Caustic system

S oda—Anthraquinone. A few mills worldwide use soda pulping of hardwoods. In such cases, the addition of anthraquinone is immediately justifiable in terms of increased yield and upgraded pulp quaHty. The conversion of existing kraft mills is not as simple because AQ contributes no alkalinity to the process as sulfide does, and most kraft causticizing systems would have to be expanded by about 33%. This conversion is probably not justifiable in terms of the yield gain. The greatest benefit from AQ is for new mills in which expenditures for air pollution abatement devices can be reduced. 

In this paper we report first the spontaneous emulsification mechanisms in the petroleum sulfonate and caustic systems. This is followed by the kinetics of coalescence in alkaline systems for both the Thums Long Beach (heavy) crude oil and the Huntington Beach (less viscous) crude oil. Measurements of interfacial viscosity, interfacial tension, interfacial charge and micellar aggregate distributions are presented. Interrelationships between these properties and coalescence rates have been established. 

Spontaneous Emulsification Mechanisms in Sulfonate and Caustic Systems... 

NaOH and 1% NaCl (Caustic System). The process of emulsification was observed through the Nikon LKe Interference Phase Microscope (magnifications of 400 and 1,000). The action was captured by high speed cine-photomicrography (64 frames/second) for the caustic system and normal speed (24 frames/second) for the sulfonate system. 

In the caustic system the mechanisms of spontaneous emulsification lead to the formation of both an oil-in-water and a water-in-oil emulsion. A representative photomicrograph has been included in Figure 2. 

Therefore, the mechanism of spontaneous emulsification in the caustic system is interfacial turbulence. The mechanisms for the sulfonate system are diffusion and stranding. 

Figure 2. Spontaneous emulsification in Long Beach crude oil-caustic system (a) initiation of fingering action and formation of water in oil droplets (h) termination of fingering action and formation of water in oil droplets (c) threads of oil droplets (d) formation of very thin strings and appearance of oil droplets in aqueous phase and (e) appearance of buds of oil at the oil-aqueous interface...

For the sulfonate system the rate of emulsification was relatively slow and yielded an unstable emulsion. The emulsification process with the caustic system was much faster and produced quite stable emulsions. 

For the caustic systems used for recovering heavy oils it has been observed that the formation of stable emulsions by spontaneous emulsification is desirable (10, 11). The stable emulsions formed during caustic flooding tend to lower injected water mobility, viscous fingering, and water channeling while improving the sweep efficiency of the injected fluids. The produced fluids from this recovery technique are emulsions and must be demulsi-fied. 

The objective of the experiments presented here is to investigate effects of sodium hydroxide and sodium chloride on emulsion stability, and to establish the dynamics of spontaneous emulsification in a caustic system. 

A series of experiments were conducted to determine the emulsion stability in caustic systems. Figures 6 and 7 show data for the kinetics of coalescence and hence emulsion stability for the crude oil from Huntington Beach (Lower Main Zone), California (oil gravity of 23°API and oil acid number of 0.65). Figure 6 shows data for a nonequilibrated system and for a very low concentration of NaOH (0.003%) and 1% NaCl. This emulsion is unstable. Figure 7 shows data for two different concentrations of... 

Figure 5. Kinetics of coalescence for nonequilihrated sample of Long Beach crude oil-caustic system...

Figure 6. Kinetics of interdroplet coalescence for caustic system...

One of the main objectives of this study has been to determine the effect of interfacial properties on coalescence, emulsion stability and oil recovery efficiency for various surfactant and caustic systems. We have recently reported (6, 19) that for a petroleum sulfonate system there is no direct correlation between rates of coalescence and interfacial tension or interfacial charge. However, a qualitative correlation has been found between coalescence rates and interfacial viscosities. 

The interfacial tension for the crude oil-caustic system was measured by the spinning drop technique. The instrument used is similar in design to the one reported by Schechter and Wade (2). 

The interfacial tension values reported for the caustic system in Figure 8 are comparable to the values reported recently in reference (22). Our experiments which have been conducted at a room temperature of about 25°C show that 0.1 to 0.4 weight percent concentrations of NaOH and 1.00 weight percent NaCl can lower the interfacial tension between the aqueous solution and the crude oil substantially below a value of 0.01 dynes/cm or that required for emulsification. We have previously discussed the stability of these emulsions (Fig. 5). In the experiments run on fired Berea cores, it was reported that a concentration of 0.1% NaOH and 1% NaCl in the caustic crude oil system resulted in a drastic reduction in residual oil saturation. The details of these tests are given in reference (22). 

Figure 7. Kinetics of coalescence for equilibrated samples of Huntington Beach crude oil-caustic systems...

Figure 13 exhibits both interfacial tension and electrophoretic mobility for the Huntington Beach Field crude oil against sodium orthosilicate containing no sodium chloride. The interfacial tension values are observed to be higher for the non-equilibrated sample in this case than for the caustic system reported in Figure 12. The minimum interfacial tension of 0.01 dynes/cm occurs at about 0.2% sodium silicate as opposed to a value of less than 0.002 dyne/cm at about 0.06% NaOH. It is interesting to note, however, that the maximum electrophoretic mobility is the same for the two systems. Once again, it should be noted that a maximum in electrophoretic mobility does not correspond to a minimum in interfacial tension for those samples which contained no sodium chloride.

The objectives of this study was to determine the changes in micellar aggregate size distributions caused by oil/water ratio, co-surfactant and three phase development in petroleum sulfonate systems and by a co-surfactant in a caustic system. 

The very low interfacial tensions reported for many crude oil-caustic systems should permit substantial reduction of residual oil saturation by the mobilization of trapped oil. We have discussed earlier that crude oil-caustic tension is low initially where reactants meet at a fresh interface but the interfacial tension increases as reaction products diffuse into the bulk phases. The technique of determining micellar aggregate size distributions could be used to study the diffusion of the reaction products into the aqueous phase. 

The reason for understanding the effect of n-hexanol on the micellar aggregate size distribution in the caustic systems are similar to those for the sulfonate systems presented earlier. 

Figure 16 shows that the presence of the co-surfactant hexanol resulted in the formation of much smaller micellar aggregates of the natural surfactant than those formed in the system which did not contain hexanol. The caustic systems contained 0.05 M NaOH (0.2% by weight) and 1% NaCl. These observations are similar to those made for the sulfonate system (19, 27). 

Figure 16. Natural crude oil surfactant micellar aggregate size distributions for Long Beach crude/caustic system. The aqueous phase containing 0.05M NaOFI without hexanol (0) and with 0.50% hexanol ( ).

The mechanisms of a spontaneous emulsification in petroleum sulfonate and caustic systems have been described. 

Preliminary results on the kinetics of coalescence of both the Long Beach and the Huntington Beach crude oil droplets in caustic systems have been presented. 

The maximum electrophoretic mobility or zeta potential corresponds to the minimum interfacial tension for the caustic systems containing 1% NaCl but this correlation is not valid for systems which do not contain NaCl. 

The interference phase contrast technique combined with high resolution optical sectioning needs to be developed further to measure the thickness of the film surrounding oil droplets in caustic systems. The film thickness and the molecular packing in the film need to be correlated with the stability of an emulsion system. Preliminary results obtained to date are quite encouraging. 

---