spent caustic treatment refinary and petro

Spent Caustic Treatment: The Merits of PRETREAT™ Technology

Applications for the Refinery/Petrochemical Industries

Joseph M. Gondolfe and Sabah A. Kurukchi, PhD

Copyright © All Rights Reserved Shaw Stone & Webster

Joseph M. Gondolfe and Sabah A. Kurukchi

Shaw Stone & Webster

Originally prepared for presentation at the Eleventh Ethylene Forum

May 14-16, 1997 The Woodlands, Texas

13 Shaw Stone & Webster

Intentionally Blank


Intentionally Blank

1 Shaw Stone & Webster 1. Patent Pending

Today's refiner and petrochemical producer is forced by more stringent environmental regulations to better monitor and control their wastewater to/from biological treatment. One of the key contributors to relatively high chemical oxygen demand (COD) and biological oxygen demand (BOD) is from the acid gas (both CO2 and H2S) removal system(s) typically using dilute caustic soda (NaOH) as the active reagent – the resultant waste stream is otherwise known as spent caustic.

Typical sulfidic spent caustic derived from acid gas removal in either a petrochemical or refinery facility can contain significant contaminants comprised of dissolved hydrocarbon, polymers and active polymer precursors in addition to the well defined levels of sodium salts and free caustic. These lesser known contaminants can grossly inhibit the conversion level of sodium sulfide to its highest oxidation state – sodium sulfate.

A unique approach to (pre)treat these aforementioned contaminants has been developed, otherwise known as PRETREAT™ Technology¹, which now allows a cost effective method of achieving virtually complete oxidation of spent caustic using the following steps:

(1) (Pre)Treatment (2) Bulk Wet Air Oxidation (3) Advanced Chemical Oxidation (4) Post-Treatment

Extensive pilot plant tests have confirmed that the process configuration illustrated in Figure 1 affords high flexibility to address the specific demands of today's petrochemical/ refinery facility including upgrading existing treatment units to better satisfy current (or future) federal and state regulatory requirements. This unique approach to spent caustic treatment includes innovations never before applied in the manner prescribed.

Moreover, the use of PRETREAT™ Technology can now be used as a singular method to condition raw spent caustic for virtually complete removal of all oligomers and organics from solution to yield a value-added “product” for sale to the pulp & paper industry as sulfidic/alkali make-up (i.e. Na2O equivalency).

Process Configuration

Figure 1: Process Configuration

Spent Caustic Treatment: The Merits Of PRETREAT™ Technology


Neutralization

Bulk Wet Air Oxidation

Advanced Chemical Oxidation

Polymer & Hydrocarbon

Removal

(Pre)Treatment Oxidation Post-Treatment


Fundamentals of Acid Gas Removal

In ethylene plants, hydrocarbon feedstock and steam are pyrolyzed at elevated temperatures to produce the desired primary olefinic products, ethylene and propylene

Ref.1,2. Also produced are a series of other

aliphatic, aromatic, and, to a much lesser extent, oxygenated hydrocarbon compounds. H2S is produced as a result of sulfur-hydrogen reaction; CO2 is also produced by water-gas reactions and reforming.

The steam cracker effluents are cooled, then quenched and compressed from near atmospheric conditions to high pressure (400-500 psia) in a multi-stage gas compressor. The acid gases (CO2 and H2S) are removed from the gas phase in an absorber typically located between the compression stages as shown in Figure 2. The absorber removes the CO2 and H2S from up to a few hundred ppm to < 1ppm in order to satisfy the ethylene and propylene product specifications and to prevent both poisoning of the downstream hydrogenation catalyst with H2S and solidification of CO2 in the cryogenic section of the ethylene train.

The stoichiometric reactions that are fast and irreversible between CO2 and H2S with excess NaOH are:

CO2 + 2NaOH → Na2CO3 + H2O

H2S + 2NaOH → Na2S + 2H2O

When the caustic tower operates under conditions of NaOH deficiency, the following reversible reactions also occur:

CO2 + NaOH ↔ NaHCO3

H2S + NaOH ↔ NaHS + H2O

Na2S + H2S ↔ 2NaHS

Because the above reactions are reversible, and in order to insure the removal of acid gases from the cracked gas stream, the caustic tower is operated with excess NaOH.

The caustic tower can remove other gases such as hydrogen cyanide (HCN) and mercaptans (RSH) to the limit of their solubility in the caustic solution due to the reversible nature of the reaction:

RSH + NaOH ↔ RSNa + H2O

Also absorbed in the caustic tower are the oxygenates formed during steam cracking, including, but not limited to, the carbonyls, ketones, and organic acids, which are all soluble in the caustic solution.

Basic Ethylene Unit Diagram

Figure 2: Basic Ethylene Unit Diagram


Conclusions

A unique new spent caustic (pre)treatment process known as PRETREAT™ is now available through Shaw Stone & Webster which affords a powerful solution for essentially complete removal of all dissolved organics. PRETREAT™ Technology includes treating the raw spent caustic with hydrogenated aromatic rich gasoline in a liquid-liquid extractor followed by (indirect) steam stripping. The (pre)treated spent caustic is environmentally safe and can be sold for its sulfidic-alkali content.

There are synergistic effects between industries -refinery/petrochemical and pulp & paper -wherein industrial waste (as spent caustic) can now be (pre) treated to provide a value-added “product” sold for its sulfidic-alkali content. The use of PRETREAT™ in this context truly represents a unique cross-cutting technology.

Other obvious benefits are: • (Pre)treatment of spent caustic enhances simple wet air oxidation by

20-40% compared with the oxidation of the untreated spent caustic; and,

• Any oxidation chemistry using ozone-only can be enhanced by 10-20% conversion of sulfide to sulfate; and,

• Refinery waste caustic may be (pre)treated then neutralized to liberate H2S for elemental sulfur recovery and further stripped for ultimate hydrocarbon removal before biological treatment; thus, enhancing final wastewater quality.

References

(1) Hallee, L.P., Zdonik, S.B. & Green, E. J. “Ethylene Worldwide #18”, The Oil & Gas Journal November 24, P. 96 (1970)

(2) Hallee, L.P., Zdonik, S.B. & Green, E. J. “Ethylene Worldwide #19”, The Oil & Gas Journal April 27, P. 94 (1971)

(3) Asomaning, S., Watkinson, A. P., The Canadian Journal of Chemical Engineering, Volume 70, 444 – 451, June 1992

(4) Georgeiff, K.K., The Canadian Journal of Chemical Engineering, Volume 62, 367-373 June 1984

(5) Handy, C.T. and Rothrock, H.S., J. Am. Chem. Soc., 80, 5306-8, 1958

(6) Small, K., “Performance of Low Pressure Wet Air Oxidation Unit” AIChE Spring National meeting, Houston, TX, April 17 – 21, 1994

(7) Matthews, R.W., AIChE 8th Ethylene Producers Conference; New Orleans, Louisiana; February 27, 1996

(8) Kirk – Othmer, “Encyclopedia of Chemical Technology” 3rd edn, Volume 19, P. 402, 1982


Removal of Residual Hydrocarbons

The raffinate (treated spent caustic) from the liquid-liquid extractor is saturated with the components from the extracting solvent; the dissolved hydrocarbons in the raffinate would require removal if the spent caustic is to be sold for its alkali content, or if it is to be oxidized by simple wet air chemistry or other methods.

The dissolved hydrocarbons can be steam stripped. The stripper is operated at such a pressure and temperature that ensures the removal

of the hydrocarbons to the desired low levels. The extracting solvent should be chosen to be free of any heavy residual hydrocarbon tail that prevents simple stripping as an effective removal step of these residual organics.

The overhead of the stripper is essentially water vapor with residual hydrocarbon; this stream is purged. The bottom of the hydrocarbon stripper is now fully (pre)treated spent caustic. The (pre)treatment unit, including the Liquid-Liquid Extractor and the Hydrocarbon Stripper, is shown in Figure 7.

(Pre)Treatment

Figure 7: (Pre) Treatment Unit


Spent Caustic Speciation

To better comprehend those compounds present in typical spent caustic from an ethylene plant or refinery, an effort has been made to speciate this waste stream to qualitatively and quantitatively define the constituents that may negatively effect oxidation of the principal compound of interest, sodium sulfide. The results of this analytical work are shown in Table 1.1 and 1.2.

Spent caustic composition is variable and depends on the type of feedstock, cracking severity, sulfur content in the feed and caustic utilization. Typical spent caustic composition ranges are:

NaOH 1 – 2 wt% Na2S 0.5 – 5 wt% Na2CO3 1 – 10 wt% NaRS 0 – 0.2 wt% Cyanides 0 – 0.05 wt% Dissolved H/C 0.1 – 0.3 wt% COD 20,000 – 50,000 mg/l BOD 5,000 – 15,000 mg/l pH 13.5 – 14

Polymer Formation

To fully appreciate the positive impact of PRETREAT™ Technology, one must revisit the fundamental sources of polymer formation within the acid gas removal system itself.

Two distinct types of polymers are formed: Aldol Condensation Polymers and Free Radical Addition Polymers.

Aldol Condensation

Oxygenated compounds, including carbonyl compounds, are formed during steam cracking. The amount of carbonyls formed, such as aldehydes and ketones, can vary widely from 50 to 500 ppm in the cracked gas stream depending on the type of feedstock and severity of cracking.

In the caustic tower most of the carbonyls are absorbed in caustic solution. These oxygenates, specially acetaldehyde, will undergo Aldol Condensation Polymerization in the presence of the strong alkali (NaOH) and at the temperature and pressure conditions in the caustic tower.


NaOH

2CH3CHO acetaldehyde

→ CH3CH(OH)CH2CHO B-hydroxyaldehyde (formed but not isolated)

- H2O

CH3CH(OH)CH2CHO B-hydroxyaldehyde

→ CH3CH = CHCHO 2-Butenal

NaOH

CH3CH = CHCHO + CH3CHO → CH3CH = CHCH(OH)CH2CHO

-H2O

CH3CH = CHCH(OH)CH2CHO → CH3CH = CHCH = CHCHO

The Aldol Condensation Polymer (Red Oil) formed is partially soluble in the caustic solution and insoluble in water.

In the caustic tower, the resulting polymer typically settles on tower internals, leading to fouling and plugging, unless measures are taken to remove the polymer from the system at a rate equal to its rate of formation.

It is estimated that some 70% of the carbonyls in the cracked gas are absorbed by the caustic solution in the tower. The amount and extent of polymer formation in the caustic tower depends upon the temperature and pressure conditions in the tower.

Free Radical Addition

Highly unsaturated compounds in the cracked gas, such as acetylenes and dienes, are appreciably soluble in the caustic solution, and at the temperature and pressure conditions in the caustic tower would undergo polymerization by free radical addition mechanism as shown in Figure 3.

At an appropriate temperature, a peroxide decomposes to yield free radicals. In the presence of monomer, the free radicals add to the monomer and thereby initiate chain growth that can extend in three dimensions and eventually result in a high molecular weight, insoluble polymer that drops out of the solution and causes fouling and plugging of the equipment.

Among the possible polymeric reaction free radicals (initiators) are peroxides, azo compounds, oxygen, and metal oxides, all of which may be present in the caustic tower and equipment handling and processing the spent caustic.

Diolefin Polymerization Mechanism

Initiation R• +

Polymer Growth R Figure 3: Diolefin Polymerization Mechanism

•


Optional Solvent Regeneration

The solvent from the liquid-liquid extractor is contaminated with all components in the spent caustic stream including light hydrocarbons, polymers, polymer precursors and heavy hydrocarbons.

The solvent regenerator is a simple distillation column with partial condenser; the regenerated solvent is withdrawn as a heart-cut side

stream. The light volatile contaminants are purged with the overhead vapor product and the heavy polymeric materials are removed with the net bottoms.

Small make-up solvent is added to replace the solvent lost in the net overhead and bottoms.

Solvent Recovery – Closed Loop

Figure 6b: Solvent Recovery – Closed Loop


The concentrated Black Liquor is directed to a recovery furnace operated at 1250°C where the organics are burned and the inorganic salts are recovered in the form Na2CO3 and Na2S. Make-up sodium sulfate is added to the furnace feed to compensate for the loss of alkali in the recovery cycle written as:

Na2SO4 + 2C → Na2S + 2CO2

The flue gas effluent from the furnace is utilized to generate steam while the inorganic (Na2CO3 and Na2S) residue smelt at 1000°C runs out of the furnace to a water quench dissolver to form the Green Liquor.

The Green Liquor is causticized with slacked lime as follows: Na2CO3 + Ca(OH)2 → 2NaOH + CaCO3

The mixture is clarified to separate the insoluble CaCO3. The clarified solution is the desired White Liquor.

The spent caustic generated in ethylene plants can be used in the pulp & paper mills as make-up alkali in the Kraft Recovery System, but environmental regulations and both public and management awareness of the hazardous nature of aromatic hydrocarbons as well as the presence of polymers and polymer precursors tend to inhibit this practice. The use of PRETREAT™ Technology as prescribed herein frees all hydrocarbons, polymers and polymer precursors to < 10 ppm

in the spent caustic; thus, spent caustic now becomes very attractive for use in pulp & paper mills while converting a hazardous waste in the ethylene plants to a valuable byproduct. Other similar use of (pre) treated spent caustic is found in noble metal catalyst manufacture and metal treatment processes.

Intimate Contact With a Selective Solvent

Pilot plant tests have demonstrated that these polymer precursors and their monomers in the spent caustic can be removed by extraction with organic solvents. The solvent of choice must exhibit favorable distribution coefficients for the removal of polymer precursors and monomers. Suitable extracting solvent would be a hydrogenated aromatic rich stream that may be available at the plant site. The liquid-liquid extraction is carried out in a multistage countercurrent extractor sized and operated at conditions and rates that are effective in removing these polymer precursors and monomers to negligible concentration levels.

The solvent or extract from the extractor is sent for purification by solvent regeneration and simply recycled (as new) to the liquid-liquid extraction. The solvent can be either returned to the primary process of interest (Figure 6a) or properly conditioned in a dedicated solvent regenerator (Figure 6b).

Solvent Recovery – Open Loop

Figure 6a: Solvent Recovery – Open Loop


Infiltration of Air (as Oxygen) Within the Acid Gas Removal System

As is known in ethylene plant serviceRef.3,4

for deethanizer and depropanizer columns, pyrophoric polymer formation has been well proven. Reactive compounds such as butadiene and other conjugated dienes in the presence of small quantities of air react with oxygen to form trans-1,4 and 1,2 butadiene polyperoxides

Ref.5.

CH2 = CHCH = CH2 + O2 → [-CH2CH = CHCH2OO-]χ + CHCH200- CH = CH2

γ

2trans – 1,4 form

Polymerization in The Spent Caustic by Other Initiators

The spent caustic stream leaving the tower will contain both the condensation and addition types of polymers formed, as well as the unreacted monomers and precursors that would continue to polymerize and cause fouling of equipment handling the spent caustic if the temperature and pressure conditions in this equipment are conducive to polymer formation. Ingress of oxygen to the spent caustic will accelerate polymerization of spent caustic.

Still other known polymer precursors such as methyl acetylene/propadiene and vinyl/ethyl acetylene are highly solubilized in this alkaline solution; therefore, at elevated temperatures these alkyne and allene compounds only aid in the overall polymerization.

The quality of any fresh caustic soda can be paramount to the concentration and rate of formation of the aforementioned polymers. These unwanted compounds include heavy metals and other known initiators such as mercury, iron, nickel, magnesium, silica, calcium, lead, copper and still others.

These metallic elements react directly with sulfur and give rise to compounds referred to as metal chalconides and act as polymer promoters or initiators. To avoid potential formation of metal chalconides within the spent caustic, a (fresh) caustic soda with the minimum of initiators is obviously most preferred. If logistically accessible and economically justifiable, the preferred choice of (fresh) caustic soda (50% aqueous solution) is Rayon Grade as opposed to Diaphragm Grade. It is well known that Rayon Grade caustic soda typically has about half the carbonates, chlorides and heavy metals as compared to Diaphragm Grade caustic soda.

The qualitative conclusion of importance is that the selection of Rayon Grade with low chlorates and metals content as opposed to Diaphragm Grade is the preferred choice to be used in caustic tower services.

Positive Impact of PRETREAT™ Technology

The spent caustic from the absorber is generated as a result of the intimate countercurrent liquid/vapor contact of the dilute caustic solution with the Cracked Gas (CG) in the tower. The acid gases react with the sodium hydroxide to form sodium salts; the spent caustic simultaneously becomes saturated with an array of hydrocarbon components in the CG including heat sensitive polymer precursors and monomers such as carbonyls, dienes and styrenics.

The polymerization rate of polymer precursors and their monomers increases with increasing temperature. Polymerization of the carbonyls by aldol condensation reactions is catalyzed by free sodium hydroxide in solution.

Polymers continue to form within the spent caustic solution as long as the solution contains dissolved polymer precursors at elevated temperature. The presence of these species and other organics in the spent caustic proper act as a poison to appreciably retard the preferred oxidation chemistry to produce sodium sulfate. The presence of organics in the spent caustic makes the solution environmentally hazardous and limits its use for integration within the pulp & paper industry and other metal treatment processes.

Therefore, it is essential to free the spent caustic from dissolved polymer precursors and their monomers if polymer formation is to be avoided prior to any oxidation reaction or if the spent caustic is to be used for its alkali content.

Proper (pre)treatment of spent caustic, as well as heavy organic compounds, in the manner prescribed herein offers a unique solution to removing known polymers and their polymer precursors with relative ease and simple operation. Although these polymers and their precursors are present in relatively low concentrations, nonetheless, they remain as heat sensitive gums in solution that cause fouling and plugging thus rendering simple distillation impractical.


Through the use of liquid/liquid extraction, the transfer of the solute from one liquid phase (spent caustic) into a second liquid phase (extracting solvent) allows transfer of the solutes from the spent caustic to the solvent due to the inherently higher solubility in the extracting solvent. The preferred attributes of the “solvent of choice” are: • ready availability within the overall plant configuration at a

reasonable cost; and, • good selectivity in extracting the solutes from the caustic solution;

and, • a relatively small density difference and interfacial tension between

the spent caustic and solvent of choice to allow successive ease of mixing with minimum shear between liquid phases thus facilitating good separation of the phases subsequent to intimate contacting.

The extracted spent caustic becomes saturated with the components present in the extracting solvent and these can be removed by stripping. The stripped spent caustic with <10 ppm total hydrocarbon content becomes suitable to: (a) enhance oxidation rates through simple wet air chemistry, as the presence of hydrocarbon contaminants has been shown by pilot testing

Ref.6 and plotted in Figure 4, to grossly inhibit the conversion level

of sodium sulfide to its highest oxidation state --- sodium sulfate.

(b) sell to the pulp & paper mills and other industrial users for its alkali salts content without having an environmentally unacceptable hydrocarbon content.

Effect of Organics on Sulfate Yield in LPWAO

Figure 4: Effect of Organics on Sulfate Yield in LPWAO (Low Pressure Wet Air Oxidation)


Industrial Waste to Value-Added “Product”

To fully appreciate the significance of PRETREAT™ Technology, a proper understanding of this processing scheme as a cross-cutting technology between the refinery/ petrochemical producer and the pulp & paper industry is necessary; therefore, for the purpose of demonstrating to the reader the synergism between industries, a broad overview of the fundamentals of the Kraft Recovery Process as applied to the pulp & paper industry is briefly described hereinafter.

In a typical pulp & paper mill, NaOH/Na2S solution (White Liquor) is used in the Kraft Recovery System

Ref.8 (Figure 5) to digest steamed

wood added in a ratio of liquor/wood ≈ 4/1 in a continuous digester operated at 170°C and 150 psig; this White Liquor has the following typical specifications:

• 18% Na2O (equivalent NaOH & Na2S) • 25% Sulfidity (% Na2S/Na2O)

The pulp is washed with water and separated. The spent liquor (Black Liquor) is drained and now has 12-15 wt% solids.

In order to prevent the loss of the volatile sodium bisulfide and mercaptides, the Black Liquor is oxidized with air for the stoichiometry as written:

2NaHS + 2O2 → Na2S2O3 + H2O

2CH3SNa + O2 + 2H2O → CH3SSCH3 + 2NaOH + ½O2 + H2O

The oxidized Black Liquor is evaporated in a multi-effect evaporator to 40-50 wt% solution. The solution is further concentrated to 65-70 wt% solid in a forced circulation concentrator.

Schematic of Kraft Recovery System

Figure 5: Schematic of Kraft Recovery System

Digester and Blow

Tank

Washer

Oxidation

Dissolving Tank

Multiple-effect Evaporator / Concentrator

Slaker and Causticizer

White-Liquor

Clarifier

Recovery Furnace

150 psig 170°C

Steamed Wood

Black Liquor and pulp

Wash Water

Pulp

Weak Black Liquor (12-15 wt% solids)

Air

Vapor Water

Ca(OH)

White Liquor

NaOH/ Na2S

Lime Mud CaCO3

Green Liquor (Na2S, Na2CO3)

Smelt

1000°C

Flue gas for heat recovery

Makeup Na2SO4

Concentrated Black Liquor, 65 wt% solids

(Pre) Treated Spent Caustic

Addition

1250°C

Na2S, NaOH, CaCO3


The concentrated Black Liquor is directed to a recovery furnace operated at 1250°C where the organics are burned and the inorganic salts are recovered in the form Na2CO3 and Na2S. Make-up sodium sulfate is added to the furnace feed to compensate for the loss of alkali in the recovery cycle written as:

Na2SO4 + 2C → Na2S + 2CO2

The flue gas effluent from the furnace is utilized to generate steam while the inorganic (Na2CO3 and Na2S) residue smelt at 1000°C runs out of the furnace to a water quench dissolver to form the Green Liquor.

The Green Liquor is causticized with slacked lime as follows: Na2CO3 + Ca(OH)2 → 2NaOH + CaCO3

The mixture is clarified to separate the insoluble CaCO3. The clarified solution is the desired White Liquor.

The spent caustic generated in ethylene plants can be used in the pulp & paper mills as make-up alkali in the Kraft Recovery System, but environmental regulations and both public and management awareness of the hazardous nature of aromatic hydrocarbons as well as the presence of polymers and polymer precursors tend to inhibit this practice. The use of PRETREAT™ Technology as prescribed herein frees all hydrocarbons, polymers and polymer precursors to < 10 ppm

in the spent caustic; thus, spent caustic now becomes very attractive for use in pulp & paper mills while converting a hazardous waste in the ethylene plants to a valuable byproduct. Other similar use of (pre) treated spent caustic is found in noble metal catalyst manufacture and metal treatment processes.

Intimate Contact With a Selective Solvent

Pilot plant tests have demonstrated that these polymer precursors and their monomers in the spent caustic can be removed by extraction with organic solvents. The solvent of choice must exhibit favorable distribution coefficients for the removal of polymer precursors and monomers. Suitable extracting solvent would be a hydrogenated aromatic rich stream that may be available at the plant site. The liquid-liquid extraction is carried out in a multistage countercurrent extractor sized and operated at conditions and rates that are effective in removing these polymer precursors and monomers to negligible concentration levels.

The solvent or extract from the extractor is sent for purification by solvent regeneration and simply recycled (as new) to the liquid-liquid extraction. The solvent can be either returned to the primary process of interest (Figure 6a) or properly conditioned in a dedicated solvent regenerator (Figure 6b).

Solvent Recovery – Open Loop

Figure 6a: Solvent Recovery – Open Loop


Infiltration of Air (as Oxygen) Within the Acid Gas Removal System

As is known in ethylene plant serviceRef.3,4

for deethanizer and depropanizer columns, pyrophoric polymer formation has been well proven. Reactive compounds such as butadiene and other conjugated dienes in the presence of small quantities of air react with oxygen to form trans-1,4 and 1,2 butadiene polyperoxides

Ref.5.

CH2 = CHCH = CH2 + O2 → [-CH2CH = CHCH2OO-]χ + CHCH200- CH = CH2

γ

2trans – 1,4 form

Polymerization in The Spent Caustic by Other Initiators

The spent caustic stream leaving the tower will contain both the condensation and addition types of polymers formed, as well as the unreacted monomers and precursors that would continue to polymerize and cause fouling of equipment handling the spent caustic if the temperature and pressure conditions in this equipment are conducive to polymer formation. Ingress of oxygen to the spent caustic will accelerate polymerization of spent caustic.

Still other known polymer precursors such as methyl acetylene/propadiene and vinyl/ethyl acetylene are highly solubilized in this alkaline solution; therefore, at elevated temperatures these alkyne and allene compounds only aid in the overall polymerization.

The quality of any fresh caustic soda can be paramount to the concentration and rate of formation of the aforementioned polymers. These unwanted compounds include heavy metals and other known initiators such as mercury, iron, nickel, magnesium, silica, calcium, lead, copper and still others.

These metallic elements react directly with sulfur and give rise to compounds referred to as metal chalconides and act as polymer promoters or initiators. To avoid potential formation of metal chalconides within the spent caustic, a (fresh) caustic soda with the minimum of initiators is obviously most preferred. If logistically accessible and economically justifiable, the preferred choice of (fresh) caustic soda (50% aqueous solution) is Rayon Grade as opposed to Diaphragm Grade. It is well known that Rayon Grade caustic soda typically has about half the carbonates, chlorides and heavy metals as compared to Diaphragm Grade caustic soda.

The qualitative conclusion of importance is that the selection of Rayon Grade with low chlorates and metals content as opposed to Diaphragm Grade is the preferred choice to be used in caustic tower services.

Positive Impact of PRETREAT™ Technology

The spent caustic from the absorber is generated as a result of the intimate countercurrent liquid/vapor contact of the dilute caustic solution with the Cracked Gas (CG) in the tower. The acid gases react with the sodium hydroxide to form sodium salts; the spent caustic simultaneously becomes saturated with an array of hydrocarbon components in the CG including heat sensitive polymer precursors and monomers such as carbonyls, dienes and styrenics.

The polymerization rate of polymer precursors and their monomers increases with increasing temperature. Polymerization of the carbonyls by aldol condensation reactions is catalyzed by free sodium hydroxide in solution.

Polymers continue to form within the spent caustic solution as long as the solution contains dissolved polymer precursors at elevated temperature. The presence of these species and other organics in the spent caustic proper act as a poison to appreciably retard the preferred oxidation chemistry to produce sodium sulfate. The presence of organics in the spent caustic makes the solution environmentally hazardous and limits its use for integration within the pulp & paper industry and other metal treatment processes.

Therefore, it is essential to free the spent caustic from dissolved polymer precursors and their monomers if polymer formation is to be avoided prior to any oxidation reaction or if the spent caustic is to be used for its alkali content.

Proper (pre)treatment of spent caustic, as well as heavy organic compounds, in the manner prescribed herein offers a unique solution to removing known polymers and their polymer precursors with relative ease and simple operation. Although these polymers and their precursors are present in relatively low concentrations, nonetheless, they remain as heat sensitive gums in solution that cause fouling and plugging thus rendering simple distillation impractical.


NaOH

2CH3CHO acetaldehyde

→ CH3CH(OH)CH2CHO B-hydroxyaldehyde (formed but not isolated)

- H2O

CH3CH(OH)CH2CHO B-hydroxyaldehyde

→ CH3CH = CHCHO 2-Butenal

NaOH

CH3CH = CHCHO + CH3CHO → CH3CH = CHCH(OH)CH2CHO

-H2O

CH3CH = CHCH(OH)CH2CHO → CH3CH = CHCH = CHCHO

The Aldol Condensation Polymer (Red Oil) formed is partially soluble in the caustic solution and insoluble in water.

In the caustic tower, the resulting polymer typically settles on tower internals, leading to fouling and plugging, unless measures are taken to remove the polymer from the system at a rate equal to its rate of formation.

It is estimated that some 70% of the carbonyls in the cracked gas are absorbed by the caustic solution in the tower. The amount and extent of polymer formation in the caustic tower depends upon the temperature and pressure conditions in the tower.

Free Radical Addition

Highly unsaturated compounds in the cracked gas, such as acetylenes and dienes, are appreciably soluble in the caustic solution, and at the temperature and pressure conditions in the caustic tower would undergo polymerization by free radical addition mechanism as shown in Figure 3.

At an appropriate temperature, a peroxide decomposes to yield free radicals. In the presence of monomer, the free radicals add to the monomer and thereby initiate chain growth that can extend in three dimensions and eventually result in a high molecular weight, insoluble polymer that drops out of the solution and causes fouling and plugging of the equipment.

Among the possible polymeric reaction free radicals (initiators) are peroxides, azo compounds, oxygen, and metal oxides, all of which may be present in the caustic tower and equipment handling and processing the spent caustic.

Diolefin Polymerization Mechanism

Initiation R• +

Polymer Growth R Figure 3: Diolefin Polymerization Mechanism

•


Optional Solvent Regeneration

The solvent from the liquid-liquid extractor is contaminated with all components in the spent caustic stream including light hydrocarbons, polymers, polymer precursors and heavy hydrocarbons.

The solvent regenerator is a simple distillation column with partial condenser; the regenerated solvent is withdrawn as a heart-cut side

stream. The light volatile contaminants are purged with the overhead vapor product and the heavy polymeric materials are removed with the net bottoms.

Small make-up solvent is added to replace the solvent lost in the net overhead and bottoms.

Solvent Recovery – Closed Loop

Figure 6b: Solvent Recovery – Closed Loop


Removal of Residual Hydrocarbons

The raffinate (treated spent caustic) from the liquid-liquid extractor is saturated with the components from the extracting solvent; the dissolved hydrocarbons in the raffinate would require removal if the spent caustic is to be sold for its alkali content, or if it is to be oxidized by simple wet air chemistry or other methods.

The dissolved hydrocarbons can be steam stripped. The stripper is operated at such a pressure and temperature that ensures the removal

of the hydrocarbons to the desired low levels. The extracting solvent should be chosen to be free of any heavy residual hydrocarbon tail that prevents simple stripping as an effective removal step of these residual organics.

The overhead of the stripper is essentially water vapor with residual hydrocarbon; this stream is purged. The bottom of the hydrocarbon stripper is now fully (pre)treated spent caustic. The (pre)treatment unit, including the Liquid-Liquid Extractor and the Hydrocarbon Stripper, is shown in Figure 7.

(Pre)Treatment

Figure 7: (Pre) Treatment Unit


Spent Caustic Speciation

To better comprehend those compounds present in typical spent caustic from an ethylene plant or refinery, an effort has been made to speciate this waste stream to qualitatively and quantitatively define the constituents that may negatively effect oxidation of the principal compound of interest, sodium sulfide. The results of this analytical work are shown in Table 1.1 and 1.2.

Spent caustic composition is variable and depends on the type of feedstock, cracking severity, sulfur content in the feed and caustic utilization. Typical spent caustic composition ranges are:

NaOH 1 – 2 wt% Na2S 0.5 – 5 wt% Na2CO3 1 – 10 wt% NaRS 0 – 0.2 wt% Cyanides 0 – 0.05 wt% Dissolved H/C 0.1 – 0.3 wt% COD 20,000 – 50,000 mg/l BOD 5,000 – 15,000 mg/l pH 13.5 – 14

Polymer Formation

To fully appreciate the positive impact of PRETREAT™ Technology, one must revisit the fundamental sources of polymer formation within the acid gas removal system itself.

Two distinct types of polymers are formed: Aldol Condensation Polymers and Free Radical Addition Polymers.

Aldol Condensation

Oxygenated compounds, including carbonyl compounds, are formed during steam cracking. The amount of carbonyls formed, such as aldehydes and ketones, can vary widely from 50 to 500 ppm in the cracked gas stream depending on the type of feedstock and severity of cracking.

In the caustic tower most of the carbonyls are absorbed in caustic solution. These oxygenates, specially acetaldehyde, will undergo Aldol Condensation Polymerization in the presence of the strong alkali (NaOH) and at the temperature and pressure conditions in the caustic tower.


Fundamentals of Acid Gas Removal

In ethylene plants, hydrocarbon feedstock and steam are pyrolyzed at elevated temperatures to produce the desired primary olefinic products, ethylene and propylene

Ref.1,2. Also produced are a series of other

aliphatic, aromatic, and, to a much lesser extent, oxygenated hydrocarbon compounds. H2S is produced as a result of sulfur-hydrogen reaction; CO2 is also produced by water-gas reactions and reforming.

The steam cracker effluents are cooled, then quenched and compressed from near atmospheric conditions to high pressure (400-500 psia) in a multi-stage gas compressor. The acid gases (CO2 and H2S) are removed from the gas phase in an absorber typically located between the compression stages as shown in Figure 2. The absorber removes the CO2 and H2S from up to a few hundred ppm to < 1ppm in order to satisfy the ethylene and propylene product specifications and to prevent both poisoning of the downstream hydrogenation catalyst with H2S and solidification of CO2 in the cryogenic section of the ethylene train.

The stoichiometric reactions that are fast and irreversible between CO2 and H2S with excess NaOH are:

CO2 + 2NaOH → Na2CO3 + H2O

H2S + 2NaOH → Na2S + 2H2O

When the caustic tower operates under conditions of NaOH deficiency, the following reversible reactions also occur:

CO2 + NaOH ↔ NaHCO3

H2S + NaOH ↔ NaHS + H2O

Na2S + H2S ↔ 2NaHS

Because the above reactions are reversible, and in order to insure the removal of acid gases from the cracked gas stream, the caustic tower is operated with excess NaOH.

The caustic tower can remove other gases such as hydrogen cyanide (HCN) and mercaptans (RSH) to the limit of their solubility in the caustic solution due to the reversible nature of the reaction:

RSH + NaOH ↔ RSNa + H2O

Also absorbed in the caustic tower are the oxygenates formed during steam cracking, including, but not limited to, the carbonyls, ketones, and organic acids, which are all soluble in the caustic solution.

Basic Ethylene Unit Diagram

Figure 2: Basic Ethylene Unit Diagram


Conclusions

A unique new spent caustic (pre)treatment process known as PRETREAT™ is now available through Shaw Stone & Webster which affords a powerful solution for essentially complete removal of all dissolved organics. PRETREAT™ Technology includes treating the raw spent caustic with hydrogenated aromatic rich gasoline in a liquid-liquid extractor followed by (indirect) steam stripping. The (pre)treated spent caustic is environmentally safe and can be sold for its sulfidic-alkali content.

There are synergistic effects between industries -refinery/petrochemical and pulp & paper -wherein industrial waste (as spent caustic) can now be (pre) treated to provide a value-added “product” sold for its sulfidic-alkali content. The use of PRETREAT™ in this context truly represents a unique cross-cutting technology.

Other obvious benefits are: • (Pre)treatment of spent caustic enhances simple wet air oxidation by

20-40% compared with the oxidation of the untreated spent caustic; and,

• Any oxidation chemistry using ozone-only can be enhanced by 10-20% conversion of sulfide to sulfate; and,

• Refinery waste caustic may be (pre)treated then neutralized to liberate H2S for elemental sulfur recovery and further stripped for ultimate hydrocarbon removal before biological treatment; thus, enhancing final wastewater quality.

References

(1) Hallee, L.P., Zdonik, S.B. & Green, E. J. “Ethylene Worldwide #18”, The Oil & Gas Journal November 24, P. 96 (1970)

(2) Hallee, L.P., Zdonik, S.B. & Green, E. J. “Ethylene Worldwide #19”, The Oil & Gas Journal April 27, P. 94 (1971)

(3) Asomaning, S., Watkinson, A. P., The Canadian Journal of Chemical Engineering, Volume 70, 444 – 451, June 1992

(4) Georgeiff, K.K., The Canadian Journal of Chemical Engineering, Volume 62, 367-373 June 1984

(5) Handy, C.T. and Rothrock, H.S., J. Am. Chem. Soc., 80, 5306-8, 1958

(6) Small, K., “Performance of Low Pressure Wet Air Oxidation Unit” AIChE Spring National meeting, Houston, TX, April 17 – 21, 1994

(7) Matthews, R.W., AIChE 8th Ethylene Producers Conference; New Orleans, Louisiana; February 27, 1996

(8) Kirk – Othmer, “Encyclopedia of Chemical Technology” 3rd edn, Volume 19, P. 402, 1982


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1 Shaw Stone & Webster 1. Patent Pending

Today's refiner and petrochemical producer is forced by more stringent environmental regulations to better monitor and control their wastewater to/from biological treatment. One of the key contributors to relatively high chemical oxygen demand (COD) and biological oxygen demand (BOD) is from the acid gas (both CO2 and H2S) removal system(s) typically using dilute caustic soda (NaOH) as the active reagent – the resultant waste stream is otherwise known as spent caustic.

Typical sulfidic spent caustic derived from acid gas removal in either a petrochemical or refinery facility can contain significant contaminants comprised of dissolved hydrocarbon, polymers and active polymer precursors in addition to the well defined levels of sodium salts and free caustic. These lesser known contaminants can grossly inhibit the conversion level of sodium sulfide to its highest oxidation state – sodium sulfate.

A unique approach to (pre)treat these aforementioned contaminants has been developed, otherwise known as PRETREAT™ Technology¹, which now allows a cost effective method of achieving virtually complete oxidation of spent caustic using the following steps:

(1) (Pre)Treatment (2) Bulk Wet Air Oxidation (3) Advanced Chemical Oxidation (4) Post-Treatment

Extensive pilot plant tests have confirmed that the process configuration illustrated in Figure 1 affords high flexibility to address the specific demands of today's petrochemical/ refinery facility including upgrading existing treatment units to better satisfy current (or future) federal and state regulatory requirements. This unique approach to spent caustic treatment includes innovations never before applied in the manner prescribed.

Moreover, the use of PRETREAT™ Technology can now be used as a singular method to condition raw spent caustic for virtually complete removal of all oligomers and organics from solution to yield a value-added “product” for sale to the pulp & paper industry as sulfidic/alkali make-up (i.e. Na2O equivalency).

Process Configuration

Figure 1: Process Configuration

Spent Caustic Treatment: The Merits Of PRETREAT™ Technology


Neutralization

Bulk Wet Air Oxidation

Advanced Chemical Oxidation

Polymer & Hydrocarbon

Removal

(Pre)Treatment Oxidation Post-Treatment

Shaw Stone & Webster

Originally prepared for presentation at the Eleventh Ethylene Forum

May 14-16, 1997 The Woodlands, Texas


Intentionally Blank

Spent Caustic Treatment: The Merits of PRETREAT™ Technology


Joseph M. Gondolfe and Sabah A. Kurukchi, PhD

Copyright © All Rights Reserved Shaw Stone & Webster

Joseph M. Gondolfe and Sabah A. Kurukchi

spent caustic treatment refinary and petro

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