Over­view of Mem­brane Sepa­ra­tions with in­sight on ce­ramic mem­branes

Ma­gan Khakharia1, Sachin Jad­hav2, Arvind Sikar­war2, Bhaskar Tho­rat2

Chemical Industry Digest - - What’s In? -

– Ma­gan Khakharia, Mi­cro­filt In­dia Pvt Ltd; Sachin Jad­hav, Arvind Sikar­war, Bhaskar Tho­rat, ICT-Mum­bai

Ar­ti­cle pro­vides a quick re­view of the devel­op­ments in mem­brane sepa­ra­tions, par­tic­u­larly on mem­brane ma­te­ri­als with a brief on the lat­est in­tro­duc­tion, ce­ramic mem­branes.

Ab­stract

Mem­brane sepa­ra­tions have pro­gressed very fast, over­tak­ing many of the con­ven­tional sepa­ra­tions like dis­til­la­tion, evap­o­ra­tion etc. The ad­van­tages of mem­brane sepa­ra­tions are many and it is also find­ing in­creas­ing ap­pli­ca­tions in newer ar­eas. With ad­vances in ma­te­ri­als de­vel­op­ment, new mem­brane ma­te­ri­als like ce­ramic mem­branes are in­creas­ing the ap­pli­ca­tions band­width to a wide range of process pa­ram­e­ters apart from longer life of the mem­brane.

In­tro­duc­tion

Mem­branes

are or­ganic or in­or­ganic por­ous ma­te­ri­als that al­low the sep­a­ra­tion of dif­fer­ent sub­stances (sol­u­ble or not) present in a liq­uid or gas. The sep­a­ra­tion is per­formed by ap­ply­ing the pres­sure gra­di­ent be­tween the feed side and the per­me­ate side of the mem­brane. It re­sults in the pas­sage of par­ti­cles or mol­e­cules smaller than the mem­brane pore size.

His­tor­i­cal chronol­ogy

Year Evo­lu­tion 1748: Dis­cov­ery of semi per­me­able mem­brane by Ab­bott Nol­lec

1900: Pro­duc­tion of ar­ti­fi­cial mem­brane

1950: Sea water de­sali­na­tion pro­gram in the USA, Ura­nium en­rich­ment pro­gram in the France by the CEA with the help of min­eral mem­brane 1960: Dis­cov­ery of cel­lu­lose ac­etate asym­met­ric mem­brane by Loeb and by Souri­ra­jan

1970: Many in­dus­trial units for sea water de­sali­na­tion 1971: Be­gin­ning of UF ap­pli­ca­tion for the ex­trac­tion of milk pro­tein

1980: In­dus­trial pro­duc­tion of the first In­or­ganic mem­brane in the world CARBOSEP ap­plied to UF for the food and bev­er­ages and bio ap­pli­ca­tion

In­ter­est of mem­brane sep­a­ra­tion and do­mains

The re­ten­tion of mol­e­cules are re­al­ized with­out any phase change as against the other ubiq­ui­tous unit op­er­a­tions such as evap­o­ra­tion, crys­tal­liza­tion and liq­uidliq­uid ex­trac­tion. In ma­jor­ity of the cases, the in­ter­ested

solute is ele­gantly sep­a­rated from the source with­out any dis­tor­tion or di-ori­en­ta­tion of mol­e­cules as it may hap­pen in other sep­a­ra­tion tech­niques such as chro­matog­ra­phy.

Mem­brane pro­cesses can be roughly clas­si­fied as: mi­cro-fil­tra­tion (MF), ul­tra-fil­tra­tion (UF), nano-fil­tra­tion (NF), re­verse os­mo­sis (RO), per­va­po­ra­tion (PV) and elec­tro-dial­y­sis (ED). Out of these,the mirco-, ul­tra-, nano-fil­tra­tion, and re­verse os­mo­sis work on size ex­clu­sion prin­ci­ple. On the other hand, per­va­po­ra­tion is based on mem­brane affin­ity to­wards the com­po­nents and elec­tro­dial­y­sis makes use of charge based sep­a­ra­tion. A de­tailed Os­mon­ics Chart is pre­sented in Fig­ure 1 to un­der­stand the dif­fer­en­ti­a­tion be­tween the above men­tioned fil­tra­tion pro­cesses.

Hy­dro­dy­nam­ics of mem­brane process

Mem­brane pro­cesses can be largely clas­si­fied as Dead End Fil­tra­tion and Cross Flow Fil­tra­tion. The dead end fil­tra­tion is a batch process where pres­sure is ap­plied on one side of the mem­brane to draw fil­trate on the other side. Here, the fil­tra­tion layer forms over the mem­brane over the pe­riod of time, thus in­creas­ing the pres­sure drop across. This type of fil­tra­tion sys­tem is pop­u­lar in lab­o­ra­tory scale fil­tra­tion stud­ies. Most in­dus­trial mem­brane pro­cesses uses cross flow mech­a­nism and hence it is also known as cross flow fil­tra­tion or tan­gen­tial flow fil­tra­tion. These fil­tra­tion equip­ment can be then fur­ther scaled up and uti­lized at com­mer­cial scale. Fig­ure 2(a) and 2(b)de­picts the op­er­a­tional fea­tures of cross flow and dead end fil­tra­tion, re­spec­tively.

Mem­brane ma­te­ri­als clas­si­fi­ca­tion

A wide range of syn­thetic mem­branes are avail­able for a va­ri­ety of sep­a­ra­tion as­sign­ments. They can be pro­duced from or­ganic ma­te­ri­als such as poly­mers and liq­uids, as well as in­or­ganic ma­te­ri­als. The most of com­mer­cially uti­lized syn­thetic mem­branes in sep­a­ra­tion in­dus­try are made of poly­meric ma­te­ri­als. Poly­meric mem­branes lead the mem­brane sep­a­ra­tion in­dus­try mar­ket be­cause they are very com­pet­i­tive in per­for-

mance and eco­nom­ics. They can be clas­si­fied based on their sur­face chem­istry, bulk struc­ture, mor­phol­ogy, and pro­duc­tion method. Fig­ure 3 shows the clas­si­fi­ca­tion of the mem­branes. The most com­mon poly­mers that are em­ployed com­mer­cially are cel­lu­lose ac­etate, ni­tro­cel­lu­lose, and cel­lu­lose es­ters (CA, CN, and CE), poly­sul­fone (PS), polyether sul­fone(PES), poly­acriloni­trile (PAN), polyamide, poly­imide, poly­eth­yl­ene and polypropy­lene (PE and PP), poly­te­traflu­o­roethy­lene (PTFE), polyvinyli­dene flu­o­ride (PVDF), polyvinylchlo­ride (PVC). Car­bon, ce­ramic and SS mem­branes are the ma­jor types of in­or­ganic mem­branes.

Ce­ramic mem­branes are ma­jorly pro­duced from zir­co­nium, Ti­ta­nium or Alu­mina. The pores are ob­tained by the su­per­im­po­si­tion of sev­eral lay­ers of por­ous me­dia of de­creas­ing gran­u­lom­e­try. The lay­ers are bound to­gether by sin­ter­ing. The val­ues of thick­ness of the lay­ers are spec­i­fied be­low:

1. 1st por­ous me­dia – 1-5 nm

2. 2nd por­ous me­dia – 5-100 nm

3. 3rd por­ous me­dia – 0.1-1.5 μm

4. 4th por­ous me­dia – 5-10 μm

Fil­tra­tion layer types

Syn­thetic mem­branes can also be cat­e­go­rized based on their mor­phol­ogy, namely, sym­met­ric mem­branes and asym­met­ric mem­branes. As the names sug­gest the sym­met­ric mem­branes con­sists of ho­mo­ge­neous sym­met­ric layer of pores through­out the mem­brane ma­te­ri­als as shown in Fig­ure 4. On the other hand, the asym­met­ric mem­branes are com­posed of a fil­tra­tion layer (or ac­tive layer) sup­ported by an­other layer called as sup­port layer (Fig­ure 5). Both of these lay­ers are essen­tially made up of dif­fer­ent ma­te­ri­als. The sup­port layer is usu­ally por­ous in na­ture and offers low mass trans­fer re­sis­tance.

The Fig­ure 6 shows the sur­face prop­er­ties of a sam­ple mem­brane. The mul­ti­chan­nel mem­branes are the most used ce­ramic mem­branes. The chan­nel di­am­e­ter can range from 1.5 mm to 6 mm hy­draulic di­am­e­ter. The avail­able di­am­e­ter of tube can be from 10mm to 45mm. The num­ber of chan­nels in one mul­ti­chan­nel mem­brane can vary from 7 to 93.The area for mem­brane can vary from 0.1 m2 to 1 m2. Fig­ures 7a and b should pro­vide a bet­ter un­der­stand­ing of the macro struc­tures.

Ta­ble 1 also shows the com­par­i­son be­tween the or­ganic and ce­ramic mem­branes. The sev­eral ad­van­tages of­fered by ce­ramic mem­branes are high work­ing tem­per­a­tures, large work­ing pH range, bet­ter sol­vent ox­i­da­tion prop­er­ties and longer life span.

Fun­da­men­tals

Con­sider Fig­ure 8 to un­der­stand the fun­da­men­tals of mem­brane fil­tra­tion.

Let, P =Re­ten­tate pres­sure car­ton in­let; P = Re­ten­tate 1 2 pres­sure car­ton out­let; P =Per­me­ate back pres­sure

3

The ax­ial pres­sure drop is given by,

ΔP= P – P

1 2

The av­er­age pres­sure over mem­brane is given by, P = Av­er­age pres­sure = (P + P )/2

m 1 2

The pres­sure that is needed to push the fil­trate through the mem­brane is called Trans mem­brane pres­sure (TMP). The TMP is de­fined as the pres­sure gra­di­ent of the mem­brane, or the av­er­age feed pres­sure mi­nus the per­me­ate pres­sure. The Trans­mem­brane pres­sure is there­fore given by,

TMP = P – P

m 3

As the fil­tra­tion pro­gresses, per­me­ate is re­cov­ered from the fil­trate side and the re­ten­tate is re­cy­cled to the feed tank. This re­sults in the de­creased feed vol­ume over the en­tire fil­tra­tion process un­less the fresh feed is in­tro­duced to the feed tank. Here, the vol­ume cor­rec­tion fac­tor is ap­plied to re­move the in­con­sis­ten­cies given as fol­lows: Ini­tial feed vol­ume Vol­ume con­cen­tra­tion fac­tor =

Fi­nal re­ten­tate vol­ume

The ve­loc­ity along the mem­brane is a very crit­i­cal pa­ram­e­ter. It is rec­om­mended to keep it high so as to main­tain the mem­brane sur­face clean. Typ­i­cal ve­loc­i­ties can vary from 2 m/s to 76 m/s, de­pend­ing on the foul­ing ten­dency of the mem­brane

The mem­brane sys­tem can have dif­fer­ent con­fig­u­ra­tions such as:

- Sam­ple batch sys­tem

- Batch sys­tem with re­ten­tate re­cy­cling

- Batch sys­tem with re­ten­tate re­cy­cling and feed­ing - Feed batch sys­tem

- Multi stage sys­tem for con­tin­u­ous op­er­a­tion.

Fig­ure 8. Schematic of cross flow fil­tra­tion process with pos­si­ble con­fig­u­ra­tions Global man­u­fac­tur­ing and mar­ket share

World­wide con­tri­bu­tion of ce­ramic mem­branes is less than 15%. The rest is contributed by or­ganic/poly-

meric mem­branes. Ma­jor ce­ramic mem­branes man­u­fac­tur­ers are:

- PALL (Ger­many)

- Frana (Nor­way)

- FAME in­dus­tries (France)

- Atech (Ger­many)

- Ji­uwuHitech (China)

- CTI (China)

The fol­low­ing num­bers give the breakup of man­u­fac­tur­ing cost.

- Labour – 7%

- Raw ma­te­rial – 75%

- Other man­u­fac­tur­ing cost – 18%

Mar­ket share by ge­og­ra­phy

- North Amer­ica – 22%

- China – 15%

- Europe – 28%

- Ja­pan – 10%

- South East Asia – 10%

- In­dia < 5%

- Rest of world < 10%

Mar­ket share by ap­pli­ca­tion:

- Biotech­nol­ogy and Phar­ma­ceu­ti­cals – 27% - Chem­i­cal In­dus­try – 28%

- Food and Bev­er­age – 17%

- Waste and Waste Water Treat­ment – 24%

- Oth­ers – 4%

Ap­pli­ca­tions and pro­cesses:

- Biotech­nol­ogy and Phar­ma­ceu­ti­cals

- Ex­trac­tion clar­i­fi­ca­tion of en­zymes

- Pro­duc­tion of lac­tic bac­te­ria

- Ex­trac­tion/clar­i­fi­ca­tion of An­tibi­otics - Ex­trac­tion/clar­i­fi­ca­tion of amino acids - Py­ro­gen re­moval from water

- Con­cen­tra­tion pu­rifi­ca­tion of whole cell vac­cine Food and bev­er­age:

- Glu­cose syrup clar­i­fi­ca­tion

- Whey con­cen­tra­tion

- Milk con­cen­tra­tion and bac­te­ria re­moval

- Milk pro­tein stan­dard­iza­tion

- Wine/juice clar­i­fi­ca­tion

- Plant ex­tract/tea/cof­fee clar­i­fi­ca­tion En­vi­ron­ment:

- Treat­ment of coat­ing ef­flu­ents

- De­greas­ing bath treat­ment

- Print­ing ef­flu­ent treat­ment

- Dye pen­e­tra­tion test ef­flu­ent treat­ment

- Iron re­moval from water

Chem­i­cal in­dus­try:

- Strong cor­ro­sive acid/al­kali sep­a­ra­tion at high tem­per­a­ture or pres­sure

- Salt chem­i­cals, petro­chem­i­cals, fine chem­i­cals, chloro-al­kali in­dus­try.

Future prospects of ce­ramic mem­branes

The ce­ramic mem­brane mar­ket is pro­jected to reach a mar­ket size of 5.1 bil­lion USD by 2020. It has recorded a sig­nif­i­cant an­nual growth of ap­prox­i­mately 11% on an av­er­age over last few years.The Asia–Pa­cific re­gion will be the big­gest mar­ket for ce­ramic mem­branes due to emerg­ing economies like In­dia, Ja­pan and China (ma­jor fo­cus on China). Waste water treat­ment is es­ti­mated to be the largest mar­ket for ce­ramic mem­branes. This is due to ris­ing is­sue of water scarcity and the grow­ing de­mand for clean water. In In­dia, var­i­ous lead­ing in­sti­tutes like ICT, CGCSMRI Kolkata, IIT’s, and CSMRI Bhav­na­gar are al­ready work­ing to de­velop in­dige­nous ce­ramic mem­branes.

Fig­ure 1. Size ex­clu­sion chart for var­i­ous mem­brane fil­tra­tion pro­cesses

Fig­ure 2. Schematic of cross flow fil­tra­tion and dead end fil­tra­tion

Fig­ure 3. Clas­si­fi­ca­tion of mem­brane ma­te­ri­als

Fig­ure 7(a): Ce­ramic mem­brane ge­om­e­try; (b): Ce­ramic mem­branes in hous­ings

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