Microreactor Technology and Process Intensification - ACS Publications

Chapter 23

Downloaded by IMPERIAL COLL LONDON on September 3, 2013 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch023

Process Intensification in Water-in-Crude Oil Emulsion Separation by Simultaneous Application of Electric Field and Novel Demulsifier Adsorbers Based on Polyhipe Polymers 1,2

1

1

G. Akay , Z. Z. Noor , M. Dogru 1

Process Intensification and Miniaturization (PIM) Centre, School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom Institute of Nanoscale Science and Technology, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom (http://www.Newcastle.ac.uk/PIM)

2

A novel process intensification technique has been demonstrated in the demulsification (separation) of highly stable water-in-crude oil emulsions through the superimposition of an electro-static separation field with micro-porous demulsifier adsorbers which are prepared through a high internal phase emulsion polymerization route and subsequently sulphonated. These materials are also known as PolyHIPE Polymers (PHP). The hydrophilic version of PHPs is also an excellent ion exchanger and therefore during the demulsification process, they not only remove the surface active species from the crude oil, but they also remove ionic moieties thus acting as a combined demulsifier and adsorber. Although these materials can be used to demulsify stable emulsions very effectively, they are ineffective in highly stable emulsions. The same is also true for electro-static

378

© 2005 American Chemical Society

In Microreactor Technology and Process Intensification; Wang, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


379 emulsion separators. The combination of these two techniques appears to create synergy which results in the intensification of the separation process. Experiments were carried out using a model emulsion with highly viscous crude o i l and model sea -water aqueous phase. A n electro-static separator was used and the effects of emulsion flow rate and electric field strength were evaluated using 0.5 gram PHP demulsifier adsorber in 1 kg 50:50 water-in-oil emulsion. Complete demulsification was achieved at high flow rates or low electric field strengths when electro-static separation was not effective without the P H P demulsifier adsorber.

Introduction There are circumstances when the separation of emulsions is required. This process is known as demulsification. Demulsification is readily achieved in coarse emulsions but in highly stable, fine emulsions, demulsification process can be complex. Demulsification is most important in crude oil / water emulsions which can contain up to 90% water but yet it is still economically viable to remove water in the recovery of oil. Another important application of demulsification is in the nuclear re-processing when a highly stable and viscous water-in-oil emulsion (called interfacial crud) is formed which reduces the heat transfer rates as a result of equipment fouling. Stable emulsions are also produced in process industries which can cause environmental problems. Demulsification can also allow the recycling of the emulsion phases. In this study we only consider the demulsification of water-in-crude oil emulsions. There are a number of well established mechanical, chemical and electrical demulsification techniques available, including the use of hydrocylones, centrifuges, p H adjustment, steam/air stripping, membrane filtration, electric field and the addition of chemicals known as demulsifiers (1-11). The common purpose in these techniques is to cause the coalescence of the dispersed phase droplets which are stabilised by the surface-active materials present in the emulsion. In water-in-crude oil emulsions, the type and concentration of indigenous surface active materials are dependent on the oil-field. These indigenous surface active materials are responsible for the formation of relatively stable emulsions as a result of mixing of crude oil with water as this mixture is pumped from the oil well. Most of the crude oil demulsification techniques cited above have been developed for on-shore separation where the availability of space and storage facilities are not restricted and the oil viscosity is not high. However, this is not the case in offshore crude oil production, although oil-water separation at source of production has several advantages. Most importantly,



380 separation at source can utilise the potential energy of the crude oil (namely, high temperature and pressure) in destabilising the water-in- crude oil emulsion which in turn reduces the cost of pumping of the produced water and subsequent re-heating o f the crude oil for separation. Thermal de-stabilisation of emulsions is well known. Recently, it was shown that the emulsion stability decreases with increasing pressure (10) and hence separation at source (either on the off-shore oil platform or on the sea-bed or down hole) can be economically desirable, socially acceptable and technologically safe. Conventional methods of demulsification may take days, due to large hydraulic residence time. Due to the large volumes of crude o i l and slow rate of separation, the mechanical systems such as gravity settling, hydrocyclones, centrifugal separators and stripping columns are large and costly as the concentration of water in the emulsion increases due to the advancing age of the oil wells. Therefore, the current technology can not be used in off-shore applications especially when the emulsions are highly stable. What is needed is an intensified process which requires extremely fast demulsification rate with small processing volume in a continuous process. Recently, such a process has been disclosed (9). This process consists of two elements. Firstly, it was discovered that hydrophilic micro-porous polymers are highly effective as demulsifiers for the removal of surface active species from emulsions (interfacial crud) produced during the nuclear re-processing of spent radio-active fuel. This was also true for water-in-crude o i l emulsions. These micro-porous polymers are prepared through a high internal emulsion (HIPE) polymerisation route and are known as PolyHIPE Polymer (PHP) (1115). Secondly, these polymers are also very effective for the removal of metal ions through an ion exchange mechanism (11, 14). Furthermore, they also adsorb organic toxins due to the presence of hydrophilic and hydrophobic domains on the walls of the porous structure. Consequently, these polymers are named as demulsifier adsorbers. A very useful hydrophobic P H P is based on cross-linked styrene-divynil benzene. These materials can be manufactured over a wide range of pore size, D ( 0.5 μηι < D < 5000 μηι) and interconnect size, d (0 < d/D < 0.5). Pores with size above ~ 200 μιη are obtained through a coalescence polymerisation route as described previously (13-15). Post polymerisation chemical functionalization (such as sulphonation) can be achieved by incorporating the necessary chemical species in the oil or aqueous phase during the emulsification stage in order obtain uniform chemical structure (14). This technique is especially useful and economically necessary in the sulphonation of monolithic materials. PolyHIPE Polymers have been used in several chemical process intensification as well as support for animal cells and bacteria in tissue engineering and bioprocess intensification (16-19). They are also useful in the separation of tar and water from gases produced through the gasification of



381 biomass (20). PolyHIPE polymers are used as templates for the manufacture of nano-structured micro-porous metals used in intensified catalysis (11). The imposition of nano-structure in the PolyHIPE Polymer is also possible (13) where the micron-thick pore walls have nano pores present. These nanostructured micro-porous polymers therefore provide enhanced surface area which is accessible to reactants as a result of the pore size gradient. The use of electrostatic separation of water-in-crude emulsions has several advantages such as low power consumption due low electrical current across the dispersion (3). This technique is independent of moving parts so it is free from mechanical breakdown. Combining electrical separation technology with various other techniques such as centrifugal separation (5-7), heating (8) and addition of chemicals was found to create synergy. Demulsifiers have become an increasingly popular method in destabilising emulsions especially in oil and petroleum industries. Demulsifiers can also be used as pre-treatment to inhibit emulsification process by adding them to the crude oils (21). Demulsifiers are blend of several chemical compounds with different chemical structures (22,23) and broad distribution of molecular weight. Normally, demulsifiers used for crude oil déstabilisation are polymeric. Since sulphonated PolyHIPE Polymers appear to selectively remove surfactants (2426) thus causing demulsification (9), in this study we investigate the use of sulphonated PolyHIPE Polymer as demulsifier adsorber in the presence of high voltage electric field in order to intensify the demulsification of water-in-crude oil emulsions.

Material and Experimental Method

Materials The crude oil (from Harding Field in the North Sea) was supplied by B P Amoco. It has a specific gravity o f 0.80 mg/L. The crude oil is Newtonian over the experimental shear rate range (up to 1000 s" ) with viscosity (measured by H A A K E VT550 viscometer at 25°C) 153 mPa.s. A model sea water was used as the internal phase which contained C a C l , M g C l and NaCl. These chemicals were obtained from Merck and they were used as received. De-ionised water was used in the preparation of the model sea-water. The PolyHIPE Polymer demulsifier adsorber used in this study was previously developed (9). The preparation of this material is described in (13). It is a sulphonated P H P in the form of granules with the size range of700 to 1000 μηι. 1

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382 Emulsion Preparation Water-in-crude oil emulsions had equal phase volumes of oil and water phases. The aqueous phase contained: 28.1 g/L NaCl; 0.6 g/L C a C l ; 5.0 g/L M g C l ; and had a density of 1.021 g/mL. N o synthetic surfactant was used in the preparation of the emulsions since the crude oil has indigenous surfactants present. The model emulsion used in this experiment was prepared in a stainless steel, laboratory scale mixer at room temperature. The mixer had a diameter of 12 cm and an approximate nominal capacity of 550 ml. The temperature of the vessel was controlled by water circulation. The stirrer was powered by an electric motor allowing rotation speeds over a range of 200 to 2000 rpm. The mixing was conducted using two 100 mm diameter flat paddles at 90 degrees to each other. Details of the equipment are given in (9,10). The emulsions were prepared at a volume of 500 ml (i.e. 250 ml crude oil and 250 ml aqueous phase). First, the oil phase was placed into the vessel and aqueous phase was pumped with in five minutes while mixing at 2000 rpm. Mixing was continued for another 15 minutes and the resulting emulsion was transferred to a holding container where it was mixed with the P H P demulsifier adsorber. In order to avoid any aging effects, this emulsion was used immediately in the separation experiments. The viscosity of the emulsion was slightly shear thinning at the highest shear rates available in the Haake Viscometer. A t the shear rate of 1000 s' 50:50 water-in-crude oil emulsion had viscosity of 1030 mPa.s. Its conductivity was 0.8 μ β / α η at 25°C and the emulsions showed no sign of separation within 4 weeks of preparation. 2


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1

Oil/Water Separation The oil-water separation experiments were carried out using a Perspex electric field separation cell. The cell consists of two blocks held together with six plastic screws and bolts (Figure 1). The emulsion is fed into the circular central flow channel (2 cm diameter) from the central inlet port and the samples are collected from the top and bottom outlets after passing through the stainless steel electrodes as shown in Figure 1. A rubber seal between the blocks stops the leakage of the emulsion. Before the separation under high voltage electric field, 0.5 g demulsifier adsorber was added to 1 kg emulsion. This mixture was constantly stirred using a magnetic stirrer through out the experiment. The bottom electrode was earthed and the top electrode was connected to high positive potential. The latter electrode was fully insulted using several layers of lacquer providing a vertical electric field. High voltage (up to 30kV) D C electric field was available to apply to the top electrode by gradually increasing from zero. A n ammeter was present


383

Oil outlets Γ2Χ)

1 Top electrode connected


to high voltage Circular flow channel 203 cm

Inlet for emulsion

Bottom electrode

9.3 an

Figure 1. Schematic diagram of the water-in-oil emulsion separation cell. The electrodes have two slits to allow fluid flow through two outlets located in each block of the separation cell

on the electrical equipment scaled to 15 mA. For safety reason, the power supply would automatically cut off when the current reaches 15 mA. Due to the current leakage at very high potentials, maximum level of applied potential was 5kV. Since this system is potentially hazardous, as a result of flammable oil and high voltage, after the unit that was turned off, it was left standing for at least ten minutes before further handling of the equipment to ensure there was no residual electrical charge. Samples from the top and bottom electrodes were collected in glass measuring cylinders and the emulsion was allowed to separate into oil and water phases and overall degree of separation was determined. The



384 degree of separation was determined immediately after collecting total of400 ml sample (within ten minutes of separation) or alternatively after 1 hour or 24 hours. The separation of the emulsion was conducted under constant electric field of 2.5kV (field strength of 312 V/cm) while varying the emulsion flow rate; or alternatively, at constant flow rate ( 60 mL/min) while varying the electric field strength. These experiments were repeated without the addition of any demulsifier adsorber in order to evaluate the effect of the polymer. A t the end of the experiment, the demulsifier / adsorber was only present in the aqueous phase from where it was collected, dried and used in the Scanning Electron Microscopy (SEM) analysis. Scanning Electron Microscopy and ED AX The demulsifier / adsorber that have been used in the separation experiment was then analysed using Scanning Electron Microscope (SEM). Dried samples were mounted on aluminum specimen stubs, and coated with carbon and examined under S E M (Cambridge s240) with Energy Dispersive Analysis with X-rays ( E D A X ) facility. Scanning electron micrograph of the demulsifier / adsorber is shown in Figure 2.

Figure 2. Scanning electron micrograph of the demulsifier adsorber used in the experiments.


385 Results and Discussion


Oil Water Separation Under Constant Electric Field Strength The enhancement of electrical field separation by the use of P H P demulsifier was tested under two conditions: 1) varying flow rate of emulsion while voltage was held constant and; 2) varying voltage while flow rate of emulsion was held constant. This section described the result of the former condition. During this experiment, the voltage was held constant at 2.5 k V . In Figure 3, the variation of degree of separation immediately after the passage of the emulsion through the electric separator is shown as a function of flow rate. As shown in Figure 3, in the presence of P H P demulsifier, the separation efficiency is not reduced significantly with increasing flow rate where as, without PHP, there is no immediate separation when the flow rate is ca. 90 mL/min. However, at low flow rates, the effect of PHP is not as significant. Figures 4 and 5 illustrate the separation efficiency after 1 and 24 hours of standing respectively in the presence and absence of PHP demulsifier. A s shown in these figures, as the standing time increases from 10 minutes to 24 hours, the effect of PHP demulsifier / adsorber decreases.

Figure 3. Variation ofpercent phase separation with emulsionflowrate throug the electro-static separator at 2.5 kV applied voltage immediately after emergingfromthe separator in the presence or absence ofPHP demulsifier.



386

Figure 4. Variation ofpercent phase separation with emulsionflowrate through the electrostatic separator at 2.5 kV applied voltage after lhour of standing in the presence or absence ofPHP demulsifier adsorber.

100.0

80.0 \

-Without PHP -With PHP

60

70

80

90

100

Emulsion Flow Rate ml/mln

Figure 5. Variation ofpercent phase separation with emulsion flow rate through the electro-static separator at 2.5 kV applied voltage after 24-hours standing in the presence or absence ofPHP demulsifier adsorber.


387 Oil/Water Separation Under Constant Flow Rate


The effect of electric field strength on the separation efficiency is shown in Figures 6 when the flow rate is kept constant at 60 mL/min. Similar to the previous case illustrated in Figures 3-5, at this emulsion flow rate 100% separation takes place within 10 minutes of leaving the electro-static separator even i f the electric field strength is low ( l k V ) while no separation is observed without the P H P demulsier / adsorber at this electric field strength. There is a slight improvement on the separation efficiency after an hour of standing without the polymer (data not shown).

Figure 6. Variation ofpercent phase separation with electricfieldstrength at constantflowrate of 60 mL/min immediately after emergingfromthe separator in the presence or absence ofPHP demulsifier adsorber.

S E M and E D A X Analysis S E M micrograph in Figure 7 illustrates the appearance of spent demulsifier / adsorber, indicating the collapse of the porous structure due mechanical compression during the pumping of emulsion followed by drying. However,



388

when these dried samples are put into water, they swell again, recovering their original properties. Nevertheless, we only used fresh polymer in each experiment. Energy Dispersive Analysis with X-rays ( E D A X ) is summarised in Table 1 while the E D A X spectra of the carbon coated spent PolyHIPE Polymer demulsifier adsorber is shown in Figure 8 which indicates that the polymer contains several additional elements that were not present in the polymer and in aqueous phase. These additional elements are silica, aluminium, phosphorous which were removed from the crude oil. We conclude that the demulsifier adsorber is also able to remove several other compounds from the crude oil, and therefore P H P demulsifier adsorber is useful in reducing the metal loading of the crude oil.

Figure 7. Scanning Electron Microscopy ofspent demulsifier adsorber indicating its collapsed structure after use and de-hydration.


389 Table 1: Energy Dispersive Analysis with X-rays ( E D A X ) giving the molar concentration of metals adsorbed by the PolyHIPE Polymer during demuhification. Although carbon and oxygen appear in the X-ray spectrum (Figure 8 below) they were excluded from the analysis. Main elements adsorbed by the demulsifier are: Na, M g , A l , P, S, CI, and Ca.


*** E D A X Analysis *** elem/line

P/B

Β

F

c(100%) coofid.Jh_

Na K-ser

43.1

0.99937

1.00680

13.18

+-3.21

Mg K-ser

20.0

1.00255

1.01096

5.13

+-1.32

Al K-ser

11.5

1.00547

1.01852

2.65

+-0.8

Ρ K-ser

22.4

1.01066 1.04885

4.05

+- 1.13

S K-ser

150.8

1.01300 1.03648

24.61

+-4.13

CI K-ser

233.9

1.01518 1.01085

37.84

+- 5.61

83.4

1.02104 1.01120

12.54

+- 2.04

Ca K-alpha

100.00

standardless

[2s] Incident energy: 25Uktf Measure time: 60 β Pulse rate: 1541 cpB Advanced Chemical anc MatertafeAnaiyels

Na

LI 0.0

Ό.5 Ί . 0 1.5

2.0

2.5

3Λ

'3.5

4.0

U.5

Microreactor Technology and Process Intensification - ACS Publications

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