silica pre-treatment

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TP1058EN.doc Feb-10

Reverse Osmosis Pre Treatment of High

Silica WatersAuthors: Matthew J. White, Account Engineer, Ion-ics, Jorge L. Masbate Jr., Technologist, Pilipinas ShellPetroleum Corporation and Simon G. Gare, SeniorApplications Chemist, Ionics

Note: Ionics was purchased by GE Water & ProcessTechnologies in 2005.

Introduction

A Shell petroleum refinery in the Philippines experi-

ences high silica in its feedwater, the silica has beendetermined to be a combination of Reactive andColloidal. They had previously been treating theirdemineralization plant with a Hot Lime Softener(HPS). The decision to evaluate other forms of pretreatment to the demineralization plant at the refin-ery was taken by the process team after evaluatingcosts of both operation and maintenance of theHPS unit, costs to the refinery through lost produc-tion caused by HPS outages, the general unreliabil-ity of the unit, and finally the increasedenvironmental difficulties that were being encoun-tered with regards to discharging the lime wasteof the plant.

The refinery therefore embarked on a study toexplore other alternatives to the current system toproduce demineralized water. Other technologiesalong with Reverse Osmosis were taken into con-sideration, but after taking all into account theydecided that the most effective, reliable and eco-nomic way of producing water at the plant wasto replace their troublesome HPS unit withReverse Osmosis as a pre treatment to their demin-

eralization plant.

After evaluating both capital purchase and out-sourcing, they decided that the outsourcingoption would be of more economic benefit to thecompany rather than installing a capital system. In

addition, it would give them the benefit of a leadingwater treatment company such as Ionics (now GE)taking care of such a precious commodity.

This paper will discuss why Reverse Osmosis andthe selected pre treatment to the Reverse Osmosisunit was chosen as the best suited technology totreat the water source at the refinery. It will alsohighlight the problems that can be encounteredrunning Reverse Osmosis units on high silica andhigh hardness water. We will then go on to discussthe performance of the installed system at therefinery, and the benefits that the refinery is cur-rently enjoying as a result of installing the unit.

Refinery Location and Water Supply

The oil refinery was first commissioned in 1968 andhas a capacity of 165,000 barrels per day. It islocated in Tabangao, Batangas City, which is about62 miles (100 km) south of the capital city of thePhilippines, Manila. The refinery lies in the southernpart of Batangas, which is generally characterizedby moderately sloping to rolling terrain with slopesbetween 8 and 18%. It lies within the floodplains of2 rivers, Malitam and Tabangao, and on a slopingterrain underlain by quaternary pyroclastics.

The location has two pronounced seasons, wet anddry. The wet season generally extends from June toDecember while the dry season from Jan to May.Annual rainfall is about 70 inches (178 cm), themean monthly rainfall during wet season is9.8 inches (25 cm), and over dry season 1.97 inches(5 cm). The mean annual temperature in the Philip-

pines is 81.5°F (27.5°C), with a peak temperature of94.6°F (34.8°C) in April and minimum temperature of70.5°F (21.4°C) in February. The mean annualhumidity is 78%.



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The refinery has three deep wells within its bound-ary, each of these have been sunk 55 yards(50 meters) in depth. It is from these wells that therefinery draws its water for various needs; theseinclude jacket cooling water for the cooling towersand sanitary needs. They also use the above sourcein the production of high-pressure (50 Bar) steam,

which is used for generating power in the steamturbine, compressing gas in the refineries plat-former and other various needs around the refinery.The specification for this boiler feed water is0.1 µS/cm and 20 ppb total silica.

The bore hole water requires special attention if it isto be treated to become boiler feed water; this isdue to the fact that it is constantly high in both col-loidal and reactive silica (up to 120 ppm) and totalhardness (up to 400 ppm). The ionic make up of thiswater is due to the geological characteristics of the

Philippines, which are of volcanic origin.

Demineralization Plant Configuration

Previously the feed water for the production ofsteam was pretreated with a Hot-lime Process Sof-tener (HPS). This was then followed by a standardSAC/SBA/MB demineralization system. The HPS sys-tem was used due to the high silica and hardnesslevels. The theory behind this process is that lime isadded to the feed water at high temperature, thisprecipitates out the hardness and silica in the feed

water, this is then fed to the demineralization plantfor polishing to the required specification.

The problems that are encountered with the opera-tion of this unit are high in number, these include,high maintenance of HPS unit due to discharge ofsludge waste, intensive manpower to operate dueto the possibility of lime carryover to the deminer-alization system, increasing difficulty in disposing ofthe sludge waste produced by the system, and theloss of energy caused by the need to cool the waterto below 95°F (35°C) before it enters the deminerali-zation plant. The final point is essential when HPS ispre treating a standard SAC/SBA/MB demineraliza-tion plant due to the temperature specifications ofthe resin in the plant, which leads to the feedwater having to be heated twice before enteringthe boiler!

Problems Encountered with SilicaContamination

If silica is not removed before entering a plantssteam system, the consequences to the life cycle ofthe plant can be quite considerable. Common prob-lems associated with silica contamination are

as follows:1. Contamination of product.

2. Reduced efficiency of steam turbines due tosilica depositing on turbine blades.

3. Reduced efficiency of boiler due to scaling.

4. High maintenance of boilers due to scaling.

5. High maintenance of turbines due to scalingwhich leads to vibration of the turbine.

Many problems encountered by silica contamina-tion could have been understood better if only col-

loidal silica and reactive silica had beendifferentiated at the offset.

Reactive and Colloidal silica

Total silica is the term given to the sum of colloidaland ionic silica found in water. Silica exists in a widerange of structures, from a simple silicate to a com-plicated polymeric material. The polymers structurecan persist when the material is dissolved in surfacewaters. The size of the silica polymer can be sub-stantial, ranging up to the colloidal state. Colloidal

silica can be found in surface waters but not usuallyat significant concentrations, high concentrationsare found in well waters that derive from areas ofvolcanic activity.

Detection of Colloidal and Reactive Silica

The polymeric form of Silica does not complex in thestandard molybate based colorimetric test for silica- this form is termed non reactive. The polymericform of silica is not thermally stable and whenheated in a boiler reverts back to the basic silica

monomer, this is reactive with molybate. Analysis ofa boiler feed water may reveal little or no silica,while the boiler blow down measurements mayshow high levels. High boiler water silica and lowfeed water silica values are often a sign that colloi-dal silica is present in the make-up feed water.

Colloidal silica is difficult to measure directly and isusually done by measuring the total silica in thefeed water and then subtracting the amount of



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ionic silica. The balance is the total amount of col-loidal silica.

Removing Silica from Feed Water

The varied forms of silica in water different tech-nologies need to be applied to remove them. Reac-

tive silica dissolved in water dissociates to form aweak acid, the slight negative charge results in anaffinity to Strong Base Anion resin and is thereforeremoved effectively using demineralization units.However, the colloidal form is a little more difficultto remove from feed water due to its neutralcharge, and has in some cases had a negativeimpact on the demineralization unit itself. There aresome macroporus resins on the market that arecapable of removing colloidal silica however theresin choice must be tailored to the water source asthe colloid will only be absorbed into a narrowrange of pore sizes. One of the biggest problemswith this is that if the feed water compositionchanges the plant can become inefficient and unreliable.

Colloidal silica can also contaminate cation resinwithin a mixed bed as a result of poor separationthis can lead to an increase in silica content to theboiler feed water.

Reverse Osmosis is a technology that is effective atremoving silica in both its reactive and non-reactivestate. The following are examples of the typical %

rejection values of Reverse Osmosis, utilizing PolyAmide thin film composite membranes (PA).

1. Reactive Silica > 80%

2. Colloidal Silica > 99.8%

Pretreatment of Reverse Osmosis onHigh Silica Waters

Silica is a sparingly soluble compound, pretreat-ment of the RO feed must therefore be consideredwhen designing a system to run on high silica

waters. Two ways to effect the concentration of sil-ica sustainable in the feed are to increase the tem-perature or the pH. Increasing the temperature ofthe feed may be impractical when considering theenergy cost and also polyamide RO membraneshave upper temperature limits of ~100°F (38°C).Increasing the pH however is a viable as polyamidemembranes have a wide tolerance for pH. In theory,it is predicted that at a pH of 10 the level of silica

that can be maintained in the concentrate streamof an RO unit is around 400 ppm. (Figure 1)

Figure 1: Graph of pH against dissolved silicaconcentration

Other options include the use of Antiscalents, whichpromote super-saturation of silica and hence

increases it’s solubility, or limiting the % recovery ofthe unit and hence the concentration factor. Thelater is rarely used due to environmental constraintsand raw water cost.

It is important to note at this stage that Silica foul-ing of membranes is practically irreversible as thereis no known method to clean RO membranes oncethey are scaled with silica. Hence, as this occur-rence is only rectified with membrane replacement,the choice of pretreatment when running an ROon high silica water is one that should not betaken lightly!

Design Choices

Due to the composition of the feed water availableat the refinery, there were limitations on the con-figuration of the equipment GE could use:

Antiscalent / RO

Although there was adequate performance data forthe Antiscalent system we did not pursue thisoption due to the inability to actually monitor the

concentration of antiscalent present in the systemon-line. The only way to control the concentrationof antiscalent in the system is by proportional dos-ing. The problems identified here were the possibil-ity of the dosing pump losing prime and it not beingdetected, or changes in feed water composition towhich proportional dosing would not be able toreact to. The result of either of the above situationwould be catastrophic; there would be no way theunit could be stopped except manually.



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Softener > NaOH addition > Reverse Osmosis

After evaluating all options, it was decided that themost reliable way of pretreating the RO and elimi-nate the potential of silica scaling the membraneswas to increase the pH of the RO feed water toapproximately 9.5 by dosing 50% NaOH. We wouldthen be able to sustain a silica concentration in theRO concentrate of around 400 ppm and hence beable to run the system at a recovery of around 65%based on a silica concentration in the feed of ~120ppm. The benefit of using this method was that itwould give us the opportunity to monitor the pH ofthe feed water and RO concentrate in-line andhence set alarm points in the RO control panel toshut down the unit if the NaOH dosing failed forwhatever reason.

To utilize the system above, it was necessary toeliminate any hardness in the feed water with sof-tener pretreatment. If pH is increased to above 9.5without softener pretreatment, the system wouldnot work due to the inevitable fouling of mem-branes due to large concentrations of hard ionswhich would form scaling compounds such asCaCO3 etc. Although this can be cleaned off themembranes, the presence of such high levels ofhardness in the feed water would have led to clean-ing on an all too frequent basis, which was bothinefficient and uneconomic. The RO would also beunable to tolerate even the smallest amount ofhardness over a long duration due to possibility ofirreversible silica scale forming on the bed ofreversible hardness scale. To combat this weinstalled a polishing softener unit downstream ofthe softener unit to reduce the total hardness in thefeed water of the RO to below 0.1 ppm. Again thiscould be monitored with an in-line hardness moni-tor linked to the control panel of the RO, making thesystem full proof in the event of hardness leakagefrom the softeners.

System Configuration

The unit at the refinery is designed to produce aflow of up to 14,500 gpm (55 m3/h) on a continuousbasis, with a 95% service factor. The quality of wa-ter that the unit is designed to produce is a >95%rejection of the Average Feed TDS, which can becalculated as follows.

Average feed TDS = (RO incoming feed TDS + ROConcentrate feed TDS) / 2

The system is made up of the following components:

Activated Carbon Filtration

Two five-foot diameter activated carbon vesselswere placed in parallel upstream of the unit toeliminate any oxidants in the water, along with anyhydrocarbon contamination occasionally found inthe ground water supply.

Primary Softener unit

The softener unit comprises of six five-foot diametervessels loaded with cation resin in the sodium form.These units are set up in parallel and run as twostreams of three vessels, which allowed us to oper-ate one stream in service with the other in regen-eration / standby mode. The unit was designed torun at a flow rate of 110 m3/h constantly with afeed total hardness of up to 400 ppm. The unit is

regenerated automatically on a counter and can beregenerated manually in emergencies.

Polishing Softener

The polishing unit comprises of two five-foot diame-ter vessels loaded with cation resin in the sodiumform. These units are set up in parallel and aredesigned to run at a flow rate of up to 24,000 gpm(90 m3/h), it is regenerated manually and has a runlength of ~52 million gallons (~195,000 m3) basedon an inlet feed hardness of 1ppm. The unit is

reduced to 50% production for the regeneration ofthe polishing softeners, which takes 1 hour per vesseland takes place every three months.

NaOH Addition

NaOH is added to the system using a controlledfeed system linked to incoming pH of the feedwater to maintain a pH of 9.5 in the RO concentrate.The pH monitor is linked to a shutdown systemshould the pumps or lines fail in any way.

Reverse OsmosisThe Reverse Osmosis unit on site contains PolyAmide thin film membranes in a 4 x 2 array. The unitis designed to run at a minimum recovery of 63%,although as we gain experience treating the waterat the refinery we hope to increase this to themaximum 70% recovery.



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Operational Data

The system has now been supplying the refinery forthe past 8 months. In that time we have seen aslight increase in the pressures in the system overthe baseline data. This increase is approximately

6%, not enough to require cleaning, there has beenno reduction in permeate flow or recovery of theunit. Currently we are recording around a 64%recovery for the reverse osmosis unit at site. Beloware the ranges of operational performance.

Reverse Osmosis % Recovery = 60% to 65%

Reverse Osmosis % Rejection = 94% to 97%

Feed water pH after NaOH addition = 9.5 to 10.5** **Dependent on silica content of feed water

Tables 1, 2 and 3 represent the feed, product andconcentrate concentrations from the system. Also agraph of differential pressure is presented andin-out recovery. (Figure 3)

Figure 2: Recovery and Rejection (March to June)

Table 1: Feed Concentrations

Table 2: Product Concentrations

Table 3: Concentrate Concentrations

Conclusion

Since installing the unit at the refinery, PilipinasShell has benefited both technically and economi-cally. They have eliminated the troublesome HPSsoftener as a pre treatment to the Demineralizationplant and hence the waste associated with the unit.All waste from the softener / RO system are routedto drain, although the high pH waste stream

(pH~10) is blended with another wastewater streamat the plant to reduce the pH and silica concentra-tion. This method of lowering the pH was decideddue to the fact that if acid is added to lower thepH there would also have been the possibility ofsilica precipitation due to the concentration being220 ppm - 335 ppm1 at 77°F (25°C).



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Another benefit to the refinery was the fact that thewater produced by the RO system was of a muchhigher quality than that produced by the HPS plant.

As the TDS of this water was much lower than theHPS water the run lengths of the Demineralizerwere increased dramatically which led to majorcost savings on commodity chemicals. This was dueto a reduction of chemicals such as H2SO4 andNaOH, which are used for demineralizer regenera-tions. There was also a reduction in maintenancecosts of the demineralizer unit due to reduction inits workload, and the fact that there was no morecarry over from the HPS, which sometimes blockedthe beds and damaged the resin. Other cost sav-ings, as said earlier were through energy savings,these were substantial and were the major factor ininfluencing the refinery to install the reverse osmo-sis system. Now, the feed water had only to beheated once before the boiler, instead of the previ-ous set up which involved heating the water for theHPS process (which utilized steam), cooling thewater down to meet the demineralizer temperaturespecification 100°F (38°C), and finally reheatingbefore the deaerator.

Last but by no means least, one of the major bene-fits that has been recognized by the refinery is theincreased reliability of the new technology. After

eliminating a particularly troublesome piece ofequipment, which at times caused reduction in theproduction capability of the plant, the refinery nowenjoys the benefit of increased inventories oftreated water. Pilipinas Shell Petroleum Corporationhave entered the new millennium knowing that thisprecious resource is in the hands of a first classwater treatment company.

References

1. Dyson, Mark G. How can colloidal silica and TOC

reduction increase the life cycle of power sta-tions, 1999.

2. Threlfall, David P. The use of membrane tech-nology for the production of DI water in envi-ronmentally sensitive areas, 1999.

3. Miller, William S. Reverse Osmosis membranetechnology for make up systems.

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