G. Agar and P. Clewis, Agar Corp., Houston, and C. Spencer, Litwin Engineers & Constructors,Inc., Houston

Energy absorption instrumentation is rapidlyemerging as the preferred method of interfacecontrol for separation processes. This high-fre-

quency electromagnetic measurement technique accu-rately senses volume percentages (not level) in phaseseparations such as water and oil. Instead of searchingfor or assuming a clean interface, the instruments mon-itor percentages of water at points in the system, andcan measure either water in oil or oil in water mixtures.This sensitivity gives the operator “vision” inside the sys-tem and consequently, more reliable control.

Now unit operations can effectively monitor andreduce their oily-water releases. Reducing the workload on existing wastewater treatment systems lessensoil-grease levels in effluent water. Tighter hydrocar-bon-release monitoring can bring discharges into com-pliance and diminish overall emission levels.

A need is met. This new technology confronts one ofmany problems associated with pollution source-pointcontrol: detection.

Because operators cannot see through vessel walls,they must rely on other methods that show fluid lev-els. The emulsion’s nature further complicates leveldetection and adds to the dilemma. For most emulsions,the interface is not a clean-cut line. Rather it is a hydro-carbon/water transition zone where component con-centration varies especially with vertical position. Con-sequently, traditional level control techniques have notacknowledged this phenomenon. Thus they often gavefalse information that ultimately released hydrocar-bons into wastewater.

New solution. State-of-the-art source-reductionsolutions that are both effective and economical are

often attainable through improved instrumentation andprocess control. New technology for process control inseparation processes must help in achieving compli-ance with increasingly rigid EPA regulations such asBenzene NESHAPS. Other interface control methodsprocedures for separation process control such as sightglasses and capacitance probes have been ineffectivein detecting the hydrocarbon/water interface. Result—undercarry of hydrocarbons (and benzene) in wastewaterstreams. A new solution, energy absorption technology,measures hydrocarbon concentration in water, ratherthan the interface. This highly reliable method greatlyreduces hydrocarbon undercarry.

Two case studies. A midwestern U.S. refinery and alarge petroleum bulk terminal in Taiwan demonstratethe benefit of energy absorption. Both facilities expe-rienced significant improvement in loss control andreductions in effluent treatment costs from this tech-nique. The refinery met the latest Benzene NESHAPstandards and reduced total benzene discharged 82%.Other project benefits included reduced capital costsfor the same project. This refinery spent approximately$4 million on a source reduction project (less than$400,000 was spent on related instruments) and avoidedinvesting over $70 million on a wastewater treatmentunit (WWTU) project to meet the same regulatory com-pliance. A petroleum bulk terminal in Taiwan achievedsimilar results and cut oil-discharge concentrations toless than 10 ppm.

Source reduction—prevention is better than cure.Numerous studies detail the advantages of source-reduc-tion over treatment programs.1–3 A good example iscompliance with the NESHAP Benzene Waste Opera-tions regulation (40 CFR 61, Subpart FF—revised Jan-uary 7, 1993, 58 FR 3072). This regulation requires thatall facilities discharging 10 metric tons per year (mtpy)or more total benzene must treat all wastestreams con-taining 10 ppm or more benzene.

In a refinery, benzene originates from hydrocarbonundercarry in wastewater streams. Therefore, a source-reduction program that segregates total wastewater

Reprinted from HYDROCARBON PROCESSING® magazine, August 1993 issue, pgs. 55-59. Used with permission.

Energy absorption probescontrol oily-water discharges

New technology monitors oil-emulsionlayer in water separation processesand reduces pollution source-points

ENVIRONMENT

flow into individual streams by using advanced controlstrategies can meet the required 10 ppm limit. Segre-gated low-concentration (< 10 ppm) streams would beexempt from treatment. Even in instances where streamexemption is not the goal, lessening the load on theWWTU justifies source reduction.

Defining sources. These EPA references show sepa-ration processes as the most significant contributors togenerated wastewater. These can include both batchprocesses (tank dewatering, batch separators, etc.) andcontinuous processes (desalters/dehydrators, in-lineseparators, etc.).

When manually controlled, these processes offer thegreatest opportunity for improvement. Separators witholder technology can contribute significant pollutants towastewater. Typical oily-water contributors for a refin-ery are:

• Desalters—40%• Storage—20%• Slop oil recovery and tanks—15%• Other processes—25%.Thus, by cutting hydrocarbon undercarry from the

primary contributors, one can achieve fewer losses andmuch less pollution.

Traditional approaches. Separation control schemeshave traditionally been designed to control the height orlevel of a supposed clear-cut interface between a hydro-carbon phase and an aqueous phase in a separator. Withvery light hydrocarbons that rapidly and cleanly sepa-rate from the aqueous phase (e.g., gasoline and water),and without any turbulence, this approach is a capa-ble, acceptable control form. However, in processeswhere mixing energy and physical properties play agreater role, the phases tend to mix, inter-disperseand/or emulsify. When this occurs, the concept of levelbecomes meaningless, because no distinct point of phase

change (i.e., no clear-cut interface) exists. Instead, atransition zone or rag layer exists between the phases.

Sight glass. A basic level-reporting technology, thesight glass, is intended to give visual indication of theinterface. This method rarely, if ever, shows the pres-ence or size of an emulsion that may exist in the ves-sel. If the emulsion is positioned between the upper andlower sight glass connections, it cannot enter the sightglass and, therefore, cannot be detected. Also, poor fluidexchange between the sight glass and the tank ensureslonger residence time. Even if some of the emulsion doesenter the sight glass, this level is not indicative of theactual level.

Other technologies indicate a supposed interface levelbased on differential specific gravity. Some examplesare floats, displacers and differential pressure cells.However, all these methods give false indications of aclear-cut interface when there is an emulsion. Theseindications are unreliable in emulsions, and the hydro-carbons dispersed in the water will not measurablyaffect their output.

Capacitance probe. Another traditional technol-ogy is measurement of the liquid’s capacitance or dielec-tric with a capacitance probe. This technique’s benefitsare direct process contact and no dependence on spe-cific gravity. However, when installed vertically thecapacitance probe acts as an averaging device, reportingtotal water and oil along the active antenna but pro-viding no information as to the distribution of the twophases. For example, if it is immersed in a 50% emul-sion, it could give the same reading as if it were halfimmersed in water and half in oil, with a clear-cut inter-face. When installed horizontally the capacitance probecan sometimes act as a point alarm, but it suffers froman inability to detect the presence of hydrocarbons inthe aqueous phase. A capacitance probe relies on aninsulating media (e.g., oil or oil with water droplets)between the capacitor’s plates. When process water(which is conductive) becomes the continuous phase,even if there is significant hydrocarbon in the water,the capacitance will short-circuit and the output willpeg at full scale and falsely indicate 100% water.

Instrumentation requirements. When evaluatingcontrol instrumentation to minimize effluent under-carry, and detect/control emulsions and dispersions,certain guidelines must be considered:

• Direct contact with the process• Measurement of 0% to 100% hydrocarbon/water

concentration (not level) in both oil-continuous (water inhydrocarbon) and water-continuous (hydrocarbon inwater) phases.

• Local or point measurement, instead of averagingover a large area. This method avoids errors due tohydrocarbon/water distribution or rag layer.

• Minimal effect on measurement from fluid proper-ties (specific gravity, pressure, temperature, viscosityand coating buildups).

Innovative technique. A new technology, known asenergy absorption, has been developed specifically tomeet the previously described requirements. The outputof energy absorption instruments is expressed in units

HYDROCARBON PROCESSING / AUGUST 1993

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TransformerOutletcollector

Inter-facelevel

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Fig. 1. Bottom injected desalter or dehydrator.

of volume percentages (concentration, not level) of thewater in the near vicinity of the probe’s antennae. Theinstruments are positioned to penetrate the vessel atpoints where the measurement is desired. Consequently,the instruments not only serve to monitor the positionof an interface, but also to track changes in the size,rate of growth or shrinkage and water content of raglayers, emulsions and dispersions.

The energy absorption probe transmits a band of veryhigh frequency electromagnetic energy impulses intothe fluid around its antenna and measures the energyabsorbed. Energy absorption allows for remarkablyaccurate measurement under varying process condi-tions of the hydrocarbon-in-water phase. Its ability tomeasure a small amount of hydrocarbon in water makesthe most significant contribution when controlling theseparation processes without allowing hydrocarbonundercarry.

Example—desalter control. Energy absorption tech-nology has been used with great success in refineriesand petrochemical operations throughout the world.4Control applications have varied from the relativelysimple storage tank dewatering processes to complexdesalter control systems. The typical control strategyfor a low velocity desalter is shown in Fig. 1.

In the desalter control system, probes provide con-tinuous 4 to 20 mA output signals that are proportionalto the water concentration at their locations. Probe 1controls the brine outlet valve, using its ability to mea-sure small amounts of oil in water to maintain a veryhigh (and unstable) percentage of water several feetabove the bottom of the vessel. This allows suspended oilin the water phase to separate, thus inhibiting oil under-carry (as a primary control function). While probe 1establishes this lower limit for the emulsion layer, probe2 monitors the water content below the lower electri-cal grid to detect and alarm on emulsion growth (whichmust, by control, occur in the upward direction). Thismonitoring function allows the operator to avoid down-stream upset by advance warning of such growth, andallows time for corrective measures preventing under-carry or carryover. Probe 3, installed on the crude oilfeed line near the tank farm, monitors the line for exces-sive water in the feed to the unit (also providing anadvance warning). Probe 4 monitors the water phaseof the desalter below the distributor, alarming on thepresence of suspended oil that does not readily sepa-rate and threatens contamination of the brine (“reverse”emulsions).

Case studies. A U.S. midwestern refinery is an exam-ple of an economic source reduction program using theadvanced technology. The refinery’s total level of ben-zene discharge was nearly 17 mtpy. Initially additionalstripping capacity for WWTU was considered andplanned. However, the new stripping system wouldreduce the benzene discharge by 41% and cost severalmillion dollars. A project team consisting of companyengineering and refinery personnel and a major engi-neering firm evaluated the available level control tech-nologies and selected energy absorption to bring therefinery into compliance more economically. Fig. 2 shows

a typical block diagram for refining. A review of the pri-mary sources contributing to the refinery’s benzene dis-charge before modifications is summarized in Table 1.

At times, the existing ref inery control systemsallowed undercarry of free hydrocarbons from desalteroperations because of their inability to accurately detectthe interface between the hydrocarbons and aqueousphases. This project used two methods to improve theundercarry quality: a hydrocarbon detection instru-mentation system and the addition of recycled water tothe desalters. Testing showed that energy absorptioninstrumentation was able to detect the first traces ofsuspended hydrocarbons above the water draw-off inthe desalters that virtually eliminated free-hydrocar-bon discharge in the undercarry.

Results. To control costs, all crude and product stor-age tanks were modified to use the energy absorptionprobes as a portable system. The probes would beinstalled on the vessels for tank-bottom-draw opera-tions. This method significantly lowered total capitalcosts on the project. End-of-project results yield therefinery these benefits:

• Total benzene discharge level dropped to approxi-mately 3 mtpy

• Improved operations yielded a 82% decrease in totalbenzene emissions

• Project investment cost less than 5% of the origi-nal estimate for the additional stripping capacity.

Consequently, the refinery avoided a capital waste-water treatment project estimated over $70 million.Total capital investment for energy absorption instru-mentation was less than $400,000.

Bulk storage. Another rigorous test for tank-dewa-tering control was conducted at a large petroleum bulkstorage operation in Taiwan. Average hydrocarbon

HYDROCARBON PROCESSING / AUGUST 1993

Finishedproduct

tank farm

Crudetankfarm

Desalter Refining

Tankdewatering

systemno. 2

Tankdewatering

systemno. 1

Tankdewatering

systemno. 3

Recoveredoil

Slopoil

tanks

Skimmer controlsystem no. 5

EffluentWastewater APIsep-

arator

Product

Tank dewateringsystem no. 4

Recoveredslop oil

Fig. 2. Oily-water process flow diagram.

Table 1. Refinery benzene sourceSource Contribution,%Crude and product storage tanks (50 total) 45Crude unit desalters (2) 45Slop oil tankage and others 10

Article copyright © 2000 by Gulf Publishing Company. All rights reserved. Printed in USA.

undercarry in effluent water from mixed-crude tank-age was several percent. Sidewalls of the tanks werehot tapped to allow entry of two energy absorption con-centration control instruments. The instruments wereinserted at a 45-degree angle downward to allow foradjustment of the points of measurement (antenna loca-tions). The setpoints that closed water-drainage valves

had a range of 80% to 90% water (in high-water-con-tinuous or oil-in-water regime). The range control wouldshut down the system at the first signs of hydrocarbonmixture nearing the vessel’s effluent discharge point.A series of tests were conducted by the EnvironmentalInspection Division, an independent auditing groupthat found oil and grease concentration in the effluentwater decreased from percentage levels to a residual of7 to 8 ppm.

The challenge. Refining and petrochemical industriesmust balance environmental responsibility, tougher eco-nomic competition and increasingly rigid regulationsgoverning air and water discharge limits. In some areas,new and/or larger units and other large-scale capitalprojects may be required to achieve the legislated pol-lutant removal levels. However, in many systems, thebest place to start is the source(s) generating those pol-lutants. Advanced technologies such as energy absorp-tion allow control approaches that can eliminate manystreams as pollutant sources. Only after reviewing thepotentials for source reduction is there a certainty thatcost-effective compliance can be achieved.

LITERATURE CITED1 Internal and Environmental Audits of the Industrial and Transportation Operations

Can Identify Areas that Need Improved Control Management, 1987.2 U.S. EPA, Development Document for Interim Final Effluent Limitations Guidelines

and New Source Performance Standards for the Significant Organics Products Seg-ment of the Organic Chemical Manufacturing Point Source Category, EPA 440/1-75/045, 1975.

3 U.S. EPA, Development Document for Effluent Limitations Guidelines for thePetroleum Refining Point Source Category, EPA 440/1-79/014b, December 1979.

4 Putman, “What’s the Best Way to Control an Interface When an Emulsion Tends toForm Between the Phases?” Control Magazine, July 1992, pp. 47–49 .

The authorsGideon Agar is president of Agar Corp. Mr.Agar holds a BS degree in computer sciencefrom Brown University, Providence, RhodeIsland.

Paul Clewis is regional applications managerat Agar Corp. and has 12 years of experiencein petrochemical and production markets. Hehas extensive experience in refining unit oper-ations, specializing in the chemical and equip-ment technologies of oil/water treatment andperipheral operations. Mr. Clewis holds a BSdegree in chemical engineering from Rice Uni-versity, Houston.

Calvin Spencer is technology director at LitwinEngineers & Constructors, Inc. He has 21 yearsof multi-media environmental regulatory andtechnology experience in refining, petrochemi-cal and polymer industries. Mr. Spencer holds aBS degree in chemical engineering from theUniversity of Texas, Austin.