innovative qw treatment - by s kuruchi

Innovative Quench Water Pretreat Technology For Ethylene Plants

Paper No. 97D

By

Sabah Kurukchi, Ph.D. and Joseph Gondolfe Stone & Webster, Inc. A Shaw Group Company

Houston, Texas

Presented at the 14th Ethylene Producers� Conference AIChE 2002 Spring National Meeting, New Orleans, Louisiana

March 10 � 14, 2002

UNPUBLISHED

Copyright of Stone & Webster, Inc. Sabah Kurukchi, Ph.D. and Joseph M. Gondolfe

The AIChE shall not be responsible for statements or opinions contained in its publications

INNOVATIVE QUENCH WATER PRETREAT TECHNOLOGY

FOR ETHYLENE PLANTS

SABAH KURUKCHI, PH.D. AND JOSEPH GONDOLFE

Stone & Webster, Inc., A Shaw Group Company

14th Annual Ethylene Producers� Conference AIChE 2002 Spring National Meeting Page 1 of 10

Introduction Base Petrochemicals such as ethylene and propylene are principally produced by steam cracking of saturated hydrocarbons (H/C). In the endothermic cracking process hydrocarbon is decomposed by free radical reactions between 750°C and 900°C, In addition to the primary base petrochemicals so formed many other co-products are also produced in varying quantities including: hydrogen, paraffins, olefins, acetylenes, diolefins, cyclic, aromatic compounds and coke together with CO, CO2, H2S and a series of organic sulfur compounds. The composition of the furnace effluent cracked gas (CG) varies with feedstock composition and cracking severity. In ethylene plants cracking gaseous feedstocks (ethane, propane and butanes), a Quench Oil Tower (QOT) is not required because only small amounts of C5+liquids are produced. In these plants, only a Quench Water Tower (QWT) is used to cool the effluent gas from the TLE. The CG cools in the bottom of the QWT to its adiabatic saturation temperature causing condensation of the tar and heavy components. The CG continues its cooling by contacting the recirculating quench water (QW) as it flows up the QWT, thereby condensing most of the dilution steam and heavy hydrocarbons. The recirculating QW leaving the QWT carries all condensed hydrocarbon components both dissolved and as a separate phase in the form of tars, oils, and coke. Although water is highly suitable for quenching purposes, since it is both an effective and inexpensive heat transfer media, it has one great disadvantage. The oil, tar and coke materials in the QW form complex gels of and aqueous hydrocarbon emulsions. The major feed to the QWT is the furnace cracked gas. Unfortunately, the QWT serves as a convenient disposal for many other recycled streams both continuous and intermittent, which can cause changes in the surface properties of the water, as well as its pH. A low pH (<4.5) or a high pH (>9.5) make it difficult to separate the emulsified oil. In addition, a low pH raises corrosion concerns while a high pH increases foaming tendencies and causes difficulties in oil/water separation. Spalled coke and coke fines, from furnace transient (decoking) conditions, reach the QWT and float in both the oil and water phases. Tars and heavy oil in furnace effluent streams are contained in the bottom section of the QWT where they settle out. In the upper section of the QWT, lower molecular weight hydrocarbons condense and separate as light oil. The combination of the tar, heavy oil, and polymers together with the coke fines creates a gummy agglomerate that causes fouling and in certain cases blockage of the tower internals. The QW from the bottom of the QWT contains about 2000 � 6000 ppm oil as an emulsion The stability of the emulsion is due, at least in part, to a mutual affinity between the unsaturated hydrocarbon components in the dispersed oil phase and the continuous aqueous phase. Thus, the

Innovative Quench Water Pretreat System


emulsion will resist efforts to separate sharply into its various phases. The QW settles in an Oil-Water Separator, which typically has three compartments separated by weirs; the heavy tar and solids are withdrawn from the bottom of the first compartment, the raw QW from the second, and the pyrolysis gasoline from the third, respectively. The raw QW, from the oil/water separator, containing dissolved, emulsified and some separated oil together with some tar and solids is strained to filter out solid particles >600 micron size. Most (90-95%) of this raw QW (at about 90°C) is recirculated for low-level heat recovery in the plant before returning to the QWT. The rest of the raw QW is the net QW, which is either purged to battery limits as an open-loop system, Figure (1), or used to regenerate dilution steam for steam cracking as a closed loop system, Figure (2). Net QW Treatment This net QW can be processed to remove the residual suspended solids, as well as free and emulsified oil, in order to prevent or reduce fouling in a downstream dilution steam generation system. On the other hand, if the excess raw water were simply purged to battery limits, it would still be necessary to remove organic impurities (e.g. benzene, dienes, and other carcinogens) to such an extent that it could be discharged into local streams without causing pollution. Because ethylene plants cracking gaseous feedstocks do not have a QOT prior to the QWT, quench water streams in these plants are characterized by being more fouling and more susceptible to emulsion formation than their counterparts in liquids cracking plants. A particular problem is the entrainment of fouling species in the quench water slipstream to the dilution steam generator (DSG). Such species include unsaturated reactive polymer precursors such as styrenes, indene, and dienes, which have appreciable solubility in the aqueous phase. These tend to polymerize when exposed to the high temperatures encountered in the bottom of the QWT, low pressure water stripper (LPWS), and particularly the high pressure DSG. Debutanizer bottoms have been used as a raw gasoline wash to improve the separation between the QW and the hydrocarbons, by increasing the density differential between the water and the hydrocarbon phases. However, the presence of dienes and other polymer precursors in this raw gasoline wash stream increases the risk of fouling in the downstream water stripper and DSG. Commercially Available QW Treatment Systems To minimize heavy oil/tar carryover with the QW to the LP water stripper and DSG one or more of the following traditional systems are typically used: • Filter and/or Hydrocyclone • Coalescer • Dispersed Oil Extractor (DOX) system • Induced Gas Floatation (IGF) system



Cartridge filters are typically used to separate out the residual fine solid particles from the net QW that do not settle out in the oil/water separator. If the solid content in the net QW is high, hydrocyclones are considered when the cartridge replacement cost becomes excessive. Coalescers are used to transform small size droplets (0.2-50 microns) into a suspension of enlarged droplets in the size range (500-5000 microns) by passage of the liquid through multi-layer coalescer media; beginning with very fine structure, and gradually opening to allow for void space for the coalescing droplets. Glass fiber coalescer media have been used in this application, and work well for emulsions that have an interfacial tension (∆ σ) > 20 dyne/cm. Guyot et al. 2000, reported that Pall Corporation has developed a coalescer constructed with a formulated polymer/fluoropolymer media that is effective for emulsions with ∆σ as low as 0.5 dyne/cm. Both pilot and actual plant data taken at Naphthachimie, Lavera, France (liquid cracker) showed that the Pall coalescer reduced the hydrocarbon content from a range of 950-3500 ppm down to a range of 230-490 ppm. The residual hydrocarbon content in the net QW is mainly dissolved hydrocarbons. Cockshutt et al. 1992, discussed the difficulties experienced at Nova E2 ethylene plant (ethane cracker) at Joffre, Alberta, Canada. The plant had an oil/water separator, and an IGF unit followed by a carbon filter. The QW treating system performed poorly from 1986-89 causing fouling of the LPWS every 3 months. The performance of the unit improved by the addition of C5+ solvent wash (DC bottoms) to the top and bottom of the QWT. The IGF unit demonstrated that an oil and grease (O&G) removal rate as high as 95% and an average O&G removal rate of 75% could be achieved. The carbon filters downstream of the IGF unit plugged frequently with solids and tar. The activated carbon was subsequently replaced with a mixture of crushed pecan and walnut shells. These filters were periodically soaked with C5+ solvent to remove O&G attached to the media. The crushed shell media filters increased the overall O&G removal to 80-90%. Plant tests at the Joffre plant site using a pilot size DOX unit, showed that the DOX unit performs well removing O&G down to <15 ppm. Mullenix et al. 1993, reported that the performance of the DOX unit, at Vista Chemical Co., Lake Charles, Louisiana, declined after 6 months of operation, thus increasing fouling in both the LPWS and the DSG, as a result of heavy oil coating of the DOX media. Coating (fouling) of the coalescing media reduced the area available to agglomerate the oil droplets. The media, which was covered with dark coke, had to be replaced after 18 months in service. Present quench water treating processes, including the traditional filter/coalescer, DOX and IGF, demonstrate reasonable success in the removal of the free insoluble oil from the quench water. All well designed units are capable of removing the free oil from about 500 wppm free oil content down to between 20 and 50 ppm free oil. None of these commercially available



processes, however, are capable of removing the dissolved oil that has a larger content of unsaturated hydrocarbons and polymer precursors from the QW. QW Pretreat Technology. Because of the tendency of dissolved oil in the QW to contain polymer precursors which in turn cause fouling of the downstream LPWS and DSG systems, Stone & Webster has developed a process that provides an effective lower-cost solution to this problem. The invention, Quench Water Pretreat1, presents an innovative and cost effective approach to QW cleanup that takes advantage of the high solubility (almost infinite) of the oil droplet, tar and polymer precursors in aromatic rich gasoline compared to their limited solubility in water. The Pretreat Unit consists of an extractor, regenerator and stripper, refer Figure (3). The net QW that has been previously filtered through a strainer, is filtered again to remove solid particles >20-micron size, as these particles may deposit within the unit. The filtered QW is contacted with stabilized hydrogenated aromatic rich gasoline (C6 - C8 cut), or preferably toluene, to extract the polymer precursors (styrenes, indene, and dienes) that would polymerize when exposed to high temperatures in the downstream LPWS and the dilution steam reboilers. The net QW is fed to the Liquid/Liquid Extractor. Regenerated gasoline from the gasoline regenerator enters the liquid/liquid extractor on flow control in proportion to the QW flow rate. The deoiled QW is countercurrently contacted with the gasoline under conditions that minimize emulsion formation. The function of the extractor is to transfer the polymerizable styrenes, indene, dienes, and heavy organics from the aqueous phase to the aromatic gasoline phase. Pilot plant data has shown that in the extractor, more than 95% of the polymeric materials and polymer forming styrenes, indene, dienes, and aromatic vinyl compounds are removed. Spent solvent (gasoline) leaves the top of the extractor and is routed to the gasoline regenerator for recovery. The extracted QW exits the bottom of the extractor on interface level control and flows to the top tray of the LPWS. The net QW, while in contact with the gasoline in the extractor, becomes saturated with the light aromatic components of the gasoline which are stripped out easily in the LPWS. Steam stripping of the extracted QW results in the removal of essentially >99.9% of the benzene and lighter materials, and > 99% of the toluene. The stripper column utilizes low-pressure steam as the vapor phase for stripping. The stripper is operated at a suitable bottom temperature to maximize the VOC's removal. The pressure of the stripper is sufficient for recycling the overhead vapor (steam + HC) back to the QWT. The stripped bottoms (treated QW) is pumped to the DSG in a closed loop system, or discharged to battery limits in an open loop system. Any heavy organic materials are not removed by stripping. Consequently, heavy hydrocarbons and polymers that are formed during the stripping process are removed from the DSG through a continuous purge stream. 1 Patent Pending



The spent gasoline from the top of the extractor is routed to the gasoline regenerator. Stabilized hydrogenated gasoline (C6 - C8 cut), or preferably toluene, feeds the gasoline regenerator that is reboiled with desuperheated MP steam in a vertical thermosiphon reboiler and a water-cooled condenser. Regenerated gasoline is withdrawn from the tower as a heart cut. In the gasoline regenerator, both the recirculating gasoline from the extractor and the makeup gasoline are fractionated to remove the extracted light hydrocarbon precursors with the overheads and the heavy hydrocarbons including polymers with the bottoms. The overhead vapor purge is routed to the QWT and the bottoms to the tar drum for disposal. The regenerated gasoline is withdrawn as a liquid side stream from the upper section of the gasoline regenerator and pumped to the bottom of liquid/liquid extractor. Simulation of Quench Water Pretreat Unit was performed using S&W proprietary LLE equilibrium data of the polymer precursors (dienes, styrenes and indenes) between aqueous and the hydrocarbon phases. QW feed stream from a typical gas cracker plant having a flow of 300 gpm was simulated. Simulation results are presented in Table (1). The removal of the polymer precursors from the extractor raffinate stream down to a total of 10 ppm allows the LPWS to be operated without fouling. The stripper removes benzene to non-detectable levels and can be operated at higher bottom temperatures (up to 140° C) to achieve higher removal of the residual trace hydrocarbon contaminants. Since the QW feed to the DSG system is free of fouling contaminants, only a boiler and a knockout drum are required to generate the dilution steam. The DSG system water purge would contain any heavy hydrocarbon tail carried from the extracting solvent as well as the product of polymerization of the trace polymer precursors that are carried with the QW raffinate stream. Advantages of QW Pretreat Technology. Cleaning the oily net QW by this Pretreat system offers the following advantages: • Eliminates the need for the expensive coalescing equipment (DOX and IGF) • Contacting with C6-C8 aromatic rich gasoline removes polymer precursors, phenols and

heavy hydrocarbons from QW; the extracted QW is readily stripped in the LP water stripper to the required benzene specification.

• Eliminates fouling of the LPWS and DSG units • The stripped QW has only trace amounts of heavy hydrocarbons; hence the need for a DSG

tower is eliminated. • Dilution steam is generated by the use of knockout drum and boiler only. • Water blow down is significantly reduced because all the heavy hydrocarbon is extracted.

The amount is controlled by the salt content in the water and not by the oil content.



• Both COD and BOD of the dilution steam generator blow down are significantly lowered;

thus, the duty of the wastewater treatment (WWT) unit is reduced. • In an open-loop system, the net water discharge meets the environmental regulations

regarding benzene, dienes and other carcinogenic contaminants. • Eliminates the risk of coking precursors returning with the dilution steam to the furnaces. References Koenig et al.,�Quench Water Cleanup�, AIChE 3rd Annual Ethylene Producers Conference Proceedings, New Orleans, LA, March 31-April 3, 1992. Mullenix et al., �DOX Operating Experience in Ethane Cracker�, AIChE 5th Annual Ethylene Producers Conference Proceedings, Houston, TX, March 29-April 1, 1993. Guyot et al., �Increase Ethylene Processing Capacity and Efficiency with Improved Liquid/Liquid Separation�, AIChE 12th Annual Ethylene Producers Conference Proceedings, Atlanta, GA, March 3-7, 2000. Biography Sabah Kurukchi is a Senior Technologist at Stone & Webster, Houston, specializing in the design and evaluation of separations systems. He can be contacted at 281-368-4008 and at [email protected].



Table �1 Material Balance

STREAM ID 2 10 12 16 22 32+12 28 30 36 38 48 50

STREAM NAME Extractor Extractor Extractor Extractor Solvent Regen. Regen. Regen. Stripper Stripper DSG DSG

QW Feed Solvent FD Extract Raffinate Make-up Feed OVHD BTMS OVHD BTMS OVHD BTMS

COMP.,WPPM

1 H2O 996934 1759 2087 999381 0 2081 165439 0 987626 999969 999968 999996

2 C2 and Lighter 65 0 106 11 0 106 52928 0 249 0 0 0

3 C3's 28 28 28 28 28 28 28 0 28 0 0 0

4 BUTADIENE 51 2 104 0 0 104 50784 0 0 0 0 0

5 C4's SATURATED 4 4 4 4 4 4 4 0 4 0 0 0

6 CYCLOPENTADIENE 32 53 113 <2 0 113 30170 0 24 <2 <2 0

7 1PENTENE 6 2 12 0 0 12 5444 0 0 0 0 0

8 2METHYL HEXANE 0 1 1 0 0 1 58 0 0 0 0 0

9 BENZENE 524 61541 61885 188 0 61870 487422 413 4188 0 0 0

10 TOLUENE 78 935962 930598 346 54255 930584 184822 422490 7696 0 0 0

11 OXYLENE 105 272 478 0 923460 500 0 29901 2 0 0 0

12 C9-400F 1454 46 2918 0 11253 2912 0 375355 0 0 0 0

13 LFO C14-C24 468 0 924 0 0 922 0 120723 0 0 0 0

14 HFO C26+TAR 90 0 177 0 0 177 0 23115 0 0 0 0

15 ETHYL BENZENE 4 246 253 0 0 275 1 4204 1 0 0 0

16 STYRENE 21 35 77 <2 11000 77 0 5453 1 <2 0 <2

17 PHENOL 53 8 40 36 0 40 0 4312 72 31 31 3

18 ISOPRENE 24 22 68 1 0 68 22897 0 12 0 0 0

19 INDANE 4 0 9 0 0 9 0 1139 0 0 0 0

20 NAPHTHALENE 6 0 12 0 0 12 0 1536 0 0 0 0

21 CARBONYLS 4 17 17 4 0 17 3 0 97 0 0 0

22 INDENE 44 1 88 <2 0 88 0 11359 0 <2 0 <2

RATE, LB/HR 147684 74392 74781 147295 150 74931 150 572 6620 157374 157194 180

TEMP, F 178.2 191.2 178.4 179.9 100.0 184.7 141.9 290.0 242.1 246.4 373.1 374.0

PRES., PSIA 70.0 70.0 50.0 68.0 25.0 20.0 18.0 20.0 26.0 28.0 180.0 182.0

MOLE WEIGHT 18.06 90.49 90.46 18.02 91.52 90.46 46.64 111.52 18.19 18.02 18.02 18.02

WT FRAC VAPOR 0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.0 1.0 0.0 1.0 0.0



CRACKING FURNACES

HEAT EXCHANGER

QWT

OIL / WATERSEPARATOR

HP STEAM(FOR POWERGENERATOR)

BFW

QUENCHED CRACKED GAS TO

RAWQW

TAR PYROLYSISGASOLINE

QUENCH WATERHEAT

RECOVERY(USERS)

DILUTION STEAM

FIGURE 1 CRACKED GAS -WATER QUENCHING

OPEN LOOP SYSTEM

QWT = QUENCH WATER TOWERLPWS = LOW PRESSURE WATER STRIPPERDSG = DILUTION STEAM GENERATORQWPT = QUENCH WATER PRETREAT

FEEDSTOCK

MAKEUPGASOLINE

CW

SPENTGASOLINE

QW - W/ODISSOLVED

OIL

QW - WDISSOLVED

OIL

TRADITIONALFREE OIL

SEPARATORSYSTEM

LPWSEITHER

S&WQWPT

NET

QW

PURGE WATER TO BATERY LIMIT

LPWS TO QWT



CRACKING FURNACES

HEATEXCHANGER

QWT

OIL / WATERSEPARATOR

HP STEAM(FOR POWERGENERATOR)

BFW

QUENCHED CRACKED GAS TO

RAWQW

TAR PYROLYSISGASOLINE

QUENCH WATERHEAT

RECOVERY(USERS)

DILUTION STEAM

FIGURE 2

CRACKED GAS -WATER QUENCHING AND DILUTION STEAM GENERATING SYSTEM

CLOSE LOOP SYSTYEM

QWT = QUENCH WATER LPWS = LOW PRESSURE WATER DSG = DILUTION STEAM QWPT = QUENCH WATER

FEEDSTOCK

MAKEUPGASOLINE

CW

SPENTGASOLINE

QW - W/ODISSOLVED

OILDILUTIONSTEAM

BLOWDOWN

QW - WDISSOLVED

OIL

TRADITIONALFREE OIL

SEPARATORSYSTEM

DSGLPWSEITHER

S&WQWPT

NET

QW



DEOILED QW

LIQUID/LIQUID EXTRACTORM AKE-UP GASOLINE

LP STEAM

LOW PRESSURE WATER STRIPPER

GASOLINE

REGENERATOR

DILUTION STEAM TO FURNACES

QW BLOWDOWN

DILUTION STEAM DRUM

M P STEAM

LP STEAM

STRIPPER OVHDTO QUENCH TOWER

SPENT GASOLINETO QUENCH TOWER

OVHD PURGETO QUENCH TOWER

FROM OIL/WATERSEPARATOR

QUENCH WATER PRETREAT UNIT

FIGURE 3

innovative qw treatment - by s kuruchi

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