Practical Aspects of Emulsion Stability - American Chemical Society

2

Downloaded by NORTH CAROLINA STATE UNIV on December 7, 2012 | http://pubs.acs.org Publication Date: May 5, 1992 | doi: 10.1021/ba-1992-0231.ch002

Practical Aspects of Emulsion Stability Ε . E . Isaacs and R. S. Chow Alberta Research Council, Oil Sands and Hydrocarbon Recovery, Edmonton, Alberta, Canada T6H 5X2

The formations of emulsions by droplet breakup mechanisms is de scribed in relation to the rheological properties of the dispersed and continuous phases. Once emulsions are formed, their stability is largely determined by the molecular, electric double-layer, steric, and hydrodynamic forces. The application of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory to successfully predict the stability of oil-in-water emulsions is described. The stability of waterin-crude-oil emulsions can be monitored with electrokinetic sonic analysis; the change in size of the water droplets is indicated by the change in the ultrasound vibration potential signal. With this devel opment, the dewatering characteristics of chemical demulsifiers can be assessed rapidly. For water dispersed in conventional crude oil, a combination of oil-soluble and water-soluble demulsifiers gave the best results.

Emulsion Formation T h e r m o d y n a m i c C o n c e p t s . G e n e r a l l y , t w o m e t h o d s are u s e d to p r e p a r e dispersions, n a m e l y , b u i l d i n g u p particles f r o m m o l e c u l a r units (nucleation a n d growth) a n d s u b d i v i s i o n o f larger b u l k materials i n t o s m a l l e r units ( g r i n d i n g o r e m u l s i f i c a t i o n ) . T h e process o f e m u l s i f i c a t i o n , that is, d i s p e r s i o n o f l i q u i d s i n l i q u i d s , is g o v e r n e d b y the surface forces. T h e free energy o f f o r m a t i o n o f droplets f r o m a b u l k l i q u i d ( A G ) is i l l u s t r a t e d i n F i g u r e 1 a n d is given b y f o r m

aG

form

= AAy -TàS m

conÎ

0065-2393/92/0231-0051 $07.75/0 © 1992 American Chemical Society

In Emulsions; Schramm, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

(1)

52

EMULSIONS IN THE PETROLEUM INDUSTRY

Ο 1 ° ο Ο ο ο ο Ο ο

Dispersion


Flocculation and coalescence Figure 1. Schematic representation of emulsion formation (Adapted from reference 1.)

and breakdown,

w h e r e Δ Α is t h e increase i n i n t e r f a c i a l area, 7 is t h e i n t e r f a c i a l t e n s i o n between the two liquids, and T A S is t h e e n t r o p y c o n t r i b u t i o n r e s u l t i n g f r o m t h e increase i n c o n f i g u r a t i o n a l e n t r o p y w h e n a large n u m b e r o f d r o p lets is f o r m e d . U s u a l l y Δ Α 7 » T A S , a n d h e n c e e m u l s i f i c a t i o n is a n o n spontaneous process. H o w e v e r , t h e energy r e q u i r e d f o r e m u l s i f i c a t i o n is orders o f m a g n i t u d e larger t h a n t h e t h e r m o d y n a m i c energy ( Δ Α 7 ) f o r c r e a t i n g a n e w surface. T h i s larger energy r e q u i r e m e n t results f r o m t h e a d d i t i o n a l effect o f c r e a t i n g a c u r v e d interface w i t h a d i f f e r e n t r a d i u s . T h e a d d i t i o n a l energy r e q u i r e d c a n b e expressed i n t h e Y o u n g - L a p l a c e e q u a t i o n , 1 2

c o n f

1 2

c o n f

1 2

ΔΡ =

Ύ ΐ :

1 1 — + — η

(2)

r

2

w h e r e Δ Ρ is t h e L a p l a c e pressure d i f f e r e n c e a n d r a n d r are t h e p r i n c i p a l r a d i i o f c u r v a t u r e . T h e presence o f surfactant ( w h i c h lowers 7 ) lowers t h e energy r e q u i r e d f o r e m u l s i f i c a t i o n . x

2

12

Droplet Breakup. A s described b y the Young-Laplace equation, b e f o r e c r e a t i o n o f a n e w surface c a n take p l a c e , d e f o r m a t i o n o f the d i s p e r s e d phase is r e q u i r e d . I n l a m i n a r flow, this d e f o r m a t i o n is p r o d u c e d b y viscous forces exerted b y t h e s u r r o u n d i n g b u l k l i q u i d , that is, ^ ( d u / c b ) , w h e r e % is the viscosity o f the c o n t i n u o u s phase a n d dv/dz is t h e v e l o c i t y gradient. T h e energy i n p u t to create t h e necessary v e l o c i t y gradient c a n b e o f the o r d e r o f 1 0 times t h e t h e r m o d y n a m i c e n e r g y (ày ). T h e excess energy is d i s s i p a t e d as heat. 3

l2

T h e p h e n o m e n o n o f d r o p l e t b r e a k u p is o f great i m p o r t a n c e i n t h e p r e p a r a t i o n o f e m u l s i o n s . I f a stream o f l i q u i d is i n j e c t e d w i t h little t u r b u l e n c e i n t o another l i q u i d w i t h w h i c h i t is i m m i s c i b l e , t h e c y l i n d e r that m a y f o r m is unstable, breaks d o w n i n several spots, a n d breaks u p i n t o d r o p l e t s ( F i g u r e 2a). I f t h e i n j e c t i o n rate is s u c h as to p r o d u c e t u r b u l e n c e , t h e d i s r u p t i o n is faster, a n d m a n y s m a l l e r droplets are p r o d u c e d ( F i g u r e 2 b ) . I f i n a d d i t i o n t h e l i q u i d i m p i n g e s against a surface, m a n y s m a l l e r d r o p l e t s w i l l be f o r m e d .



2.

ISAACS AND CHOW

Practical Aspects of Emulsion

Stability

53

Ο ο

a) Low turbulence

On

b) High turbulence Θ ο Ο

Figure 2. Schematic of the breakup of a liquid stream injected into another immiscible liquid. (Reproduced with permission from reference 2. Copyright 1983 Dekker.) I n actual situations several processes o c c u r s i m u l t a n e o u s l y . T h e details o f any p a r t i c u l a r d i s p e r s i o n processes are also a f f e c t e d b y the viscosity o f each phase, the shear i n the system, the i n t e r f a c i a l energy, the pressure o f s o l i d particles, a n d d i s s o l v e d substances. I n n o n u n i f o r m shear flow (e.g., t u b u l a r P o i s e u i l l e flow), f o r example, d r o p l e t b r e a k u p can b e r e l a t e d t o the b u l k r h e o l o g i c a l p r o p e r t i e s o f the d i s p e r s e d a n d c o n t i n u o u s phases a n d the c r i t i c a l W e b e r n u m b e r ( W e ) as s h o w n i n F i g u r e 3 (3). T h e W e is a d i m e n sional g r o u p d e f i n e d b y c

W e = ^ £ Ί


(3)

54


Ύ12


y Droplet breakup

-

No droplet breakup

1

10 "

1

> I 1

III

10 -3

4

1

1 1 1 1 I III

ΙΟ"

2

1

1

,

l-.l

1

III!

1

t

f

1

ι 1 1n l

1

1

t i l l

101

10°

10-1

Figure 3. Droplet breakup as a function of viscosity ratio depicted schemati cally from the work of Chin and Han (3). The solid line represents the critical Weber number value above which droplet breakup will occur. w h e r e y is t h e rate o f extension (shear rate m u l t i p l i e d b y d e f o r m a t i o n p a r a m e t e r ) ; R is the radius o f the p a r t i c l e ; a n d η a n d η are the viscosities o f the c o n t i n u o u s a n d d i s p e r s e d phases, respectively. A t a g i v e n Ύ] /η , l o w e r i n g y 12 (through t h e use o f surfactants) lowers t h e energy ( d e s c r i b e d b y the W e ) r e q u i r e d f o r d r o p l e t b r e a k u p . F i g u r e 3 shows that t h e greater the viscosity ratio (171/172), the easier it is to f o r m the e m u l s i o n . T h i s c o n c e p t provides a n explanation f o r t h e observations that i n h e a v y - o i l reservoirs (high % / % ratio), w a t e r - i n - o i l e m u l s i o n s are p r o d u c e d i n p r e f e r e n c e to o i l - i n - w a t e r emulsions. e

1

2

ι

2

c

Stability in Oil-in-Water Emulsions Stable e m u l s i o n s often f o r m d u r i n g i n d u s t r i a l p r o c e s s i n g . O n t h e m i c r o scopic scale, t h e reasons that t h e droplets r e m a i n d i s p e r s e d f a l l i n t o t w o b r o a d categories: (1) p h y s i c a l barriers to coalescence a n d (2) e l e c t r i c a l r e p u l s i o n b e t w e e n d r o p l e t s . A n e x a m p l e o f a p h y s i c a l b a r r i e r is t h e p r e s e n c e o f finely d i v i d e d solids at t h e o i l - w a t e r interface. O f p r i m a r y c o n c e r n , h o w ever, is t h e c o n s i d e r a t i o n o f e l e c t r i c a l forces because t h e i r i n f l u e n c e is significant at relatively l o n g e r distances. E l e c t r i c a l r e p u l s i v e forces arise


2.

ISAACS AND CHOW


Stability

55

w h e n the d o u b l e layers s u r r o u n d i n g c h a r g e d droplets o v e r l a p , a n d thus the " t h i c k n e s s " o f t h e d o u b l e layer i n r e l a t i o n to the size o f t h e p a r t i c l e is an important parameter.


A p p l i c a t i o n o f D L V O T h e o r y . S o m e o f the concepts a n d expres sions o f D e r j a g u i n , L a n d a u , V e r w e y , a n d O v e r b e e k ( D L V O ) t h e o r y o f c o l l o i d stability have b e e n d e s c r i b e d i n C h a p t e r 1, o r c a n b e f o u n d i n m a n y d i f f e r e n t textbooks (4, 5). T h e a p p l i c a t i o n o f D L V O t h e o r y to o i l - i n - w a t e r colloids w i t h s p e c i a l r e f e r e n c e to the stability o f b i t u m e n - i n - w a t e r e m u l s i o n s w i l l b e discussed h e r e . Theoretical

T h e basic c o n c e p t o f D L V O t h e o r y is that the

Aspects.

stability o f a c o l l o i d c a n b e d e s c r i b e d i n terms o f the r e p u l s i v e a n d attractive interactions b e t w e e n d r o p l e t s : V (/ ) = V ( / ) + V (/ ) lot

l

rep

l

attr

(4)

i

where V , V , and V are the total, r e p u l s i v e , a n d attractive interactions, respectively; a n d h is the separation distance b e t w e e n the particles. P r e v i ously, w h e n D L V O t h e o r y was a p p l i e d to b i t u m e n - i n - w a t e r e m u l s i o n s (6), it was assumed that t h e r e p u l s i v e i n t e r a c t i o n o r i g i n a t i n g f r o m t h e o v e r l a p p i n g d o u b l e layers s u r r o u n d i n g t w o droplets o f radius a a n d c o u l d b e expressed by: t o t

r e p

a t t r

V^h)

=

M

™

n

f - l \ ^ { - K h )

(5)

and 7 = tan

h

ζβζ

(6)

Âkf.

(7)

ekT

w h e r e n is the n u m b e r o f ions p e r u n i t v o l u m e , k is the B o l t z m a n n constant, Γ is the absolute t e m p e r a t u r e , z is t h e v a l e n c y o f the i o n , e is t h e e l e c t r o n i c charge, e is t h e p e r m i t t i v i t y o f the c o n t i n u o u s phase, ζ is t h e zeta p o t e n t i a l o f the d r o p l e t , a n d κ is the D e b y e - H i i c k e l f u n c t i o n that characterizes t h e extension o f t h e d o u b l e layer. T h e o r i g i n o f t h e attractive i n t e r a c t i o n was a s s u m e d to b e t h e v a n d e r W a a l s o r d i s p e r s i o n force, w h i c h can b e expressed as 0

V

a t t r

(/l) =

-Aa

Λ

1

5.32/i In λ

1 +

5.32/i


(8)

56


w h e r e A is t h e H a m a k e r constant that c a n b e c a l c u l a t e d f r o m L i f s h i t z t h e o r y (7, 8) a n d λ is t h e L o n d o n w a v e l e n g t h o f r o u g h l y 100 n m (9). T h e H a m a k e r constant u s e d i n t h e study was 1.7 Χ 1 0 " J (6).


20

Determination of the Electrophoretic Mobility, T o evaluate t h e e q u a t i o n f o r t h e d o u b l e - l a y e r i n t e r a c t i o n ( e q 5), t h e zeta p o t e n t i a l , ζ, must be k n o w n ; it is c a l c u l a t e d f r o m t h e e x p e r i m e n t a l l y m e a s u r e d e l e c t r o p h o r e t i c m o b i l i t y . F o r e m u l s i o n s , the most c o m m o n t e c h n i q u e u s e d is p a r t i c l e elec trophoresis, w h i c h is s h o w n s c h e m a t i c a l l y i n F i g u r e 4 . I n this t e c h n i q u e t h e e m u l s i o n d r o p l e t is subjected t o a n electric field. I f t h e d r o p l e t possesses i n t e r f a c i a l charge, i t w i l l migrate w i t h a v e l o c i t y that is p r o p o r t i o n a l to t h e m a g n i t u d e o f that charge. T h e v e l o c i t y d i v i d e d b y the strength o f the e l e c t r i c field is k n o w n as t h e e l e c t r o p h o r e t i c m o b i l i t y . M o b i l i t i e s are generally deter m i n e d as a f u n c t i o n o f electrolyte c o n c e n t r a t i o n o r as a f u n c t i o n o f s o l u t i o n pH. M a n y d i f f e r e n t c o m m e r c i a l i n s t r u m e n t s c a n b e u s e d for p a r t i c l e e l e c t r o phoresis. T h e y c a n generally b e d i v i d e d i n t o t w o categories: (1) those i n w h i c h t h e v e l o c i t y o f t h e p a r t i c l e is d e t e r m i n e d b y o b s e r v i n g t h e p a r t i c l e t h r o u g h a m i c r o s c o p e a n d d e t e r m i n i n g t h e v e l o c i t y b y t i m i n g the m o v e m e n t over a g r i d o f k n o w n d i m e n s i o n , a n d (2) those that d e t e r m i n e the v e l o c i t y b y the D o p p l e r shift o f scattered r a d i a t i o n f r o m t h e m o v i n g p a r t i c l e . I n s t r u ments i n t h e s e c o n d category have t h e advantage o f m e a s u r i n g t h e v e l o c i t y o f a large n u m b e r o f particles i n a short t i m e , b u t they are generally m o r e expensive.

ι

®

1

U: Electrophoretic Mobility, fiB / J L sec/ cm Figure 4. Schematic diagram of particle

electrophoresis.

Calculation of the Zeta Potential. T h e conversion o f electropho retic m o b i l i t y t o zeta p o t e n t i a l is c o m p l i c a t e d somewhat b y the existence o f the e l e c t r o p h o r e t i c relaxation effect. F i g u r e 5 shows a schematic d i a g r a m o f this effect. A s w e expose a n e m u l s i o n d r o p l e t a n d its s u r r o u n d i n g d o u b l e layer to an e l e c t r i c field, t h e d o u b l e layer distorts to t h e shape s h o w n i n t h e figure. T h i s d i s t o r t e d d o u b l e layer n o w creates its o w n electric field that


2.

ISAACS AND CHOW


57

Stability


Ê

Counterion Cloud Figure 5. A schematic diagram of the electrophoretic relaxation effect. The distorted ion cloud around the particle generates its own electric field that opposes the motion of the particle.

opposes t h e m o t i o n o f t h e p a r t i c l e . T h e m a g n i t u d e o f t h e o p p o s i t i o n is r e l a t e d to t h e thickness o f t h e d o u b l e layer i n r e l a t i o n to t h e size o f t h e p a r t i c l e . T h i s r e l a t i o n s h i p c a n b e expressed b y t h e p a r a m e t e r κα, w h e r e κ is as d e f i n e d f o r e q 7 a n d a is t h e radius o f t h e p a r t i c l e . I n t h e l i m i t s o f s m a l l κα (κα « 1) a n d large κα (κα » 3 0 0 ) , s i m p l e equations c a n convert t h e e l e c t r o p h o r e t i c m o b i l i t y , U, to zeta p o t e n t i a l . T h e s e are the H u c k e l a n d S m o l u c h o w s k i equations, respectively. T h e y m a y be expressed b y H u c k e l e q u a t i o n (κα « 1) S m o l u c h o w s k i e q u a t i o n (κα

300)

17 = — — 1.5η 17

(9) (10)

V w h e r e η is t h e viscosity o f t h e c o n t i n u o u s phase. T h e l i m i t e d range o f a p p l i c a b i l i t y o f these equations, h o w e v e r , leaves a large area (i.e., 1 » κα » 300) w h e r e t h e extent o f the relaxation effect w o u l d n e e d to b e a c c o u n t e d for. F o r example, i n a s o l u t i o n c o n t a i n i n g 0.01 M u n i v a l e n t electrolyte, κ = 3.31 Χ 10" m " , a n d f o r Ι.Ο-μ-m radius d r o p l e t s , κα = 330, a situation just barely covered b y the Smoluchowski equation. 8

1

O J B r i e n a n d W h i t e (JO), t a k i n g i n t o account t h e relaxation effect, d e r i v e d t h e r e l a t i o n s h i p b e t w e e n e l e c t r o p h o r e t i c m o b i l i t y a n d zeta p o t e n t i a l . T h e results o f the t h e o r e t i c a l c a l c u l a t i o n are s h o w n i n F i g u r e 6. F i g u r e 6 shows t h e r e l a t i o n s h i p b e t w e e n e l e c t r o p h o r e t i c m o b i l i t y , 17, a n d zeta p o t e n t i a l f o r t w o values o f κα, 114 a n d 2 8 5 . E a c h value o f κα has its o w n r e l a t i o n -


58



Ka=285

50

150 -C.mV

250

Figure 6. An example of the relationship between electrophoretic mobility zeta potential.

and

ship b e t w e e n zeta p o t e n t i a l a n d e l e c t r o p h o r e t i c m o b i l i t y ; p r a c t i c a l l y this means that k n o w i n g b o t h electrolyte c o n c e n t r a t i o n a n d p a r t i c l e size is vital. I n a d d i t i o n , the obvious m a x i m u m i n m o b i l i t y makes it d i f f i c u l t to d e t e r m i n e a u n i q u e value o f zeta p o t e n t i a l w h e n the m e a s u r e d m o b i l i t y is close to the m a x i m u m v a l u e . T h e c o m b i n a t i o n o f these t w o factors c a n make it d i f f i c u l t to d e t e r m i n e a zeta p o t e n t i a l w h e n d e a l i n g w i t h e m u l s i o n s that c o m m o n l y possess a w i d e d i s t r i b u t i o n o f p a r t i c l e sizes. W e w i l l assume that w e are d e a l i n g w i t h an e m u l s i o n that is p o l y d i s p e r s e a n d has p a r t i c l e that range i n radius f r o m 2.0 to 5.0 μπι. T h e electrophoresis measurements are b e i n g c o n d u c t e d i n a s o l u t i o n c o n t a i n i n g 3.0 Χ 1 0 M N a C l . T h e value for κα i n these situations is 114 for a 2-μτη d r o p l e t a n d 285 for a 5-μπι d r o p l e t . I f the m e a n e l e c t r o p h o r e t i c m o b i l i t y m e a s u r e d was 9.9 μηι/s'cm/V w i t h a v a r i a t i o n o f ± 5 . 0 % , w i t h o u t k n o w l e d g e o f the size o f the droplets (almost a certainty because o f the d a r k field i l l u m i n a t i o n u s e d i n most p a r t i c l e electrophoresis apparatus), the zeta p o t e n t i a l c o u l d range f r o m - 1 1 5 to - 2 0 0 m V . -4

I n r e f e r e n c e to the example o f the stability o f b i t u m e n - i n - w a t e r e m u l sions, p r o d u c t i o n samples w e r e o b t a i n e d f r o m the A l b e r t a R e s e a r c h C o u n -


2.

ISAACS AND CHOW


cil's 1 5 0 - c m p h y s i c a l s i m u l a t o r . T h e s e w e r e a c q u i r e d as steam i n j e c t i o n experiments w i t h the s i m u l a t o r w e r e b e i n g c o n d u c t e d o n A t h a b a s c a o i l sands. T h e e m u l s i o n was d i l u t e d a n d e l e c t r o p h o r e t i c m o b i l i t i e s w e r e deter m i n e d w i t h a R a n k B r o t h e r s M k l l electrophoresis apparatus, i n w h i c h the v e l o c i t y is d e t e r m i n e d b y t i m i n g the m o v e m e n t o f the particles. M e a s u r e ments w e r e c o n d u c t e d as a f u n c t i o n o f electrolyte c o n c e n t r a t i o n i n the p r e s e n c e o f N a C l , C a C l , a n d A l ( S 0 ) . M o b i l i t i e s w e r e c o n v e r t e d to zeta potentials b y u s i n g the S m o l u c h o w s k i e q u a t i o n (eq 10) a n d are s h o w n i n F i g u r e 7. A l l curves s h o w s i m i l a r trends i n the zeta p o t e n t i a l i n that the p o t e n t i a l slowly b e c o m e s m o r e electronegative w i t h i n c r e a s i n g electrolyte c o n c e n t r a t i o n a n d t h e n slowly decreases towards zero. T h e existence o f this m i n i m u m has b e e n r e p o r t e d p r e v i o u s l y a n d is not e x p e c t e d f r o m d o u b l e layer theory. C u r r e n t l y this is an area o f research (11, 12). T h e b i t u m e n droplets also s h o w r e d u c e d zeta p o t e n t i a l i n the p r e s e n c e o f either C a or A l ion. 2


59

Stability

2

4

3

2 +

3 +

Interpretation of the Energy Diagrams. O n c e the values o f zeta p o t e n t i a l are d e t e r m i n e d , e q 9 c a n be evaluated. T h e results o b t a i n e d are s u m m e d to those o f the v a n d e r W a a l s i n t e r a c t i o n (eq 8) a n d p l o t t e d as a p o t e n t i a l energy o f i n t e r a c t i o n as a f u n c t i o n o f separation distance b e t w e e n the d r o p l e t s . T h e energy is o f t e n expressed i n terms o f kT units, i n o r d e r to b e t t e r relate the energy o f the i n t e r a c t i o n to the t h e r m a l energy o f B r o w n i a n

10-4

10-3

10-2

10-

1

Electr. cone, M Figure 7. The zeta potentials for bitumen-in-water emulsions as a function of electrolyte concentration. (Reproduced with permission from reference 6. Copyright 1982,)


60


m o t i o n . D L V O t h e o r y p r e d i c t s that i f an energy o f m o r e t h a n 15fcT exists i n the energy d i a g r a m , u n d e r the c o n d i t i o n s o f no flow, the e m u l s i o n w o u l d be stable. T h i s stability is d u e to the s m a l l p r o b a b i l i t y that one d r o p l e t w o u l d possess this m u c h energy. F i g u r e 8 shows the c a l c u l a t e d energy diagrams for the b i t u m e n - i n - w a t e r e m u l s i o n s as expressed i n e q 4. T h e figure shows that i n solutions c o n t a i n i n g 20 m M C a C l there is a net negative energy o f i n t e r a c t i o n at a l l separation distances; that is, as b i t u m e n d r o p l e t s meet, they w i l l coagulate a n d coalesce. I f the droplets are i m m e r s e d i n a s o l u t i o n o f 10 m M C a C l , an energy m a x i m u m o f SkT is s h o w n at 3.5 n m . T h i s e n e r g y " b a r r i e r " is not o f sufficient h e i g h t to result i n a stable e m u l s i o n b u t w i l l s l o w the rate o f c o a g u l a t i o n . T h e p r e d i c t i o n for 3 m M C a C l is a stable e m u l s i o n as a large positive energy is e x p e r i e n c e d at 13 n m . I n solutions c o n t a i n i n g N a C l , the energy diagrams show that the e m u l s i o n w i l l b e stable i n solutions c o n t a i n i n g 10 m M N a C l . A s the c o n c e n t r a t i o n is i n c r e a s e d , h o w e v e r , the large positive energy ap pears to shift to shorter distances, b u t the m o r e i n t e r e s t i n g feature is the appearance o f a energy m i n i m u m . W i t h 300 m M N a C l , the m i n i m u m is o f sufficient d e p t h ( ~ l l £ T ) that droplets c o u l d b e c o m e closely associated b u t w o u l d still r e m a i n 3 - 4 n m apart. I n this situation the e m u l s i o n w o u l d appear to flocculate b u t c o u l d be easily r e d i s p e r s e d b y m e c h a n i c a l agitation. 2


2

2

T o c o n f i r m the a p p l i c a b i l i t y o f D L V O theory, c o a g u l a t i o n tests w e r e p e r f o r m e d o n the p r o d u c t i o n samples. T h e results, s h o w n p h o t o g r a p h i c a l l y

0

20

0

20

h, nm Figure 8. The energy diagram for bitumen-in-water emulsions in the presence of NaCl and CaCl . In the presence of 300 mM NaCl the emulsion should flocculate, and in the presence of the 20 mM CaCh the emulsion will coagulate. (Reproduced with permission from reference 6. Copyright 1982.) 2


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i n F i g u r e 9, i n d i c a t e r a p i d c o a g u l a t i o n i n a 20 m M s o l u t i o n o f C a C l a n d fiocculation a n d c r e a m i n g i n the p r e s e n c e o f 300 m M N a C l . T h e s e results c o n f i r m the a p p l i c a b i l i t y o f D L V O t h e o r y f o r b i t u m e n - i n - w a t e r e m u l s i o n s .


2

Ionizable S u r f a c e - G r o u p M o d e l . O n e o f the m e c h a n i s m s to d e v e l o p i n t e r f a c i a l charge is the i o n i z a t i o n o f f u n c t i o n a l groups o n the surface o f the d i s p e r s e d phase. H e a l y a n d W h i t e (13) d e v e l o p e d this c o n c e p t i n t o a m o d e l to p r e d i c t the e l e c t r i c p r o p e r t i e s o f interfaces. W h e n this m o d e l was a p p l i e d to the b i t u m e n - w a t e r interface, it was assumed that the surface charge o r i g i n a t e d f r o m the dissociation o f carboxy g r o u p that b e l o n g to n a t u r a l surfactants p r e s e n t i n the o i l (14). T h i s w o r k was e x t e n d e d i n a later study (15) w h e r e the zeta potentials as c a l c u l a t e d b y the m o d e l w e r e u s e d i n c o n j u n c t i o n w i t h D L V O t h e o r y to p r e d i c t the stability o f the b i t u m e n - i n water e m u l s i o n s . A g r e e m e n t b e t w e e n the e x p e c t e d a n d observations f r o m coagulation tests was excellent over a w i d e range o f s o l u t i o n p H a n d e l e c t r o lyte c o n c e n t r a t i o n . T h e m o d e l can also be a p p l i e d to the c o n v e n t i o n a l c r u d e - o i l - w a t e r interface (16, 17). T h o s e studies d e m o n s t r a t e d that it was necessary to i n v o k e the dissociation o f u p to t h r e e d i f f e r e n t types o f f u n c t i o n a l groups to successfully use the m o d e l . A n o t h e r i n t e r e s t i n g finding was that the e l e c t r o -

Figure 9. The behavior of bitumen-in-water emulsions in the presence of 300 mM NaCl (right side) and 20 mM CaCl . The emulsions are behaving as predicted by DLVO theory. 2


62


phoretie mobility with p H curve for conventional crude-oil-in-water e m u l sions w o u l d change w i t h t i m e after d i s p e r s i o n . W h e n a p p l y i n g t h e m o d e l , this aging process was t h o u g h t to b e d u e to c e r t a i n f u n c t i o n a l groups l e a v i n g the interface.


Non-DLVO Forces. A l t h o u g h D L V O theory w o r k e d v e r y w e l l f o r the e l e c t r o l y t e - i n d u c e d coagulation o f b i t u m e n - i n - w a t e r e m u l s i o n s , i t c a n not b e a p p l i e d i n some cases. Polymer-Induced Flocculation. Polymer-induced flocculation is the most c o m m o n l y u s e d t e c h n i q u e f o r b r e a k i n g w a t e r - i n - o i l e m u l s i o n s i n the p e t r o l e u m i n d u s t r y . T h i s t o p i c w i l l b e c o v e r e d i n m u c h m o r e d e t a i l i n C h a p t e r 9, b u t w i l l b e b r i e f l y c o v e r e d i n this section. F o r the p o l y m e r to b e effective, i t must adsorb to t h e interface a n d m a i n t a i n a c e r t a i n c o n f i g u r a t i o n . T h u s t h e f o l l o w i n g d i s c u s s i o n describes various e x p e r i m e n t a l techniques u s e d f o r the study o f a d s o r p t i o n density a n d c o n f i g u r a t i o n o f p o l y m e r at t h e interface. A f t e r a d s o r p t i o n occurs, t h e m a i n m e c h a n i s m s o f flocculation are d u e to t h e a d s o r p t i o n o f a single p o l y m e r m o l e c u l e o n separate particles, i n t e r a c t i o n t h r o u g h t h e interpénétration o f a d s o r b e d p o l y m e r , a n d interactions d u e to t h e loss o f f r e e d o m o f m o v e m e n t o f the p o l y m e r chains. Experimental Techniques for the Study of Polymer Adsorption. T h e theory o f p o l y m e r a d s o r p t i o n a n d c o n f i g u r a t i o n is still n o t f u l l y d e v e l o p e d because t h e difficulties e n c o u n t e r e d i n d e s i g n i n g e x p e r i m e n t s are i m m e n s e . A t e c h n i q u e is r e q u i r e d to m e a s u r e t h e c o n f i g u r a t i o n o f a p o l y m e r m o l e c u l e at t h e interface a n d thus o b t a i n c o n c e n t r a t i o n also. A l t h o u g h t h e c o n f i g u r a t i o n o f t h e a d s o r b e d p o l y m e r cannot b e seen d i r e c t l y , some e x p e r i m e n t a l t e c h n i q u e s can give an i d e a o f the c o n c e n t r a t i o n a n d c o n f i g u r a t i o n . F o r example, b o u n d p o l y m e r c o n c e n t r a t i o n c a n b e deter m i n e d t h r o u g h m e a s u r i n g t h e c o n c e n t r a t i o n o f p o l y m e r i n s o l u t i o n after t h e i n t r o d u c t i o n o f a n e m u l s i o n o f k n o w n surface area. V a r i o u s spectroscopic t e c h n i q u e s ( i n f r a r e d , e l e c t r o n s p i n resonance, n u c l e a r magnetic resonance) m a y d i s t i n g u i s h t h e loss o f rotational a n d translational f r e e d o m w h e n a p o l y m e r segment adsorbs onto a n interface, a n d thus the a m o u n t o f b o u n d segments o f a p o l y m e r ean b e estimated. A d s o r b e d layer thickness c a n b e i n d i r e c t l y o b t a i n e d b y e i t h e r d e t e r m i n i n g t h e increase o f h y d r o d y n a m i c radius o f the droplets o r b y m e a s u r i n g t h e d i f f u s i o n coefficient o f the e m u l sion droplets. T h i s i n f o r m a t i o n c a n also b e o b t a i n e d b y e l l i p s o m e t r y , assum i n g sufficient refractive i n d e x d i f f e r e n c e b e t w e e n t h e d r o p l e t , a d s o r b e d layer, a n d b u l k s o l u t i o n . Interaction of Droplets with Adsorbed simplest case o f p o l y m e r - i n d u c e d flocculation,

Polymer Layers. I n the a single p o l y m e r m o l e c u l e


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adsorbs onto two separate d r o p l e t s , a n d the result is flocculation. T h i s type o f situation is f a v o r e d b y l o w coverage o f p o l y m e r o n the surfaces a n d the c o n t i n u o u s phase b e i n g a relatively g o o d solvent. I f these c o n d i t i o n s are met, it is possible f o r the p o l y m e r to adsorb to the surface a n d r e m a i n e x t e n d e d i n t o the s o l u t i o n i n o r d e r to attach to another d r o p l e t . As two p o l y m e r - c o a t e d droplets a p p r o a c h each other, the a d s o r b e d layers w i l l b e g i n to interact. T h e extent o f the i n t e r a c t i o n c a n be i n t e r p r e t e d t h r o u g h the free energy o f t w o d i f f e r e n t t e r m s , the interpénétration t e r m and the m i x i n g t e r m . T h e interpénétration t e r m is r e p u l s i v e a n d e n t r o p i e i n nature; that is, the loss o f f r e e d o m o f m o v e m e n t as the layers i n t e r p e n e t r a t e results i n a r e p u l s i o n t e r m . T h i s r e p u l s i o n t e r m c a n be o v e r c o m e , h o w e v e r , b y a l t e r i n g the m i x i n g t e r m , w h i c h can be r e p u l s i v e o r attractive i n nature. T h e m i x i n g t e r m has b e e n h a n d l e d the same as f o r the d i l u t i o n o f p o l y m e r i n b u l k s o l u t i o n . T h e m a g n i t u d e o f the t e r m is d e p e n d e n t u p o n the n u m b e r o f p o l y m e r m o l e c u l e s i n the overlap r e g i o n , v o l u m e fractions o f the solvent a n d p o l y m e r i n this area, a n d a p a r a m e t e r k n o w n as the F l o r y - H u g g i n s interac t i o n p a r a m e t e r that takes i n t o account the e n t h a l p y o f m i x i n g a n d v o l u m e o f m i x i n g effects. B y a l t e r i n g the s o l u t i o n c o n d i t i o n s , a situation c a n be c r e a t e d i n w h i c h the c o n t i n u o u s phase b e c o m e s a p o o r e r solvent f o r the a d s o r b e d p o l y m e r ; flocculation results. T h i s situation c a n b e c r e a t e d b y c h a n g i n g the t e m p e r a t u r e , a d d i n g a p o o r e r solvent to the c o l l o i d , or a d d i n g electrolyte. A s p e c i a l case to c o n s i d e r is the existence o f w h a t is t e r m e d d e p l e t i o n flocculation. T h i s t e r m o r i g i n a t e d f r o m the o b s e r v a t i o n that the a d d i t i o n o f a s m a l l a m o u n t o f n o n a d s o r b i n g p o l y m e r w i l l cause flocculation i n a system. T h e reason for this effect is that, as the particles a p p r o a c h each other, the m o b i l e chains o f n o n a d s o r b i n g p o l y m e r are s q u e e z e d out f r o m b e t w e e n the particles. A s the particles a p p r o a c h to v e r y close distances, almost p u r e solvent exists b e t w e e n the particles, a n d at a g i v e n separation, the o s m o t i c pressure that results f r o m this p u r e solvent drives it out i n t o the b u l k s o l u t i o n a n d t h e r e b y causes flocculation. Hydration and Hydrophobic Forces. A s surfaces a p p r o a c h e a c h o t h e r to distances less t h a n 10 n m , a force exists that is not a c c o u n t e d f o r i n c o n v e n t i o n a l D L V O theory. T h i s force c a n be r e p u l s i v e or attractive i n nature a n d c a n be o f m a g n i t u d e greater t h a n e i t h e r the d o u b l e - l a y e r o r v a n d e r W a a l s interactions. T h i s force was d i s c o v e r e d b y I s r a e l a c h i v i l i a n d c o w o r k e r s (18-20) u s i n g a u n i q u e apparatus that they d e v e l o p e d that d i r e c t l y m e a s u r e d the force b e t w e e n t w o m i c a surfaces. F o r s o l i d m i c a surfaces i m m e r s e d i n t o water, the force was r e p u l s i v e a n d oscillatory a n d e x p o n e n tially d e c a y e d as a f u n c t i o n o f distance f r o m the surface (see F i g u r e 10). T h e o r i g i n o f this force is t h o u g h t to be the m o d i f i c a t i o n o f the o r i e n t a t i o n o f w a t e r m o l e c u l e s i n the v i c i n i t y o f a surface to f o r m a s t r u c t u r e . A s surfaces a p p r o a c h each other, a d d i t i o n a l energy is r e q u i r e d to d e c o m p o s e this structure. T h e oscillatory p r o p e r t y o f the force was t h o u g h t to be d u e to


64



10

5

ε

I°

1 4

10

3

0

1

2

3

4

D, nm Figure 10. Schematic representation of the force as a function of distance between two mica surfaces as reported by Israelachwili and Adams (18). the various " l a y e r s " o f w a t e r m o l e c u l e s . A c o m p l e t e t h e o r y o n the o r i g i n a n d quantitative b e h a v i o r o f this force is n o t available. It is a n area o f c u r r e n t research effort {21, 22). A l t h o u g h the p r e d i c t e d b i t u m e n - i n - w a t e r e m u l s i o n stability c a n b e ac c o m p l i s h e d w i t h o u t i n v o k i n g this force, r e c e n t research has s h o w n (23) that this force exists b e t w e e n l i q u i d bilayers i m m e r s e d i n aqueous a n d n o n a q u e ous l i q u i d s . W i t h some types o f oils, i t m a y b e i m p o r t a n t to c o n s i d e r this force.

Thin-Film Stability in Water-in-Oil Emulsions Importance of Thin Films.

I n o r d e r f o r coalescence to result i n

u l t i m a t e separation o f the w a t e r droplets f r o m the c o n t i n u o u s m e d i u m ,



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t h i n n i n g a n d d i s r u p t i o n o f the l i q u i d film (called " t h i n film" p r i o r to r u p ture) b e t w e e n the droplets m u s t take place. A g a i n this process is g o v e r n e d b y the surface forces (van d e r W a a l s , d o u b l e - l a y e r , a n d steric forces) that operate i n the l i q u i d lamellae b e t w e e n the d r o p l e t s . T h e r u p t u r e o f the t h i n film is u s u a l l y the result o f t h e r m a l o r m e c h a n i c a l fluctuation, w h i c h results i n s t r e t c h i n g o f the l i q u i d surface w i t h f o r m a t i o n o f surface waves that w i l l g r o w i n a m p l i t u d e a n d result i n d r o p l e t coalescence. A n y f o r c e that " d a m p e n s " the f o r m a t i o n o f such waves w i l l r e d u c e or p r e v e n t coalescence. I n systems c o n t a i n i n g surfactants, wave d a m p e n i n g occurs as a result o f the soc a l l e d M a r a n g o n i effect, w h i c h arises f r o m the p r e s e n c e o f the surfactant film. I f a film is s u b j e c t e d to l o c a l s t r e t c h i n g as a result o f thickness fluctua t i o n , the c o n s e q u e n t increase i n surface area causes a l o c a l increase i n i n t e r f a c i a l tension (decrease i n the surface excess o f the a d s o r b e d surfaceactive agent i n that region) that opposes the s t r e t c h i n g (see F i g u r e 11). B e c a u s e a finite t i m e is r e q u i r e d for surfactant m o l e c u l e s to diffuse to this r e g i o n o f the interface to restore the o r i g i n a l surface t e n s i o n ( M a r a n g o n i effect), the fluctuations w i l l t e n d to d a m p e n rather than g r o w (i.e., r u p t u r e is prevented).

Persistent Films. T h e stability o f w a t e r - i n - c r u d e - o i l e m u l s i o n s a n d the factors c o n t r i b u t i n g to that stability are l o n g - s t a n d i n g p r o b l e m s o f i m p o r t a n c e i n the p r o d u c t i o n o f o i l f r o m u n d e r g r o u n d reservoirs. A l t h o u g h a great d e a l o f effort has b e e n e x p e n d e d i n the investigation o f the d e s t a b i l i z a t i o n o f w a t e r - i n - o i l e m u l s i o n s , the actual m e c h a n i s m s are still not w e l l u n d e r s t o o d . A l t h o u g h n a t u r a l surfactants p r e s e n t i n the c r u d e o i l c a n i n t h e i r o w n right stabilize the e m u l s i o n s , o t h e r types i n d i g e n o u s m a t e r i a l i n the o i l t e n d to gather at the interface a n d p l a y a significant role i n h i n d e r i n g the t h i n n i n g a n d r u p t u r e o f the l i q u i d films a n d act as a s t r u c t u r a l b a r r i e r to coalescence o f the water droplets. A s p h a l t e n e s a n d p o r p h y r i n i c c o m p o u n d s m a y be the s t a b i l i z i n g agents. F u r t h e r m o r e , the p r e s e n c e o f finely d i v i d e d

Figure 11. Photograph of bitumen drop in aqueous NaOH showing low- and high-tension sites due to periodic diffusion of natural surfactants from the oilwater interface to the aqueous phase in a spinning drop tensiometer.


66



solids s u c h as sand, wax crystals, a n d clay particles c a n stabilize e m u l sions (24). A l l these " n a t u r a l s t a b i l i z i n g a g e n t s " p r e v e n t t h i n n i n g o f the t h i n film a n d e x p l a i n w h y the c r u d e - o i l films are so persistent. A n example o f a persistent film is s h o w n i n F i g u r e 12, w h i c h is a p h o t o m i c r o g r a p h o f the b o t t o m layer o f a w a t e r - i n - o i l ( L e d u c crude) e m u l s i o n treated w i t h d e m u l s i f i e r . E v e n after 3 days, the w a t e r droplets are s t i l l e n v e l o p e d b y a t h i n c r u d e - o i l film that w i l l not d r a i n any f u r t h e r w i t h o u t a d d i t i o n a l treat ment. R e c e n t w o r k (25-27) has s h o w n that surfactants o r m e d i u m - c h a i n alco hols that m o d i f y the r i g i d i t y o f the film, i n c o m b i n a t i o n w i t h d e m u l s i f i e r s ( w h i c h act m a i n l y to flocculate the w a t e r droplets) c a n c o n s i d e r a b l y s p e e d u p the separation process (see the section e n t i t l e d " E f f e c t o f D e m u l s i f i e r Mixture").

Use of Ultrasonic Vibration Potential To Monitor Coalescence. T h e c o m p l e x c h e m i c a l nature o f c r u d e oils makes it d i f f i c u l t to relate the d i s p e r s i o n b e h a v i o r to the p h y s i c o c h e m i c a l p r o p e r t i e s at the c r u d e - o i l - w a t e r interface. I n a d d i t i o n , the n o n p o l a r a n d nontransparent nature o f the o l e i c phase p r o v i d e s significant obstacles for studies o f the interactions o f the s u s p e n d e d w a t e r droplets i n real systems. R e c e n t d e v e l o p m e n t (28, 29) o f electroacoustical t e c h n i q u e s has s h o w n c o n s i d e r a b l e p r o m i s e f o r e l e c t r o k i n e t i c m e a s u r e m e n t s o f c o l l o i d a l systems a n d the d i r e c t m o n i t o r i n g o f the rate a n d extent o f c o a g u l a t i o n (flocculation a n d coales cence) of water droplets i n nontransparent water-in-oil media. T h e electroacoustic m e a s u r e m e n t for c o l l o i d a l systems i n n o n p o l a r m e d i a is based o n the u l t r a s o u n d v i b r a t i o n p o t e n t i a l ( U V P ) m o d e , w h i c h involves the a p p l i c a -

Figure 12. Photomicrograph of settled bottom layer of a water-in-oil emulsion taken 3 days after addition of demulsifier (100 ppm of Duomeen C). (Reproduced with permission from reference 27. Copyright 1990 Elsevier.)


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67

Stability

t i o n o f a sonic field a n d the d e t e c t i o n o f a n e l e c t r i c field. A s c h e m a t i c d i a g r a m o f the p r o b e a n d the p r i n c i p l e o f U V P are r e p r e s e n t e d i n F i g u r e 13. W h e n voltage U is a p p l i e d at the transducer, a s o u n d wave propagates i n t o the c o l l o i d . I f the densities o f the d i s p e r s e d a n d c o n t i n u o u s phases d i f f e r , relative m o t i o n b e t w e e n the c o l l o i d a l particles a n d t h e i r d o u b l e layer w i l l result. T h e c o m b i n e d relative m o t i o n w i l l generate an e l e c t r i c field, w h i c h is d e t e c t e d as voltage U b e t w e e n the electrodes. T h e m e a s u r e d signals are p r o p o r t i o n a l to the h i g h - f r e q u e n c y e l e c t r o p h o r e t i c m o b i l i t y μ(ω). A s d e r i v e d b y B a b c h i n et a l . (28), the f r e q u e n c y - d e p e n d e n t e l e c t r o p h o r e t i c m o b i l i t y , μ(ω), for the case o f l o w potentials, c a n be expressed b y 2


1

μ(ω)

(ID

=• •V(6TTT|R)

2

+

-o)p R ei(

2

with (12a)

δ

Pe!(=Po +

2ηρ 4R

1 =

2R

(12b)

9δ.

w h e r e e, 17, a n d ρ are the d i e l e c t r i c p e r m i t t i v i t y , viscosity, a n d d e n s i t y o f the c o n t i n u o u s phase, respectively; p is the d e n s i t y o f the p a r t i c l e ; p is the effective density o f a sphere i n oscillatory m o t i o n ; δ is the thickness o f a fluid layer s u r r o u n d i n g the p a r t i c l e that influences l i q u i d flow a r o u n d the p a r t i c l e 0

e f f

Figure 13. Schematic diagram showing the principle of UVP. (Reproduced with permission from reference 27. Copyright 1990 Elsevier.)


68


[δ = (2η/ρω) ];

ζ is the e l e c t r o k i n e t i c p o t e n t i a l ; / ( K R ) is the H e n r y f u n c t i o n ;

ΙΛ

κ is the D e b y e - H u c k e l f u n c t i o n that characterizes

the extension o f the

d o u b l e layer; R is the p a r t i c l e r a d i u s ; a n d ω is the f r e q u e n c y . T h e s u p p l e m e n t a r y phase angle φ(ω) b e t w e e n the a p p l i e d e l e c t r i c

field

a n d the p a r t i c l e v e l o c i t y response, at a fixed f r e q u e n c y ω is g i v e n b y ^ φ(ω)


η

(13)

= - ^ & β 9 ηR

T h e m a g n i t u d e o f the p o t e n t i a l d i f f e r e n c e b e t w e e n the electrodes, άΨ , 0

in

the c i r c u i t U is g i v e n b y x

UVPM =

Φ Δ

Ρ^ν ( ) μ

(14)

ω

w h e r e Φ is the v o l u m e f r a c t i o n o f d i s p e r s e d phase, Δ ρ is the d e n s i t y d i f f e r e n c e b e t w e e n the d i s p e r s e d a n d c o n t i n u o u s phases, c is the s o u n d v e l o c i t y i n the e m u l s i o n , K * is the c o m p l e x c o n d u c t i v i t y o f o i l , a n d Gj is a g e o m e t r i c a l factor d e p e n d e n t o n the g e o m e t r y o f the electrodes. E q u a t i o n s 1 1 - 1 4 c l e a r l y s h o w that an increase i n the effective p a r t i c l e r a d i u s , p r o m o t e d b y a c o a g u l a t i o n process, w i l l result i n the d i m i n u t i o n o f the U V P signal a n d a shift i n the phase angle. I n a d d i t i o n , the l o w v a l u e o f the c o m p l e x c o n d u c t i v i t y o f o i l , K * , acts as a n a t u r a l a m p l i f i e r to p r o v i d e f o r a significant Δ Ψ that makes it easy to m o n i t o r U V P e v e n f o r s m a l l values o f 0

μ(ω).

F i g u r e 14 shows that the U V P signal is sensitive to the w a t e r c o n t e n t i n

the e m u l s i o n s . F i g u r e 15 shows the sensitivity o f the U V P signal to the c o a g u l a t i o n process. P h o t o g r a p h s t a k e n at 3 , 1 2 , a n d 24 m i n s h o w that as the d r o p l e t size grows, the U V P signal decreases. T h u s b y m e a s u r i n g the U V P signal, c o a g u lation can be monitored. B y m o n i t o r i n g b o t h the U V P signal a n d phase angle φ, changes i n zeta that effect o n l y the U V P signal c a n be d i s t i n g u i s h e d f r o m changes i n p a r t i c l e radius that e f f e c t b o t h the U V P a n d φ{ω)

Effect of Demulsifier Mixture. m e e n C , w h i c h was effective i n c a u s i n g

(30). I n p r e v i o u s studies (27) flocculation

Duo-

o f the w a t e r d r o p l e t s ,

was not v e r y effective i n b r e a k i n g the i n t e r f a c i a l film f o r m e d b e t w e e n the w a t e r d r o p l e t s , w h i c h i n h i b i t s c o a l e s c e n c e . ( D u o m e e n C is a m i x t u r e o f m a n y types o f surfactants; the g e n e r a l classification is a fatty a c i d ester n i t r o g e n derivative.) H o w e v e r , D u o m e e n C i n c o m b i n a t i o n w i t h docusate s o d i u m ( A e r o s o l O T ) , a h y d r o p h i l i c surfactant, was m u c h m o r e effective i n c a u s i n g w a t e r separation c o m p a r e d to the i n d i v i d u a l c h e m i c a l s . T h i s effect is s h o w n i n F i g u r e 16 f o r a 6 v o l % w a t e r - i n - o i l ( L e d u c c r u d e ) e m u l s i o n i n w h i c h b o t h the U V P signal (20 m i n after c h e m i c a l a d d i t i o n ) a n d the v o l u m e


2.

ISAACS AND CHOW


τ—~

Practical Aspects of Emulsion 1

τ

π

69

Stability 1

1

Γ

Volume of emulsified water, % Figure 14. Sensitivity of UVP signal to water content in the emulsions.

o f w a t e r r e c o v e r e d b y c e n t r i f u g a t i o n are p l o t t e d against the w e i g h t p e r c e n t o f D u o m e e n C i n the m i x t u r e . A s expected, A e r o s o l O T , t h e w a t e r - s o l u b l e surfactant, b y i t s e l f h a d p r a c t i c a l l y n o effect o n e i t h e r t h e U V P signal o r t h e w a t e r separation. D u o m e e n C alone also h a d l i t t l e effect o n the a m o u n t o f w a t e r r e c o v e r e d b y c e n t r i f u g a t i o n . T h e change i n U V P signal t h e r e f o r e , l i k e l y r e f l e c t e d D u o m e e n C ' s a b i l i t y t o flocculate t h e d r o p l e t s . T h e m i x t u r e o f t h e t w o c h e m i c a l s , h o w e v e r , p e r f o r m e d i n a synergistic m a n n e r , a 1:1 m i x t u r e o f c h e m i c a l s b e i n g most effective. A l s o , a d i r e c t c o r r e s p o n d e n c e is apparent b e t w e e n t h e m i n i m u m i n U V P signal a n d m a x i m u m i n w a t e r r e c o v e r y b y centrifugation.

Film Drainage and Demulsifier Adsorption. T o e n h a n c e the coagulation process, a c o m m o n p r a c t i c e is to use c h e m i c a l d e m u l s i f i e r s that are b e l i e v e d to




70

Figure 15. Sensitivity of the UVP signal to the coagulation process. Demulsifier was added at 4 min. (Reproduced with permission from reference 27. Copyright 1990 Elsevier.) 1. p r o m o t e the

floeculation

o f the d r o p l e t s b y w e a k e n i n g the

r e p u l s i v e forces that stabilize the e m u l s i o n 2. e n h a n c e the drainage o f the i n t e r f a c i a l film b e t w e e n the floc culated droplets. T h e c h o i c e o f c h e m i c a l is u s u a l l y b a s e d o n t r i a l - a n d - e r r o r p r o c e d u r e s ; h e n c e , d e m u l s i f i e r t e c h n o l o g y is m o r e o f an art t h a n a science. I n most cases a c o m b i n a t i o n o f c h e m i c a l s is u s e d i n the d e m u l s i f i e r f o r m u l a t i o n to achieve b o t h efficient floeculation a n d coalescence. T h e type o f d e m u l s i f i e r s a n d t h e i r effect o n i n t e r f a c i a l area are a m o n g the i m p o r t a n t factors that i n f l u e n c e the coalescence process. T i m e - d e p e n d e n t i n t e r f a c i a l tensions have b e e n s h o w n to b e sensitive to these factors, a n d the r e l a t i o n b e t w e e n t i m e d e p e n d e n t i n t e r f a c i a l tensions a n d the a d s o r p t i o n o f surfactants at the o i l aqueous interface was c o n s i d e r e d b y a n u m b e r o f researchers (27, 31-36). F r o m studies o f the t i m e - d e p e n d e n t tensions at the interface b e t w e e n or ganic solvents a n d aqueous solutions o f d i f f e r e n t surfactants, Joos a n d co w o r k e r s (33-36) c o n c l u d e d that the a d s o r p t i o n process o f the surfactants at the l i q u i d - l i q u i d interface was not o n l y d i f f u s i o n c o n t r o l l e d b u t that adsorp t i o n barriers a n d surfactant m o l e c u l e r e o r i e n t a t i o n w e r e i m p o r t a n t m e c h a -



2.

ISAACS AND CHOW

OQU Ο


1

1

25

50

71

Stability

* 75

«-J 100

Weight percent of Duomeen C in the mixture Figure 16. Comparison of the coalescence process using mixtures with a total concentration of 100 ppm. nisms d e p e n d i n g o n the system. F o r surfactant a d s o r p t i o n f r o m the o i l phase to t h e o i l - w a t e r interface, a r e o r i e n t a t i o n process at t h e interface was t h o u g h t to b e t h e r a t e - c o n t r o l l i n g step; a d s o r p t i o n o c c u r r e d at a m u c h s l o w e r rate t h a n that o b s e r v e d f o r a p u r e l y d i f f u s i o n - c o n t r o l l e d s i t u a t i o n . V o g l e r (31 ) d e v e l o p e d a m a t h e m a t i c a l m o d e l to d e r i v e semiquantitative k i n e t i c parameters i n t e r p r e t e d i n terms o f transport a n d a d s o r p t i o n o f surfactants at t h e interface. T h e m o d e l was fitted t o e x p e r i m e n t a l t i m e dependent interfacial tension, and empirical models o f concentration-de p e n d e n t i n t e r f a c i a l t e n s i o n w e r e c o m p a r e d t o t h e o r e t i c a l expressions f o r t i m e - d e p e n d e n t surfactant c o n c e n t r a t i o n . A d a m c z y k (32) t h e o r e t i c a l l y r e l a t e d the m e c h a n i c a l p r o p e r t i e s o f the interface t o the a d s o r p t i o n k i n e t i c s o f surfactants b y i n t r o d u c i n g t h e c o m p o s i t i o n a l surface elasticity, w h i c h was d e f i n e d as t h e p r o p o r t i o n a l i t y coefficient b e t w e e n a r b i t r a r y surface d e f o r mations a n d t h e r e s u l t i n g surface concentrations. A l t h o u g h t h e expressions to d e s c r i b e the a d s o r p t i o n process d i f f e r e d f r o m one another, i t was d e m o n strated that t h e t i m e - d e p e n d e n t i n t e r f a c i a l tensions m i r r o r e d t h e change o f surface-active substances at t h e interface.


72


F o r studies w i t h r e a l systems, Isaacs et a l . (27) u s e d t h e s i m p l i f i e d a p p r o a c h o f e x a m i n i n g changes at t h e o i l - w a t e r interface w i t h o u t s p e c i f y i n g a d s o r p t i o n m e c h a n i s m s o r pathways. B a s e d o n m e a s u r e m e n t s o f t i m e - d e p e n d e n t i n t e r f a c i a l tensions, t h e f o l l o w i n g expression ( t e r m e d the s p r e a d i n g rate p a r a m e t e r ) s e r v e d to c h a r a c t e r i z e t h e relative a d s o r p t i o n p e r f o r m a n c e of demulsifiers o r demulsifier combination: _


s p r e a d i n g rate p a r a m e t e r =

where 7 and 7 0

s / e

Ύ θ ~ 7s/e

(15)

àt

are t h e steady-state v a l u e o f o i l - a q u e o u s i n t e r f a c i a l t e n s i o n

i n t h e absence a n d p r e s e n c e o f a d d e d c h e m i c a l , respectively; àt represents the t i m e r e q u i r e d t o r e a c h t h e steady-state t e n s i o n , 7

s / e

. A schematic o f t h e

t e c h n i q u e u s e d to measure this p a r a m e t e r is s h o w n i n F i g u r e 17 t o g e t h e r w i t h t h e d e p i c t i o n o f t h e a d s o r p t i o n o f D u o m e e n C f r o m t h e o i l to t h e o i l w a t e r i n t e r f a c e a n d A e r o s o l O T f r o m t h e w a t e r to t h e w a t e r - o i l i n t e r f a c e . T o u n d e r s t a n d t h e reasons f o r t h e d e w a t e r i n g effectiveness r e s u l t i n g f r o m t h e i n t e r a c t i o n s b e t w e e n t h e t w o surfactants, t i m e - d e p e n d e n t i n t e r f a c i a l tensions w e r e m e a s u r e d t o e x a m i n e t h e transfer o f t h e surfactants f r o m the b u l k to t h e i n t e r f a c e . B a s e d o n these m e a s u r e m e n t s , F i g u r e 18 shows a p l o t o f t h e a p p a r e n t s p r e a d i n g rate p a r a m e t e r , w h i c h is a measure o f b o t h

_^ (r

«

Capillary tube

Aqueous phase

4-

Oil drop

\

C H NH - C H - N H — • R - N H 12

25

2

4

2

2

I R-SO3--

H

• C 0 — CeHf7 2

SO3-C;

" C H — C 0 — CeH-j 7 2

Oleic

2

Aqueous

Figure 17. Schematic of the spinning drop capillary technique depicting the adsorption of oil-soluble and water-soluble surfactants from the bulk to the interface. The molecular structures of the two additives are also given.



2.

ISAACS AND CHOW

0

u

,

0


73

Stability

I

ι

I

25

50

75

i_i 100

Weight percent of Duomeen C in the mixture Figure 18. Apparent spreading rate as a function of the ratio of Duomeen C and Aerosol OT concentrations in the mixture. The total concentration is 100 ppm. the ease o f d e f o r m a t i o n o f the interface a n d t h e speed o f a d s o r p t i o n o r mass transfer o f m a t e r i a l t o the interface, as a f u n c t i o n o f the ratio o f D u o m e e n C a n d A e r o s o l O T concentrations i n t h e m i x t u r e . C l e a r l y , t h e m a x i m u m s p r e a d i n g rate occurs at a 1:1 ratio o f t h e reagents. F i g u r e 19 shows a n excellent a g r e e m e n t b e t w e e n t h e s p r e a d i n g rate p a r a m e t e r a n d b o t h t h e w a t e r recovery b y c e n t r i f u g a t i o n a n d the final U V P signal. T h e d i r e c t c o r r e l a t i o n b e t w e e n results o f d y n a m i c i n t e r f a c i a l tensions a n d t h e results o f coalescence o r d e w a t e r i n g e f f i c i e n c y is a n e w p h e n o m e n o n that has t h e p o t e n t i a l f o r quantitative analysis a n d t a i l o r - m a k i n g demulsifiers f o r a p a r t i c u l a r c r u d e - o i l e m u l s i o n system.

Conclusions T h e p r o c e d u r e s f o r b r e a k i n g o i l - i n - w a t e r a n d w a t e r - i n - o i l e m u l s i o n s are v e r y d i f f e r e n t . F o r o i l - i n - w a t e r e m u l s i o n s , t h e i n t e r f a c i a l charge contributes t o



74


0

4

8

12

0

4

8

12

Spreading rate, mN/m-sec χ 10

3

Figure 19. Comparison of electroacoustic analysis and dewatering efficiency of the mixed demulsifier as a function of the apparent spreading rate.

the stability; i n w a t e r - i n - o i l e m u l s i o n s , the strength o f the i n t e r f a c i a l film o f o i l that forms b e t w e e n t h e w a t e r droplets is o f p r i m e c o n c e r n . I n this discussion the p r e s e n c e o f solids at the interface was n o t c o n s i d e r e d ; i f they are present, h o w e v e r , the a d d i t i o n a l stability i n t h e i r p r e s e n c e w o u l d r e q u i r e attention. T h e m o r e c o m m o n e m u l s i o n f o r m e d i n t h e p e t r o l e u m i n d u s t r y is t h e w a t e r - i n - o i l type. T h e sensitivity o f e l e c t r o k i n e t i c sonic analysis t o coagula t i o n - c o a l e s c e n c e processes i n w a t e r - i n - o i l m e d i a is o f great i m p o r t a n c e . It allows f o r r a p i d selection a n d o p t i m i z a t i o n o f d i f f e r e n t c h e m i c a l d e m u l s i f i e r s . I n a d d i t i o n , as a research t o o l , i t supports t h e d e v e l o p m e n t o f a f u n d a m e n t a l u n d e r s t a n d i n g o f c h e m i c a l treatment sions.

of water-in-oil emul


2.

ISAACS AND CHOW


Stability

75

Acknowledgments W e thank the A l b e r t a R e s e a r c h C o u n c i l f o r financial s u p p o r t a n d C a r o l H o p p e r f o r h e l p i n t y p i n g this m a n u s c r i p t . T h i s c h a p t e r is A l b e r t a R e s e a r c h C o u n c i l C o n t r i b u t i o n N o . 2025.


List of Symbols a A c dv/dz e f( R) K

G k n

f

Henry function g e o m e t r i c a l factor d e p e n d e n t o n g e o m e t r y o f electrodes B o l t z m a n n constant n u m b e r o f ions p e r u n i t v o l u m e principal radii o f curvature p a r t i c l e radius absolute t e m p e r a t u r e electrophoretic mobility voltage

0

R Τ U v V

p a r t i c l e radius H a m a k e r constant sound velocity i n the emulsion v e l o c i t y gradient e l e c t r o n i c charge

attractive i n t e r a c t i o n repulsive interaction

a t t r

rep

v We ζ

total i n t e r a c t i o n Weber number

t o t

c

valency o f the i o n

Greek δ

thickness o f fluid layer s u r r o u n d i n g a p a r t i c l e

ΔΑ AG ΔΡ AS At

increase i n i n t e r f a c i a l area free energy o f f o r m a t i o n o f droplets L a p l a c e pressure d i f f e r e n c e increase i n c o n f i g u r a t i o n a l e n t r o p y t i m e r e q u i r e d to reach steady-state t e n s i o n

Δ7 Δρ

f o r m

c o n f

1 2

ΔΨ 7o 7 7^ 1 2

0

t h e r m o d y n a m i c energy density d i f f e r e n c e b e t w e e n d i s p e r s e d a n d c o n t i n u o u s phases p o t e n t i a l d i f f e r e n c e b e t w e e n electrodes steady-state i n t e r f a c i a l t e n s i o n i n absence o f a d d e d c h e m i c a l i n t e r f a c i a l t e n s i o n b e t w e e n two l i q u i d s steady-state i n t e r f a c i a l t e n s i o n i n p r e s e n c e o f a d d e d c h e m i c a l



References 1. Tadros, T h . G . L’actulaite Chim. 1987. 2. Lissant, Κ. J. Demulsification: Industrial Applications; Dekker: N e w York, 1983. 3. C h i n , H. B.; H a n , G. D. J. Rheol. 1980, 1. 4. Heimenz, P. C . Principles of Colloid and Surface Chemistry; Dekker: N e w York, 1977. 5. Shaw, D . J. Introduction to Colloid and Surface Chemistry, 3rd ed.; Butterworth: London, 1980. 6. Takamura, K . ; Chow, R. Energy Process./Can. 1982, 9, 29. 7. Overbeek, J. T h . G . Colloid Science; Elsevier: Amsterdam, Netherlands, 1952. 8. Vincent, B. J. Colloid Interface Sci. 1973, 42, 270. 9. Gregory, J. J. Colloid Interface Sci. 1981, 83, 138. 10. O’Brien, R. W.; White, L . R. J. Chem. Soc. Faraday Trans. 2 1978, 74, 1607. 11. Chow, R. S.; Takamura, K . J. Colloid Interface Sci. 1988, 125, 226. 12. Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: N e w York, 1981. 13. Healy, T. W . ; White, L . R. Adv. Colloid Interface Sci. 1978, 9, 303. 14. Takamura, K.; Chow, R. Colloids Surf. 1985, 15, 35. 15. Takamura, K . ; Chow, R.; Tse, D . L . In Flocculation in Biotechnology and Sepa ration Systems; Attia, Υ. Α., Ed.; Elsevier: N e w York, 1987. 16. Chow, R. S.; Takamura, K . J. Colloid Interface Sci. 1988, 125, 212. 17. Takamura, K . ; Buckley, J.; Morrow, N . SPE Reservoir Eng. 1987, 62, contribu tion no. 16964. 18. Israelachivili, J. N.; Adams, G . E. J. Chem. Soc. Faraday Trans. 1 1978, 74, 975. 19. Israelachivili, J. N.; McGuiggan, P. M . Science 1988, 241, 795.


2.

ISAACS AND CHOW

20.

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21. 22. 23. 24. 25.


26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.


Stability

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RECEIVED for review December 18, 1990. ACCEPTED revised manuscript June 3, 1991.


Practical Aspects of Emulsion Stability - American Chemical Society

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