ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 602 (2009) 809–813

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/nima

RPC gas recovery by open loop method

Avinash Joshi a,�, S.D. Kalmani b, N.K. Mondal b, B. Satyanarayana b

a Alpha Pneumatics, 11, Krishna Kutir, Madanlal Dhigra Road, Panch Pakhadi, Indiab Tata Institute of Fundamental Research, Mumbai, India

a r t i c l e i n f o

Available online 4 January 2009

02/$ - see front matter & 2009 Elsevier B.V. A

016/j.nima.2008.12.137

esponding author.

ail address: alpha_pneumatics@hotmail.com (

a b s t r a c t

RPC detectors require to be flushed with small but continuous flow of gas mixture. Dealing with large

number of detectors, gas consumption to very large volumes. Gas flow is a running expense and

constituent gases are too expensive to be treated as consumables. Exhaust gas mixture from detectors is

a potential environmental hazard if discharged directly into the atmosphere. Storage of gases on a large

scale also leads to inventory- and safety-related problems.

A solution to these problems is the recovery and reuse of exhaust gas mixture from RPC detectors.

Close loop method employs recirculation of exhausted gas mixture after purification, analysis and

addition of top-up quantities. In open loop method, under consideration here, individual component

gases are separated from gas mixture and reused as source. During open loop process, gases liquefiable

at low pressures are separated from ones liquefiable at high pressure. The gas phase components within

each group are successively separated by either fractional condensation or gravity separation.

Gas mixture coming from RPC exhaust is first desiccated by passage through molecular sieve

adsorbent type (3A+4A). Subsequent scrubbing over basic activated alumina removes toxic and acidic

contaminants such as S2F10 produced during corona (arcing) discharge. In the first stage of separation

isobutane and freon are concentrated by diffusion and liquefied by fractional condensation by cooling

upto �30 1C. Liquefied gases are returned to source tanks.

In the second stage of separation, argon and sulphur hexafluoride, the residual gases, are

concentrated by settling due to density difference. SF6 is stored for recovery by condensation at high

pressure while argon is further purified by thermal cracking of crossover impurities at 1000 1C followed

by wet scrubbing.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Radiation detectors in general and RPCs in particular usemixture of gases as dielectric medium. A precise combination andratio of component gases is maintained to enhance performanceof RPC. Mixture of gases with freon R134a, isobutane, argon andsulphur hexa fluoride as components gases in various concentra-tion is most commonly used. Even though the gases participate inradiation detection none are consumed in the process. However, asmall fraction may undergo a permanent chemical change anddoes need to be replaced.

A small flow of gas mixture is to be maintained to flush outcontamination, it is usually equivalent to 4–6 volume changes perday. The gas mixture after its passage through an RPC is discardedand let into the atmosphere. This practice is apparently veryconvenient for few RPCs. But when the number of RPCs is large,

ll rights reserved.

A. Joshi).

the quantity of gases thrown out assumes large volume and thisposes the following problems:

(1)

Cost of gases: Some of the constituent gases are veryexpensive. Exhausting them to the atmosphere will contributeto running cost. Typically RPCs having 2 m�2 m-size detec-tors will use 150 g of freon per day, which is equivalent toUS$3.5 at prevailing prices. Running cost of set of RPCs,therefore, could become prohibitively high.

(2)

Pollution: Hazardous and toxic gas products may be generated(Table 1) due to corona discharge inside an RPC. The toxicradicals are harmful to health and unsafe to be let out withoutadsorbing or converting it into safer compounds.RPC exhaust contains SF6 and freon R134a, which have globalwarming potential (green house effect—Table 2) 22,400 and4200 times greater, respectively, than carbon dioxide. They arehighly stable and remain in the atmosphere for a very longtime, closer to the ground.

(3)

Storage hazards: Large space is needed for storage of gascylinders to cope up with consumption. Near detector bank,space is at a premium.

ARTICLE IN PRESS

A. Joshi et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 809–813810

(4)

TablCoro

Prod

S2F1

SOF2

SO2F

S2F2

CF4

HF

WF4

H2SO

SO2

TablImpo

Prop

Mol.

Stru

Gas

Liqu

Visc

Boili

Heat

Criti

Criti

Glob

Pote

Puri

Impu

Pressure Instability: If the gas mixture is discharged into theatmosphere, RPC pressure regulation system gets affected bychanges in the atmospheric pressure.

(5)

Performance: To minimize vent loss, the number of volumechanges is always kept to a bare minimum, and might resultin degradation of performance.

A solution to the above mentioned problems is to reuse theexhausted gas mixture from RPC detectors. Presently, the closed-loop technique is the only available method for this purpose.In this method RPC exhaust gas mixture is re-circulated withan elaborate pressure/flow control system. In situ gas analysis isperformed, unwanted components are removed and lost quan-tities are topped up. This is a very effective technique, but asequipment involves extensive instrumentation, it is too expensivefor laboratory scale setup. As an alternative to re-circulationmethod open loop method is being developed. Yield in the rangeof 95% and purities of 1000 ppm are targeted in the initial phase.

VAPOR PRESSURE OF GASES

25

30

R A

SF6

2. Principle of operation

In this method gases are separated, purified, stored and reused.Separation of gases from the mixture is accomplished by usingdifferences in their physical and chemical properties. Table 2 listsimportant properties of commonly used RPC gases and thedifferences in their properties are quite evident. For example:isobutane and freon R134a can be liquefied at temperatures above�30 1C, whereas argon and SF6 are still in the gas phase at thattemperature. R134a is a highly saturated compound but isobutaneretains substantial chemical affinity. In gas phase SF6 is 21

2 timesheavier than argon (Fig. 1).

Differences between characteristics of gases are employed toseparate gases from one another and purify for reuse. At firstisobutane and R134a are separated by condensation. The suitable

e 1na discharge radicals of SF6 gas.

uct Hazard

0 Extremely toxic, TLV:o1 ppm

Highly toxic and corrosive, TLV:100 ppm

2 Toxic, attacks respiratory system, TLV: 1000 ppm

Reacts with trace hydrogen to produce H2S

Component very difficult to separate

Highly reactive to MOC of RPC

, SiF4 Reaction products of HF with glass and SS316

4 Highly corrosive

Highly corrosive for metallic parts

e 2rtant gas properties.

erty Unit R134a Isobutane

wt. 102.3 58.12

cture Ring Ring

density kg/m3 4.25 2.82

id density kg/m3 1206 593

osity cP 0.012 0.006

ng pt. 1C/1 bar abs �26.3 �11.7

of vapourization kJ/mol 22.021 23.300

cal temp. 1C 101.1 134.9

cal press. bar abs 40.6 36.84

al warming

ntial CO2 ¼ 1 4200 –

ty level used % 99 99.9

rities O2, H2O, N2, CF4 CH4, H2, H2O,

temperature–pressure for condensation is given by the Clausius–Clapeyron equation for boiling point of a vapour–liquid system as

TB ¼fRðlnðP0Þ � lnð101:325Þg

DHvapþ

1

T0

�� �1

where TB is the boiling point (K), R is the ideal gas constant(8.314 J/K mol), P0 is the vapour pressure of gas at giventemperature (kPa abs), DHvap is the heat of vapourization (J/mol)and T0 is the gas temperature.

Based on the data from Table 2 for isobutane and R134a,pressure and liquification temperature curves are plotted (Fig. 2).The curve represents vapour–liquid transitions with reference topartial pressures and temperatures. If a working point lies abovethe curve, liquid phase exists in equilibrium with gas phase. Forany point below the curve, no liquid can exist.

During separation of condensable gases, parameters such asthe pressure, temperature and surface activation are maintainedin such way that only one gas is adsorbed, condensed and trapped.For isobutane this temperature is typically �13 1C at 0.5 bar. Afterremoval of isobutane the residual gas mixture flows to the nextzone where conditions (�26 1C and 0.5 bar) are set for freon R134to condense. Liquified freon is in equilibrium with gas at partialpressure of 1 bar at �26 1C. The gas mixture coming out of thefractional condensation column is concentrated in non-condensablegases such as argon and SF6 with washed-off vapours ofcondensed gases.

In the next stage argon +SF6 mixture is allowed to settle underlow turbulence condition. As densities (refer Table 2) of Ar and SF6

Argon SF6

39.948 146.05

– Ring

1.78 6.27

1880

0.02 0.015

�185.8 Triple pt.: �49.4 1C at 2.2 bar, Sub. pt.:�63.9 1C

6.43 23.681

�122.13 45.5

48.98 37.59

– 22,400

99.999 99.9

N2 N2, O2, H2O, HC H2O, O2, CF4

0

5

10

15

20

-60TEMPERATURE (°C)

PRES

SUR

E B

A

R134A ISOBUTANE SF6

ISOBUTANE

R134A

-40 -20 0 20 40

Fig. 1.

ARTICLE IN PRESS

OPEN LOOP GAS RECOVERY

PURIFYER UNIT

Ar

EXTRACTION COLUMN

CAPILLARY IMPEDENCE0.2 Bar@50 SCCM

RECEIVER

CHAMBER

BASIC ACTIVATED ALUMINA

VACUUM SCRUBBER COLUMN

MOLECULAR SIEVES 3A

PRESSURE

CHAMBER

TO SF6

RECOVERY

SAFETYBUBBLER

ISOLATION

BUBBLER

FROM RPC

EXHAUST

UNCONDENSED

GASES

TO L.T.

CONDENSER

RECYCLED

BACKFLOW

VENT

EXHAUST

A

0.5 BAR

BACK PRESSURE

CHECK VALVE

C

B

ACTIVATED CARBON

MOLECULAR SIEVES 4A

SF6 SETTLING COLUMN

INDICATOR

BUBBLER

DRY TYPE DIAPHRGM VACUUM PUMP

TO VENT

EXHAUST

Fig. 2.

A. Joshi et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 809–813 811

are quite different, heavier gas (SF6) settles to the bottom of tank.Argon is collected from top. Argon is subsequently adsorbedin column of molecular sieve type 4A. When dealing with smallquantities of gas a batch purifier (with low purge loss) isrecommended. For continuous recovery of large flow rates ofargon, dual column tandem-operated purifier is suitable. In eithercase molecular sieves are regenerated by thermal swing.

Remaining volume of gases consists of traces of SF6 and ofcondensable gases. It may be discharged into the atmosphere orsent to the cracking furnace for chemical breakup (which may be

plasma assisted), passed through activated carbon to trap and/orchemical scrubber.

3. Equipment

3.1. Separation of gases

Process equipment employed for open gas recovery consists of:(a) gas purifier (Fig. 3) and (b) gas condenser (Fig. 4). The gases

ARTICLE IN PRESS

OPEN LOOP GAS RECOVERY

CONDENSER UNIT

LIQUIFIED ISOBUTANE

SHUTOFF VALVES

TO FREON

LOOP

TO ISOBUTANE

LOOP

LIQUIFIED

R134A

+20C

ACTIVATED PALLADIUM

CATALYST

TRANSFER

CHAMBERS

TO A

FROM B

ACTIVATED

COPPER

GRANULES

NC SOLENOID

VALVES

PRESSURE

REGULATORS

EE

E

+27C

E

E E

TO C

REFRIGERATED CHAMBER

C62-C51-

Fig. 3.

A. Joshi et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 809–813812

coming out of the RPC enter the purifier unit through an isolationbubbler which serves as a flow counter. The gas flows further intoa chamber which provides capacitance to compensate fluctua-tions in flow rates. An impedance in the form of a micro-borecapillary connects the gas to a receiver chamber maintainedat 700 mbar absolute pressure (vacuum). Capillary impedanceregulates flow into the receiver and does not allow a negativepressure to develop inside the RPC. A safety outlet is provided inthe form of a bubbler to avoid backpressure built-up inside RPC.The vacuum column is charged with activated alumina whichreacts with and fixes radicals present in the gas stream (Table 2).Oil free diaphragm pumps gas mixture from vacuum column and

feeds it to the pressure chamber. The vacuum chamber is fittedwith a vacuum transmitter and switch set at 690–710 mbar abs.The pressure chamber is also equipped with a pressure transmit-ter and switch set at 1.5–1.6 bar abs. Pressure chamber alsocontains molecular sieve type 3A to remove water vapour downto 2 ppm.

Gas mixture flows into refrigerated chamber where twocolumns are maintained at �13 and �26 1C, respectively. Danfossmake compressor model SC12 CL with freon R404 is used to createsource temperature of �35 1C. First column contains activatedpallidium catalyst (on alumina). Adsorption surface is typically200 m2/g. Isobutane is extracted to 10 ppm in this column. The

ARTICLE IN PRESS

Fig. 4.

A. Joshi et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 809–813 813

second column contains copper granules and is maintained at�26 1C; freon condenses in this column. A check valve adjustedat 1.5 bar absolute pressure is placed at the outlet on the gasline. This valve maintains sufficient back pressure essential for

condensation.Overflow gas returns to purifier and enters argon extraction

column. Here argon (in absence of moisture) is absorbed inmolecular sieve type 4A. This column is to be thermallyregenerated for removal of adsorbed argon. Residual gas is settledin a container with activated carbon. At the lower end SF6 istapped out for further concentration and recovery. The unrecov-ered gas flows out to the vent exhaust through the indicator-cum-isolation bubbler.

All columns except activated alumina are to be regeneratedafter a certain usage, time of which depends on concentration of

gases. Activated alumina needs to be replaced as it gets consumedin the process of cleaning of radicals.

3.2. Reuse of extracted gases

Isobutane and freon R134a are collected at �13 and �26 1C intheir respective columns. To transfer the liquids into theircontainers a sequence of valve operations is carried out. Valvesare all metal solenoid type suitable for low-temperature applica-tion. At start, a valve located inside the refrigerated chamber isopened. At the same time the release valve connecting the transferchamber outlet to the vacuum chamber is also opened. Under apressure differential liquefied gas flows into the insulated transferchamber. After few seconds both valves are closed.

The insulated chamber is heated to about 27 1C (+10 1C aboveroom temperature) by electric heaters. Pressure measuring about4 bar abs (for isobutane) and 7 bar abs (for R134a) develops insidethe transfer chamber(s). At this point, the solenoid valveconnecting the transfer chamber to the liquefied gas container isopened. Gas pressure inside the container is 3 bar abs (isobutane)and 5 bar abs (R134a) at 17 1C, which is typical room temperature.Under the pressure gradient liquefied gas flows from the transferchamber into the liquid container. The lower valve is closed, andthe released valve is opened. Evaporated gas flows back into thevacuum chamber and joins the incoming RPC exhaust for nextcycle of recovery.

4. Experimental results

4.1. Volumetric method for estimating gas recovery

The three identical bubblers connected in gas stream indicatethe amount of flow rate passing through. The number of bubblescounted by isolation bubbler in a given amount of time should beequal to sum of bubbles coming out of safety and indicatorbubblers, in absence of leakage through the system. At this stagerefrigeration is started. At around �24 1C, the flow in the indicatorbubbler starts dropping. As soon as equilibrium is reachedbetween pressure, vacuum and temperature the flow in theindicator bubbler drops to less than 10% of its original value.The measurement indicates that more than 90% gas is beingcollected inside the recovery system. As there is no pressureincrease the collected gas must be in liquid phase.

4.2. Isobutane measurement

Isobutane content in the gas mixture is about 1–5%. Removal ofisobutane, therefore does not significantly change the totalvolume of gas. Matheson gas detector model 8057 with minimum100 ppm sensitivity for isobutane was used to estimate thepresence of isobutane at test points during its passage throughthe recovery system. It is found that isobutane was completelyabsorbed by activated carbon and that isobutane is completelyextracted from 1% to 100 ppm by condensation.