I, -.' 9 DESALTER OPERATIONS Troubleshooting the refinery desalter operation* Donald L. Kronenberger and Deborah A. Pattison' i 1 I '' [ - The increasing oil weight, sulfur content, and addition ot impurities in production challenge the basic design ot the chem-electric desalter and complicate its tunctioning as a pressurized-heater, gravrtational settling, and oil-water resolution device. The increasing oil weight leads to greater o i l carry-under and the increased impurities act as emulsion stabilizers, making the resolution of the oil-water emulsion more ditlicult. These problems call l o r a better understanding ot the basic tactors allecting successlul desalter operations, e.g., mechanical. electrical, gravrtational. and chemical variables and their interactions wrth one another. A clearer understanding ot these varrabies better equip the refiner to handle the problems assocrated with charging dilficulttedesalt teedstocks. OIL SUPPLIES ARE BECOMING HEAVIER. more sour. and dirtier. At the same time, secondary and tertiary recovery methods are Introducing agents that often cause produced 011to be tightly emulsified. The result is that increasmg oil welgnt and complicating impurities in raw crude state-of-the-art methods of refinery feedstock are challeng~ng ?lectrical. chemical. and chem-electric desalting. Heavier crudes contain higher percentages of res~duurn than lighter crudes. This residuum consists of larger molecules whose constituent atoms are more closely packed than those of the lighter crudes. The heavier molecules are not only more dense, but impart higher viscosity to the crude because of their large size. Often this residuum is heaver than desalter wash water and contradicts the baslc concept of the desalter Vessel as a differential gravity settler, with dry oil rising to the lop and saline water falling to the bottom. I n referring to the percentages and American Petroleum Institute (APIr1' grav~tiesof the residuum components in varlous weights of crude oil. it is interesting that crudes in the 32 to 38 API range average 20 to 30010 residuum contents having API gravities in the 12 to 20 range. However. crudes in the heavier 20 to 30 API range averaged 30 to 50% residuum content with weights in the 8 to 13 API range. Thus, approximately 'Presented during CORROSION186. Paper No. 209, NACE, Houston. TX, 1986. 'Kronenberger Company, P.O. Box 580351, Houston. TX 77258. "'American Petroleum Institute, 1220 L. Street, N.W., Washington. D.C. 20005. - Introduction WORLDCRUDE half of the constituent hydrocarbon molecules of some heavy crudes challenge the fundamental physics and p r ~ n c ~ p of le operation of current desalter technology. The predictable result is stable. interface layers of solids-stabilized emulsion in the desalter vessels and oil carryunder in the desalter effluent water. Another problem characteristic of heavier. more viscous crudes is that they hold on to suspended particulate matter such as sand. silt, and drilling mud solids longer than the lighter, less viscous crudes do. This causes particulate matter to fall more slowly through a more viscous, heavy oil phase In the desalter, making solids separation and removal more dlfficult. While increasing oil weight does not necessarily imply increasing sulfur content, generally the heavier crudes are more sour. In fact, some 85% of the known remaining crude 011 reserves are sour. Hydrogen sulfide, mercaptans, carbonyl sulfide, sulfur oxides, and other sulfur compounds present a complicated chemistry both to the oil and wash water phases in the desalter. Depending on temperature, pressure. and other components in the crude oil and wash water, sulfur cornpounds partition themselves between the oil and water phases of a desalter according to complex equilibria. This distribution often results in such adverse effects as undesirable pH ranges and added emulsion stability. In an 011-water contact process, such as the typical refinery crude oil desalting system, "pure oil" and "Pure water" can be emulsified and demulsified quite easily. It is the .'irnpur~ties" in the oil and water that tend to stabilize emulsions. These "impurities" can either be normal components 01 the petroleum resulting from its natural evolution, or they can be introduced into thecrude oil via drilling and production processes. transportation. storage. and corrosion products associated with these activities (Table 1). The animal and vegetable matter from which petroleum 1 s derived contains protein, fat, carbohydrates, cellulose, lignin, and metal salts. Many of these components contaln oxygen, nitrogen, and sulfur in addition to carbon and hydrogen. Such integral constituents of crude can cause problems in the desalter. For instance, compounds derived from hemin and chlorophyl, from the decomposition of plant and animal prote~n,have properties as emulsion stabilizers. These compounds are basically hydrocarbon structures. but they also contain polar groups bearing oxygen, nitrogen, and sulfur. AS a result, these compounds tend to situate themselves at the oil-water interface of emulsified water droplets causlng them to be stabilized. There ere also those impunties that become m~xed into !he crude 011 duriilg the process of extracting it from its natural habitat and producing, transporting, and processing it On Its ~94,1~uss100~7j?1~300~0~ ,Q 1986 National Association of Corrosion Eng~neers . According t o this trouble shooting guide.1% or less water and less than one pound per thousand barrels of salt. and sulfate COnCentration. 3. Sometimes desalter upset can be directly related to One category. These processes add chemicals. Once accurate records of the desalter's normal operating parameters are obtained. With the possibility of water carry-over comes the likelihood of high salt carryover out of the desalter. realistic approach to develop an action plan (i. to maintain a proper oil-water interface level. Mix valve delta p. 6. ana breaker Sample collecters/coolers: For trycocks. that serve as emulsion stabilizers. the desalter's level control floats. (2) electrical (Table 3): (3) gravitational (Table 3). This makes it easier to define problems when desalter upsets occur. chemelectric desalter transformed petroleum refining from a semi-batch process to a true continuous on-stream process. I f the water level becomes too high. each category will be discussed ~ndividually. sourness. (These variables affect f e e d . or chemical. and natural and artificially introduced emulsion stabilizers put modern desalters to a difficult test. Water level in the desalter.-surfactants. The automated. Conversely. water dump control valve and desalted crude exit valve need t o be funct~oning satisfactorily. dirtiness. the satisfactory operation of a desalter can be addressed under four c a t e gories: (1) mechanical (Table 2).t ~ f f l u e n t exchangers and not the desalter operation itself.e. the internal water level will rise too high or fall too low. and (c) solids concentration. Modern two stage desalting systems. For instance. (b) or1 concentration. When an upset occurs. The reason is that the entire desalting technology as currently practiced is based on two premises: (1) Oil is lighter than water: and (2) emulsions that are stab~lized by surface actwe agents can be destabilized by the action of either: (a) An electric field. "rag" or "cuff" thickness. Mechanical Mechanical defines the proper functioning and operation of the desalter hardware as it was designed t o operate. These include drilling mud solids: pipeline. Step 3-If all mechanical parameters are within range. a water level that is too low TABLE 2 . When these basic premises are severely tested by charging heavy. emulsion-stabilized crude oils. The key to learning about the s desalter during satisfactory operation IS logging ~ t operating history. (b) total hardness. 9.Impurltlrs In crude oil Salts: Sand Sill Clay Drllling mu0 sol~ds HydrophoMcK)Wphilic (Tendingto oll rolubillty or affinity) Sullur cmmnas: Hydrogen sull~de Carbonyl sull~de Merca~tans Organic solldssem~-soltds: asp hall^ Heavy reslauum I Contalnlng nltrogen and oxygen way to consumer end uses. successful operating conditions: 1. Nature of interface layer. attempt t o identify the most probable source of the problem as electrical. Interface. heaviness. Effluent water quality: (a) pH. Top oil quality: (a) salt content.~ust steam wing LOWlevel safety float Trycockslsamole pomt take-011s at various levels Bafllefsr andfor spreader plate@): posstble. 2 Amperage across the electrodes. there is the danger of an amperage overload and shorting out of the electrodes. gravitational. total bicarbonate and carbonate content. sour. ammeters. Distributor gap delta p (if variable). it becomes imperative to understand the desalter hardware and chemistry to extract the best performance possible from the system under difficult circumstances. Percent wash water added to the crude charge. inline. The most essential element in an enlightened approach to desalter management during upsets is to learn about the desalter in good times. and keeping good records. if functionmg properly. (b) percent BS[W (bottom settlings and water). Otherwise. a threestep trouble shooting gulde can be employed to narrow and identify the problem(s) encountered during a desalter upset: Step 1-Identify devlation(s) from the desalter's normal "vital sign" ranges. 8. Equally important are the emulsion stabilizers introduced in the secondary and tertiary recovery processes and enhanced oil recovery methods in general. ana effluentwaler Mixing valve Wash water meler Desaltsd crude ex11 valve Water Oumo~elfluent control valve aesaltea cruae out - - - - - - - - Materials Performance . and (4) chemical (Table 3). 11. However. Voltage across the electrodes.. make the appropriate adiustment or repair. and polymers. one can be prepared with an orderly sequence of screening techniques to define as narrowly as possible what has changed to cause the upset. such as caustic. but no! common Translormers Voltmeters. but the problem is often such that it is a result Of several categories acting simultaneously and often synerg~stically However. tanker and barge corrosion products: ballast water: and products resulting from bacterial action on the crude.Basic desalter mechanical equipment Electroaes Insulators and entrance DuShtngS D~str~butortsl Interlace level controllerllloat Sediment wasn~sludge. Wash water quality: (a) pH. Such carry-over could lead to fouling in the crude tower.TABLE 1 . This defines baseline operational parameters and tolerable variances for acceptable operation. It is necessary to know and log the desalter's "vital signs" during normal. 10.andlor (b) chemical demulsifiers. making the right observations.) 5. considering what can be expected from the desalter under the circumstances and helping it to deliver the maximum of which it is capable). 7. 4. Step 2-If an obvious operating andlor mechancal parameter is incorrect. preheat exchangers and could result i n overhead upset or low pH's from the hydrolysis of these salts to acids. can consistently turn out crude oil with 0. This enables the refiner to use a factual. Typical curves of salt content and BS&W vs July 1986 . so does salt carry-over. The delta p is then raised a small increment. One should then examine the delta pIsalUBSBW curve and set the mix valve a safe distance back from the point where the BSCW started to increase (usually about 2 Ibs delta p). usually not more than 2 Ibs. This occurs because. is the installation of baffles andlor spreader plates as an integral part of the desalter design. These could function to increase the travel time of the emulsion through the desalter and could give the dispersed water droplets more opportunity to coalesce. The mix valve is first set at a value that is expected to be too low (e. salt removal continues to improve until a point is reached where the shearing action of the mix valve creates an emulsion that the applied electric field and the demulslfier cannot resolve. At this point. allowing them more opportunity to collide and coalesce in a shorter amount of time. 3 Ibs delta p). The key to a successful pressure arop here is to achieve the greatest possi- ble mixing while minimizing shearing forces to avoid creating an emulsion that cannot be resolved into its original. it is best to consult the desalter's operational manual to determine the current level control settings for which a particular desalter was designed. Another related mechanical variable is the regulation of the desalter mix valve delta p. and flow pattern at which the emulsion enters the desalter vepel. This is best a o complished by running a delta plsalUBSBW profile (Figure 1). oil and water components. These steps are repeated until the analyses reveal an increasingly high salt content and percentage BSCW. FIGURE 1 . as available crudes become more and more difficult to desalt. A mechanical parameter that may be given more consideration in tlre future. It is used to alter the velocity. separate.are used to determine these values. This point is considered the optimum mix valve setting. After an hour and a half at this settlng. for it allows maximum salt removal without the risk of forming an emulsion too tough to be resolved. A few mechanical features are best monitored (and aa- OP1IMUM MIXING PRESSURE INCREASING MIXING PRESSURE mixing valve pressure drop. A stabilization period of about an hour and a half is again allowed.TABLE 3 .. as the mix valve delta p is increased. The mix valve setting is chosen that allows the maximum salt removal from the oil with a mlnimum of saline water carry-over. Laboratory analyses. and. Since a target interface level varies depending on the particular desalter type and design being considered. Another mechanical consideration of the desalter is the distributor gap delta p (if it is variable). as water carry-over increases. shear. The turbulence and rolling action brought about as the emulsion moved through these mechamcal obstacles would increase the particles' Browntan movement. and another desalted crude Sam ple is collected and analyzed. This valve is generally situated to contact the crude oil just before it enters the desalter vessel. Once this point is reached and the curve "breaks upward. Its purpose IS to provide maximum contact between the wash water and the salts in the crude oil and yet produce an emulsion that can be broken in the desalter. a desalted crudesample is collected and analyzed for salt and BSCW. but its purpose remains the same. the percentage of BSCW in the desalted crude increases sharply.g. such as Salt content (ASTM-D3230)and BSBW (ASTM-96)." the experiment can be terminated. The distributor(s) design and posltion vanes from desalter to desalter. These laboratory analpes may be combined with an iw field testing method to determine the optimum mix valve pressure drop for a particular crude charge.Darrltlng drrlgn conrldrrrtlonr Slobas' Law h w e n c e by r a M m collisions and d~tferenl~al veloc~ty -1-1 propenles ot 011 an0 wrler thal allect phase separatm: Dens~ty -Vlrcarlly - ~mgur~tlos In 011 and water that atrest phase separation: -ACIOS -Bases -Sollor ' often leads to oil carry-under out the desalter and subsequent difficulties in the API. when processing crudes below approximately 28 to 30 API. As an additional electrical control. etc. = density of water of oil Po = dens~ty G = acceleration of gravity K = a constant Note that the velocity of the falling droplets is greatest when there is a largedifference in density between the water and 011. desalter operation is easiest when charging light oil of minimum viscosity that has been mixed with wash water to create an emulsion conststing of properly spaced.) Some desalter designers recom.- - - - FIGURE 2 Relationship between percent wash water added. Having all three areas accessible allows the greatest flexibility in adjusting the wash water mjection to changing emulsifying characteristics of crude charges. Unlike the above three variables. and the field becomes too conductive. 2 R~ ( - Po)G - 9 no K dZ (P. crude conductivity is a characteristic over which one has little control. This physical law describes the velocity of a falling water drop as it falls through the bulk oil phase: ( d . since the voltage across the electrodes will decline. water droplet population density. the pressure drop across the mix valve. coalescence by random collisions and by differential velocity and physical properties of oil and water that affect phase separation (dens~ty and viscosity).both before and after the crude charge pump and also just before the desalter mix valve) (Figure 3). it is wise to have several injection points ava~lable(i. corrosion. of the desalter wash water is important. For instance. and the level controller float a'nd the electrodes. - Materials Performance .g. the distributor pressure drop and crude conductivity are among the important variables relating to desalter electrical performance. when the diameter of the water droplets IS as large as posslble. Electrical Among electrical considerations involved in designing electrical and chem-electric desalters are the dipole coalescence phenomena: Water droplet distortion in the presence of an alternating electric field and attraction between polarized droplets: droplet size and population density. and the amperage will increase whenever such crudes are charged. and relative force d attraction between droplets. mend that the electrode spacing be changed from 1 to 4 in. however. entrance bushing(s). maintaining the necessary pressure drops across the mix valve and the distributor(s). Injection ahead of the crude charge pump allows maxlmum contact. as the wash water content falls below 3%) the distance between the dispersed water dropiets increases to the point that the force of attraction between these particles declines abruptly (Figure 2). high water level. the limits of coalescence and phase ratio: the impressed electric field and physical properties of oil that affect electrical performance.500 V. = dt P . controlling the desalter vessel at the proper internal temperature and pressure. the distance between them can be altered to vary the voltage to meet the particular requirements of the emulsion being treated. If the wash water content rises above 8%. is a direct and expected result of Stokes' Law. depending on the particular characteristics of the desalter and the crude being charged. The first three determine the population density of dispersed. Although a middle-of-the-roadpercentage of wash water (usually 5%) is normally used. this value may be decreased to 3% or raised to 8%. With these design parameters. and coalesced water droplets fall through the bulk oil phase to establish a bottom layer occupying the bottom one-third of the vessel. Hence. Not only the amount. In this RELATIVE FORCE OF DISTANCE BETWEEN ATTRACTION BETWEEN Gravitational Gravitational factors that should be considered include the role of Stokes' Law. emulsified water droplets. With these. and its strength or effective range altered by mechanical adjustment. and altermg the crude charge's viscosity when possible. large water droplets that fall rapidly. the applied electric field can be helf constant. this adjustment is only possible during turnarounds. way. it is necessary to add about 5% wash water to the raw crude and maintain the mix valve pressure drop in the 8 to 20 psig range to obtain water droplet diameters averaging 0. most typical desalters are designed with their electrodes spaced at about 8 in. This phenomenon. which lies at the heart of desalter design. however. If economics allow. and when the viscosity of the 011is as low as possible. the population density of the water droplets becomes too high. Highly conductive crudes should be identified.. or fouling..e. The percentage of wash water added to the crude. v = no Po)G R = radius of water droplet dxld. (With most desalter models. using the appropriate water injection points. These include such things as the condition of the electrical power ttansformerts). As one departs too far from these design boundaries. In such cases.5 pm and spaced about two droplet diameters apart in the emulsion. some desalters are equipped with adjustable electrodes. = velocity of falling droplet V = velocity of falling droplet of bulk medium (oil) no = v~scosity d = diameter of water droplet P . All should be in good physicalcondition with no signs of wear. and insulator(s). one should be careful to diagnose the source of the problem as a conductive crude rather than assuming these electrical changes to be symptomatic of high water carry-over. allowing an adequate residence time for the emulsion to be resolved. and carry an electrical load of about 16.justed or replaced if necessary) during turnaround periods. The desalter vessel is in essence a gravity settler in which lighter oil rises to occupy the top two-thirds of the vessel. These conditions are obtalned by maintaining the proper wash water injection rate. they congregate too closely. such as conductivity. and (- EUULSIOM HATER DROPLETS WATER D R O P L E T S - - -- -. but its shearing action cannot be closely control!ed. but also the injection point. the applied electric field becomes hard pressed to perform its intended function (e. and their average particle diameter and the distance between their centers. The goal is to create droplets with as large diameters as posslble that are spaced about two droplet diameters apart to maxlmlze the force of attraction between them. the operator must weight two opposrng considerations: (1) How efficient a desalting operation is desired. but this may vary depending upon the particular desalter design being considered. a highly viscous crude can be blended wcth one of lower viscosity in order toobtain a charge having a more tolerable viscos~ty:second. i f not halted. A generally accepted operational temperature range is from 250 to 270 F (121 to 132 C). high solids. i f not impossible. increasing the temperature is not a "miracle cure. to add diluents or solvents to the desalter feedstock t o lower its viscosity. or settling time. C ' . etc. Another option is to inject the wash water just prior t o the desalter mix valve. it is generally not advised to steam-treat. This approach can be enhanced by improved attention to tank farm management and by identifying especially troublesome crudes that display a preponderance of poor qualities: Low API grav~ty.Injection points for wash water and demulsifier chemical. Here. Chemical C. altering the percentage of wash water to each according to the degree of mixing andlor water washing desired. An additional approach is to use several of these injection points simultaneously. or gases) July 1986 . high concentration of emulsion stabilizers. (This assumes that the desalter floor was originally clean. or sludge-bust. An obvious advantage to operating the desalter at a small applied pressure (usually about 20 psig above the crude charge's vapor pressure at the desalter's operational temperature) is to prevent boiling of the desalter contents. the refiner can make rational adjustments in operating conditions. and (2) how long a residence time is economically feasible to maintain the desired crude unit throughput.high viscosity." and has limitations. the desalter for extended periods of time. Two other methods may also be considered: First. Both of these changes create more favorable conditions for rapid water settling. high metals.g. Along this same line of reasoning. depending on the crude. This pressure drop. By identifying such "troublemakers" before they reach the desalter(s).according to the crude feed rate and the volume capacity of the desaiter. liquids. it may create an emulsion too tight to be resolved in the desalter. Increasing the desalter temperature is one means to reduce the oil viscosity to a more desirable lower value.. a powerful shot of steam. For most crude charges. etc. if not altogether economical..) An extended or continuous application of steam results in an excessive stirring of the vessel contents that makes normal coalescence and gravity settling difficult. is more than adequate to prevent sludge or sediment build-up on the bottom of the desalter. However. Another operations' variable that influences the effectiveness of Stokes' Law is the length of residence. Injecting the wash water at the discharge side of the crude charge pump eliminates this problem and helps to reduce or eliminate fouling in the crude preheat train by providing water washing there. This natural occurrence. along with other factors such as the nature of the crude charge and its emulsifying tendencres. the refiner must either install a larger vessel or realistically accept the consequences of higher water and salt carry-over from the smaller vessel because of insufficient settling time. the applied pressure drop and subsequent mixing can be closely controlled and over-emulsification can be avoided. The desalter's internal pressure is another important variable. that the emulsion spends in the desalter vessel.I fall FIGURE 3 . The same preparedness tactics can also be practiced for tank switching periods (when solids and BSBW percentages may rlse temporarily) and when slop oil is charged through the desalters. demulsifier formulations and rates. after a turnaround.:emical considerations that affect the desalter operation ~nclude any chemlcal species (solids. It is essential that the proper pressure drop across the desalter mix valve be maintained. it may be necessary. but most practitioners recommend that this period be at least 30 min. dirty. in advance. as and available crudes become ever increasingly heavy. increasing the desalter temperature reduces the oil's viscosity and widens the density differential between oil and water. v~scous. e. Most refiners are buffering the effects of increasing weight of crude supplies by blending heavy stocks into lighter stocks that are still available to them. If throughput is given priority. In this way. This is expressed mathematically as follows: where: F = force of attraction between dispersed water droplets K = a constant E = voltage gradient a = radius of water drop d = distance between drop centers This allows for determination of the mix valve setting that creates the optimum water droplet size and population denslty necessary to maxlmlze Stokes' Law settling. Regulating the desalter's internal temperature has two positive effects that increase the success of Stokes' Law. would greatly disrupt Stokes' Law gravity settling. This value changes. determlnes the population dens~ty and size of the emulsified water droplets formed in the desalter feedstock. lasting a mere 15 to30 slday. Some c o r n pounds give very dry oil but provide no help in water-wetting solids. and 12 ppm) and two different simulated mix valve shears (7 and 14 psi delta p). 6. but care should be taken that the chosen chemical formulation is injected into the correct desalter phase at the correct injection point. Before a series time investment is made in bench testing demulsifiers. at injection rates that are too high. 9. sediments. a water-soluble demulsifier is usually added in or with the wash water (Figure 3). aspnaltenes. or 0. An oil-soluble demulsifier is usually added to the crude immediately after the charge pump so that it has a chance to become thoroughly homogenized in the oil phase before the 5% wash water is added. For these reasons. and iron sulfides. the standard industrydosage rate is usually 6 ppm. and finely divided solid particles such as iron. oxygenated sulfur and nitrogen compounds. The main supplies needed are a mlxing device. as its heat source. In other words. but most are polar in nature and distribute themselves directly at the oil-water interface of the dispersed droplets. others give clean effluent water but are slow acting. Mixing and settling can be accomplished in the same vessel and heat can be applied using a modified water bath.Examples of demulsifier chemicals Attrfbutes Fast water drop Good interface destabilization General Clean effluent water Fast water drop Excellent solids wetting Fast water drcp Good solids wetting Good solids wetting DernulsHkr chemistry Alkanolamine condensates Ethoxylated alcohols Ethoxylated amines Ethoxylated phenol1 formaldehyde resins Ethoxy lated resins Oxyalkylated phenols (primarily ethoxylated nonyl phenol) Polyglycols P O I Y ~ I ~ C O Iesters General Slow water drop Fast interface destabilization Clean effluent water General Fast water drop Good solids wetting General Polyhydric alcohols Sulfonic acid salts Sulfonated castor oil ( whether soluble. the connate water or the wash water. and in the proper dosage rate or concentration relative to the desalter throughput. A minimum effective dosage rate is 3 ppm. "rag" or "cuff" layers that resist resolution. If the experimenter does not have access to any of this equipthea~d ment. several lOOmL graduated. a simple demulsifier comparison can be run w ~ t h of a separatory funnel. insoluble.TABLE 4 . A meaningful bench screening procedure need not involve elaborate equipment. rather than the hot water bath. One compromise could reduce the number of test specimens to 24. silica. The selection of a demulsifying chemical is complicated. The bench test procedure should simulate the actual operational parameters of the desalter as closely as possible. care should be taken to obtain a representative oil sample of the chargqs) in clean. Both oil-soluble and water-soluble varieties are available. On the other hand. Also important are clays. others enhance solids wetting and removal but leave the effluent water oily and the desalted crude wet. Emulsion stabilizers situate themselves a t t h e oilwater interface of the dispersed water droplets and comprise a physical barrier that prevents the water drops from getting close enough for the intermolecular forces of water-towater attraction t o be of sufficient strength to bring about coalescence.75 ppm of pure demulsifier: the maximum dosage rate usually accepted by the industry is 12 ppm. the adaed chemical may work in favor of the action it was intended to resolve if too much is added. Most demulsifier compounds are formulated as a 25% active product. Not only is the choice of demulsifiers important. oil-soluble organic acids. For instance. The heavier crudes cause desalter problems because they contain higher than usual percentages of both naturally and artificially introduced solids and emulsion stabilizers that form tenacious sludge. The same 12 demulsifier formulations could be tested at Materials Performance . Dosage rates beyond this maximum value can create more problems than they correct. Another beneficial device may be a mini-electrode setup designed to simulate the desalter's electric field. zinc and aluminum sulfates. this device usually employs the heat sink. The main objective of a chemical demulsifier (or destabilire0 is t o attack the tough stabilizing film surrounding the s and tenacidispersed water droplets and reduce ~ t thickness ty t o the point where the interface film can be ruptured by random collision or contact with other droplets or through the action of an alternating electric current or both. a possible scenario would be the testing of 12 demulsifier formulations at four dosage rates (3. from the many different chemicals available under the broad heading of "demulsifiers" (Table 4). This scheme leads to a total of 96 test specimens. Special injection nozzles are not needed (a simple nipple and valve injection system is adequate) since the cold (or preheat) train of exchangers provide ample mixing. calcium carbonate. Three basic rules should be applied to this testing: 1. a chemical demulsifier choice is often likely to be a compromise. Screening demulsifiers involves making some compromises between all the possible variables so that the experimentation can be completed within a realistic time frame. These stabilizers may include such materials as asphalt. Thecrude(or crude blend) sample that is tested should be as representative as possible of the normal desalter crude charge. To answer the question. "How much demulsifier and of what formulation should I add?. For such a product. and some porphyrins. dry sample containers from a sample line that has been thoroughly purged. It is also desirable to shake the sample well before pouring off the d e sired test aliquot(sl. 2. Consequently. most commercial demulsifier formulations are blends of several effective materials of various molecular weight ranges. The reason is that demulsifiers often suffer a role reversal as a function of concentration (i. demulsifiers often behave as emulsifiers). or 3 ppm of pure chemical.e. conical tubes: and a hot water bath or heat sink." one should devote time to bench testing various demulsifier formulations. or 1. resins. 3. glass. One must decide what is most wanted and then accept the negatives that may inherently come along with that choice. The oil sample should be as fresh as possible because rapid aging of some emulsions alters their susceptibility to treatment. or dispersible in oil and/or water that may be present in the oil.. Some compounds tend t o p r o duce fast water drop but leave the effluent water oily.5 ppm of pure (100% active) demulsifier. For safety reasons. however. " at the Interface. 6. no1 dlrs~tly con. high percentage of solids and metals.5 to 8. Most commonly a variable speed blender is used. it should have a low total hardness.0 range. The attainment of a smooth desalter operation is difficult because of the decreased ava~lability of easily desalted charge stocks. This is because of the formation of sodium napthenates. Place the filled tube in a hot bath or heat sink. [A water bath with a thermostatic control set in a range of 150 to 180 F (66 to 82 C) works well. the available choices are quite straightforward: Correct the upstream operating problem. 7. In addition. For example. If such a well is not adequately purged before production is collected for market. When wash water quality is a problem. 1. A further time reducing c o r n promise would be to screen these twelve demulsifiers at a 9 wmdosage rate and an 11 psig mix valve simulation. Also observe the "brightness" of the top oil. Clean glass walls can be a sign of a well resolved emulsion. either individually or in combination. When practical. Most of today's world crude supplies are especially troublesome because they display a high preponderance of poor desalting qualities: A low API gravity. Another guideline is to maintain the desalter wash and effluent waters within a 5.0 pH range. gravitational settling device. that are essentially aromatic soaps. trollaale C 11. especially when this water is used in feed-teeffluent exchangers. observe the water drop rate until the total 5% wash water has been recovered.5 to 8.19 Bench test demuls~f~ers to oblaln one lhal m~n~m~zes 011carrpunder: a~lute heaq ends crudes July 1986 . an extremely acidic charge can be sent to the desalters.) 3. it should have a pH in the 5. ammonia and hydrogen sulfide levels in desalter wash water can fluctuate suddenly if stripped sour water is used as desalter wash and the stripper becomes upset. Discussion There are several theoretical considerations to be reviewed to achieve a satisfactory desalter operation.75 lo 3 ppm 01 pure chemcal) 13 R a m to opt~mum level $4 Run della ~1saluBSbW prcflle 10cetermlna the optmum pressure arop settlng 5 Adjust l o proper level beneath electrodes 1415 Malntam a1 oplmum j a m t 20 psq above charge s vapor pressure) to prevent gasslng andor Wtltng In the desalter 16 Blend heavy crude w ~ l h lighter stocks andlor diluenls: rna~ntam temperatures as high as safety and economics allow 17 Allow adequale selllmg by currlng throughpu~ rate or Duytng a larger vessel 6 If it's too lhick lower level to a safe disunce below electrwes 7.Desalter problem solving gulde X X X X X X 1 VOIIS ArnOs MIX valve aella p interlace level lnlerface Ihlckness Percent wasn water Demuls~t~er formulation Demuls~l~er dosage rate Tempuature Pressure VlSCOSliy Restdence t~me Effluent water 2 3 5 6 9 X 14 1. observe thecleanliness of the tube's glass wall. A resulting symptom is to find the desalter effluent water with a much lower pH than the added wash water.12 Bench lest to determ~ne the b e t formulat~on and dosage rate tor Dart~cular charge (acceptable do-ge range I S normally 0. some practi.wett~ng and droo: dump lnlerface 11II rernalns too tn~clvbad 9-10 Adlust 1nwt10n rale to opt~mum of 5% 16. Stir for the required time at this speed to obtain the desired delta p simulation. Caustic is simllar to the demulsifier chemical in that if it is added at too high an injection rate. directly challenge the desalter's role as a chem-electric. o r 5 min intervals. and sulfate concentration. pressurizedheater. 4. Note the clarity or opaque ness of the dropped effluent water as a measure of otl carryunder andior the creatlon of an inverse (oil-in-water)emulsion in the effluent. Add the desired aliquot of demulsifier (sized to the volume of oil being tested) while stirring with a low blender speed for approximately 10 s. Prewarm the crude oil sample i n an oven or hot bath. Observe and record the percentage of water drop at 1.dosage rates of 6 and 12 ppm with a middle-of-theroad mix simulation of 11 psig. and numerous emulsion stabilizers. This temperature range compmmises actual desalter operating temperatures with the i safety limitations of open. These factors. This provides baseline data with which to compare subsequent water analyses on samples taken during upsets. modify the water by acid (usually sulfuric acid) or base (usually sodium hydroxide) i f pH is the continual problem or change water sources partially or completely.] j' 2. sodium hydroxide. This property can be another sign of dry oil and good demulsification.8 Bencn test demuls~liers to sotam more sol~ds. Another chemcal now rarely used in the desalting process is caustic. usmg whichever interval is appropriate for "the drop rate being observed. reducing the number of test specimens to 12. In addition. bicarbonate and carbonate content. Increase the blender stir speed to the desired level and add 5% wash water. It is also advisaMe to record the sharpness of the oilwater interface and the presence or absence of any unresolved emulsion. as related to enhancing the desalter operation. (This device is needed to simulate the shearing and mixing action of the crude charge pump and/or mix valve. TABLE 5 . The best way to determine the maximum allowable dosage rateof caustic is to bench test different injection concentrations using a representative crude. emulsion stabliization results. noting the appearance of the water-wet glass at the water layer and the oil-wet glass in the oil layer. m e general testing procedure should be performed in a manner similar to the following: 1. Another chemlcal consideration is the monltorlng and mainteance of a proper wash water quality.2 Symplomal~c of problems elsewhere. It is used to regulate the desalter wash water pH to create an optimum pH range for rapid emulsion separation or resolution. ~f it is something like an upset sour water stripper. Cease stirring and transfer a 100mL aliquot of the emulsion to a graduated conical tube. Transfer a known volume of the test oil to the mixing device. nonpressurized bench testing. If the wash water is to be preheated through exchangers before it is added to the crude charge. tioners like to use a plpet to withdraw an oil sample from a p proximately 15 mm below the surface to determine the percentage BS&W by centrifugation. high viscosity. but a general rule is to never exceed a 30 ppm dosage rate. Desalter pH fluctuations Can be a problem because strong acids are sometimes used to acidify oil wells to increase their production. j ' A perlodic complete water analysis on the desalter wash water is helpful. "rag" or -cuff. 5. As a further check of the top oil quality. bench lesl lo lmd a beller sOl~d+wetl~ng lormulalmn a\ Change bl Operate sludge ousterr cl Repa~r a1 Increase aemulsifler Interlace lh~ckness A. Stabillzed emuls~on D. Stabilized emulsion E. hameter of aispersba water droplels too srrull Vollage Varies wllh Iransformer lap anaor voltmdler design LOW v0llagcl 1 .weltmg totmulat~on I' a1 Cower lntedace level b) Bench test aemuls~tlers Cower mlr valve della P 4 tntedace level AI mlddle trycock Too hlgh or 100 low A. 0. To help to incorporate the theoretical considerations of desalter operation into practical relations between desalter symptoms and solutions. there are only a few controllable factors that are economically realistic. 6 to 12 in.. gravitational. Float level problems) a) Weights inappropr1al6 b) Floal dirty andlot sluck c) Level controtter Iallure 0 a) Increase Q t m u l S ~ f W dosage bl If (a) IS not successful. and chemical factors and recommends the likely remedy. the practical and controllable variables become limited in number. It relates essential desalter "vital signs" problems to electrical. and chemical remedies to desalter problems. Conduclive crude D. Although Some nvrllllcatlons to the desalter hardware systems and OPeratlonul Darameters show promise in improving the refiners' abil~ty 10 cope with heavier crudes.' Repair ' Percent wash water Flow conlroller (allure ^walr Materials Performance - - - - - - - . Interlace layer too thick andlor loo high F.m and lighter waterrising to the top of the desalter). raising the desalter temperature may not be possible without the addition of new heat exchangers. two problem solving tables are p r e vided. there is a c. electrical. bench test lo lmd a b t t e r sollds. increasing the desalter residence time would involve either cutting throughput or installing a larger vessel. the expeaad result and when and where this result should appear. Demulsilier inleclion pump failure 4 S ' dosage b) I f (a) IS not successfuL bench lest lo fmd a beller sol~ds-welllng lormulal~on 0 Reson to "to aump or not to aump Iwl' increase wetlfng agent concentralron In OemulSlfler tormulabon . the ratltlor must realize that sometimes the upset is by doing nothing. Aspa~r Reoa~r Tolerate or Oknd charge with less conduclive cruae I' a) Increase d e m u l ~ ~ l ~ e r dosage bl If (a) IS not ruccesSluL bencn test t o find a better sorrlrwectlng lormulal~on t a) Lower ~nleflace level b) Bench l e s ~ aemulsifiers t Cower mtr vawe aelta P 4 ' Amperage Low velocity design0 10 10 A High veloclty des~gnOt050A Flucluating amps High amps A. Conductive crude C. A. Diameter o f 01spetUM water drODlels too S K I a l l * Increase desalter pressure R Tolerate or Blend charge wbth less conduct~ve cruae a\ Increase aemulslher dosage h) l l (a) IS not successlul. Slable emulsion-wde inlerface layer B. Transformer failure 8. 12 l o 24 ~ n .t troubleshooting the desalter should unless you know the reason for be: D O not change a W o r t ~ e t e r the change. f o r example. Interlace layer loo lhlck andlor loo high E. Insulator failure C. When such Costly remedies are eliminated. TABLE 8 . too much should not be expected lrom the desalter when crude weight and water weight draw bo closely together as to challenge the basic physics and P r l r r l ~ l e s of desalter operation. Very stable emulslon C . t o be-expected phenomem (such as heavier-than-water hydro carbons falling to thenmr. Conse quently. Table 5 illustrates the assumption that all mechanical variables of the desalter hardware are functional. Slable emulsion 8.Although there are some obvious mechanical. High solids ~ncharge D. Crude gassing at current desalter temperature 8. gravitational. Table 6 shows a more detailed problem solving grid that relates spec~fic upset symptoms to their possible cause and gives remedial measures. When faced with desaltbr svmptoms resulting from natural. Similarly.Desalter problem solving grid A. A best way to react to a llos~lter good general principle r1.uttmen9urate need for engineers and operators to more fully 0.nmprehend the function of the many desalter variables 10 gal11 s clearer understanding of what they realistically can and r:ot~not expect each alternative to do. drop tho tog of the interfaw to the third andor m~ddle sample tap by lowermg me watu level. If rag layer renulns stable and does not at least bepn to narrow. Daaap. Triple the demulsifierdosage rate for 1 h (usually 18 ppm or 3 gtsllOOO bbt crude). Lower dosage rate D. Bench lest l o find a m e appropriate formulal~on € Lower tmperature . For 2 stage desallingc l plb For 1 stage desaltlng<2 ptb High D. Low API gravity crude f hervy resoduum B. h C. July 1986 .l0h BS6W Out desalter <6 l o 8 in. a) Blend w ~ t h lighter APII lighter crude 0) Switch crudes Tolerate B. Blend lhis Crude with a more manageable one 11 possible. MIXvalw delta p too high ' wash water >7% 8. c. Uude v~scosily loo high E. Increase to 5% wash water D. then run a delta DIBSLWIS~I~ prof~le to determ~ne the optlmum mix valve settlng B. d. * hwash watef < 4% D.<0. or 2 qlYlOOO bbl crude). b. too much shoum not be expected of any demulsifler formulal~o:. "rag" or "cuff" layer exceed8 12 in. Increase lemwrature A. Ccnage rate loo high C.rwntagl of BS in Qenrtea crude High wrwnlage of water In uerrttadcrude > 100 ppm oil w t in effluent water A. Reduce lo 5% wash water C. Obaewe 2 l o 12 h for response. lhen sol the demulsifier rate at W the normal dosage (usually 12 ppm. High Ya O in crude charge C. inter1 ace >6 l o 8 In. dump the water ana mledace layers) and tmgin lo reestablish a new waler level. a) Blend wltn hgnter crude b) Swttch crudes C) Tolerate C. Temwrature loo high.. dump the desalter contents to the clean 011 level (1. Temporar~ly lower delta D.. more viscous and high solids bearing crudes. resulting in ~nsulf~c~ent contact of salts w ~ t h wash water and water and salt carwmer A. when crude we~ght and water we~ght draw so closely together as to challenge the basic pnysics of oil and water separation. Temperature loo low High eoncnntralion of O i l in effluent water < 100 ppm > 100 Ppm A. 5 % active product C. a Leave the demulsifier rate at 2X Ihe normal dosage rate tor as long as the problem crude 1 s being chargsd. interfaw BbnCh test demukifiers lo find best O r u lor c r w Dsing charge < 100 ppm al W I In effluent water 1 g u 1 m bblS or 8 ppmbased on 25% acttve product Hlgh COncentral'in ol salt in desalted crud. A . Dernuls~fier w a g e loo high D. Increase 10 1 UUIOOO bbbb s e d on 25% active proaucl b W on 2 .e. rate tao low 1 pU1000 bbt B. <3ppmor A. Chemlcal injFtion pump ta~bre Mix vake delta p loo low. Rcpav For 2 stage desaltlng>1 ptb For 1 stage desalting> 2 Dtb Run a delta plBSLW1sait profile to determine the optrmum m u vatvo setting Insufficient wetting agent in demulaifier formulation Bench test demulsifiers to find one with gaM soiidc wetting and drop performance A . a) Blend with a less v~scous crude b) Increase temperature E. The refmer shoUKl keep in mlrd that demulsifier chem~cals-regardless of the~r lormulation-are lust one 01 many components ot a successful desalter operation. Wh11e11is lrue lhal the proper selection of chemical demutsifying agents and wetting agents can exlend the effectivedemulsitymg ranCe Into somewhat haav~er. > 12 ppm or 2 ~IslldOO bbl 8. Demulsifier formulation wrong for particular charge E. increasmg solubility of oil in watef ~~ - ~ ~ " ' ' 1 0 Qumpor no! to dump" test: r Il the interface.
Report "Troubleshooting the Refinery Desalter Operation"