Single-stage Vacuum Deaeration Technology forAchieving Low Dissolved Gas in Process Wateri

GLENN HARBOLD AND JONATHAN PARKGasTran Systems, Cleveland, OH

IWC-08-22

KEYWORDS: Deaeration, Degasification, Decarbonation, Dissolved oxygen, Membranes, Rotating Packed Beds, VacuumTowers, Carbonated Soft Drink Packaging

ABSTRACT: A novel mechanical approach to single-stage vacuum deaeration has been developed in order to removedissolved gases to extremely low levels in process water.The technology uses process intensification principles to improve mass transfer processes involving gases and liquids. Byimparting high shear and centrifugal forces on a liquid to create extremely small droplets, a large surface area is exposedthrough which efficient gas absorption and gas removal can occur. Once the small droplets are created, a vacuum pump isused to remove dissolved gases to low levels not achievable with other vacuum deaeration designs.Applications for the technology include dissolved gas removal from liquids for beverage bottling, ion exchange, ultrapureapplications, downhole water injection, boiler feedwater, and many other process water pre-treatment applications.

TECHNOLOGY BACKGROUND

ROTATING PACKED BEDS Rotating packedbeds (RPBS) have been around for more than threedecades attempting to use large gravitational fieldsto improve mass transfer efficiency. Thefundamental differentiating idea behind RPBs wasgenerating high gravitational forces by rotating apacked rotor at high speeds, instead of relying on thesingular force of gravity in packed or trayedcolumns.The origin of RPBs goes back to the 1970’s andColin Ramshaw’s work in the chemical processindustry at ICI, most notably in trying to find analternative to conventional distillation towers. Sincethen, a significant body of research has growninvolving the use of rotating packed beds. Themajority of this work consists of fundamental studiesof RPBs, while very little has been written aboutindustrial-scale installations. Partly, this is due tothe fact that there have been relatively few industrialinstallations of RPBs. As well, some installationshave been shrouded in corporate secrecy and othershave failed to uncover true value for customers.GASTRAN TECHNOLOGY The currentgeneration of designs can be configured to run eitherusing a stripping gas such as air or nitrogen, orplaced under vacuum to enable the removal of

dissolved gases. The current generation of RPBs isshown in Figure 1.

Figure 1: Vacuum Deaeration Technology

The spinning rotor is driven by a motor andgenerates large centrifugal forces. Liquids fed intothe center of the rotor are forced through the porouspacking material, shearing the water into nano-sizeddroplets and sending it into the outer chamber. It isduring this extremely short period of time that thewater droplets are exposed to the vacuum, enablingthe deaeration to occur. The dissolved gases in thewater are evacuated out the top of the chamber to a

RotorChamber

Rotor

vacuum pump. The deaerated water collects in thechamber and then drains out the bottom.When a stripping gas is used, the set up is verysimilar. A gas is fed into the rotor chamber andevacuated out the top of the unit. The flow of thegas is counter-current to the flow of the liquid,maximizing the mass transfer efficiency.In either set up, the key to good deaerationefficiency is the intense shearing action creating thetiny droplets. This exposes large surface area to thevacuum or stripping gas in order to promote highmass transfer coefficients.

VACUUM DEAERATION PERFORMANCEMEASUREMENT

Laboratory work has been performed to study thetechnology’s performance and key parametersinvolved in vacuum removal of dissolved oxygenfrom water. The work has led to an optimizeddesign. The testing has also lead to the developmentof an overall model that is used to predictperformance at customer installations.EXPERIMENTAL SETUP The experimentalwork was performed on a laboratory scale systemwith a maximum rated flow of 5 gallons per minute(gpm). The system set up is shown in Figure 2. AnAirTech oil recirculating vacuum pump was used toachieve the low levels of vacuum required for thetesting. The dissolved oxygen levels were measuredwith a Rosemount 499A TrDO. Liquid flowratesduring the testing ranged from 1 to 5 gpm with anincoming dissolved oxygen level of 5 to 8 parts permillion (ppm). City water was fed continuouslythrough the system during the experiments attemperatures ranging from 6 to 20°C.

FE

GasTran Unit

Vacuum Gauge

LE

12 gal holdingtank

TE

Vacuum Pump

Trap

City Water Supply

Exhaust

To Drain

Dissolved Oxygen Sensor

Figure 2 - Experimental Set Up

DERIVATION OF PERFORMANCE Duringthe experimentation, system performance wascharacterized using the Number of Transfer Units(NTU). The NTU is a unit of measure that ignoresthe difference between a stagewise process (traytower) and the continuous contacting process thistechnology uses. It is similar to the concept ofequilibrium stages but is used especially when theequilibrium distribution curve is straight (as is thecase with oxygen and water). A higher number oftransfer units means a higher overall level of masstransfer occurs in the system. It is defined by theequation below:

2

1* 1

1ln

211

2xx

xxdx

NTUx

x

where x is the concentration of oxygen in the waterand x* is the equilibrium concentration of oxygen inwater at the system temperature and pressure.Because this is a dilute solution the equation can befurther simplified to:

1

2

*

x

x xxdx

NTU

Because the system is under vacuum and theequilibrium does not vary much across the rotorchamber the NTU can be approximated using theequation below:

mequilibiruoutO

mequilibriuinO

CC

CCNTU

2

2ln

where C is the concentration in mg/L

RESULTS Some results from these experimentsare shown in Figure 3. This data was taken fromruns that were performed at an optimizedconfiguration on the 5 gpm system. The incomingwater from these runs was 8 ppm dissolved oxygenat 8°C. The performance of the technologyincreases as deeper vacuum levels are achievedleading to NTU values of 4.3 – 4.5 and dissolvedoxygen levels of 235 ppb. The technology isachieving this performance level across only a fewinches of rotor packing.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 20 40 60 80 100 120 140

Vacuum Level (Torr Absolute)

NT

U

0

500

1000

1500

2000

2500

Dis

solv

edO

xyg

enC

on

cen

trat

ion

(pp

b)

NTU

Dissolved Oxygen Levels

Figure 3 - DO and NTUs by Vacuum Level

This data led to the optimization of larger scaleinstallations. These systems are running at waterflowrates up to 300 gpm and are operating in the 3.5– 4.0 NTU performance range, delivering water thatcontains 200 – 400 ppb dissolved oxygen at 8°C.

THE BENEFITS OF VACUUM DEAERATION

A variety of industries and applications require lowdissolved gas content in their process water. Oneindustry that is currently making significant stridesto improve their standards for dissolved oxygen(DO) content and subsequent product quality is thecarbonated soft drink (CSD) bottling and canningindustry.CSD FILLING PROCESS CSD packaging plantscommonly follow the process flow for bottle or canfilling that is shown in Figure 4.SIDE-EFFECTS OF HIGH DISSOLVEDOXYGEN IN CSD The presence of DO in theprocess water produces several consequencesdetrimental to the control of the CO2 level in thefinal package, as well as the accurate control ofbottle fill levels while minimizing product waste.

Due to the interaction between dissolved O2 and CO2

in solution, dissolved oxygen acts to increase thepartial pressure of CO2 required to carbonate to thedefined specification. This interaction decreases thereal carbon dioxide solubility in water and also actsas a promoter for increased carbon dioxidedegasification during mixing and rapid pressurechanges.To increase the overall solubility of CO2, manybottling lines compensate with higher pressures inthe carbonation step and/or lower watertemperatures. This causes several unintendedconsequences in the process.COMPENSATING WITH HIGHERPRESSURES The main problem with increasingpressures to obtain carbonation requirements are thehigher pressure drops as the liquid transitions fromprocess pressure to atmospheric pressure. Taken byitself, a larger pressure drop is not a significantissue. But when DO is still present in solution, mostfillers will suffer from increased foaming as the CO2

rapidly expands out of solution. This escape of theCO2 often is seen as a “volcanic” eruption out of thebottle as a foamy liquid. When the foam escapes,the result can be a “low fill” condition in the bottle,meaning the liquid volume does not meet theminimum specification. These bottles get rejected asan expensive yield loss after capping. The foamitself is problematic in part because it ends uploading the plant’s wastewater stream with sugar.Wastewater streams are most commonly dischargedfrom the plants to a municipal treatment and highsugar content can result in fines or surcharges backto the plant.COMPENSATING WITH LOWERTEMPERATURES The other compensationmethod for carbonation control is to decrease theliquid temperature. This increases the solubility ofcarbon dioxide by lowering the necessary partialpressure required to dissolve the carbon dioxide.The CO2 is thus more soluble and more stable whenDO is present. The obvious problem with this is thehigh cost of cooling.

Figure 4 – Overview of CSD Filling Process

TRADE-OFFS BETWEEN CARBONATIONAND FOAMING In an effort to reduce foamingand minimize cooling costs (especially in warmerlocations), the tempting choice can be to maintainlower pressures and not cool the water. The likelyresult can be low carbonation levels, which couldlead to off-spec product or reduced shelf life, sincethe primary limiter to shelf life in CSD products iscarbonation leakage (i.e. “flat” product).

CO2 STRIPPING TO LOWER DO LEVELSUsing CO2 stripping towers to achieve low DOlevels has been considered one way to achieve bettercarbonation control in the final product. Givenenough CO2, dissolved oxygen in these towers canbe lowered to 1,000 ppb or less. High CO2 costs andpoor stripping efficiency, however, make this optionan expensive one. With increasing awarenessaround controlling carbon emissions, the carbondioxide off-gas from these strippers becomes aneven less cost-effective and environmentally-friendly alternative.This option is also problematic for proportional flowcontrol blending systems, such as thoseincorporating mass flow metering. Coupled with aCO2 stripper, these blenders fail to achieve theirprescribed benefits, since CO2 bubbles lower theliquid density and disturb the flow metering.MEMBRANE DEAERATION An alternativeapproach to deaeration starting about five years agohas been to use hollow-fiber, hydrophobicmembranes. The process water passes through themembranes, while a vacuum pressure applied to thecore of the membrane removes the dissolved gases.The gas molecules are small enough to fit throughthe holes in the membrane material, while the watermolecules are not. Sometimes this vacuum pressureis applied together with a sweep gas of either carbondioxide or nitrogen in order to reduce theequilibrium concentration of the stripped gas.Several practical problems have preventedmembranes from delivering sustained deaerationperformance below 1 part per million DO. Primaryamong them is rapid degradation and fouling of themembrane material from inconsistent incomingwater quality and dissolved solids.Another major drawback to membranes is theircleanability. Most CSD bottling plants run sometype of regular Clean In Place (CIP) procedure toclean and sanitize bottling equipment, but installedmembranes systems have repeatedly shown rapiddeterioration with frequent exposure to high CIP

temperatures (typically 82 °C or higher) and pHfluctuations (from caustic and acid washes). Onoccasion, even membrane housings havepermanently deformed to the point of catastrophicfailure when exposed to high CIP temperatures.Some installers of membrane systems attempt toisolate the membranes during CIP cycles in order toslow deterioration, but this solution prevents themembrane system from being adequately cleanedand leaves it vulnerable to biological growth.Despite large investments by CSD bottling plantsand costly replacements of cartridges over the lastseveral years, membranes have failed to deliver asignificant and sustained lower DO to the rest of thebottling line.FOUNDATION TO BETTER BLENDING ANDCARBONATION Given the challenges CSDbottlers face with carbonation and syrup blending,sustaining low DO below 500 ppb in the deaerationstep provides a solid foundation for carbonationstability, overall product quality, and increased lineefficiency. Furthermore, achieving low DO withoutthe addition of CO2 prior to carbonation is the mostlogical and best economic option, leaving vacuumdeaeration as a clear choice.Installing a vacuum deaeration system capable ofsustained low DO does not by itself guaranteesuccess, however. Making the transition to betterquality and faster bottling speeds requires a holisticapproach to the production line. Each step affectsthe subsequent parts of the process. Equipment andoperating parameters will require re-tuning andadjustment both prior to and after installation, untilthe entire line begins to work in harmony.SYSTEM SPECIFICATIONS The dimensionsfor an entire 300 gallon per minute (gpm) skiddedsystem incorporating the vacuum technology,including water handling and controls, areapproximately 4’x5’x12’, meaning it fits easily intoexisting plant floors.Figure 5 shows a picture of a skidded system at aCSD plant. The technology including the rotorhousing is at the top of the picture.System performance is monitored and data isrecorded. No degradation in performance has beenobserved over more than two years of continuousoperation across multiple installations. Thetechnology does not have wear parts requiringregular replacement.The system is sanitized as part of the regular CIPprocedure for each plant.

Figure 5 - Vacuum Deaeration System

ADDITIONAL MARKETS FOR VACUUMDEAERATION

Besides the successful applications of the technologyin CSD, many additional markets provide a good fitwith the core strengths of the technology.DECARBONATION FOR DEIONIZEDWATER PRE-TREATMENT Many ultrapurewater applications require very low levels ofdissolved ions in their water. This requirement isnecessitated for a variety of reasons includingcorrosion and scaling reduction as well as processcontamination.Decarbonation using a vacuum deaerationtechnology effectively lowers dissolved CO2 contentthat otherwise would consume ion exchange resincapacity and hurt electrodeionization (EDI) andmixed bed resin performance.The technology provides consistent decarbonationperformance without fouling. Its compact size andlow profile provides a significant advantage overvacuum towers in terms of pressure loss and spacerequirements.DEAERATION FOR ONSHORE ANDOFFSHORE OIL RECOVERY Offshore andonshore secondary oil recovery commonly utilize awaterflood approach to wash oil from depletedreservoirs using seawater or brackish water. In order

to minimize corrosion of the downhole piping andbacterial contamination of the piping and reservoir,degasification technologies are utilized to removedissolved oxygen, carbon dioxide, and hydrogensulfide (H2S).The vacuum deaeration technology of this paperprovides an enormous size and weight advantageover conventional vacuum towers. This advantageprovides a cost benefit because the lower weight andspace requirements translate into direct savings inthe upfront cost of the platform.ADDITIONAL SET UPS FOR LOWERDISSOLVED GAS REQUIREMENTSAdditional set ups are possible using the vacuum toobtain even lower dissolved gas concentrations.Among these options is the use of a stripping gas incombination with the vacuum. A stripping gas suchas nitrogen can be introduced into the rotor chamberin small quantities to lower the equilibriumconcentration and provide a driving force forremoving the dissolved gases.

SUMMARY

A novel mechanical approach to vacuum deaerationhas been developed out of earlier work done onrotating packed bed concepts.Laboratory studies were completed to optimize thedesign and develop models for predictingperformance at different operating conditions.The technology has been scaled up to 300 gpm sizesand obtains approximately 4.0 NTUs at maximumrated flows. This enables the technology to producedeaerated water with dissolved oxygen content of200 – 400 ppb at 8°C in CSD plants, depending onthe incoming DO concentration.The compact size, performance in removingdissolved gases, and the sustained performance ofthe technology give it a distinct advantage overalternative technologies for deaeration in a variety ofindustries.

i Presented at the International Water Conference – SanAntonio, TX October 2008