API-36-204 Oil Treating Methods

8/12/2019 API-36-204 Oil Treating Methods

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Oil-TreatingMethods

The purpose of this paper is to present some of themore recent developments in crude-oil dehydration prac-tice. The highly technical subject of emulsion theor ieswill be discussed no more than enough to elucidate theproblems involved in the separation of water fromcrude-oil emulsions.

ABSTRACT

CHARACTER OF EMULSIONS

A brief description of some of the known character-istics of crude-oil emulsions precedes the discussion ofoil-treating methods. I t is suggested that emulsifica-tion be reduced, whenever possible, by maintaining wellpumps and line pu mps i n good condition, an d by elimi-nating sources of agi tation in pipe lines. Various meth-ods of dehydrating crude-oil emulsions ar e listed. Th echemical and electrical methods a re the most successfulof those in use. Preliminary treatment of the emulsionoften is similar in the chemical and electrical processes.The flow-line system of tre atment is prefe rred over thetank (batch) system in both methods, fo r the emulsionbecomes more stabilized as i t ages i n wet-oil tanks. De-mulsifying chemical is usually injected in the flow lineat th e well in chemical-method plants, and in th e feed-

An emulsion is a stable dispersion of one liquid inanother. C. H. M. Roberts state s tha t th e factors whichappear indispensable for the formation of an emulsionare: 1 that the two liquids be mutually immiscible; 2that there be suitable agitation to cause the dispersionof one liquid in the other; and, 3, tha t th ere be presentin one liquid-or in both-some substance or substanceswhich have the property of protecting the liquid-liquidinterface-so a s to prevent contact and coalescence ofthe droplets of the dispersed phase.' This protectivesubstance is called the emulsifying agent.

crude-oil emulsion may be one of three types waterdispersed in oil, oil dispersed in water, or a combinationof these two. The presen t discussion deals with thewater-in-oil, o r normal, type. The other two types ar enot often found in oil-field emulsions. Fig. 1 2 and 3illus trate typical crude-oil emulsions. Fig. 1 shows alow-gravity crude with a high percentage of loose emul-sion; the sample in Fig. 2 contains less loose emulsionand more tight emulsion; and Fig. 3 is an exampleof a tight gas-blown emulsion.

pump suction in tank systems of electrical dehydratiThe flow-line type of electric dehydrator seldom requichemical. Waste heat f rom gas engines is sometimutilized, when auxiliary heat is required. Firebox boiproduction heaters app ear most satisfactory f o r chemiplants handling fairly large volumes. Thennosyphheaters are adaptable to chemical plants. Tub ular boiheaters ar e reported to have rather high thermal eciencies. Small, low-priced, vertical boiler heaters widely used for smaller volumes. Flow-line electric hydrato rs often tr eat emulsions a t well temperaturTh e pape r describes typical chemical an d electrical treing plants, showing advantages to be gained by moderzation of old plants.

Barnsdall 011 Co.. os Angeles. Calif.Bignres refer to bibliography o p. 224.

San Joaquin Valley oil-gravity, 14.5 deg. API;water content, 40 per cent.

Microphotograph of Loose Emulsion in Low-GravityCrude Oil.

FIG.


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FORMATION OF MULSIONS

Gravity of oil, 14 5 deg. API water content, 24per cent.Microphotograph of Crude-Oil ~A u l s i o n ith Lower

Ratio of Loose Emulsion.FIG. 2

Gravity of oil, 28 deg. API; water content, 24per cent.

Microphotograph of Tight Gas-blown Emulsion.FIG 3

Immiscible LiquidsIn a wet-oil well we have the first factor necessary

for the fo rmation of a n emulsion: the immiscible liquids,oil and water. The term, wate r is used ra the r looselyto refer either to pure water or, most commonly, to anaqueous solution of mineral and/or organic materials.If the oil and water are from separate sources, thisfac tor sometimes can be eliminated by well-repair work.

gitationAgitation should be eliminated as f a r a s possible con-

sistent with the necessity of int imately mixing chemicalwith the emulsion when a demulsifying chemical is used.There may be some emulsification as the oil and waterenter the hole from formation, but t his is believed t o be'negligible. The well pump appear s to be the main sourcein pumping wells. Emulsions almost always ar e moreeasily broken down when well pumps are maintained ingood condition. The tubing, tub ing foot-piece, o r otherdevices produce agitation-par ticularly in flowing andgas-lift wells. The flow bean, when used, is a potentagitator. Emulsions may be formed, or their stabilityincreased, in various surface facilities through whichthe production must pass. Line pumps especially ar eoffenders in this respect. I t is reported t ha t positive-displacement pumps cause less violent agitation thancentrifugal or other types of pumps. The pump used,regardless of type, should be kept in good repair sothat a minimum amount of slippage occur^ ^

The foregoing discussion of agitation may be thoughtto be superfluous, but is included because of the growingrealization of th e importance of preventative measuresin the treatmen t of emulsions. I n one case, which mayhe cited as an example, the treating costs were cut inhalf by an alteration in pipe-line connections.Emulsifying gent

The thi rd factor involved in the formation of a n emul-sion appe ars to be the l east understood. Numeroustheories have been advanced regarding the nature andaction of -the emulsifying agent, t he substance o r sub-stances which form the interfacial films or envelopessurrounding the dispersed droplets. Fo r purposes ofthis paper, it will suffice to say that there are knownmethods fo r weakening the fiims and facilitating coales-cence of the dispersed particles-although the exactmanner in which these methods accomplish their resultsmay not be understood completely. Chemicals whichhave th is function ar e available. Properly-controlledsubjection of the emulsion to the influence of a high-potential electric field is another means.

TREATING METHODSPractically al l crude-oil-emulsion treat ing methods

utilize the force of gravity, by reducing the viscosityof the oil and coalescing the water particles into largerdroplets so tha t they will settle to the bottom.


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Some of the methods which have been used a re asfollows1 Sq ue e z e t r np : The water is removed in a squeeze

trap , often in conjunction with chemical treatment, be-fore the oil and water have a chance to emulsify moretightl y by p assage th roug h t he flow _bean. Thi s methodis applicable ~)arti cularlyo emulsions fro m gas-lift andflowing wells. I t is not used very ge nerally a t present.2 . Se t t l i ~ l y t e tho d with heat applied to reduce vis-cosity of th e oil: Th e emulsion usually is heated in atank, either with steam coils or live steam. The water

diagrammatically a typical chemical treating plant.Two types of electrical plants ar e shown in Fig. and6. Preliminary treatment of the emulsion often is simi-la r in the chemical and electrical processes, a nd s o prac-tices and plant units more or less common to the twomethods will be discussed first.

EFFECT OF AGEIt is well known that emulsions tend to stabilize a sthey age. F o r this reason th e trend in both the chemicaland electrical processes is towards flow-line treatment.

Typical Chenlical Treating Plant.FIG. 4

is then allowed to settle out. Chemical is often required.The practic e of rolling a tank of emulsion with livesteam h as been discarded in most plan ts because of itstendency to f orm a tighte r emulsion, although in somecases th e agitation caused by the s team may be desirablefo r mixing chemical with t he emulsion. The settlingmethod is adapt abl e to coarse emulsions in heavy crudes,whkre the main problems ar e to increase the gravit y dif-ferential between t he oil and water, a nd t o decrease theviscosity of th e oil so th at th e wat er will settle out.

3 Distiblutio?~ rte t lzod:The principle of fraction al dis-tillation ha s been applied t o th e deh ydrati on .of crude-oil emulsions, but has not proved very successful. It isusually expensive, increases th e viscosity of t he oil, andleaves in t he oil the salts from th e water. The methodmay be said to be limited, therefore, to emulsions inwhich the water is relatively soft.

4 . C e ~ ~ t r i f 7 ~ y eLarge centrifuges have been used toclean en~ uls ion s n commercial quan tities. The methodhas not gained popu larity, due to it s high cost and limi-ted applicability.

5 Fil te r presses : Some emulsions are broken downwhen forced thr oug h filters. Th e method does not ap-pear to be practical on a larg e scale.6 Cllen~iccclm e t ho d : The fundamental characteristic

of this method of dehydration is the use of selectedchemicals for weakening the interfacial films surround-ing the dispersed droplets.

7 . E l e e t ~ i c a l z e tl ~ o d:This method utilizes the effectof a h igh-potential electric field to coalesce th e disperseddroplets.

The chemical an d electrical methods a re t he two mostcommonly used in the dehydration of crude-oil emul-sions, and will be described more fully. Fig . 4 illustrates

L A

Diagram of Flow-Line Method of Electric DehydrationPlant.

FIG. 5

Diagram of Tank-Nlethod Electric Dehydration Plant.FIG 6


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Decided savings in treating costs have been effected bychanging from the tank, or batch, system to the con-tinuous-flow system, in which the emulsion is given nochance to age while standin g in a tank awaiting treat-ment. Where i t is necessary to gath er t he emulsion ina wet-oil tank, the sto rage period should be reducedto a minimum. An illustra tion conveniently a t han d isthat of a sample sent to the laboratory for chemicaltre atm ent . chemical-oil rat io of 5,000 (by volume)cleaned the emulsion when i t was two hours old. Threedays late r a ratio of :3,000 was necessary to break it.

CHOICE OF CHEMIC LThe first step in th e dehydration of a n emulsion, par-

ticularly in the chemical method, is the choice of thedemulsifyin g chemical. Th e electrical method usuallyrequires chemical in relatively sinall quantities, if. atall. It has been fo und th at the chemical chosen a s beingmost efficient for a given emulsion ordinarily will be theone best suited to t ha t emulsion throughout the l ife ofthe well. Exceptions a re encountered, especially whena decided temperature change occurs in the productionof a well. The n too, new chemicals ar e being developed,an d a new one inay be found to be more suitable. Thechemical to be applied usually is selected in th e labora-tory . Sometimes a chenlical is picked by actu al fieldt r ials f rom a small g roup previously selected in thelaboratory.

INJECTION OF CHEMIC LIn the tre ating system in which the emulsion is gath-

ered in tanks before tre atme nt (hereafter called thetank system ) the chelnical is sometimes added to the

emulsion in the tanks. The more common practice inthis system is to in ject the chemical at some point inthe l ine between the ta nk and th e treatin g unit , usuallyin the treater feed-pump suction in the electrical-dehy-drati on method. One adva ntag e of placing the injectora t the feed-pump suction is t ha t this permits additionof chemical a t one point to emulsions from various tankson th e property. Also, if a pu mp chemical feeder isused, i t ma y be actuated by the tr eate r feed pump.

Bettei. treat ing resu lts a re obtained, a s a rule, whenth e chenlical i s introduced a t th e well, eith er betweencasi ng and tubing, o r in the flow line. This does notalways hold true; for, in a few cases, some re-emulsi-fication occurs before the production reaches the treat er.Injection a t the wells may increase the investment inequipment, but this increase genera lly is more than com-pensated .f or by th e increas ed pla nt efficiency whenchemical cost is a n apprec iable item, a s in th e chemicaltreat ing method.

In inany wells it is advisable, from a mechanicalstan dpo int, to introduce chemical down the hole. Somegas-lift wells produce such a viscous enlulsion th at thega s pressure required to l if t i t is more than can besafely produced by th e coinpressor plant. In man y wellsof thi s nat ur e th e introduction of chemical down thehole has reduced the viscosity of the emulsion and, in

turn , decreased the g as pressure required. n exampleof a similar problem in a pu mping well is th at of a\\.ell p ump ing abou t OO bbl. gross production per day-water content approximately 90 per cent, with a :


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Chemical Feeder Pump, Driven by j/4-Hp. Electric

Motor.FIG. 7

Chemical Feeder Pump, Driven by WalkingBeam.FIG 8

a s 10-gal. or more per day. They are designed to operateagainst a pressure as high as 600 lb. per sq. in. Thecost of the pump unit i s approximately 20. The uniwith electric motor will cost probably 60. Larger feedepumps a re designed to pump a s much a s 1,000 gal. perday agai nst a . pressure of 2,000 lb. per sq. in.

ADMIXTION OF CHEMICALFlow lines, tubing, or other facilities through which

the emulsion and chemical pass will cause, normallysufficient mixing. In some instances a special mixer i

A Thermostat valveB Pressure-relief valveC. Gravel columnD. ConductorE. a s h tankF. Vapor condenserChemical Treating Plant.

FIG. 9necessary. Manufactured mixers a re available, but inmost cases the mixers a re made in the field shop. Onetype of shop-made mixer is constructed by extendingthe up-stream side of the flow line into a shell. The endof the flow line is plugged, and the section inside theshell is perfora ted wi th small holes. The down-streamside is similarly constructed.

The gravel-column is the more common, and usuallymore efficient, type of mixer. I t consists of an uprighsection of pipe, 8 in. or more in diameter, filled with peagravel. The emulsion and chemical enter a t the bottompass upward through t he gravel, and flow out a t thetop of the column.

The chemical treating plant shown in Fig. 9 wouldnot clean the emulsion properly until the gravel column

C) was installed. Several similar cases are known.


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HE TERSHea t must be applied to many of the crude-oil emul-

re they will break down. The older styles ofpressure heating units, which do not require firemen inconstant attendance. Elimination of high-pressure boilerplants also reduces maintenance costs.

Plan ts which tre at emulsions a t higher temperaturesshould be designed to avoid gravity and volume losses.Table 1, compiled by William Woelfin, indicates thatthe gravity loss in handling hot oil depends upon thegrav ity of the crude oil, the temperature, and the man-ner in which the oil enters the tank.3

The chart in Fig. 11, taken from the sameshows that an appreciable volume loss accompanies thegravity loss.

Gas Engine HeatersWaste heat from gas-engine exhausts and circulating

waters have been found adequate for efficient operationof a number of trea ting plants. Results obtained byR. L. Pettefer in tests on gas-engine exhaust and cir-culating water heaters are shown in Table 2. Theseheaters, to function satisfactorily, should be insulated.Heat-transfer efficiency of gas-engine exhaust heaterstested is said to be from 64 per cent to 70 per cent, thatof circulating water heaters 39 per cent to 8 per cent.

Older Sryle of Dehydration Plant.FIG 10

rs to heat the emulsion. Fig. 10 illustrates one ofre boiler plant. High-pressure steam is still used,

Present opinion, however, favors low-T BLE 1ravity Losses

API Gravity at 60 Deg. F.Centrifuge- Dehy-

Chemical drated-Gravity Oil Shipping Loss inof Wet Oil Grav ity Gravity Gravity

Temperatureof Oil

to Tank(Deg. F.150

Method by which Oil Entersthe TankNo. FieldSignal Hill Oil enters through ell a t openmanhole a t top of t an k. .

Oil en ters through ell a t openmanhole a t top of t ank ..

Oil enters throug h ell a t openmanhole a t top of ta nk.

Oil enter s throug h ell a t openmanhole a t top of t ank .Oil enters a t bottom of tank, open

anhole at . top ..

Signal HillSignal HillSignal HillSignal HillHuntington

Beach Oil enters a t top of vapor-tighttank.Oil enter s top n ear open manhole.Oil enter s top n ear open manhole.Oil to bottom a t center of t ank

through over-shot. Open 6-in.gage hole a t top ..

Oil enters a t bottom of vapor-tight tanks.Oil enters a t bottom of vapor-

tight tanks.Oil enter s a t top of vapor-tight

tanks.

TorranceTorranceSignal Hill

Signal HillSignal HillVenice


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Relation Betw een Gravity Loss in Deg. API and Pe rCent Loss by Volume.FIG. 11

Firebox Boiler HeatersOne popular type of production heater is illustratedin Fig. 12. An old firebox boiler is used. The fireboxends of t he tubes ar e welded to the tube sheet and the

stack. Thi s type of hea ter i s controlled by an intgra ted thermostat in which the temperatu re elemeand th e gas valve ar e separat e units. Thermostats this design may be purchased for from about 5070, and will control the temperature within a rangeapproximately 5 deg. F. The thermostat bulb shoube inserted i n the boiler on the side of th e barrel afai rly near t he crown sheet. The ga s valve a balancvalve may be placed a t an y convenient point in t

Firebox Boiler Production Heater.FIG. 12

fuel line. It is actuated through a flexible capillary tufr om the bulb. Firebox boiler hea ter s ar e equippeusually with low-pressure relief valves. These shoube of t he hooded type with th e outlets piped awa y frothe boiler to avoid fire hazard.The thermal efficiency of firebox boiler heate rs rported to be about 50 to 60 per cent may be somewhlower tha n tha t of certa in other types of heaters. spite of this they seem to be the most successful of theate rs so f a r used in chemical treat ing plants proably because of a washing action which occurs in tboiler.

fusible plug is replaced by a cast-iron or steel plug.The emulsion is conducted into both sides a t t he back Tubular Boiler Heatersand n ear th e base of the mud ring. The outlet in theunit illustrated is fr om the dome but gr ea te r efficiencyis obtained if it is placed on top of th e boiler nea r theTubular boilers are used as heaters in a differeway. Emulsion passing directly through t he boiler shewould deposit sediment upon the bottom of the barre

TABLE 2Heaters Uti l iz ing the Waste Heat from Gas Engines

Circulating CirculatingWater Water Exhaust ExhausType of Heater A B t c 1 i DProduction of wet oil bbl. per day.. 300 110 120 252Water in wet oil per cen t.. 70 83 45 16Temp era ture of wet oil to hea ter deg. F . 121 105 100 97Tempera ture of wet oil out of heater deg. F . . 129 129 134 116Tempera ture increase deg. F 8 24 34 9Tempera ture of circulating wat er to heater deg. F 141% 161Tempera ture of circul ating wa te r out of heater deg. F . 146 137Circulating water bbl. per hour .. 53.5 6.15


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Tubular Boiler Production Healer

OIL TREATING ETHO S

FIG. 13

t tr ans fer from the tubes to the emulsion.Fig. 3 illustrates a tubular boiler heater similar toe one described above. Instea d of hav ing the hea t ex-

ll. Low-pressure steam gene rated i n th e boiler passes

er f alls back into t he boiler.Investment costs are higher for tubular boiler heat-s than for firebox boiler heaters, although thei ral efficiency is said to be greater. Only soft wat erused in the boiler proper. Very littl e make-up wate r

The temperature control is not as accurate as that ofthe more expensive thermostat previously described, thetem per atu re variatio h being about 20 deg. t o 25 deg. FOne style of smal l thermostat is designed to give agraduated gas-volume control, and is more sensitivetha n ones which tu rn on the full ga s volume a t once.The vertical boiler heaters are adaptable to plantstr ea ti ng individual wells. The ir initial costs ar e low.No da ta regard ing their efficiency a r e available.Pipe Heaters

2

The pipe heater shown in Fig. 5 is an uncommontype of heater, suitable to tank-system plants whichtreat large volumes of emulsion. Its initial cost prob-

ge. Instea d, the boiler is used a s a low-The upper halves of the tube sheets2 in. to in.). Ex changer tubes of ad-metal ar e then rolled into these upper sheets.ch i s re-built to fa ll below th e coils. Baffledn the tube sheets form ret urn passes. The

isby low-pressure steam generated in the lowerrt of the boiler. The tubula r boiler type of heate r ised to have a high the rmal efficiency, probably a nge of SO per cent, and as high as 92 per cent. Thisciency is said to be due primarily to the g rea ter

Vertical Boiler HeatersA popular type of small heater is shown in Fig. 14The emulsion enters the bottom of a vertical boiier,Passes upward through coils, and emerges at the top.The t emper ature is controlled by a thermostat insertedin the top of the boiler. I n the small thermostats used

with this type of heater the bi-metallic element and thega s valve are both in one unit. The maximum fuel-gaspressure on these small thermostats should not be inexcess of 5 Ib. per sq. in., a nd so a gas-pressure regula-to r is placed in the ga s line. The cost of th e thermo-stat and pressure regulator is approximately 25.


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ably is rather high, but its thermal efficiency shouldalso be high.Thermosyphon Heaters

Thermosyphon hea ters of various designs are in opera-tion in chemical-method tre ating plants. They ar e oftwo general designs, the open system and the closed

system. In the open system, the water i n the bottomthe settl ing tank circulates directly through the heaIn the closed system, fresh water from the heate r pasthrough coils in the settling tank, and ret urns to heater. The advantage of the closed system' is tha t osoft water passes through the heater, thus reducheate r repairs. The disadvantage of the closed sysis that if repairs to the coils in the settling tank necessary, the tank must be emptied.

Thermosyphon heaters, by their nature, must be signed so that the line carrying the hot water risesa n even grade to the t ank. The return line shouldlevel, or on a down-grade to the heater.

The heating unit in a thermosyphon heater mayany one of several forms. Firebox boilers have bused in some plants. One plant utilizing a fireboiler in a closed thermosyphon system reported system to be very satisfactory. I t has not beenoperation long enough, however, to indicate how wthe tank coils will withstand corrosion.

Fig. 16 shows an open-system thermosyphon headesigned and installed commercially. The small vecal heater is set 25 ft. or more from the settling taThe hot-water line, well insulated, is flanged to the ta t a height of about two-thirds t ha t of the tank. tank shown is set within an excavated fire wall, andthe cool-water line rises from the bottom of the tto the heater. This is not usually desirable, for i t serto decrease the rat e of water circulation. A check vais placed in the cool-water line at a point just outsthe heater. No thermostat is used.

Many heaters of thi s design are being used on smproperties, usually with a 750-bbl. tank, approxima15 ft . in diameter and 25 ft. high. Results repoobtained with these heaters are given in Table 3 Tfuel gas is seldom metered. Check tes ts indicate a fugas consumption of from 1,500 to 30,000 cu. ft. per dwith most units consuming between 5,000 and 1 0 , 0 0ft. pe r day. The heate r installed, including labor fittings, costs about 300.Other Heating Methods

Almost any source of heat availab le may be used heating the emulsion. In one chemical tre ati ng plthe production from two hot wells (175 deg. F.

A Emulsion inletB. Emulsion outle tC ThermostatD. Gas-pressure regulator

Vertical Boiler Production Heater.FIG. 14

TABLE 3Some Results Reported -Obtained With Open Th ennosyphon System Using a Vertical Heater

Plant A 1 B C D 'Net production of oil, bbl. per day.. . . . . . . . . . . . .Water in wet oil, per cent. . . . . . . . . . . . . . . . . . . . . . .Dry gravit y of oil a t well, deg. A PI . . . . . . . . . . . . .Temperature at well, deg. F . . . . . . . . . . . . . . . . . . . .Settling-tank temperature, deg. F . . . . . . . . . . . . .Temperatu re of cleaned oil, deg. F . . . . . . . . . . . . . .Chemical consumption, qt. per day.. . . . . . . . . . . . . .

. . . . . . . . .aximum water in cleaned oil, per cent.Gravity of cleaned oil, deg. A P I . . . . . . . . . . . . . . . .

* Oscar V Burris


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A. Gas engine driven feed pumpsB. Pipe heaterC. Heat exchangerD. Relief va l ves

Tank System Electric Dehydration Plant.FIG. 5

Open System Thermosyphon Heater.FIG. 6

mixed with th at from four or five cooler wells 100 deg.).The resulting temperature of the water in the settlingtank is from 140 deg. to 145 deg. F

I t has been suggested tha t the hot water drainin gfrom the treater be utilized for preheating the emulsion.

CHEMICAL TREATING METHODThe primary difference between the chemical treating

plant and the electrical treating plant is th at the formermakes use of a settl ing tank to clean the oil, whereasthe latter uses an electric dehydrator.

Either the tank or the flow-line system may be usedin the chemical treating method, but the flow-line sys-tem is preferred. In the typical chemical treating plant(Fig. 4) the chemical is injected i n the flow line a t thewell. The chemical then has a better chance to mix withthe fre sh emulsion, particularly in th e ga s- eparator.The emulsion is heated a fte r i t leaves the sep arator, ifext ra heat is necessary. The firebox type of heate r isbelieved to be best suited to the chemical plant, as pre-viously stated, especially if a fairly large throughputis handled, as is usually the case in a tank system. Theemulsion goes from the heater to the settling tankt,hrough a conductor pipe or boot. The settl ing tankremoves the water from the emulsion. The clean oil flowsby gravity to the stock tanks. In a very few cases anemulsion will not break completely in the settling tank


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unless an excessive amount of chemical is used. I nsuch cases the remaining water is removed by causingthe emulsion t o flow through a n excelsior tank o r col-umn, commonly called hay tanks and columns.

The clean oil, if very hot, should e nter a t the bottomsof the stock tanks. Otherwise gravity and volume lossesmay occur (see Table 1 .

The production of each well is metered with a dis-placement or weigh meter equipped with sampler whenone chemical plant operating on the flow-line principletre ats t he production from several wells. If only, onewell produces through the plant, the clean oil is gagedin the stock tanks. The operator may meter the water,if he desires a record of the water production.A chemical treating plant is practically fully auto-

matic, requiring very little attention. Total treatin gcosts are of lit tle comparative value because of varia-tions in accounting methods. The cost of chemical var iesfrom less than 2 cent per barrel of net oil to as muchas 2 cents per barrel. I t averages probably less tha n 1cent per barrel . The cost of ga s fuel is very low. I t isseldom necessary to heat the emulsion to a temperatureabove 140 deg. F., although in a few plants the oper-ati ng temperat ure is a s high as 180 deg. F. Mainte-nance costs ar e low. Amortizat ion costs will dependupon the individual operator s accounting system.

The chemical treating method has been successfullyapplied to emulsions which ca n be cleaned by other meth-ods only with gr ea t difficulty. Finely-powdered solidsmay act a s emulsifying agents. Therefore, emulsionscontaminated by drilling mud are very hard to clean.In one case a very substan tial recovery was obtained bycleaning up drilling and sump oil cutting from 25 percent to 60 per cent, with a h igh mud content. Theemulsion was heated to about 200 deg. F., and waspumped to a 2,000-bbl. settl ing tank . Chemical wasadded a t the pump. The emulsion was washed through

ft. of wate r in the settling tank, a nd was then allowedto stand one to th ree days. The water content of thecleaned oil was 1 per cent. The operation was quiteprofitable, even though the amoun t of chemical used wasseveral times the normal requirement, because theemulsion could not be cleaned economically in anyother way.

SETTLING TANKThe settling tank, also called the wash tank, is made

up of five main pa rts : the conductor pipe, the spre ader,the tank itself, the automatic water drain, and theclean-oil outlet.

The conductor, o r boot, serves two purposes. I t feedsthe emulsion to the settling tank more steadily by re-ducing surging, a nd it removes a ny remaining vapors.The vapors go over the top of the conductor, and areusually returned to the oil through a line extending intothe clean oil in the top of the settling tank . Sometimesthe vapors ar e taken off by a vapor-recovery system.The conductor may also help to mix the chemical withthe emulsion.

The conductor pipe is about 8 in. to 6 in. in diameter,and extends 5 ft. o r 6 ft. above the top of the se ttling

tank (see Fig. 17) . I t may be either internal or external to the tank. The emulsion enters it at , or a littleabove, the level of the tank top.

The emulsion goes fro m the bottom of the conductopipe to a spreader inside the settling tank.

Emulsion spre aders generally a re constructed by eachcompany to fit its own ideas. The simplest is a n elbowturned upward on the end of t he inlet line to the tankSome companies run a perforated or slotted lateral toeach side of the inlet. Fig. 18 illustrates a perforatedcone spreader, which may be used with or without the

External Conductor PipeFIG 17

cone bottom. If the cone bottom is used, th e total ar eaof the holes in the conical spreader should be adequateto insure easy passage of the emulsion. Otherwise someof t he emulsion may go over the top of the conductorpipe and through the vapor line to the clean oil.

The design of the spreader shown in Fig. 9 is basedupon the idea th at the dispersed water particles, ifforced to drag along a plate while rising through thewash wat er in t he bottom of the ta nk, will have more ofa tendency to coalesce with each othe r and with the washwater.

The settling tank itself may be any size, but shouldbe fairly high in proportion t o its diameter. b n e of themore popular sizes is the 750-bbl. tank, 24 ft. high, andabout 15 ft. in diameter. The water-oil level is usuallymaintained about midway between the spreader and


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- -GRADE EOARDSS ND CLE N OUT

Conical spreader: Flat Bottom Tank w i t h Perforated Cone.FIG. 8


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the clean-011 outlet. Thi s allows plenty of water fo r agood wash action, and enough time for the water drop-lets to settle out of the oil between the time it leavesthe wash water and the time it passes out the clean-oilline. I n some plant s the oil is clean by the time i t leavesthe wash water. In such cases the tank might better becalled a wash tank.

The clean-oil outlet should be sufficiently high toallow the oil to flow by grav ity to the stock tanks . Thi sis another reason for using a tall settling tank.

The water-oil level in the settling tank is controlledby an automatic water drain , or wat er leg, which ri sesfrom ne ar the bottom on the outside of the tank. Thewater-oil column inside the t ank is balanced by a water

SKETCH OFEMULS O SPRE DER

S e t plates upw rd f r o m laterals,o 4S r o m verticul

C u t % x 2% longitudin s l o tin o t tom o f e ch l ter l

Emulsion Spreader.FIG. 19

column outside. If the specific gravities of the oil andthe water ar e known, i t is a simple matte r t o calculatethe approximate height of the water leg overflow re-quired for a given water-oil level in the tank. -Theoptimum height for the water-oil level for a givenemulsion is determined, as a rule, by trial and error.I n most cases the sett ling ta nk is equipped with mani-folded sample bleeders up one side, usually above thewater-drain funnel.

The water leg is built so th at t he height of the over-flow is adjustab le. Sometimes thi s is done by means ofa swing a t the top. Fig. 2 shows a water leg which isadjus table by a manifold. The manifolded sampl6valvesa r e also visible on the right-hand side of the sett lingtank in th is picture. The clean-oil line is seen leadingfrom the top of the set tling tank toward the stocktan ks to the left. The boot in this case is internal, and

protrudes from the tank top. This particular settlingtank has a false bottom, an inverted cone, from whicha drain line runs to the sump hole.

t might be well to mention a t this time that chemicais not necessarily required in the treating plant hereincalled the chemical treat ing plant . In a few cases ofloose emulsions, the mechanical action of the wash (orsettling) tank is sufficient to clean the emulsion.

A. Flow l ineB Oil wnductorC Water legD. Sample valvesE. Clean oil l ine t stock tanks

Settling Tank.FIG. 20

EXCELSIOR COLUMNAn excelsior column is an upright cylinder filled with

packed excelsior through which the emulsion passes.Excelsior from pitch-free woods, such as aspen andcottonwood, i s recommended fo r this service. Some ex-celsior columns are fitted with several baffle plates toprevent the emulsion from channeling through the ex-celsior (Fig. 21 . Others have only two baffle plates,one below and one above the excelsior. The column il-lus tra ted is of a n older design. In those built morerecently, the baffle plates are not bolted into the shellflanges, but are held in place by a long tie rod throughtheir centers, with pipe spacers separating them.


14/21

The dir ty oil enters the excelsior column ne ar its base,travels upwards through the excelsior, and the cleanoil flows out near t he top. I t has been found th at mostof the water collects above the upper baffle plate, afterthe oil and water have passed ou t of the excelsior. Thewater is bled off by manual control, but the volume is so

Excelsior Column.FIG 21

The capacity of an excelsior column will depend uponSmall columns, 8 in. o r 10 in. in diameter,

le 40 bbl. to 50 bbl. per day. Diam eters in excess8 in. a re recommended, however. A column 4 ft. in

bbl. per day. The relation of the sizes and positions ofexcelsior columns with respect to the stock tanks isshown in Fig. 22.A small amount of tig ht emulsion sometimes remainsin some oils after they have been treated in a settlingtank . This emulsion cannot be removed except bygreatly increasing the chemicals added, or by raisingthe tempera ture of the emulsion. Excelsior columnshave cleaned this residual emulsion very successfully.Two cases are cited a s examples:

Case A : Approximate well production, 450 bbl.wet oil per day; water content, about 90 per cent;qt. of chemical used per day. Wa ter content af te rleaving sett ling tank, 4.0 per cent; wate r contentafter leaving excelsior column, 0.2 per cent.

A Settling tanksB Excelsior columnsC Stock tanks

Excelsior Tanks in OperationFIG 22Case B : Approximate well production, 200

bbl. wet oil per day; water content, 50 per cent;qt. of chemical used per day. Water content af te rleaving settling tank, 12.0 per cent; water contentafter leaving excelsior column, 1.0 per cent.Each of these excelsior columns is 4 ft. in diameter,

and approximately 20 ft. high. The grav ity of the oilbeing treated is about 21 deg. API.The cost of a la rge excelsior column is from 75 to125.Excelsior or ha y tanks ar e very similar to the columns,except that they require baffles to prevent channeling.

The baffles, or decks, in a hay tank slope toward theircenters, from which points the water-drain pipes leadout of the tank. A hay tank made from a 200-bbl. tankshould hand le 1,500 bbl. per day. The cost of convert-in g a 200-bbl. tank into a hay tank is about 250. Thecost of converting a 500-bbl. tank is approximately 500.

ELECTRICAL TREATING METHODFlow-Line System

The electric dehydration plant may be either the flow-line or the tank type. Fig. 5 is a diagrammatic sketchof a flow-line electric dehydrat ing plant. The wet pro-


15/21

duction flows from the ga s tr ap directly to the electricdehydrator. The wate r is removed i n the dehydrator,and oil goes to th e clean-oil stock tanks. These dehy-drator s a re in all cases automatically controlled, so th atlittle supervision is necessary. This type of plant is th elates t development in electric dehydration. Many of th eolder plants ar e being converted to thi s system. Theemulsion is treated while it is fresh, so th at very littleaid is required. Sometimes additional heat is necessary,and in a few cases a small amount of chemical about1 pint ) i s added occasionally to clear up a sludge con-dition in the dehydrator.

of concentr ic-ring type electrodes. The oil is cooled ithe he at exchanger C) before i t goes to t he stock tanksThe pressure-relief valve D) limits the maximum pressure in the dehydrator to about 20 Ib. per sq. in.The automatic-control features installed on all electric dehydrators are: 1 the automatic control of cur

re nt by means of t he choke coil; 2, the automat ic ffoatoperated switch on the transformer primary, whichopens the electric circuit a t any time t ha t the dehydrator is not full; and, 3, the relief valve or vent) othe dehydrator, which automatically limits the pressurto a predetermined value.

Tank System

Tank-System Electric Dehydration Plant.FIG. 23

A tan k syst em of electric dehydration is shown dia-grammatically in Fig. 6. The wet oil is gathered in atank, and i s then pumped through the electric dehydra-tor. Ex tr a hea t is almost always required. In the olderplants the emulsion usually is preheated by exhauststeam, heated by nigh-pressure steam heaters, andpumped by manually-controlled steam pumps. The wat erbleed is also adjusted manually. Many of these plantshave been modernized by the application of automatic-control featu res. Fig. 23 shows a typical tank-systemplant, with a 10-ft. diameter type horizontal concen-trated field type of dehydrator, high-pressure steamheaters, wet-oil feed pumps, and the standard switchhouse.A tank-system treating plant in which high-pressuresteam is not used wa s shown in Fig. 15. The pumps areoperated by gas engines A ) . The emulsion is heatedin th e pipe heater B), and passes thr ough th e electricdehydrators, each of which in this case contain two sets

Three additional controls are necessary to maketank-system .plant practically automatic: 1 automatiflow of wet oil to the dehydrator; 2 automatic temperature control; and, 3, automatic water-level control.A constant flow of wet oil to the deh ydrator a t a desired rate is usually obtained by the use of a positive

displacement type of pump with adjustable speed control. If the pump i s steam-driven, i ts speed is controlleby a governor. If th e high-pressure ste am plant habeen eliminated, th e pump is driven by a variable-speeelectric motor, a variable-speed transmission betweethe pump a nd motor, or a ga s engine whose speed cabe adjusted. I n some cases a series of by-passes fromthe pump discharge to the pump suction controls thrate of flow of the emulsion to the dehydrator.The temperature of the emulsion is controlled b

means of a thermo stat, a s previously described. Antype of heater may be used.The automatic water-level control will be described

in connection with the dehydrator.


16/21

DehydratorThe electric dehy dra tor consists of a shell, the elec-

trode system (including transformer , etc.) and the auto-matic water-level control, in addition to the featurespreviously enumerated.

The older typ es of electrodes a r e considered obsolete,and will not be discussed. Th e horizontal concentrated-field type an d th e concentric-ring t ype a re th e electrodedesigns installed in newer plants.

Electrode of Horizontal Concentrated Field Type.FIG. 24

The horizolltal concentrated-field (o r HC F) typeshown in Fig. 24 is used especially in tank-system plants.It 1s designed to deh ydr ate e~ nu ls io ns f extremely-finetype, and consists of a nozzle which jets the emulsionalo ng an electrode rod located inside a live shield.

The concentric-ring (or CR) type of electrode show11in Fig. 25 has been developed recently. It is used inflow-line plants a s well a s in some tank-treat ing plants.It consists of two sets of concentric rings, suspendedfro m supporting brackets. One set of rings is attachedto th e high-potential source, an d. he other set of ringsis grounded. Thi s drawi ng shows only one CR electrodeunit, but two units can be installed in the standard

10-ft. dehydrator shell to take acivantagk of a doubletreatment, when necessary. 1.11 he treater shown, theemulsion is introduced below the electrode unit, andrises throu gh th e high-potential field. In some units theelnulsion is introduced in a horizontal plane directlyinto the electric field, by means of a spreader cap placedon top of the input line and between the planes.of thetwo electrocles.

Concentric Ring Type of Electrotle.FIG. 25

Several methods ar e used for automatically control-ling wate r levels in electric dehydrators. The waterleg is used, a s in chemical plants, if no pressure is heldon th e dehydr ator shell. Fig. G shows flow-line sys-tem tre at er with a low-pressure shell. The water levelin the treater is automatically maintai~~edt a prede-termined level by the adjustable swing A ) a t the topof the water leg. This plant operates a t the temperaturea t which the oil is produced, and is fully automatic.

A float-operated water valve, shown in Fig. 27, is inservice on a 10-ft.-diameter pressure shell operat ing in a


17/21


18/21

A ElectrodeB Solenoid operated valve

Electrically operated Water Valve.FIG 28

A Float operated control valveB Diaphragm operated pressure control

valvesElectric Dehydrator in Flow Line System.FIG 29


19/21

Electric Dehydrator in Flow-Line System Illustrat-ing Pole Box.

FIG 30

A Float-control switcl~B. Pressure-relief valve

Electrie Dehydrator in Flow-Line System RequiringAuxiliary Heat.FIG. 31


20/21

TABLE4

.

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o

.

PaA'

Huno

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PaB

PaC

PaD

B

SaFSn

SgH

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I

-

-

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-

*-

-

Bc

Fow

Bc

Fow

Bc

Fow

Bc

Fow

T

od

ao.............HCF

C

2HC

1CR~H

HF

HCF

C

Neobpmoh...........27

27

20

:30

7

7

24$

26

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.

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.

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.

.

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.

.

7

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3

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2

9

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..

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.1

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1

9

1

1

1

1

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N

-

N

N

75

N

N

N

Ruee.................

1

N

N

N

N

N

N

.

.

.

.

.

.

.

SpngaydAP

28

28

39

39

26

20

20

20

Spncpc

.

.

.

.

.

.

.

.

.

.

.

.

15

07

03

03

08

03

17

06

Spnpccspb......

9

9

1

1

9

9

8

9

Opanmeh

pmoh..

8

7*

7

7

3

.

7

6

7

Be

eoceaoh

pmoh

8

N

7

N

3

N

6

N

Pwekwpmoh...........

2

5Noa

3

7

4

9

2

ae

L

h

p*moh.........

3

3

3

3

3

3

6

30

aR(?~

~~z

~c~~~l~~~~~~tr:~t~~ltiel~l

:

c~g

:~~l~-~~~ri~o~~t~l

el~l

*7

~

p~o

~

c

n

0

ooteemto

t~vpou

~o

eia

$no

.

1

V:~1~u~~~IIIII\v

i1

w

th

e

d~nweo>neO~to~cn

ow-npn

ea3n

~~~i~l~l~le

t10

hd

o

PaE

SgH

- Bc

~lo\

HCF

CR

16$

18

3

3

1

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N

N

N

N

24

24

09

06

8

4

sG

4

N

6

2


21/21

tric dehydrators to open autom atically t he electric cir-cuit whenever the dehydrator is' not full, and the pres-sure relief valve (B).Advantages of Flow Line System

Some of the advantages of the flow-line system ofelectric dehydration over the tank (or batch) systemhave been listed by T. N. St. Hil l6 as follows:1 Elimination of wet-oil storage tanks and dehydrator

feed pumps.2 Lower operating temperature, eliminating coolingtowers, etc.3. Elimination of chemical aid.4. No re-running of partl y-deh ydrat ed oil.5. Reduction in labor.6. Eliminat ion of grav it y losses.7. Lower plant investment cost.

Mr. St. Hill states that the changes necessary toconvert a batch pl ant t o a flow-line plan t a re relativelysimple and inexpensive. Table 4, take n from the sam epaper, illustrates the comparative operating data ofbatch and flow-line electric dehydrators. The labor re-quired for the operation of a flow-line uni t was esti-mated in th e table a t one hour per 24-hour day. Actually,;he operation cf t he dehydrat or is added to the duties ofthe pumper; so there is no increase in total amount oflabor on the lease. When these plants were operatingas batch units, it was usually necessary t o have an ex traman on the lease continuously to look after the dehy-drato r a s well as the boiler plant, which was operated tofurnish steam. for heati ng and pum ping the oil. Thisextra labor was eliminated.

Flow-line plan ts "A," B," and C (Tab le 4) op-erate a t the temperature of the oil a s it comes from thewells. Plants D and E require additional heat,but this is obtained by utilizing waste heat from thepumping engines.

The table shows the flow-line system to have an ad-vant age over the batch system in all fact ors of electric

dehydration esce pt one, the power consumption. Theavera ge cost of power f or 17 CR type flow-line uni ts wa s$12.68 per month. Th at of the four batch pla nts givenin the table was $4.80.

CONCLUSIONRecent advances in methods of tre at ing oil-field emulsions have rendered many of the older dehydrat ing pla ntsobsolete. The cost of modernizing old plan ts is usually

low, and is quickly paid out, in most cases, by reduct ionsin dehydration costs an d increases in shipping gravitiesEconomies are effected when emulsions are dehydratedwhile they ar e fresh. Prevention of emulsification ispossible to a limited extent.

ACKNOWLEDGMENTSThe writer wishes to thank J L. Port er, of the RioGrande Oil Company; E. K. Parks, of the Standard OilCompany of California; D L. Caldwell, of the Barns-

dall Oil Company; L. T. Monson;of the Tre to lit e Company of California, Ltd., and others, fo r their suggestions and contributions in the preparation of th is paperSpecial thanks are due H. C. Eddy, William Woelflinand I. L. Cooley, of the Petroleum Rectifying Companyof California; G W. Camblin and H. F. Schram, of t heTretolite Company of California, for suggestions, information, and illust rations used in the papel*.

C. H. DI Roberts. A New Theory of Emulsions," J PitusClreitl. 36, 8087 1:132).2 W,i,llinm Woelflin, P um p s for Handlirlg Crode-Oil Emolsions. Electrio Dclrgrlratc,r (l'ctroleurii ICectifyiuy Co LosAngeles, Ca1if.j 3 [3] 1 1193.1).3 Willinm Woelflin, Grnvity and \rolunic Losses wbeu Handl ingHot Crude Oil." E1cctt . i~Uclrytl~-otot. l] (1 1R21 : 3 [21 41032).R . L. Pettefer . "Uti l ix inc thc \\':lstc l -Ir:~r r,b:u Gas En-gines to I-lent Crude-nil Emulsions I~eforcElectric Dehx~lmtion.Blcctric DeA ~drolors [ I ] I; 1935. N St. Ilill . Benefits Resulting fr411rr th e Flow-Li ne Systen: nf I.:lsct .ic:~l ~)e l~ yrl r: ii(vn." l : I c r . l t . i ~ . I ~ l ~ ~ : l ~ ~ r r, r ~ . 131 1 5(1935).

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API-36-204 Oil Treating Methods

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