CHAPTER 31PETROLEUM INDUSTRY

Petroleum refining is one of the major manufacturing industries in the UnitedStates, producing more than 75% of the petroleum products consumed in thiscountry. Ninety percent of these products are fuels supplying over 46% of thecountry's energy needs. Plastics, solvents, asphalt, lubricants, and intermediatechemicals are among the more than 3000 products made from petroleum.

Petroleum refining is a continuous operation incorporating a number of inter-related process units. It is one of the largest "wet" processing industries in theUnited States.

Because it it used in the refinery for heating, cooling, and processing, water isan important factor in process operations. A refinery may draw water from a vari-ety of sources, and the treatment processes used to condition this water varyaccordingly. Little or no treatment may be required for some well waters, whileother water sources may require an extensive plant incorporating clarification,softening, and filtration. The types of water treatment problems here are muchthe same as in other industrial applications. In general, the basic water flowscheme is illustrated in Figure 31.1.

After initial processing, the water is usually divided into several streams foruse throughout the refinery. At most locations, this water can be used directly ascooling tower makeup, with little or no further pretreatment. However, an exten-sive treatment scheme is usually necessary to produce the high-quality waterrequired for boiler feed.

Figure 31.2 shows a typical water-handling system designed for a larger refin-ery utilizing a surface water source as plant makeup.

Refinery processes are net consumers of heat, to the extent that 10 to 15% ofthe heat equivalent of the incoming crude is used in the refinery operations. Thisis provided by refinery by-products, including off-gases, residual oil, and coke, inmany cases, supplemented by natural gas. A typical process heat balance is shownby Figure 31.3. This does not include heat recovered in process-heat boilers.

PROCESS OPERA TIONS

The refining of petroleum products and petrochemicals involves two basic oper-ations: physical change, or separation processes; and chemical change, or conver-sion processes. A refinery is a conglomerate of manufacturing plants, the numbervarying with the variety of products produced. The bulk of these products (ker-osenes, fuel oils, lubricating oils, and waxes) are fractions originally present in andsubsequently separated from the crude petroleum. Some of these are purified and

FIG. 31.2 Water conditioning system treating surface water for refinery usage.

Sewagetreatment

unit

Drinking

Sanitary

Hot limezeol i te Boilers

Boilerblowdown

flashto river

Clean watersewer

Flashcondensate

Once through cooling water

Fast rinseRegenerant

Processunits Sour water

from units

Stripper

Desalter

To processwater

treatment

Blowdown

Alkycooling, tower

Generalcoolingtower

Cl 2

Riverwater

FIG. 31.1 Water uses in a typical 150,000 bpd refinery.

Sani tarygpm50 50 gpm

Process350gpm

350 gpm

350 gpmDesalting

Stripped condensate

350 gpm

650,000 Ib/hr at 600 psi100,000 Ib/hr process heat at 150 psi

gpm600

Boilers

Watersupply at8000gpm 50 gpm blowdown To waste treatment

at 1700 gpm

Once-through cooling - 3500 gpm

900 gpm blowdown and losses3500gpm

Cooling towers

Fluid cat cracker -8 exchangersPlatinum reformer -12 exchangersCrude unit -6 exchangersUnsaturated gas plant-10 exchangersSaturated gas plant -10 exchangersHydrodesulfurization -4 exchangersCoker-7 exchangersSul fur plant -3 exchangersMiscellaneous ut i l i t ies.gland cooling,etc

Total heat requirement is approximately 674,000Btu/bbl of crude processed.SOURCE: From U.S. Bureau of Mines.

FIG. 31.3 Refinery process heat balance.

and all the other kinds of products that can be made would require as many as50 different processes. Figure 31.4 is an overall flow diagram for a generalizedrefinery production scheme.

As previously mentioned, refining is basically concerned with separation andconversion processes, carried out by individual unit operations. Basic to most ofthem are furnaces, heaters, and heat exchangers, and distillation and extractioncolumns. Heat exchangers are typically of shell and tube design, often utilizingincoming hydrocarbon feedstock as a cooling medium for hot products. If addi-tional cooling is required—trim cooling—this is done with water.

Distillation and extraction are usually the principal techniques in product sep-aration. Distillation separates various hydrocarbon mixtures into componentshaving different boiling points. In extraction, hydrocarbons are separated basedon their different solubilities in a specific solvent. In some unit operations, filtersare used to remove suspended contaminants from the hydrocarbon stream, suchas catalyst fines and inorganic precipitates.

Distillation

Figure 31.5 illustrates a typical distillation column (pipe still). Preheated crude ischarged into the bottom of a distillation column at a pressure slightly aboveatmospheric, and the vapors rise through the column, contracting a down flowstream (reflux). As a result, the lightest materials concentrate at the top of thecolumn, the heaviest materials at the bottom, and intermediate materials inbetween. Desired products are withdrawn at appropriate points. Because thelighter products (as vapor) must pass through the heavier products (as liquid) andmust be in equilibrium with them at each point in the column, each stream con-tains some very volatile, low-molecular-weight components (light ends).

Source of energy

Crude oilDistillate oilsResidual oilLPG

Natural gasRefinery gas

Coke

Steam (purchased)Electricity (purchased)

Percent of total heat

Nil1.69.01.3

36.235.1

13.1

1.12.6

supplemented with nonpetroleum materials to enchance their usefulness. A refin-ery with half a dozen processes, including distillation and cracking, can producegasoline, kerosene, and fuel oils. The manufacture of solvents requires two orthree more processes; lubricating oil production, at least five more; waxes anothertwo or more. Asphalts, greases, coke, gear oils, liquefied petroleum gases, alkylate,

Liquefied petroleum gas

Di-isobutylene

Gasoline

HYDRO-GENATION

2 POLY-MERIZATION

2

ALKYLATlON

BLENDING

lsobutylene

Butylene

Butane

ISOMERIZATION

lsobutone

Stabilized gasoline

Kerosine

Light furnace oil

Furnace oil

Diesel oil

Heavy fuel oils

Asphalts

[LightBLENDING

ANDPACKAGING

1 WAXREFINING

PLANT

Refined waxes

lubricating oilsHeavylubricating oils

Wax

1

DEWAXINGDECOLORIZING

GASRECOVERY

ANDSTABILIZATION

OFGASOLINE

Furnace oil ordiesel oil

Heavy fuel oils

1SOLVENTREFINING

1SOLVENTREFINING

Gas

Straight run gasoline

Gas

2

CATALYTICREFORMING

Straight run gasoline

Naphtha

Kerosine or light furnace oil

Gas oil

1

CRUDEOIL

DISTIL-LATION

Crude oil

Alternateoperations

andproducts

dependenton kind ofcrude oilbeing run

Residue

Asphalticresidue

Lubricatinaresidue

1

VACUUMDISTILLATION

1 VACUUMDISTILLATION

1ATMOSPHERICOR VACUUMDISTILLATION

Heavy gas oil

CATALYTIC CRACKING

Heavy fuel oil

Asphalts

Heavy gas oil

Lubricating distillates

Lubricating residue

FIG. 31.4 Generalized flowchart for petroleum refining. (From Shreve, 1967).

FIG. 31.5 Distillation column with sidestream steamstripper.

accumulator prior to recirculation or transfer of the hydrocarbons to another frac-tionator. The water that separates from the hydrocarbons in these accumulatorsis usually drawn off and discharged to the wastewater treatment system. Thiswater can be a major source of sulfides, especially when sour crudes are beingprocessed; it may also contain significant amounts of oil, chlorides, mercaptans,and phenols. A second significant waste source is discharge from oil samplinglines; this oil should be separable, but may form emulsions in the sewer. A thirdpossible waste source is the stable oil emulsion formed in barometric condensersused to produce a vacuum in some distillation units (Figure 31.6).

In vacuum towers a steam jet ejector is the most widely used method for cre-ating a vacuum. The steam and other vapors removed from the fractionator mustbe condensed, and the liquid removed prior to discharge of the vapor to theatmosphere.

The barometric condenser condenses the steam jet by a water spray in a closedchamber, and the water drains down the barometric leg. The organics, oils, andsteam condensate are intimately mixed in a large volume of cooling water, whichtends to form difficult-to-handle emulsions. Newer refineries use surface con-

As indicated in Figure 31.5, steam strippers are sometimes used to removelight ends from a sidestream. The sidestream is fed to the top of the stripper;countercurrent steam strips out the light ends and carries them back to the maincolumn.

The wastes from crude oil fractionation, and in general from most distillationcolumns, come from three sources. The first is the water drawn from the overhead

Disti l lat ioncolumn

Gas

Coolingwater

Side streamstripper

Crudeoil

Light naphtha

Heavy naphtha

Kerosene

Steam

Diesel

Gas oil

Preheatfurnace

Steam

Reduced crude

densers instead of barometric condensers. These units consist of a series of shelland tube exchangers in which the condensibles are removed, and the water forcooling does not come into direct contact with the condensate (Figure 31.7). Thewater discharge from the distillation operation is sent to the refinery waste plant,sometimes through a separate gravity oil separation unit.

FIG. 31.7 Tubular process steam condenser.

Sewer

Seal pit

3 4 f t

Barometr icleg

Vacuum line tovacuum tower

Primarylet

Steam

Cooling water

Steam

Secondaryjet

Non-condensiblesto f lare

FIG. 31.6 Barometric condenser.

Sewer

Seal pit

34 f t

Barometricleg

Vacuum line tovacuum tower

100 psi steam

Primaryjet

Secondaryjet Non-condensibles

to flare

Cooling water

Thermal Cracking and Related Subprocesses

In thermal cracking units, heavy oil fractions are broken down into lighter frac-tions by application of heat, but without the use of a catalyst. Production of gas-oline is low, but middle distillates and stable fuel oils are high. Visbreaking andcoking, the two major types of thermal cracking, maximize the production of cata-lytic cracking feedstocks, indirectly increasing gasoline production.

Oil feed is heated in a furnace to cracking temperatures and the cracked prod-ucts are separated in a fractionator (Figure 31.8). The heat breaks the bonds hold-ing the larger molecules together, and under certain conditions, some of the result-ing smaller molecules may recombine into larger molecules again. The productsof this second reaction may then decompose into smaller molecules dependingon the time they are held at cracking temperatures.

Visbreaking is a mild form of thermal cracking; it causes little reduction inboiling point, but significantly lowers viscosity. The feed is heated, crackedslightly in a furnace, quenched with light gas oil, and flashed in the bottom of afractionator. Gas, gasoline, and furnace oil fractions are drawn off, and the heav-ier fractions are recycled.

Residual oils may be cracked to form coke as well as the usual gaseous andliquid products. The most widely used process, known as delayed coking (Figure31.9), accounts for about 75% of the total oil coking capacity.

Thermal cracking units require cooling water and steam on the fractionatingtowers used to separate products. Some towers employ steam-stripping of a side-stream to remove light ends, requiring an overhead condenser and accumulatorsystem for product/wastewater separation. Wastewater usually contains variousoil fractions and may be high in pH, BOD, COD, NH3, phenol, and sulfides.Another important water use area is the high-pressure water sprays used for cokeremoval in delayed coking. Several refiners have instituted water recycle clarifi-cation systems to minimize water discharge from these coking units.

Catalytic Cracking

The fluid catalytic cracking process is the most widely used refining process (Fig-ure 31.10). A large mass of finely powdered catalyst contacts the vaporized oil inthe processing unit. The catalyst particles are of such a size that when aerated or"fluffed up" with air or hydrocarbon vapor, they behave like a fluid and can bemoved through pipes and control valves.

FIG. 31.8 A thermal cracking process treating topped crude (Universal Oil Products pro-cess). (From Shreve, 1967.)

ToppedCrude

Charge

Reactionchamber

Fractionatingcolumn Stripper

Condenser

WaterFlash chamberHeater

Receiver

FurnaceDistillate

Gas

Gasolineto

StabilizerFuel oil

Residuum^Lig

htcr

ack

ing

sto

ck

Hea

vycr

ack

ing

sto

ck

FIG. 31.10 Fluid catalytic cracking unit.

In the catalytic cracking process, feed and regenerated equilibrium catalystflow together into the riser reactor. The cracked vapors from the reactor passupward through a cyclone separator which removes entrained catalyst. The prod-uct vapors then enter a fractionator, where the desired products are removed andheavier fractions recycled to the reactor. Spent catalyst passes from the separationvessel, downward through a stream stripper, and into the regenerator where car-bon deposits are burned off. The regenerated catalyst again mixes with the incom-ing charge stream to repeat the cycle. On units using a CO boiler, partial com-bustion in the regenerator is used. This incomplete combustion produces apreponderance of CO over CO2. Significant amounts of hydrocarbons and othersubstances also remain unburned in the combustion gases. The CO and hydro-carbons in the exhaust make it a useful fuel. Most refineries burn this exhaust gasin specially designed carbon monoxide (CO) boilers to generate steam. The gas

FIG. 31.9 Delayed coking process.

Coke drumsOverhead

Aircooler Gas

Coke jdumping

Direct f iredheater

Feed Air cooler

Overhead Light ends t o g a s

concentration plant

Fluid ca ta lys tbed

C a t a l y s ts t r ipper

Fract ionator

Steam Light cyc le oil

C la r i f i ed oi l

Flue gas to precipitator

and CO boiler

Regenerator

Ai r forcombust ion

Compressor

Feed

C a t a l y s t f i nes

S l u r r ys e t t l e r

Heavy gas oil

Air cooler

Light gas oil

Steam

Light gasoil stripper

Naphtha

Accumulatordrum

burned in the CO boiler carries a significant amount of residual catalyst fines,which can cause furnace deposits.

Today, most units operate in a complete combustion mode in the regenerator.This reduces carbon on regenerated catalyst, improves product yields, and helpsreduce preheat fuel requirements.

Most fluid catalytic cracking units process vacuum and coker gas oils as feed.These feeds contain low levels of nickel and vanadium, which are serious catalystpoisons. "Heavy oil" units, which use part heavy oil for feed, may operate withmore than 2000 ppm Ni and V on the equilibrium catalyst. This is made possibleby using a metals passivator, which decreases the dehydrogenation poisoningactivity of Ni and V.

The catalytic cracker is one of the largest producers of "sour water" in therefinery, coming from the steam strippers and overhead accumulators on theproduct fractionators. The major pollutants resulting from catalytic crackingoperations are BOD, oil, sulfides, phenols, ammonia, and cyanide.

Catalytic Reforming

In catalytic reforming (Figure 31.11), the object is to convert straight chain intocyclic molecules (aromatics of high octane). So reforming is essential to produc-tion of high octane gasoline. Platforming, the most widely used reforming process,includes three sections: (1) in the reactor heater section, the charge plus recyclegas are heated and passed through reactors containing platinum catalyst; (2) theseparator drum separates gas from liquid, the gas being compressed for recycling;and (3) the stabilizer section corrects the separated liquid to the desired vaporpressure.

FIG. 31.11 Catalytic reforming process.

The predominant reforming reaction is dehydrogenation of naphthenes, orremoval of hydrogen from the molecule. Important secondary reactions involverearrangement of paraffin molecules. All of these result in a product with higheroctane ratings that the reactants. Platinum and molybdenum are the most widelyused catalysts, with platinum predominating because it gives better octane yields.

Hydrot reated naphtha

Cata l ys t to regenerator

No. 3

No. 2Regenerator

No. 1 reactor

Low pressure separator High pressure separator

Separator gasVapor

compressor

Pump

Direct f i red heater

Gas recyc l e

Liquid to s tab i l i ze r

Because platinum catalysts are poisoned by arsenic, sulfur, and nitrogen com-pounds, feedstocks usually are treated with hydrogen gas (hydrotreated) beforebeing charged to the reforming unit. This produces hydrogen compounds, such asH2S, which can be removed from the hydrocarbon steam.

Reforming is a relatively clean process. The volume of wastewater flow andthe pollutant concentrations are small.

Alkylation

The amalgamation of small hydrocarbon molecules to produce larger moleculesis known as alkylation (Figure 31.12). In the refinery, this reaction is carried outbetween isobutane (an isoparafnn) and propylene, pentylenes, and in particular,butylenes (olefins). The product is call alkylate. The olefm-isobutane feed is com-bined with the fractionator recycle and charged to reactor (contactor) containingan acid catalyst at a controlled temperature. Aluminum chloride, sulfuric acid,and hydrofluoric acid are common acid catalysts. The content of the contactorare circulated at high velocities to expose a large surface area between the reactinghydrocarbons and the acid catalyst. Acid is separated from the hydrocarbons in arecovery section downstream and recirculated to the reactor. The hydrocarbonstream is washed with caustic and water before going to the fractionating sections.Isobutane is recirculated to the reactor feed, and alkylate is drawn from the bot-tom of the last fraction (debutanizer).

Acid may contaminate the cooling water should heat exchangers leak. Waterdrawn from the overhead accumulators contains varying amounts of oil, sulfides,and other contaminants, but they are not a major source of waste in this subpro-cess. The wastes from the reactor contain spent acids, which refineries may pro-cess to recover clean acids or may sell. Occasionally, some leakage to the seweror cooling system does occur. The major contaminant entering the sewer from asulfuric acid alkyation unit is spent caustic from neutralization of the hydrocar-bon stream leaving the reactor.

FIG. 31.12 Alkylation process using sulfuric acid. (Adaptedfrom Hengstebeck, R. J.: PetroleumProcessing, McGraw-Hill, New York, 1959.)

Isobutane

Deisobutanizer

CausticwashSettler

Stirredreactor

Refrigerant

Chil lerSpentcaust ic

Depropanizer

Caust icwash

Freshcaust ic

Butanes-butenes

Emulsion

Acid

Freshcaustic

Spentcaustic

n-butane +alkylate

Propane

Hydrofluoric acid alkylation units do not have spent acid or spent causticwaste streams. Any leaks or spills that involve loss of fluoride constitute a seriousand difficult waste problem.

Sulfuric alkylation units usually have a chilled-water refrigeration system withseveral compressors which have critical shell-side cooling water on inter- andafter-coolers.

Hydrotreating

Hydrotreating is mild hydrogenation (Figure 31.13) that removes sulfur, nitrogen,oxygen, and halogens from a hydrocarbon feed and converts olefins (unsaturatedhydrocarbons) to saturated hydrocarbons. Petroleum feedstocks ranging fromlight naphthas to lubricating oils are hydrogen treated. The major application ofhydrotreating has been removing sulfur from feeds to catalytic reformers to pre-vent catalyst poisoning. Each of the different types of hydrotreaters, which varyin the selection of catalyst, incorporates a reactor and a separator. The oil feed,preheated to 400 to 70O0F (204 to 3710C), passes through a fixed-bed reactorwhere it combines with hydrogen in the presence of a regenerable metal oxidecatalyst at 200 to 500 lb/in2 gage (14 to 35 kg/cm2). The product stream is cooledbefore entering a separator where excess hydrogen gas is separated for use in otheroperations. After separation, the product is steam stripped for removal of residualhydrogen sulfide.

FIG. 31.13 Hydrotreating process.

Major wastewater streams come from overhead accumulators on fractionatorsand steam strippers, and sour water stripper bottoms. The major pollutants aresulfides and ammonia. Phenols may also be present, if the boiling range of thehydrocarbon feed is high enough.

UTILITY SYSTEMS

The steam-generating system is the heart of the refinery operation, since steam isa major source of energy in the refinery. It is used to drive pumps and compres-sors, to heat process streams, and to strip sour water. Few refineries generate elec-tricity, but typically produce 10% of their motive power requirement. A typicalwater balance is shown by Figure 31.14.

PreheatFeed

Hydrogen-

nchgas

ReactorPreheat

Gas

Separator

Residual H2S

Separator

WaterS t r i ppe r

Steam

Product

CW

FIG. 31.14 Process steam uses and condensate recovery in a typical utility.

The pretreatment of boiler feed water is one of the most important steps inefficient boiler operation. The specific pretreatment scheme for a steam-generat-ing system is dependent on such factors as boiler design, steam requirements, heatbalance, outside power costs, and further expansion. Many refineries use hot pro-cess softeners, filters, and ion exchange trains.

Additional steam is generated in heat recovery and process-heat boilers. Theseboilers can be found throughout the refinery at the various process units. Process-heat boilers often resemble shell- and tube-heat exchangers in design. In mostrefineries, the steam generated by the process usually condenses as an acceptablequality condensate for return to the boiler that produced it. The major concernin these systems is prevention of corrosion in the condensate system, particularlyat the point of initial steam condensation. In addition, depending on the pressureof the system, a condensate polishing unit may be installed to remove oil, corro-sion products, or both, prior to returning this water to the boiler.

COOLING WATER SYSTEMS

As stated earlier, the cooling requirements of a refinery demand a large volumeof water. It has been estimated that 80 to 85% of the total water requirements arefor cooling if cooling towers are used for water conservation. The cooling systemsin a refinery are similar to those found in many manufacturing plants, involving

To wastetreatment

DesalterSour water

stripper

40% condensate returns

Sourcondensate

Cleancondensate

Heatload

Strippingsteam

Condensate fordesuperheat

CondensingturbineBack pressure

turbines

600 psi at 70O0F

Boilerfeedwater

2 -COboilers

Processheatboiler

Aux.fuelboiler

150psi150 psi

Heatload DAH

ReceiverBFP

P r o c e s s hea t , b o i l e r s

Mam deaerator60%t reatedmakeup

once-through, open recirculating, and closed cooling circuits. While each of thesesystems can be found in the refinery, the open recirculating system serves thegreatest demand for process cooling. Once-through cooling is becoming rareexcept for coastal installations designed to use seawater.

REFINERY POLLUTION CONTROL—WASTETREATMENT

Broadly speaking, a refinery wastewater system consists of:

1. A drainage and collection system.2. Gravity-type oil-water separators and auxiliaries required to remove oil and

sediment.3. Treatment units or disposal facilities to handle segregated chemical solutions

and other process wastes and to control the effects of pollutants that have toxicproperties.

Figure 31.15 is a generalized list of wastewater sources; it also shows how theyare segregated into the various sewer systems provided to optimize reuse andreduce overall treatment volumes to a minimum. The oil-free sewer collectswaste waters that have not contacted oil and that are not subject to any other con-tamination for which treatment must be provided.

Since these waters seldom contain significant oil contamination, they maybypass API separators and some may discharge directly into the refinery outfall

FIG. 31.15 Suggested scheme of collection and treatment of refinery wastewaters.

Storm water Storm sewers

To receiv ing

streamO i l - f r ee sewers

A l t e rna tedetent ion orseparat ionOily sewers

API separator

air f l o ta t i onor both

Reuse

Finaltreatment

Strong was tes

Equal izat ionAPI separatorf l o t a t i o n

San i ta ry sewers

TreatmentTo streamSan i ta ry wastes

Stronger wastesDesa l te r was tewater

Sour condensates (excess )

B a r o m e t r i c condensate ( s e l e c t e d )Treatment plant wastes

Strong washes (caus t i c , ac id )

Once through cooling water ( C - 6 +)Trea ted cooling tower b lowdownPad drainageTank draw offBarometr ic condensate (se lec ted)

Once-through cooling water (below C-6)Non- t reated cooling tower blowdownRoof drainageWater t reatment plant washes

and f i l t r a t e s

line. However, if collected in a common sewer, this flow is usually mixed withoily waste after API separators for a common treatment.

The oily cooling water sewer system is intended to handle waters that areexpected to be subject to minor oil contamination from leaks in heat exchangeequipment or from spills. In the absence of contamination by chemicals or finesolids which tend to cause emulsions, separation of the oil from the water can bereadily accomplished. Barometric condenser cooling water subject to contami-nation by easily separable oil, but not containing emulsified oil, may also beincluded in this system.

The process water sewer system collects most wastewaters that come intodirect contact with oil or that are subject to emulsified oil contamination or tochemicals limited by the plant discharge permit. Water from the process watersewer system is treated in an oil-water separator, and pollutants remaining aftergravity separation are reduced by secondary treatment methods.

Separator skimmings, which are generally referred to as a slop oil, requiretreatment before they can be reused because they contain an excess of solids andwater. Solids and water contents in excess of 1% generally interfere withprocessing.

The sanitary sewer system collects only raw sanitary sewage and conveys it tomunicipal sewers or to refinery treatment facilities. State or local regulations usu-ally determine the sanitary disposal requirements. Raw sewage can be used forseeding refinery biological treatment units.

Special systems include those required for the separate collection and handlingof certain wastes having physical or chemical properties that cause undesirableeffects in the refinery drainage system, oil-water separators, or secondary treat-ment facilities. Spent solutions of acids and caustic, foul condensates, anddegraded solvents are examples of such wastes.

Main wastewater treatment systems separate pollutants from the water byphysical, chemical, or biological means. Primary treatment consists of physical,and often chemical, processes. Primary treatment separates the gross waste loadof oil and suspended solids from the water. Secondary treatment removes muchof the remaining organic and dissolved solid pollutants by biological treatment,which consumes and oxidizes organic matter.

There are a few physical, chemical, or biological methods known as tertiarytreatment, including activated carbon adsorption and filtration. As pollution con-trol regulations become more stringent, tertiary treatment methods will becomemore common.

Sour Water Stripping

Many wastewater effluents from petroleum refining processes originate from theuse of steam within the processes. The subsequent condensation of the steam usu-ally occurs simultaneously with the condensation of hydrocarbon liquids and inthe presence of a hydrocarbon vapor phase that often contains H2S, NH3, HCN,phenols, and mercaptans. After separation from the hydrocarbon liquid, the con-densed steam contains oil and a mixture of these contaminants. These wastewa-ters are typically called sour waters or foul waters because of the unpleasant odorcharacteristic of dissolved hydrogen sulfide.

The amounts of these contaminants in a sour water stream depend on the typeof refining process from which the stream originated, as well as the feedstock tothat process and the pressure level at which the steam was condensed within the

process. The contaminant concentrations in typical sour waters will usually be 50to 10,000 mg/L H2S, 50 to 7,000 mg/L NH3, and 10 to 700 mg/L phenolics.

The principal contaminants in sour waters are hydrogen sulfide and ammonia,ionized as HS" and NH4

+, respectively. These can be removed by single-stepsteam stripping, which is a simple form of distillation for the removal of dissolvedgases or other volatile compounds from liquids. Stripping is rather inefficient andrequires large volumes of steam because these ionized substances exert very littlegas pressure unless the pH is adjusted. Stripping also removes phenolics to someextent, but the amount of phenolics removed may vary from O to 65%.

Effluent water from a sour water stripper is often used as makeup to the desal-ter. Refiners are investigating new schemes for incorporating portions of thiswater into refinery utilities systems.

Spent Caustic Treatment

Alkaline solutions are used to wash refinery gases and light products; the spentsolutions, generally classified as sulfidic or phenolic, contain varying quantities ofsulfides, sulfates, phenolates, naphthenates, sulfonates, mercaptides, and otherorganic and inorganic compounds. These compounds are often removed beforethe spent caustic solutions are added to refinery effluent. Spent caustics usuallyoriginate as batch dumps, and the batches may be combined and equalized beforebeing treated or discharged to the general refinery wastewaters.

Spent caustic solutions can be treated by neutralization with spent acid or fluegas, although some phenolic caustics are sold untreated for their recoverablephenol value. Neutralization with spent acid is carried to a pH of 5 to ensuremaximum liberation of hydrogen sulfide and acid oils.

In the treatment of spent caustic solutions by flue gas, hydroxides are con-verted to carbonates. Sulfides, mercaptides, phenolates, and other basic salts areconverted to hydrogen sulfide, phenols, and mercaptans at the low pH conditionscaused by the flue gas stripping. Phenols can be removed and used as a fuel orcan be sold. Hydrogen sulfide and mercaptans are usually stripped and burned ina heater. Some sulfur is recovered from stripper gases. The treated solution willcontain mixtures of carbonates, sulfates, sulfides, thiosulfates, and some phenoliccompounds. Reaction time of 16 to 24 h is required for the neutralization of caus-tic solution with flue gas.

Shale Oil

Closely related to the petroleum refining industry in process technology is thedeveloping synthetic fuels industry, with its two major segments, recovery ofhydrocarbons from oil shale and conversion of coal to fuel gas, hydrocarbons, ora family of petrochemicals.

The processing of oil shale involves large-scale mining operations since theamount of oil in economically treatable shale is usually only 10 to 15% of weightof the native shale. At 15%, the production of 100,000 barrels of oil would pro-duce a residue of about 100,000 tons of spent shale.

The release of oil is accomplished by heating the raw shale in a device like akiln, although there is hope that in situ retorting may prove practical in the future.Since the heat capacity of the shale is high, this process consumes a great deal ofenergy, which is one reason for its rather slow development. The hydrocarbons

released from the shale may be processed by fractionation and extraction to yieldlight ends and heavy fractions that may be used as refinery feedstock or fuel forretorting and steam generation.

Wastewater problems are similar to those of the refineries and coal-fired utilitystations, the major one being the runoff from the large volumes of spent shaleresidue.

SUGGESTED READING

Lund, H. F., (ed.): Industrial Pollution Control Handbook, McGraw-Hill, New York, 1971.Shreve, R. N.: Chemical Process Industries, 3d ed., McGraw-Hill, New York, 1967.U.S. Environmental Protection Agency: Development Document for the Petroleum RefiningIndustry, EPA-440/1 -74-014-a, April 1974.