INGOLDLeading Process Analytics

Desaltingisakeyearly-stageprocessstepinthe

refiningofcrudeoil.Ifnotconductedcorrectly,con-

taminants remaining in the crude after desalting

causefoulingandcorrosionindownstreamprocess

operations,andreducetheheattransferefficiency

offurnacesandheatexchangers.Additionally,the

presence of oil in desalter wastewater requires

chemical treatment tomeetprocessandenviron-

mentalrequirements.Changesincrudeoilquality

anddesalterwashwatercauseprocessconditions

to vary constantly, making efficient desalting a

challenge.

Althoughoftenoverlooked,continuouspHcontrolof

desalterwashwaterisanessentialtoolforoptimiz-

ing the desalting process. This white paper ad-

dresseskeydesaltingproblemspHcontrolcanhelp

address, in particular pH’s influence on the zeta

potentialforemulsionstabilityisreviewed.

MoreOil,LessChemicalspH Control in Crude Desalting

Whi

te P

aper

BackgroundCrude oil usually contains a considerable amount of water. This can be water injected into the well as part of the recov-ery process or it can be water that is naturally present in the oil reservoir. The water is actually brine and contains vari-ous dissolved mineral salts. The crude may also contain crystalline salts, sediment and corrosion products such as rust (from cargo holds). All these contaminants need to be removed before the crude is refined as they cause fouling and corrosion of downstream equipment. Inorganic salts such as magnesium chloride and calcium chloride tend to be par-

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which, under refinery process conditions, hydrolyze and form extremely corrosive hydrochloric acid. Other contaminants may also have a direct negative impact on downstream processes, for example certain metals present in the crude poison catalysts and render them useless.

Another concern for refineries is oil under-carry in the desalter effluent as it requires additional treatment and chemical usage in order to remove it.

The salt content of crude is measured in pounds per thousand barrels (PTB) and may be as high as 300 PTB. After desalting, the crude typically has a salt content of 1 – 2 PTB.

DesaltingprocessTo dehydrate and remove the impurities, the crude is heated to 120 – 150 °C and mixed with 2 – 15 % water through a mixing valve and/or a static mixer. During mixing, the contaminants transfer to the water phase. The oil/water emulsion is then fed to one or more large holding tanks or gravity settlers where the emulsion is allowed to separate into an oil layer and a water layer. Impurities leave with the water effluent and as sludge from the bottom of the vessel, and the desalted crude is extracted from the top.

EmulsionformationAn abundance of emulsifying agents occurs naturally in most crude oils. These can be resins, asphaltenes and fatty (naph-thenic) acids or colloidal solids. It is therefore quite common that when drawn from the reservoir, the water cut of the crude and the oil are strongly emulsified. These emulsions can be very tight and difficult to break. The emulsifiers may also bind the desalter wash water into a stable emulsion with the oil and in-hibit the separation process.

To help resolve this, several aids are used during the desalting process. Demulsifying chemicals are injected to promote coagu-lation and speed up the separation of the two phases. For the same reason, gravity settlers are equipped with an electric grid that induces a 5 – 35 kV electrostatic field which concentrates and flocculates the water droplets. Generally, AC fields are used to avoid electrolysis of the water phase and to prevent galvanic corrosion of settling tanks.

Frequently, especially in the processing of heavier crudes, emul-sions can be extremely stable and separation may progress too slowly, which results in a thick interface or rag layer between the oil and water phases. If emulsion formation is excessive, operators often struggle to deal with it. The typical response is to add more demulsifiers, change mixing valve settings and reduce wash water flow. This can cause several upsets. A typical one is desalter grid overload which manifests as a voltage drop, an increase in current and finally, desalter shut down. In turn, this leads to massive water over-carry with all its consequences and to severe oil under-carry which puts an unacceptable load on effluent treatment facilities.

pHcontrolOften ignored in the desalting process is pH control, but it is important for a number of reasons:• The crude contains different organic acids or acid forming

salts. These are major causes of corrosion in the crude tower, the overhead condenser tubing and in the desalter itself. Caus-tic is usually fed to the desalter to neutralize the acidity.

• Often, ex-sour water stripper water is used to wash the crude in the desalter. This water may already have a low pH (< pH 5.5) and cause corrosion.

• If the pH is too high (> pH 8), organic acids such as naph-thenic acid in the crude ionize and produce soaps that form unbreakable oil/water emulsions.

• There is also the strong influence of the pH value on the speed of demulsifying and phase separation. This has to do with an electrokinetic property, the so called zeta “z” potential.

z-potentialIn suspensions and emulsions, particles or emulsion droplets are usually electrically charged. The charge at the droplet sur-face affects the distribution of ions or charges in the direct proximity of the droplet. It will attract counter ions or particles with the opposite charge, which form a double layer around the droplet: a bound layer attached to the droplet, and a diffuse layer which decays in concentration as distance from the droplet increases. Within the diffuse layer a shear plane can be distin-guished where, when a droplet moves, charges on the inner side of the diffuse layer stay bound with the droplet. Charges on the outer side of the diffuse layer remain with the bulk liquid. The electrical potential at the shear plane in the diffuse layer is called the ζz-potential.

3White Paper METTLERTOLEDO

The magnitude of the ζz-potential is a measure of the stability of the emulsion. When the droplets have a large ζz-potential they will repel each other and the emulsion remains stable. With a low ζz-potential London-van der Waals forces become effective and the droplets aggregate and flocculate. Pronounced coagula-tion occurs when the z-potential equals zero. Probably the most important factor affecting the surface charge and the ζz-potential is the pH value. Depending on the charge of the drop-lets, increasing or decreasing the pH of the emulsion by adding alkali or acid alters the charge of the droplets, forcing them towards the isoelectric point where the charge is zero.

pH control has economic advantages over the use of surfactants, demulsifiers and corrosion inhibitors, as acids and alkali are much cheaper. Also, less oil is lost through under-carry. Process conditions however, are such that pH measurement is very difficult.

InstrumentationDue to the continuously changing process conditions in the desalter, measuring pH by grab sampling does not really make sense. By the time the results of the analysis are available, the data will no longer apply to the current situation. In-situ measurement of process pH is the only correct method. It gives a real-time de-termination and allows for direct process control. How-ever, the oily conditions, the high temperatures and the presence of substantial quantities of sulfides cause most traditional pH probes to fail within a very short time.

The causes of failure in most cases are clogging of the sensor reference diaphragm and poisoning of the refer-ence electrode. It is the presence of crude oil and par-ticles that rapidly clogs the pores in the diaphragm on the reference side of the sensor. Poisoning occurs when sulfides diffuse through the diaphragm and react with the internal silver/silver chloride reference electrode. Both events cause a shift of the electrical potential between the pH electrode and the reference electrode and have a direct impact on the measured pH value. The result is a false pH measurement. Therefore, in practice, the pH measurement is often discarded.

Engineers who have experienced this problem before may not even install a pH measurement system into a new desalter.

As the pH measurement is often false, or totally absent, this regularly leads to uncontrolled chemical dosing of, in the main, caustic. This in itself is costly as the caustic is typically over-dosed, but also it inhibits proper oil/water separation with all of its aforementioned consequences.

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Distribution of ions around particle/droplet

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Charge as a function of distance

ElectrodesfitforpurposeThe InPro 4800i pH electrode with PTFE annular reference dia-phragm is METTLER TOLEDO’s answer to the toughest refinery applications. This highly robust electrode has a proven track record in desalter service. It resists fouling caused by oily hydro-carbon contaminants and its long helix diffusion path provides an excellent barrier against poisoning from sulfides, guarantee-ing high accuracy and fast response throughout many months of operation. It is designed to operate under high pressures and batch process temperatures. Featuring Intelligent Senor Man-agement technology, the sensor provides full diagnostics that advise when maintenance is needed or when replacement is required.

ISMIntelligent Sensor Management (ISM) is a revolutionary plat-form based on electrodes with embedded digital technology and sets a new standard in pH management. A circuit board is em-bedded within the sensor which is powered by and read through the transmitter. Critical electrode information such as self-iden-

tification, calibration data, time in operation, and process environment exposure are docu-mented. The data is utilized to continuously

monitor the condition of the sensor and using this information recommend appropriate actions based on actual sensor expo-sure. The adaptive diagnostic information directs maintenance, thereby reducing downtime and minimizing plant operational costs (see Diagnostics).

The output signal of the electrode is digital, which guarantees 100 % integrity of the measurement value, even under the tough-est conditions.

System operation and user benefits of ISM at various stages of sensor lifecycle include:• Fast, easy installation• Ability to pre-calibrate sensors• Enhanced diagnostics for trouble-shooting • Dynamic Lifetime Indicator predicts remaining sensor life• Sensor management software

PlugandMeasureInstallation of a new pH electrode typically requires calibration as the first step. This involves bringing pH buffer solutions out to the field and performing a two-point calibration. This critical step may need to be conducted outdoors under adverse weather conditions or when exposed to an uncomfortable and/or hazard-ous process environment, in addition it is a time consuming procedure.

One particularly useful ISM function is that electrodes can be calibrated offline using iSense Asset Suite software via a simple USB connection to a laptop or desktop computer. This means electrodes can be pre-calibrated in a convenient, comfortable environment such as the instrumentation workshop or labora-tory. Such functionality eliminates the need to carry cleaning agents and buffer solutions through the plant to do tedious on-site calibrations. Pre-calibrated sensors carry all configuration data on an integrated microprocessor and share it with the transmitter immediately upon connection. This avoids any pos-sible configuration errors and ensures the electrodes are imme-diately ready for measuring.

DiagnosticsAdvanced diagnostics provide a continuous flow of status and maintenance information to the transmitter. Along with moni-toring glass and reference impedance, ISM bases its diagnostics on actual process exposure history. By constantly keeping track of process pH value, temperature and operating hours, ISM cal-culates when sensor calibration, cleaning or replacement is re-quired. Any maintenance requirement is therefore recognized at an early stage. All data is easily accessible through the transmit-ter’s local operator interface. The same data is also available via HART protocol, on Foundation fieldbus or Profibus networks. ISM offers true predictive maintenance functionality.

DynamicLifetimeIndicatorIn the end, even the toughest pH electrodes will show signs of wear and tear. A major advantage of ISM is sensor lifetime monitoring. The ability to accurately forecast a sensor’s remaining lifetime, decreases maintenance costs associated with unnecessary sensor replace-ment and minimizes unexpected process shutdown due to sensor failure.

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Calibration History

CALTTMTime to Maintenance

Max. °CDays of operation

MAXMax. Temperature/ODI

ACTAdaptive Calibration Timer

DLIDynamic Lifetime Indicator

Visit for more informationMettler-ToledoAGProcess AnalyticsIm Hackacker 15CH-8902 UrdorfSwitzerland

© 02/2011

www.mt.com/pro

pH C

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ting Exposure to aggressive process conditions directly impacts the

lifetime of a chemical sensor. High, and very low pH values ad-versely affect pH electrodes, as do elevated temperatures. Since the pH electrode contains a temperature sensor for compensation purposes, historical temperature exposure, pH values and other process conditions can be used to forecast remaining useful life.

AdaptiveCalibrationTimerBy continuously monitoring reference im-pedance, pH glass impedance and temper-ature exposure, the Adaptive Calibration Timer displayed on the transmitter, predicts when calibration is next required in order to maintain measurement reliability.

TransmitterThe corresponding transmitter for the InPro 4800i is the M420, a two-wire pH analyzer that is fully certified for hazardous area use which supports HART communication and ISM diagnostics functionality.AutomatedsensormaintenanceIn cases of extreme fouling, the EasyClean 400 automatic clean-ing and calibration system guarantees continuous availability and maximum reliability of the measurement. Using the InTrac  777 retractable housing, EasyClean 400 automatically extracts the pH probe without interrupting the process and with-out the need for a slipstream. Inside the housing, the probe is automatically cleaned and subsequently calibrated, after which it is re-inserted into the process. Even organic solvents can be used as a cleaning medium. The complete EasyClean 400 system is certified for use in hazardous areas.

ConclusionAn inadequate desalting operation can have major consequenc-es in relation to downstream processes in respect of refining the crude but also in regard to effluent treatment. The importance of the ζz-potential in desalter emulsion stability is significant, as the pH level in the desalter has a direct effect on this parameter. Despite the harsh process conditions, modern process analytics equipment can be confidently utilized in monitoring desalter pH. The resulting savings on chemical use, increased speed of desalt-ing, and subsequent reduction in fouling and corrosion can be substantial.

For more information, go to:www.mt.com/pro-refiningCalibration History

CALTTMTime to Maintenance

Max. °CDays of operation

MAXMax. Temperature/ODI

ACTAdaptive Calibration Timer

DLIDynamic Lifetime Indicator