPURITIES AND THEIR EFFECTS

• IMPURITIES REMOVAL MECHANISM

FEEDSTREAM OVERVIEW

• ETHYLENE is the main raw material for producing polyethylene– ETHYLENE = C2H4

– B.P.= -185.3 OC

FEEDSTREAM OVERVIEW

• COMONOMER• HEXENE OR BUTENE - is used as a

comonomer for branching the polymer chain which directly affect the resin density– HEXENE = C6H12

– B.P. = 63.5 oC

– BUTENE = C4H8

– B.P. = -6.3 OC

FEEDSTREAM OVERVIEW

• HYDROGEN - is generally used as a chain terminating agent in the polymerization reaction to control the length of the polymer molecules or the Melt Index of the M catalyzed resin– HYDROGEN = H2

– B.P. = -252.7 O C

FEEDSTREAM OVERVIEW

• HEXANE - is used as an Induced Condensing Agent.– HEXANE = C6H14

– B.P. = 67.0 O C

FEEDSTREAM OVERVIEW

• NITROGEN - is used to carry into the reactor and as purge gas to purge the product for un-reacted hydrocarbon. It is also used to purge equipment during operation and for preparation of equipment for maintenance.– NITROGEN = N2

FEEDSTREAM OVERVIEW

• TEAL OR T2 - used as co-catalyst in the reaction process when using UCAT A or “M” catalyst.– TEAL = (C2H5)3Al

– B.P. = 186 OC

FEEDSTREAM OVERVIEW

• PROPYLENE - main raw material in the production of the polypropylene.– PROPYLENE - C3H6

– B.P. = -169.2 OC

PURIFICATION REQUIREMENTS OVERVIEW

• All reactor feed streams must be treated to remove impurities, which have an adverse effect on the catalyst. The purification facilities for each feed stream may consist of one or more operations each of which are specific to the impurities which are being removed.


• Here at JGSPC, an Ethylene Purification unit is consist of facilities for the removal of Sulfur (S2), Carbon Monoxide(CO), Oxygen(O2) and Water(H2O).


• The comonomer Purification System is consist of a Degassing Column to remove Carbon Monoxide (CO); Carbon Dioxide(CO2), Oxygen(O2) and molecular sieve Dryer to remove trace amount of Water or Moisture(H2O).


• The Hydrogen feed stream goes to a molecular sieve dryer to remove trace amounts of water(H2O).


• Nitrogen Purification unit consists of a Deoxo Vessel to remove trace amounts of Oxygen(O2) and then through a molecular sieve Dryer to remove water(H2O).


• Hexane Purification system is consist of a Degassing Column to remove Carbon Monoxide (CO), Carbon Dioxide(CO2), Oxygen(O2) and molecular sieve Dryer to remove trace amount of Water or Moisture(H2O).


• Propylene Purification unit is consist of a Degassing Column to remove Carbon Monoxide (CO), Carbon Dioxide(CO2), Oxygen(O2); and other equipment for the removal of Sulfur(S2), Methylacetylene/ Acetylene/Propadiene (MAP) and molecular sieve Dryer to remove trace amount of Water (H2O).

EFFECTS OF CONTAMINANTS

• DEACTIVATE REACTION CATALYST– React with active catalyst sites– Block polymerization sites– Cause incorrect catalyst mixing proportion

• ALTER RESIN PROPERTIES– Hinder comonomer incorporation– Change polymer chain length and branching


• ALTER REACTION OPERATING CONDITIONS– Take blow-off to remove impurities– Reduce temperature to maintain product

properties– Reduce rates

• RAISE REACTANTS QUANTITIES REQUIRED


– More components require more feed to the reactor

• INCREASE RESIDUALS IN RESIN– Ash content will increase

• CAUSE REACTOR CONTINUITY PROBLEMS– Will cause sheeting or chunking


• CHANGE SELECTIVITY AND ACTIVITY OF PURIFICATION CATALYST– Normal components or impurity for which the

purification stage was designed may not be removed due to presence of other impurities.

– Unwanted side reactions may cause catalyst to become inactive


• CAUSE DECOMPOSITION OF RAW MATERIALS– Impurities may react with materials to form

waste product and further contamination– Render materials useless

• CAUSE LOSSES OF RAW MATERIALS– May need to run excessive column vents or

tails to remove unwanted contaminants


• CAUSE SAFETY PROBLEMS– Impurities may react with purification catalyst

or raw materials and form hazardous by-products (acetylides, polymers, oil)

IMPURITY EFFECTS ON UCAT- A CATALYST REACTION

• CO is an effective kill gas for the catalyzed reaction

• CO2 significantly reduces U-A catalyst productivity

• O2, H2O, Acetylene (below 20 ppm) do not poison U-A catalyst

• MWD of U-A polymerized resin is not modified by impurities

• Slight, but unimportant decrease of MI was observed with water and oxygen

IMPURITY EFFECTS ON UCAT- A CATALYST REACTION

• In general, U-A catalyst is less affected by feed stream impurities due to presence of TEAL in the reactor

• Some impurities in very small quantities are pro-static agents and may effect the stability of the process by inducing more static charge to resin particle, than are being discharge

IMPURITIES REMOVAL MECHANISM

• CHEMICAL ADSORPTION– Impurity is adsorbed into a porous material bed

(catalyst) and becomes a third component that stays in the porous material is covered with this third compound, regeneration of the bed is necessary. Example: Deoxo Beds

– Regeneration - the process of bringing up spent catalyst to its active state. (Using heated Nitrogen).


• HYDROGENERATION– Another phase of chemical adsorption, this process

consist of the adsorption of the impurity in the porous bed, reaction with another component (H2) and de-sorption of the product caused by this reaction. Example: MAP Removal Bed where acetylene is adsorbed in the palladium catalyst and while it is adsorbed, it reacts with the hydrogen injected at the inlet of the bed. The product of this reaction,ethylene or ethane goes in the stream


• PHYSICAL ADSORPTION– Removes impurities based on size or polarity.

Example: Molecular sieves where water stays adsorbed into the sieve and is de-sorbed during the sieve regeneration. The sieve material does not react nor catalyze any reaction so regeneration is a physical process

FACTORS AFFECTING ADSORPTION

• AREA EXPOSED– Porosity

• VELOCITY– Residence time

• CONCENTRATION– Higher - Harder

• LENGTH OF REACTION ZONE(LRZ)– Longer - Better

PROCESS

• FOR GAS STREAM (ETHYLENE)– Heat up– Sulfur removal - G72D Zinc Oxide adsorbent– CO removal - G66B type 2130 zinc oxide

hydrogenation catalyst

– O2 removal - UCC1101 copper chromite on silicon deoxide

– Dryer - UOP 13 XPG molecular sieve, sodium alumino silicate

PROCESS

• FOR LIQUID STREAM (PROPYLENE)– Degassing - distillation– Cooling - prevent temperature runaway– Sulfur removal - ALCOA Selexorb COS– MAP removal - G68F– Dryer - UOP 13 XPG– Filter - particulate removal

POINTS OF CONCERN• BED TEMPERATURE ALARMS

• BED TEMPERATURE ALARMS AND MONITORING

• VESSEL PURGING AND CATALYST DEACTIVATION

• VENTS

• INSULATION OF PROCESS EQUIPMENTS AND LINES

• REGENERATION NITROGEN PIPING

CHEMICAL REACTION

• SULFUR REMOVAL– H2S + ZnO ZnS + H20

– COS + ZnO ZnS + CO2

• CO REMOVAL– CO + CuO CO2 + Cu

– CO + Cu2O CO2 + Cu

CHEMICAL REACTION

• O2 REMOVAL

– 2 Cu + 0.5 O2 Cu2O

– 2 Cu + O2 CuO

• C2 OXIDATION

– 6 Cu2O + C2H4 2 CO2 + H2O + 12 Cu

• COPPER ACETYLIDE REMOVAL– CuC=CCU + H2 Cu + Hydrocarbon

PROPYLENE

• FEED SUPPLY

• PRESSURE 340 psig/2344kpag min. 350psig/2413kpag normal 370 psig/2551kpag max.

• TEMPERATURE -10 oC min. 30 oC normal 40 oC max.

PROPYLENE

• DESIGN FLOWRATES– 45,300 lb/hr / 20,548 kg/hr Normal– 51,000 lb/hr / 23,170 kg/hr Maximum

• NORMAL OPERATING PRESSURE– 305 psig / 2,100 kpag – Column pressure controlled with a flow

controller on the steam to the reboiler

PROPYLENE• FEEDSTREAM QUALITY

– IMPURITY MAX.ppmV FINAL PRODUCT

– CO2 5 <0.1

– CO 5 NIL

– O2 5 NIL

– H2O 5 <0.1

– S2 3ppmW 0.1

– Oxygenated HC including ALCOHOL 10 0.1

– MAP 10 <0.5

PROPYLENE

• DESIGN BASIS– Distillation (Stripping) step for removal of CO2,

CO, and O2

• DESIGN OPERATION AND SAFETY CONSIDERATION– Turndown from 100% of the flow required for the

product with the maximum demand to 50 % of the flow required for the product with the minimum demand

PROPYLENE

• ADSORBENT(SULFUR REMOVAL)– ALCOA SELEXSORB COS (Promoted

Activated Alumina)

• SAFETY CONSIDERATIONS– Regeneration with hot nitrogen– No preload– 3-5 year bed life to replacement– 18 hour regeneration time

PROPYLENE

• ADSORBENT(MAP REMOVAL)– UNITED CATALYST G-68F (Palladium)

• SELECTIVITY– METHYLACETYLENE, ETHYLENE,

PROPADIENE, PROPYLENE (EXCESS O2 WILL REACT WITH UNSATURATED HYDROCARBONS)

PROPYLENE

• SAFETY CONSIDERATIONS– Non-regenerable

– Sulfur and Arsine irreversible poisons to G-68F Normal Sulfur and Arsine concentrations must be kept less than 0.1 ppmw

– High temperature shutdown normally set at 50 oC

– 3-5 year bed replacement

– H2 feed to bed stops automatically on low propylene flow

PROPYLENE

• ADSORBENT(DRYER)– UNION CARBIDE 13 XPG Molecular sieves

• SAFETY CONSIDERATIONS– Regeneration with hot Nitrogen– Controlled preload with Propylene to prevent

temperature excursions– 3-5 year bed life to replacement– 7.5 hour regeneration time

ETHYLENE

• PRESSURE– Low 2827 kpag– High 3477 kpag– High-High 3571 kpag

• DESIGN FLOW RATES– Minimum 6,000 lb/hr ( 2,722 kg/hr)– Normal 11,572 lb/hr (25,458 kg/hr)– Maximum 14,464 lb/hr (31,820 kg/hr)

ETHYLENE

• TEMPERATURE– Minimum -24 oC– Normal 30 oC– Maximum 150 oC

ETHYLENE

• EMERGENCY SHUTDOWN SYSTEM (ESD)– In the event of major emergencies such as fire

or gas release– Temperature in the purification beds is high

(125oC), two thermocouples sustained for 2 minutes

ETHYLENE

• FEEDSTREAM PURIFICATION• IMPURITY MAX. FINAL PROD.

– CO 0.2 <0.1

– O2 0.5 <0.1

– H2O 5.0 <0.1

– CH3OH 5.0 <0.1

– S2 AS H2S 0.2 <0.1

– CARBONYLS 0.1 <0.1 AS MEK

ETHYLENE

• SEQUENCE OF REMOVAL– SULFUR (G72D, ZnO)– CARBON MONOXIDE (G66B, CuO)– OXYGEN (UCC 1101, CuCr)– WATER, METHANOL, CARBONYLS (UOP

13XPG)

ETHYLENE

• EXTENDED SHUTDOWN– 12 hours to 7 days, to be depressurized to 1000

kpag– > 7 days, system to be depressurized and kept

under Nitrogen blanket

basic purification

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