Novel Aqueous Foams for Suppressing VOC Emission

Environ. Sci. Technol. 2004, 38, 2721-2728 Novel Aqueous Foams for Suppressing VOC Emission P AN KAJ S . G AU T AM AN D K I S H O R E K . M O H A N T Y* Ch em ical En gin eerin g Departm en t, Un iversity of Hou ston , 4800 Calh ou n Road, Hou ston , Texas 77204-4004 Reducing volatile organic compound (VOC) emissions from crude oil/gasoline distribution and storage facilities is important in controlling environmental pollution and enhancing w orkplace safety. Stable aqueous foam formulations are developed to provide a mass transfer barrier to the emission of VOCs during loading of gasoline. Experiments are carried out in a bench-scale foam cell using liquid hexane as oil. The foam columns of 32 cm in height w ere able to suppress the plateau concentration of hexane vapors in the effluent by 87% under experimental conditions tested. Vapor suppression increased w ith foam height but w as almost insensitive to liquid viscosity. These experiments are then upscaled from bench-scale to a vessel having an exposed surface area of roughly 2 orders of magnitude higher. Gasoline is used as oil in the upscaled experiments, and the concentrations of volatile hydrocarbons in the effluent are measured during oil loading. A 40-cm-thick foam column is found to reduce the emissions by 96% for foams prepared w ith deionized w ater and by 93.8% for foams prepared w ith 3.5 w t % NaCl brine for 10 h of oil loading. Introduction At th e loadin g an d u n loadin g term in als for gasolin e/ cru de oil, som e am ou n t of ligh t h ydrocarbon s are in variably ven ted in to th e atm osp h ere every tim e a tan ker is loaded with gasolin e/ cru de oil. Th e vap ors released from gasolin e storage an d loadin g op eration s con tain a m ixtu re of ligh t h ydrocarbon s (C4-C7), in clu din g som e arom atic h ydrocarbon s. Each loadin g can take from a few h ou rs to h alf a day. Th e vap or com p osition s for cru de oil an d gasolin e term in als are sim ilar bu t cou ld be differen t for loadin g of oth er organ ic ch em icals. Volatile organ ic com p ou n d (VOC) em ission is u n desirable sin ce it is an air p ollu tan t, a fire h azard, an d a th reat to workp lace safety. VOC em ission s from gasolin e storage an d distribution facilities are com in g un der in creased scru tin y in both th e Un ited States an d Eu rop e. U.S. En viron m en tal Protection Agen cy Stan dard 40 CFR Part 63 h as establish ed an em ission lim it of 10 m g of total organ ic com p ou n ds (TOC)/ L of gasolin e loaded. Eu rop ean Com m u n ity Stage 1 directive h as establish ed a lim it of 35 m g of TOC/ L of gasolin e loaded. Th e m ost strin gen t gasolin e em ission regu lation is set forth by th e Germ an TA-Lu ft stan dard where em ission lim it is 0.15 m g of TOC/ L of gasolin e loaded (1). In view of su ch con cern s, it becom es im p erative th at th e m eth ods to redu ce VOC em ission s be in vestigated an d evalu ated. There are two potential technologies for em ission controls vap or recovery an d vap or su p p ression . Most of th e existin g * Correspon din g author phon e: (713)743-4331; fax: (713)743-4323; e-m ail: m oh an ty@u h .edu . 10.1021/es0349599 CCC: $27.50 Published on Web 03/23/2004  2004 Am erican Chem ical Society tech n ologies for em ission con trol u se vap or recovery tech n iqu es. Vap or recovery tech n iqu es in clu de p ressu re swin g adsorp tion (2) (Sorbath en e m eth od), steam -gen erated tem p eratu re swin g adsorp tion (3), an d su bsequ en t th erm al in cin eration . Sorbath en e solven t recovery tech n ology in volves p assin g a feed stream con tain in g VOCs th rou gh an adsorben t bed. For gasolin e vap or recovery, th e stan dard Sorbath en e tech n ology n eeds to be su p p lem en ted by a com p ressor or a m ech an ical refrigeration u n it. Th e steam gen erated tem perature swin g adsorption is a useful techn ique bu t lim ited to organ ics th at are n ot th erm ally sen sitive. Addition ally, th erm al in cin eration for m an y ch em icals requ ires tail gas treatm en t an d u ltim ately releases carbon dioxide in to th e atm osp h ere. Vap or su p p ression tech n iqu es aim at creatin g a m ass tran sfer barrier th rou gh wh ich th e h ydrocarbon vap ors m u st diffu se before th ey are released in to th e atm osp h ere. Corin o et al. (4) h ave su ggested u sin g a gellin g agen t to create a roof by th e u p p er layer of oil in th e tan k to p rovide a floatin g roof of the sam e m aterial. This techn ique m ay create con siderable difficu lties in clean in g th e tan ker, esp ecially if th e tan ker h as an y p lum bin g lin es or com p artm en t walls. Polyurethan e typ e foam s can also be effective again st vap or release bu t leave beh in d n on collap sible residu e. Can evari et al. (5) were th e first to su ggest a foam ed vap or barrier to su p p ress th e release of volatile h ydrocarbon s u sin g com m on aqu eou s foam s. Con ven tion al aqu eou s foam s, h owever, seldom p ersist for m ore th an a few h ou rs an d often h ave very p oor stability in th e p resen ce of oil. Wh en a gasolin e (or cru de oil) tan ker is em p ty, an aqu eou s foam colu m n of requ ired h eigh t cou ld be dep osited on to th e bottom of th e vessel. Th e gasolin e (or cru de oil) can th en be in jected from th e bottom . Th e liqu id wou ld disp lace th e foam colu m n , wh ich in tu rn wou ld disp lace th e air to fill th e tan k. Th e ch allen ges in develop in g an aqu eou s foam form u lation for ap p lication in su p p ression of VOCs are m an y-fold. First, th e foam form u lation sh ou ld p ersist abou t 10 h or m ore for it to be viable. Th e evolu tion of foam h eigh t with tim e is a good criterion of foam stability in th is regard. Secon d, h ydrocarbon gases sh ou ld h ave low solu bility in th e foam liqu id so th at th e p erm eability of foam lam ellae to diffusin g h ydrocarbon gases is low. It is im p ortan t th at th e foam h as h igh flu idity an d covers th e su rface of gasolin e/ cru de oil com p letely. Also, th e foam blan ket sh ou ld be flexible en ou gh to retain its h eigh t wh en it is bein g p u sh ed u p ward by th e liqu id from below du rin g loadin g. Th e foam sh ou ld n ot degrade th e qu ality of ch em ical it is coverin g an d sh ou ld n ot p ose an y addition al en viron m en tal issu es. Last, bu t m ost im p ortan tly, th e foam sh ou ld be stable in th e p resen ce of gasolin e/ cru de oil. Sp ecifically, it m u st n ot ru p tu re at th e foam / oil con tact or th e foam ed barrier wou ld rap idly deteriorate. The objective of this work is to develop a foam form ulation th at m eets th e aforem en tion ed criteria an d to evalu ate th e su p p ression of VOC em ission s. In th e n ext section , foam form u lation an d stability in th e absen ce an d p resen ce of oil is discu ssed. Th is is followed by a descrip tion of th e experim en tal m ethods. The results are discussed in the fourth section , followed by th e con clu sion s. Foam Formulation and Stability Th e discu ssion on th e foam stability can be divided in to two p artssbu lk foam stability in th e absen ce of oil an d stability in th e p resen ce of oil. In th e absen ce of oil, th e stability of th e bu lk foam is typ ically con trolled by th e liqu id drain age from th e p lateau borders an d foam lam ellae in th e in itial p h ase, wh ich m ay last from a few m in u tes to a cou p le of VOL. 38, NO. 9, 2004 / ENVIRONM ENTAL SCIENCE & TECHNOLOGY 9 2721 h ou rs dep en din g u p on th e p rop erties su ch as viscosity an d bu bble size am on g oth ers (6, 7). Th e foam colu m n sh rin ks in h eigh t du rin g th is p eriod dep en din g p rim arily u p on th e in itial liquid holdup an d becom es thin n er. In the in term ediate p h ase, th e sm aller bu bbles coalesce with th e larger bu bbles du e to in ter-bu bble gas diffu sion , th u s coarsen in g th e foam (8). Th e exten ded liqu id su rface area of th e foam decreases in th is p h ase. Nish ioka an d Ross (9) develop ed a m eth od to ch aracterize foam stability based on th e total area of th e exten ded liqu id su rface. Lem lich (10) described a th eory to p redict th e evolu tion of bu bble size distribu tion du e to in terbu bble gas diffu sion . Sarm a et al. (11) sh owed th at th e p resen ce of water-solu ble p olym ers retards th e in ter-bu bble gas diffu sion , th ereby en h an cin g foam stability. Fin ally, th e foam film s th in down to a critical film th ickn ess su bject to cap illary su ction an d disjoin in g p ressu re an d ru p tu re. In th e p resen ce of oil, foam cou ld be destabilized addition ally at th e foam / oil con tact. Th e stability of foam s in th e p resen ce of oil h as been stu died by several au th ors (12-14). Sch ram m an d co-workers (15, 16) h ave discu ssed in detail th e in teraction of foam with th e oil. Th e im p ortan t p aram eters defin in g foam -oil in teraction are th e sp readin g coefficien t S () σf - σof - σo ) an d th e en terin g coefficien t E () σf + σof - σo ). Based on th ese p aram eters, th e foam -oil in teraction can be classified in to th ree categories (17). If E is n egative, th en S m u st be n egative, an d oil wou ld n eith er be drawn in to th e foam lam ellae n or sp read at th e foam liqu id-gas in terface; th u s, th e p resen ce of oil is n ot exp ected to destabilize th e foam in th is case. Th ese are called typ e A foam s. If E is p ositive an d S is n egative, oil wou ld be drawn in to th e foam lam ellae bu t is n ot exp ected to sp read at th e foam liqu id-gas in terface. Th e foam m ay or m ay n ot destabilize in th is case dep en din g u p on wh eth er oil drop s redu ce th e coh eren ce of th e foam lam ellae an d wh eth er th e oil rem ain s as a len s or is ejected ou tside th e lam ellae. Th ese are called typ e B foam s. If both E an d S are p ositive, th en oil is drawn in to th e lam ellae an d also sp reads as a film at th e foam liqu id-gas in terface. Th is cou ld seriou sly destabilize th e foam . Th ese foam s are called typ e C foam s. For a foam blan ket to be an effective m ass tran sfer barrier, th e en terin g an d sp readin g coefficien ts sh ou ld p referen tially be n egative (typ e A). Th is is p ossible wh en σf + σof < σo . Sin ce th e in terfacial ten sion between oil an d water in th e p resen ce of su rfactan ts is typ ically less th an a few m illin ewton s p er m eter, th e key to stability is decreasin g th e su rface ten sion of th e foam solu tion com fortably below th at of th e oil. Th e solu bilization of h ydrocarbon vap or in foam film s can also destabilize th em . As Bin ks et al. (18) p oin t ou t, h ydrocarbon gases affect foam stability at low su rfactan t con cen tration s. Th e foam s stu died in th eir stu dy are in th e vicin ity of th e foam in g/ n on foam in g bou n dary. We h ave stu d ied very stab le foam s with su rfactan t con cen tration s m u ch above th eir critical m icelle con cen tration . Th ese foam s last for m ore th an 1 day (n ot ju st a few m in u tes). Th ese foam s are alm ost in sen sitive to th e solu bilized h ydrocarbon gases; th ey are destabilized by th e h ydrocarbon liqu ids th at en ter th e foam film s, as discu ssed above. We h ave ch osen surfactan ts in such a way that both the en terin g an d spreadin g coefficien ts are n egative an d foam s rem ain stable in th e p resen ce of gasolin e. Experimental Section Materials. Th e foam form u lation con sisted of an aqu eou s solu tion of two su rfactan ts, a stab ilizer an d a viscosifier, sim ilar to th ose u sed by Th ach et al. (19). Th e first su rfactan t was a n on ion ic su rfactan t, Tergitol NP-10 (T). Tergitol is a n on ylp h en ol p olyeth ylen e glycol eth er from Sigm a-Aldrich . It con tain ed 97% active su rfactan t, abou t 2% p olyeth ylen e glycol, an d abou t 1% din on ylp h en yl p olyoxyeth ylen e. Th e secon d surfactan t is a fluorin ated surfactan t, F1127 (F). F1127 2722 9 ENVIRONM ENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004 FIGURE 1. Schematic of experimental setup for bench-scale experiments. is a p olyflu oroalkyl betain e from Atofin a. It con tain ed 27% active su rfactan t, 38% water, an d 35% eth an ol. Th e stabilizer is glycerol (G) obtain ed from Sigm a-Ald rich . Th e last com p on en t is an an ion ic p olym er xan th an gu m (X) from Sigm a-Aldrich . Hexan e was u sed as a gasolin e su bstitu te in som e exp erim en ts. It was su p p lied by Sigm a-Aldrich an d was 99.99% satu rated isom ers. Th e gasolin e was an u n leaded gasolin e from a local gas station . Th ese ch em icals were u sed as su p p lied. Bench-Scale Setup. Th e exp erim en tal setu p u sed for stu dyin g th e bu lk stability of foam s an d h ydrocarbon em ission in ben ch -scale is sh own in Figu re 1. Th e foam cell con sists of a glass colu m n h avin g two section ssa bottom section h avin g an i.d. of 1.4 cm an d a top section h avin g an i.d. of 5 cm . Th e len gth s of th e bottom an d top section s are 10 an d 45 cm , resp ectively. Th is kin d of design en ables u s to accu rately m easu re th e liqu id drain ed from th e foam . Th e foam cell con tain s a valve at th e bottom to regu late th e flow of su rfactan t solu tion in to th e cell. Th ere are two addition al valves in th e lower p ortion of th e cell p laced at calcu lated h eigh ts for ch argin g of n itrogen gas an d oil. All th e com p on en ts of th e foam were carefu lly m easu red, an d th e aqu eou s solu tion was th en stirred overn igh t by a m agn etic stirrer. Th e foam solu tion was ch arged in to th e cell, an d th e in itial h eigh t of liqu id in th e lower n arrow p ortion of th e cell was n oted. Nitrogen was th en bu bbled th rou gh th e liqu id at a con stan t rate. Th e in tern al diam eter of th e n ozzle (valve on th e n itrogen lin e) gen eratin g th e foam was 1 m m . Th e bu bblin g rate was kep t con stan t at 315 m L/ m in for all th e cases p resen ted in th is stu dy. Foam drain s in to th e n arrow lower p art of th e cell, an d ch an ge in th e level of drain ed liqu id is recorded with tim e. Com m ercially available h exan e was u sed as oil in th ese ben ch -scale m ass-tran sfer exp erim en ts. Prelim in ary exp erim en ts with gasolin e as th e oil su ggested th at h exan e was on e of th e m ain com p on en ts of th e volatile h ydrocarbon s an d th at m ass tran sfer was h igh er for h exan e th an for th e ligh ter com p on en ts. Th u s, h exan e rep resen ts th e worst case from th e m ass tran sfer p oin t of view. Th e oil was in trodu ced in to th e foam cell after m ost of th e bu lk liqu id drain ed ou t, wh ich is ap p roxim ately 50 m in after th e foam was gen erated in m ost cases. Th e h exan e vap ors issu in g from th e top of th e foam colu m n were swep t by th e n itrogen gas an d carried to a gas ch rom atograp h for com p osition al an alysis. Th e flow rate of n itrogen gas was con trolled by a flow m eter an d kep t con stan t at 4.8 ( 0.2 m L/ m in du rin g th ese exp erim en ts. Th e con cen tration s of th e h ydrocarbon vap ors in th e efflu en t in th e absen ce of an y foam were also m easu red u n der exp erim en tal con dition s to establish a baselin e for estim atin g th e su p p ression of h exan e vap ors in th e efflu en t in th e p resen ce of foam . Exp erim en ts were rep eated to ch eck th e con sisten cy of data. FIGURE 2. Schematic of experimental setup for the upscaled experiments. Th e viscosity of th e aqu eou s solu tion s was m easu red by a Brookfield rotation al rh eom eter. Th e su rface ten sion s of aqu eou s solu tion s an d oil were m easu red by a du Nou y rin g ten siom eter. Th e in terfacial ten sion between th e aqu eou s solu tion an d th e oil was m easu red by a sp in n in g drop ten siom eter. Foam texture/ bubble size was observed visually. Upscaled Setup. The exp erim en ts at the ben ch -scale were carried ou t in a n arrow (5 cm diam eter) cylin drical vessel. Th e flu idity of foam an d th e effective coverage of th e exp osed oil su rface b y th e foam b lan ket were n ot tested in th ese exp erim en ts. Moreover, in th e ben ch -scale exp erim en ts, th e foam colu m n m erely sat on th e top of th e oil layer an d was n ot p u sh ed from th e bottom . In field ap p lication s, th e foam colu m n wou ld be p u sh ed u p as th e cru de oil/ gasolin e is p u m p ed from th e bottom . If th e foam is n ot su fficien tly flexible/ m obile, it m igh t get p artially destroyed becau se of th e m ech an ical p ertu rbation s du rin g th e m otion . Hexan e was u sed as oil in ben ch -scale exp erim en ts. Polar organ ics, wh ich cou ld be p resen t in cru de oil/ gasolin e, are m ore destabilizin g th an th e n on p olar com p on en ts (19). A secon d set of exp erim en ts, called h ere “u p scaled”, were con du cted to evalu ate th ese effects. A cylin drical con tain er h avin g a diam eter of 46 cm an d a h eigh t of 74 cm was ch osen for th ese exp erim en ts. Th is vessel is referred to as th e “em ission cell”. Th e sch em atic of th e exp erim en tal set u p is sh own in th e Figu re 2. Th e foam was gen erated sep arately in a foam cell an d tran sferred to th e em ission cell th rou gh a h ose con n ectin g th e two. A foam colu m n 40 cm in h eigh t was sp rayed on to th e bottom of th e em ission cell. Th e em ission cell was th en covered with a lid, an d gasolin e was p u m p ed from th e bottom at a low flow rate. Th e top of th e em ission cell was con n ected by a tu be to th e gas ch rom atograp h wh ere th e com p osition of th e efflu en t was an alyzed. After a 1-in .-th ick layer of gasolin e was dep osited in side th e em ission cell, water was u sed to p u sh th is layer of gasolin e u p for th e rem ain in g du ration of th e exp erim en t. Here th e water m im ics th e in jection of gasolin e becau se it stays at th e bottom of gasolin e layer an d does n ot in teract with th e foam . Use of water redu ced th e u se of oil in th is laboratory exp erim en t an d th e associated fire-safety con cern s an d disp osal issu es. Ap p roxim ately 18 L of th e total liqu id was p u m p ed in 10 h , th e du ration of th e exp erim en t. Th e flow rate of th e gases flowin g th rou gh th e gas ch rom atograp h was ap p roxim ately 11 m L/ m in for th e exp erim en ts with foam . In th e absen ce of foam , a gas flow rate of abou t 30 m L/ m in was recorded at th e exit of th e gas ch rom atograp h . Th e resu lts of th e foam cell exp erim en ts are discu ssed n ext followed by th e em ission cell exp erim en ts. Results and Discussion Foam Cell. Several foam form u lation s were tried. Foam form u lation s with on ly su rfactan ts are n ot very stable an d FIGURE 3. Bulk drainage of foam. The foam solution viscosity is 18.1 cP, and the initial height is 0.32 m. h ave a h alf-life less th an 1 h . Foam form u lation s con sistin g of a n on ion ic su rfactan t (T), a flu orin ated su rfactan t (F), a stabilizer (G), an d a viscosifier (X) were fou n d to be rem arkably stable with a h alf-life exceedin g 1 day. Th e n on ion ic su rfactan t ten ds to in crease th e th ickn ess of foam film s; th is is con clu ded from visu al observation s of foam s. Th e flu orin ated su rfactan t redu ced th e su rface ten sion of th e aqu eou s solu tion an d stabilized th e foam colu m n in th e p resen ce of oil. Th e stabilizer (e.g., glycerol) decreased th e vap orization of water from foam film an d th u s in creased stability. The viscosifier, xan than gum , in creased the viscosity of th e aqu eou s solu tion an d slowed th e drain age of th e solu tion an d th e th in n in g of foam lam ellae, th u s p rovidin g stability to th e foam colu m n . Th e followin g n om en clatu re is u sed to sp ecify th e foam com p osition ; 1T0.4F0.4G0.16X im p lies 1 wt % Tergitol, 0.4 wt % F1127, 6 wt % glycerol, an d 0.16 wt % xan th an gu m . Th is foam com p osition was fou n d to be very stable an d h as been u sed in m ost of th e resu lts rep orted in th is p ap er. Th e su rface ten sion of th is aqu eou s foam solu tion was m easu red to be 20.5 m N/ m at room tem p eratu re (22 °C). At th e sam e tem p eratu re, th e su rface ten sion of th e oil (99.99% assay of C6-satu rated isom ers) was fou n d to be 23.3 m N/ m . Th e oil an d foam form u lation were equ ilibrated for 48 h in a volu m etric ratio of 1:2. Th e in terfacial ten sion (σof) between th e flu ids was m easu red u sin g a sp in n in g drop ten siom eter an d was fou n d to be 1.5 m N/ m . Th u s, th e en terin g coefficien t E is -1.3 m N/ m for th is system . Hen ce, th is foam falls in to the category of type Afoam accordin g to the criteria developed by Ross (17). Exp erim en tally, it was observed th at th is foam did n ot ru p tu re at th e foam / oil con tact an d exh ibited h igh degree of stability in th e p resen ce oil. Th ese foam s p ersisted for several days with ou t p ractically an y ch an ge in th e h eigh t, both in th e absen ce an d in th e p resen ce of oil. Figu re 3 sh ows th e liqu id drain age of two foam colu m n s h avin g a com p osition of 1T0.4F6G0.16Xan d an in itial h eigh t of 32 cm . Th e liqu id drain ed is p lotted as a fraction of th e VOL. 38, NO. 9, 2004 / ENVIRONM ENTAL SCIENCE & TECHNOLOGY 9 2723 FIGURE 4. Concentration of hexane vapors in the effluent w ith and w ithout foam. FIGURE 5. Concentration of hexane in the effluent w ith time for the foam columns w ith initial heights of 0.15, 0.23, and 0.32 m. The values of the parameters are µ ) 8 cP, G ) 1.03 × 103 kg/m3, σf ) 20.5 mN/m, Rb ) 10 mm, E0 ) 0.015 (L0 ) 0.15 m), E0 ) 0.009 (L0 ) 0.23 m), E0 ) 0.007 (L0 ) 0.32 m). am ou n t of liqu id foam ed origin ally. With tim e, th e fraction drain ed ten ds to a lim it sligh tly lower th an on e. Most of th e liqu id is drain ed with in 1 h . Th e am ou n t n ot drain ed is in th e foam as lam ellae an d p lateau borders. It is clear th at th e exp erim en tal data for bu lk drain age rates are fairly rep rodu cible between th e two iden tical exp erim en ts. Th ese foam s are stable for m ore th an 1 day in th e p resen ce of oil, alth ou gh th e data on con cen tration of h exan e vap ors in th e efflu en t was collected on ly for about 10 h. Visual observation con firm s th at m ost of th e foam bu bbles are of sim ilar size, th ou gh som e sm aller bubbles san dwiched between the larger bubbles are also observed. Figu re 4 sh ows th e m ole p ercen tage of h exan e vap ors in th e efflu en t with an d with ou t foam u n der exp erim en tal con dition s as described above. Th e in itial h eigh t of th e foam colu m n was 32 cm . Th e h eigh t of th e foam colu m n rem ain ed p ractically u n ch an ged th rou gh ou t th e du ration of th e m ass tran sfer exp erim en ts. With ou t foam , th e h exan e con cen tration in th e efflu en t in creases with tim e as it m ixes with n itrogen in th e cylin der an d reach es a p lateau valu e of abou t 4.5 m ol % in abou t 5 h . In con trast, th e h exan e con cen tration in th e efflu en t in th e p resen ce of foam rises to on ly abou t 0.6 m ol % at th e en d of 10 h . It is clear th at th e p resen ce of foam sign ifican tly su p p resses th e con cen tration of h exan e vap ors in th e efflu en t. If th e p lateau valu es in both th e cases are con sidered, th e p resen ce of th e foam su p p resses th e con cen tration of th e h exan e vap ors in th e efflu en t by abou t 87%. Variou s p aram eters th at cou ld affect th e p erform an ce of foam su ch as in itial foam h eigh t an d liqu id viscosity were varied exp erim en tally. Th e followin g section s discu ss th e im p act of th ese p aram eters. Effect of Foam Height. Figu res 5-7 sh ow th e effect of in itial foam h eigh t on th e con cen tration of h exan e vap ors in th e efflu en t. Th e foam h eigh t du rin g th e m ass tran sfer exp erim en ts was essen tially equ al to th e in itial foam h eigh t. 2724 9 ENVIRONM ENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004 FIGURE 6. Concentration of hexane in the effluent w ith time for the foam columns w ith initial heights of 0.15, 0.23, and 0.32 m. The values of the parameters are µ ) 18.1 cP, G ) 1.03 × 103 kg/m3, σf ) 20.5 mN/m, Rb ) 10 mm, E0 ) 0.024 (L0 ) 0.15 m), E0 ) 0.02 (L0 ) 0.23 m), E0 ) 0.016 (L0 ) 0.32 m). FIGURE 7. Concentration of hexane in the effluent w ith time for the foam columns w ith initial heights of 0.15, 0.23, and 0.32 m. The values of the parameters are µ ) 65.3 cP, G ) 1.03 × 103 kg/m3, σf ) 20.5 mN/m, Rb ) 10 mm, E0 ) 0.034 (L0 ) 0.15 m), E0 ) 0.024 (L0 ) 0.23 m), E0 ) 0.018 (L0 ) 0.32 m). Th ree foam form u lation s of viscosities 8 × 10 -3 Ns/ m 2 () 8 cP) (Figu re 5), 18.1 × 10-3 Ns/ m 2 (Figu re 6), an d 65.3 × 10 -3 Ns/ m 2 (Figu re 7) were ch osen by varyin g th e con cen tration of viscosifier in th e foam form u lation an d keep in g th e rest of th e foam com p osition u n ch an ged. Th e con cen tration of h exan e vap ors in th e efflu en t in creases with tim e an d reach es a p lateau after abou t 5 h for 15 cm colu m n an d for abou t 8 h for th e 32 cm colu m n . As exp ected, th e m ole fraction of hexan e vap ors in the effluen t decreases with in creasin g in itial foam h eigh t for a given viscosity of th e foam solu tion . Th e plateau con cen tration of hexan e also decreases with the in itial foam h eigh t. Th e aqu eou s solu tion drain s from th e lam ellae in to th e adjoin in g p lateau borders su bject to th e forces of cap illary su ction an d disjoin in g p ressu re. Th e p lateau borders form a well-con n ected n etwork in the foam colum n through which th e liqu id drain s down u n der th e action of gravity an d an op p osin g cap illary p ressu re gradien t is set u p as a con sequ en ce of th e gravity drain age. Th e p lateau borders becom e lean er in liqu id with h eigh t in a foam colu m n with th e resu lt th at th e p lateau border radiu s of cu rvatu re gets sm aller an d th e cap illary su ction h igh er cau sin g rap id drain age of th e con n ected foam lam ellae. Thus, the foam lam ellae get thin n er p rogressively with h eigh t in a foam colu m n . Sin ce th e liqu id h oldu p in th e foam film s decreases with h eigh t, th e m ass tran sfer resistan ce to th e diffu sin g h ydrocarbon vap ors also decreases. If th e liqu id h oldu p in th e foam lam ellae were u n iform (i.e., in dep en den t of th e h eigh t), th e diffu sion tim e scale of th e foam colu m n wou ld qu adru p le wh en th e foam colu m n h eigh t is dou bled assu m in g it is on ly th e liqu id in th e foam film s th at p rovides a sign ifican t diffu sive m ass tran sfer barrier. Sin ce th e m ass tran sfer resistan ce decreases with h eigh t, we exp ect th e diffu sion al tim e to in crease by less th an 4 tim es. Figu res 5-7 sh ow th at th e tim e to reach FIGURE 8. Concentration of hexane in the effluent w ith time for the foam columns w ith initial height of 0.23 m. The values of the parameters are G ) 1.03 × 103 kg/m3, σf ) 20.5 mN/m, Rb ) 10 mm, and E0 ) 0.009, 0.02, and 0.024 for µ ) 8, 18.1, and65.3 cP, respectively (1 cP ) 0.001 Ns/m2). FIGURE 9. Bulk drainage of foams of different viscosities having an initial height of 15 cm. th e p lateau con cen tration in creases by on ly a factor of abou t 8:5 (wh ich is con siderably less th an 4 tim es) wh en th e h eigh t is ap p roxim ately dou bled. Figu res 5-7 also sh ow th at th e p lateau con cen tration of h exan e vap ors in th e efflu en t is su p p ressed by abou t 65% (∼1.4-0.5 m ol %) wh en th e h eigh t of th e foam colu m n is dou bled. Effect of Viscosity. Figu re 8 sh ows th e variation of m ole p ercen tage of h exan e vap ors in th e efflu en t with tim e for foam colu m n s h avin g an in itial h eigh t of 23 cm . Th ree sets of data for th ree differen t viscosities of th e aqu eou s foam solu tion are p resen ted. It is in terestin g to n ote th at n o sign ifican t differen ce in th e con cen tration of h exan e vap ors in th e efflu en t was observed with varyin g viscosities du rin g th e tran sien t. Th e p lateau h exan e con cen tration valu e decreases sligh tly as th e viscosity of th e aqu eou s solu tion in creases. Th e in crease in viscosity slows down the drain age of liquid from th e foam lam ellae resu ltin g in th icker foam film s. Figu re 9 sh ows th e bu lk drain age of foam s h avin g an in itial h eigh t of 15 cm . Th e drain age fraction is calcu lated with resp ect to th e total am ou n t of liqu id p resen t in th e foam at th e en d of bu bblin g. Th e foam s m ade u p of aqu eou s solu tion s h avin g h igh er viscosities drain slower. Th e foam film th ickn ess was n ot exp licitly m easu red. Alth ou gh th e bu lk drain age data reflects p lateau border drain age m ore accu rately, a greater am ou n t of liqu id in th e p lateau borders im p lies weaker cap illary su ction from th e film s, th u s p rodu cin g th icker film s. If th e solu bility of h exan e is n ot affected by th e p olym er con cen tration , th e th icker foam film s wou ld h ave less p erm eability to th e diffu sin g h ydrocarbon vap ors; h en ce, th e con cen tration of hexan e vap ors in the effluen t is exp ected to decrease with h igh er viscosities. Moreover, if on e were to assu m e th at Stokes-Ein stein relation sh ip for diffu sivity in liqu id h olds for th e diffu sion of h ydrocarbon vap ors, th e diffu sivity wou ld decrease for h igh er viscosities even for th e FIGURE 10. Cumulative hydrocarbon emission w ith time per liter of gasoline loaded in the presence of foam having a composition of 1T0.4F6G0.24X and w ithout foam. sam e th ickn ess of th e foam film . Th u s, th e m ass tran sfer of h exan e sh ou ld decrease dram atically with th e in crease in viscosity. Th is does n ot ap p ear to be th e case in th is stu dy (i.e., Figu re 8). On e exp lan ation cou ld be th at in creasin g th e p olym er con cen tration n ot on ly in creases th e viscosity of the foam form ulation but also possibly in creases the solubility of th e h ydrocarbon gases in th e foam liqu id, an n u llin g an y diffu sivity decrease du e to h igh er viscosity or th icker film s. An oth er p ossibility is th at th e resistan ce p rovided by th e adsorbed su rfactan t m on olayers at th e film -gas in terface is th e dom in an t resistan ce to th e m ass tran sfer, as sh own by Qu oc et al. (20). In th at case, th e liqu id con ten t of th e foam becom es less sign ifican t an d sin ce the n um ber of m on olayers th rou gh wh ich th e h ydrocarbon m olecu les diffu se th rou gh rem ain th e sam e for a given foam h eigh t an d bu bble size irresp ective of th e viscosity, th e m ass tran sfer resistan ce of th e foam colu m n does n ot vary sign ifican tly. Em ission Cell. Scaled-u p exp erim en ts were con du cted with an u n leaded gasolin e in th e em ission cell. Figu re 10 sh ows th e cu m u lative h ydrocarbon em ission s p er liter of gasolin e loaded in th e em ission cell for th e foam com p osition 1T0.4F6G0.24X an d th e in itial foam h eigh t of 40 cm . Abou t 144 m g of total organ ic com p ou n ds are em itted p er liter of gasolin e loaded in 10 h with ou t th e foam . Abou t 5 m g of TOCs are em itted p er liter of gasolin e loaded in 10 h with th e foam . Th u s, th e su p p ression of total em ission of h ydrocarbon s is about 96.5%. This em ission , 5 m g of TOC/ L of gasolin e loaded, m eets th e U.S. EPA (10 m g of TOC/ L) an d Eu rop ean Com m u n ity (35 m g of TOC/ L) stan dards (1) in ou r em ission cell. Th e total h ydrocarbon em ission is con trolled by several p aram eters, su ch as loadin g tim e, foam h eigh t, foam form u lation , an d exp osed su rface to con tain er volu m e ratio. Th ese p aram eters can be ch osen ap p rop riately to obtain th e requ ired em ission . Figu re 11a-c sh ows th e con cen tration of th e in dividu al h ydrocarbon s in th e efflu en t for th e exp erim en t rep orted above. All th e h ydrocarbon gases h eavier th an h exan es are collectively lu m p ed as C6+, th e isom ers of p en tan e as C5, an d th e isom ers of bu tan e as C4. For th e ligh t h ydrocarbon s su ch as bu tan es, close to 100% su p p ression of th e vap ors was observed in th e p resen ce of foam , at least for 10 h for both th e foam form u lation s. For p en tan es, th e em ission is cu t down by abou t 97% in th e p resen ce of foam for th e foam form u lation 1T0.4F6G0.24X. It sh ou ld be n oted th at th e p en tan es collectively con tribu te h igh est to th e total VOC em ission s from th e gasolin e. For h exan es an d oth er h eavier gases (C6+), th e redu ction in th e em ission is abou t 71% if th e con cen tration in th e efflu en t at th e en d of 10 h is con sidered for 1T0.4F6G0.24X. Thus, these foam form ulation s ach ieve 70% to n ear 100% su p p ression in th e em ission of VOCs dep en din g u p on th e com p on en t of th e gasolin e u n der con sideration . Note th at th e em ission of C6+ is h igh er th an th at of C5, even th ou gh th e diffu sivity of C6+ is lower in th e aqu eou s solu tion . Th is im p lies th at th e solu bility of th ese organ ic vap ors in th e foam lam ellae m ay p lay an im p ortan t VOL. 38, NO. 9, 2004 / ENVIRONM ENTAL SCIENCE & TECHNOLOGY 9 2725 FIGURE 12. Comparison of foam liquid content for the foam formulation 1T0.4F6G0.24X prepared w ith deionized w ater and salty w ater. FIGURE 13. Cumulative hydrocarbon emission w ith time per liter of gasoline loaded in the presence of foam having a composition of 1T0.4F6G0.24X prepared in 3.5 w t % NaCl solution and w ithout foam. FIGURE 11. (a) Concentration of C6+ gases in the effluent w ith and w ithout foam for the upscaled experiment for identified aqueous foam formulations. (b) Concentration of pentanes in the effluent w ith and w ithout foam for the upscaled experiment for identified aqueous foam formulations. (c) Concentration of butanes in the effluent w ith and w ithout foam for the upscaled experiment for identified aqueous foam formulations. role. High er solu bility of C6+ in th e su rfactan t solu tion th an th at of C5 m ay be du e to th e p resen ce of su rfactan ts an d viscosifiers th at are organ ic. Often , th e loadin g an d u n loadin g term in als for th e cru de oil/ gasolin e are located n ear th e large salin e water bodies su ch as ocean s. Th erefore, we evalu ate th e effectiven ess of th e foam s develop ed with seawater. In th e op en ocean s, th e salin ity based on total dry solids p er kilogram of seawater typ ically varies between 3.36 an d 3.68 wt % (21). An ap p roxim ate valu e of seawater salin ity cou ld be taken as 3.5 wt %, com p osed en tirely of NaCl, alth ou gh seawater also con tain s oth er salts. In th e p relim in ary exp erim en ts with th e foam form u lation s p rep ared in salty water con tain in g 3.5 wt % of NaCl, it was observed th at th e foam colu m n ap p roxim ately retain s its in itial h eigh t for at least 10 h , alth ou gh it h as lower in itial liqu id h oldu p an d drain s faster th an th e foam form u lation p rep ared with deion ized water as sh own in Figu re 12. Th e lam ellae for th e foam p rep ared in salty water also ap p ear th in n er visibly. Th ese observation s are con sisten t with the effect of salin ity on foam stability observed by Rojas et al. (22). Th e p resen ce of electrolytes in th e foam liqu id cau ses electrostatic dou ble layer in th e foam film s to sh rin k, th ereby redu cin g th e rep u lsive forces th at stabilize th e foam film s. Th is, in tu rn , leads to th in n er foam film s in th e p resen ce of salt. Th in n er film s m ay n ot redu ce th e ability of th e foam barrier to su p p ress VOC em ission s su bstan tially 2726 9 ENVIRONM ENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004 if th e lim itin g m ass tran sfer resistan ce is p rovided by th e film -gas in terface (20). Figu re 13 sh ows th e cu m u lative h ydrocarbon em ission s p er liter of gasolin e loaded in th e em ission cell for th e foam com p osition 1T0.4F6G0.24Xp rep ared in salin e solu tion (3.5 wt % NaCl) an d th e in itial foam h eigh t of 40 cm . Abou t 9 m g of TOC is em itted p er liter of gasolin e loaded after 10 h again st 144 m g without foam . Thus, the sup p ression of total em ission of h ydrocarbon s is abou t 93.8%. Figu re 14a-c sh ows th e con cen tration of com p on en ts in th e efflu en t with tim e. As expected, the effectiven ess of the foam colum n in suppressin g hydrocarbon em ission goes down when com p ared with foam form u lation s p rep ared in deion ized water for th e sam e in itial foam h eigh t; con cen tration s of all th e com p on en ts are h igh er in Figu re 14 as com p ared to th ose in Figu re 11. However, it still sup p resses th e con cen tration of C6+ gases in th e efflu en t by 59%, of p en tan es by 84%, an d of bu tan es by 69% based on th e con cen tration in th e efflu en t at th e en d of abou t 10 h . Th e effectiven ess of th e foam colu m n can be in creased by in creasin g th e colu m n h eigh t. An ap p roxim ate m odel h as been develop ed to in terp ret th e m ass tran sfer of vap ors du rin g th e loadin g p rocess with an d with ou t foam barriers. Th is m odel con siders on edim en sion al diffu sion of dilu te VOC com p on en ts in a sem iin fin ite air colum n above the foam (or gasolin e in the absen ce of foam ). Th e distan ce (h ) h 0 - vt) between th e foam -air in terface an d th e exit of th e vessel sh rin ks as gasolin e is loaded. Th e in itial h eigh t of th e air colu m n is h 0, v is th e velocity of the gasolin e fron t, t is the tim e, an d z is the distan ce from th e foam -air in terface. Th e efflu en t con cen tration is evalu ated at h , wh ich is a fu n ction of tim e. Th e foam is m odeled with a m ass tran sfer coefficien t (k ) at th e foam -air in terface. Th e solu bilization of h ydrocarbon s in th e aqu eou s solu tion of th e foam is im p ortan t in slowin g down th e C/ Cs ) 1 - erf(zD -1/ 2t -1/ 2/ 2) - exp ((k z/ D) + (k 2t/ D))[1 - erf((k D -1/ 2t 1/ 2) + (zD -1/ 2t -1/ 2/ 2))] (2) wh ere Cs is th e con cen tration of th e com p on en t at th e gasolin e-vap or in terface dictated by th e th erm odyn am ics. If th e m ass tran sfer coefficien t is large (i.e., in th e absen ce of a foam barrier), th e last term in eq 2 is n egligible. Th is m odel h as been fitted to th e exp erim en tal data of h exan e in Figu res 11a an d 14a. Th e m ass tran sfer coefficien t (k ) estim ated for h exan e in th e 40 cm foam colu m n s stu died (from th e fittin g) are 3.9 × 10 -4 cm / s for foam with water an d 5.7 × 10-4 for foam with salt solu tion . Th ese m ass tran sfer coefficien ts dep en d on th e foam stru ctu re, solu bility in th e aqu eou s solu tion , an d diffu sivity th rou gh foam film s. Th ese factors will be stu died in detail in a fu tu re stu dy. Acknow ledgments Th is work was p artially su p p orted by th e fu n din g from Texas Hazardou s Waste Research Cen ter. Nomenclature C h ydrocarbon con cen tration in air colu m n (m ol/ L) D diffu sivity (m 2/ s) E en terin g coefficien t (m N/ m ) h h eigh t of th e air colu m n above foam (m ) k m ass tran sfer coefficien t of foam (m / s) L0 in itial foam h eigh t (cm ) Rb bu bble radiu s (cm ) S sp readin g coefficien t (m N/ m ) Greek Letters ǫ0 in itial volu m e fraction of liqu id in foam σf su rface ten sion of foam solu tion FIGURE 14. (a) Concentration of C6+ in the effluent w ith and w ithout foam for the upscaled experiment. Foam liquid composition is 1T0.4F6G0.24X and contains 3.5 w t % NaCl. (b) Concentration of pentanes in the effluent w ith and w ithout foam for the upscaled experiment. Foam liquid composition is 1T0.4F6G0.24X and contains 3.5 w t % NaCl. (c) Concentration of butanes in the effluent w ith and w ithout foam for the upscaled experiment. Foam liquid composition is 1T0.4F6G0.24X and contains 3.5 w t % NaCl. σo su rface ten sion of oil σof in terfacial ten sion between foam liqu id an d oil µ bu lk viscosity of foam solu tion F den sity of foam solu tion breakth rou gh of th e gases bu t is n eglected h ere. Th e details of diffu sion th rou gh th e liqu id-air in terface, foam film s, an d foam cells are lu m p ed in to a sin gle m ass tran sfer coefficien t. Th ese factors are im p ortan t bu t ou tside th e scop e of th is p ap er. Th e con cen tration of a h ydrocarbon com p on en t in th e air above th e foam barrier can be described by (u n der th e assu m p tion s stated above): (1) Pezolt, D. J.; Collick, S. J.; Joh n son , H. A.; Robbin s, L. A. En viron . Prog. 1997, 16, 16-19. (2) Robbin s, L. A.; Fran k, T. C. U.S. Paten t 4,857,084, 1989. (3) Skarstrom , C. W. Recen t Dev. Sep. Sci. 1975, 2, 95. (4) Corin o; et al. U.S. Paten t 3,639,258, 1972. (5) Can evari, et al. U.S. Paten t 3,850,206, 1974. (6) Weaire, D.; Hu tzler, S. Ph ys. A 1998, 257, 264-269. (7) Neeth lin g, S. J.; Lee, H. T.; Cillier, J. J. L. J. Ph ys.: Con den s. Matter 2002, 14, 331-336. (8) Magrabi, S. A.; Dlu gogorski, B. Z.; Jam eson , G. J. Ch em . En g. Sci. 1999, 54, 4007-4022. (9) Nish ioka, G.; Ross, S. J. Colloid In terface Sci. 1981, 81, 1-7. (10) Lem lich , R. In d. En g. Ch em . Fu n dam . 1978, 17, 89-93. (11) Sarm a, D. S. H. S. R.; Pan dit, J.; Kh illar, K. C. J. Colloid In terface Sci. 1988, 124, 339-348. (12) Man lowe, D. J.; Radke, C. J. SPE Res. En g. 1990, 5, 495-502. (13) Ratterm an , K. T. Proceedin gs of th e 64th An n u al Tech n ical Con feren ce of SPE; Society of Petroleu m En gin eers: Rich ardson , TX, 1989; Pap er SPE 19692. (14) Nikolov, A. D.; Wasan , D. T.; Hu an g, D. W.; Edwards, D. A. Proceedin gs of th e 61st An n u al Tech n ical Con feren ce of SPE; Society of Petroleu m En gin eers: Rich ardson , TX, 1986; Pap er SPE 15443. (15) Sch ram m , L. L.; Novosad, J. J. Colloids Su rf. 1990, 46, 21-43. (16) Man n h ardt, K.; Novosad, J. J.; Sch ram m , L. L. SPE Res. En g. Eval. 2002, 3, 23-34. ∂C/ ∂t ) D ∂2C/ ∂2z for 0 < z < ∞ (1) with th e in itial an d bou n dary con dition s: C ) 0 for all z at t ) 0 C f 0 as Z f ∞, for t > 0 -D ∂C/ ∂z ) k (C - Cs ) at z ) 0, for t > 0 where D is the diffusivity of the com pon en t in air. The solution to th e above equ ation , C(z, t), is given by ref 23: Literature Cited VOL. 38, NO. 9, 2004 / ENVIRONM ENTAL SCIENCE & TECHNOLOGY 9 2727 (17) Ross, S. J. Ph ys. Colloid Ch em . 1950, 54, 429-436. (18) Bin ks, B. P.; Fletch er, P. D. I.; Hayn es, M. D. Colloids Su rf. A 2003, 216, 1-8. (19) Th ach ; et al. U.S. Paten t 5,296,164, 1994. (20) Qu oc, P. N.; Zith a, P. L.; Cu rrie, P. K. J. Colloid In terface Sci. 2002, 248, 467-476. (21) Sp iegler, K. S. Salt-W ater Pu rification ; Plen u m Press: New York, 1977; p p 16-17. (22) Rojas, Y.; Kakadjian , S.; Ap on te, A.; Marqu ez, R.; San ch ez, G. Proceedin gs of th e 2001 SPE In tern ation al Sym posiu m on Oilfield 2728 9 ENVIRONM ENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004 Ch em istry; Society of Petroleu m En gin eers: Rich ardson , TX, 2001; Pap er SPE 64999. (23) Welty, J. R.; Wicks, C. E.; Wilson , R. E. Fu n dam en tals of Mom en tu m , Heat & Mass Tran sfer; Joh n Wiley & Son s: New York, 1984; p 307. Received for review Septem ber 2, 2003. Revised m an u script received Jan u ary 5, 2004. Accepted Febru ary 17, 2004. ES0349599