Method for removal of water soluble organics from oil process water

Abstract: A method for removal of water soluble organics from oil process water is disclosed. The method comprises a step in which free oil is removed from water to be treated by water-wet ultrafiltration to produce a water-rich filtrate stream and a later step in which water soluble organics are removed from the water-rich filtrate stream by passage of the stream through a reverse osmosis membrane. An apparatus useful in such method comprising an ultrafiltration unit (1), a reverse osmosis membrane (2) and means for communication therebetween thereby to permit filtrate from the ultrafiltration unit to be supplied to the reverse osmosis membrane.

Description: Background of the Invention 1. Field of the Invention The present invention relates to the removal of free oil and water soluble organics (WSO) from oil process water, and more particularly, but not exclusively, to the removal of certain water soluble petroleum organics as well as non-hydrocarbon organic materials from crude oil production water and from aqueous streams used in removing water and/or inorganic salts, such as sodium chloride, from crude oil, residual oil and waste oils. 2. Description of the Prior Art In the removal of crude oil or production fluid from earth formations, significant quantities of water are quite often removed along with the oil. In the Middle East, the production fluid can be virtually pure oil. However, it is not uncommon that oil well production fluids are composed of 90% or more of water and only 10% or less of crude oil. Such water is referred to as connate water and is produced along with the oil. One of the first steps after removal of the oil well production fluid is to separate the oil from the water by phase separation techniques. Separation is conventionally accomplished using a bulk separator or a free water knockout system. Virtually all of the hydrocarbon is conveniently recovered in this manner. Unfortunately, small amounts of oil (50-1,000 ppm), and certain organic compounds, as well as inorganic salts and acids, are soluble in water; and mere phase separation will not remove the water soluble compounds from the aqueous phase. Water soluble organics include certain naphthenates, phenols and lower fatty acids. Water soluble inorganics salts include sodium chloride, sodium sulfate, calcium chloride and barium chloride. While the amount of water soluble organics may be relatively small, up to 1,000 ppm, they nevertheless give rise to environmental problems, when the aqueous phase is discharged into the environment without removal of the water soluble organics. The environmental agency is concerned as these organic products register a COD and can complicate where this water can be discharged. Furthermore, the water soluble organics may be valuable substances. In order to meet the present day strict environmental standards, a process to reduce the level of water soluble organics in the discharged streams to 50 ppm or less is needed. Some known methods can reduce the content of water soluble organics to this level. For example, one could separate the water and water soluble organics by distillation or the use of biological treating ponds. However, such methods are relatively complex and/or expensive in comparison with the process of the present invention. U.S. Patent No. 4,818,410 to Bellos (a co-inventor herein) et al. shows a less expensive and simpler technique. Nevertheless, increasingly stringent environmental standards may render it desirable, in some cases, to reduce the WSO content even further. The ability to achieve such reduction is beyond the capability of commercial methods. Another method is disclosed in U.S. Patent No. 5,104,545 to Means et al. This disclosure illustrates the great expense the industry is willing to incur for greater removal of water soluble organics. The method of Means et al. further involves problems with spoliation of resin when it becomes oil wet and requires plentiful amounts of solvent or solvent regeneration. Use of certain petroleum fuel oils for specific purposes requires that such oil be treated in order to remove undesirable corrosive contaminants therefrom. For example, fuel oils used in the newer, high efficient, gas turbine power plants must meet certain strict requirements with respect to the presence of inorganic contaminants in the oil. The presence of sodium chloride and other similar inorganic salts renders the oil less than suitable as a fuel in gas turbine power plants. To upgrade fuel oil so that it is acceptable for use in the turbines, the fuel oil is commonly processed using a multistage electrostatic desalting facility. In such operation, essentially complete removal of water soluble inorganic salts from fuel oil is accomplished by a counter current water washing process in which a high electrostatic gradient is used to break the interim water-in-oil emulsion. Selective specialized chemical demulsifiers are normally used in such salt removal processes. While the process removes the inorganic salts from the fuel oil, unfortunately, the wash water removes the water soluble organics from the fuel oil as well. In view of strict environmental standards, it is of considerable importance to remove these water soluble organics present in the wash water of the inorganic salt removal processes used to render the fuel acceptable for burning in turbines. Thus, it is advantageous to provide a method that removes WSO's from oil well production fluids as well as from aqueous streams used to render fuel oil acceptable for use as fuel for gas turbine power plants. As noted, in U.S. Patent No. 4,818,410 (Bellos et al), a method for removal of water soluble organics from oil process water and which is designed for such applications was disclosed. However, there is the ever present desire and the potential for further regulation to reduce the content of water soluble organics (especially non-hydrocarbon organic material) to an even greater extent. EP-A-0 350 279 is directed to a method for treating produced water from an oil production platform comprising a settling step, a skimming step and a nanofiltration step. The combination of water-wet ultrafiltrating with reverse osmosis is neither disclosed nor suggested. FR-A-2 218 125 describes a method for treating metal-working oils by ultrafiltrating followed by reverse osmosis. Said metal-working oils are completely unrelated to oil process water. Summary of the Invention According to the present invention there is provided a method for removal of free oil and water soluble organics from oil process water, the method being a continuous-flow process comprising the steps of: a. removing free oil from the oil process water by continuous-flow, ultrafiltration employing at least one water-wet membrane to produce a water-rich filtrate stream; and b. removing water soluble organics from the water-rich filtrate stream by passage of the stream through a reverse osmosis membrane. Among the several advantages of the invention, therefore, may be noted the capability of removing water soluble organics to an even greater degree than previously accomplished. Brief Description of the Accompanying Drawing The drawing is a schematic block diagram showing the process of this invention in combination with the pre-treatment technique of U.S. Patent No. 4,818,410, wherein a production fluid is treated. The pre-treatment technique of U.S. Patent No. 4,818,410 is enclosed within dashed loop designated "'410". Detailed Description of the Preferred Embodiments In accordance with the present invention, it has been discovered that by treating oil process water in a method which is a continuous flow process involving water-wet ultrafiltration (i.e. ultrafiltration employing at least one water-wet membrane) of the water to produce a water-rich filtrate stream followed by passage of the water-rich filtrate stream through a reverse osmosis membrane, water of greatly reduced non-hydrocarbon organic material and oil content may be produced. It is preferred in this method that the oil process water be pre-treated to reduce the water soluble organics to some extent, thereby to preserve the life of ultrafiltration membrane(s) and reverse osmosis membrane. The water may be pre-treated, for example, by the method of U.S. Patent No. 4,818,410 to Bellos et al. Although such known pre-treatment methods are very effective in reducing water soluble organics, they still leave typically a measurable amount of water soluble organics, particularly non-hydrocarbon organic materials. In addition, as a result of the source of the water and the pre-treatment techniques that involve contacting the water with oil, the water generally also contains free oil dispersed therethrough. Reverse osmosis (RO) membranes are designed to reject molecules of specific molecular weight; e.g., some available RO membranes reject molecules, for example, above 125 MW. RO membranes with lower MW rejection may be employed, limited only by the RO membrane technology. It is conventional to denote the water soluble organics found in and recovered from petroleum generally as "petroleum acids" and more specifically as "naphthenic acids". Chemically, the petroleum acids are, in the main, monocarboxylic acids related to the naphthalene (alicyclic) series of hydrocarbons. However, other petroleum acidic substances are included. For example, substituted phenols, mercaptans and long chain ketones may act as petroleum acids in the sense of this process. Petroleum acids, which are natural components of crude oil and are not formed by refining, include such acids as cyclopentaneacetic acid and trans-2,2,6-trimethylcyclohexylacetic acid. A typical formula of these acids may be written as R(CH 2 ) n COOH, where R may be cyclic moiety composed of 1,2,3 or more rings and n is usually one or higher. Aromatic rings, saturated rings and fused rings are normally present. It has been found that as many as 120 or more different petroleum acids may be present in a given crude oil. Most are soluble in all proportions of hydrocarbons, but most of the acids of this series have only slight solubility in water. For example, it is known that acetic propenoic and butyric acid are present in some oilfield water. Whether they are byproducts or the result of some biogenic process is not totally known. However, as their sodium salts or other alkali metal salts these naphthenates are preferentially water soluble. It is with these water soluble organics that the present invention is concerned as well as with non-hydrocarbon organic material (NHOM). It is to be understood that, at the pH of naturally occurring aqueous compositions, the acids may exist as anions in association with counter ions e.g. of sodium and calcium. For example, naphthenic acids can be regarded as being present as naphthenate salts. Often, therefore, petroleum acids in the aqueous phase of oil production fluids are in anionic form and may be more properly termed as petroleum carboxylate salts, phenates and other salts. According to the present method, the water to be treated is supplied (preferably after pre-treatment as noted above) to employing at least one water-wet membrane ultrafiltration to remove free oil in the water to be treated. The accompanying drawing shows an embodiment of the present method including an optional pre-treatment in accordance with the teachings of U.S. Patent No. 4,818,410. The pre-treatment is shown in Figure 1 within the dashed loop labelled "'410". According to this pre-treatment, a production fluid PF is acidified by addition thereto of an aqueous solution of a strong acid (which may be a combination of mineral and organic acids) at point P. The acidified stream is then pumped to a separator S, which may be free-water knock-out (FWKO) or a low pressure separator (LPS), to remove the oil. The stream may be treated before or after oil removal to break an emulsion, if any. This determination (whether to treat before or after) for optimal performance may be made by simple experimental testing at the site. Usually, the emulsion is a reverse emulsion (i.e., an oil-in-water emulsion) and so a reverse demulsifier is used. Dashed line DEM illustrates two possible points of addition of the demulsifier. The outlet water stream may still contain some oil and so it can be further treated by a corrugated plate interceptor (CPI) or a skim tank, and a flotation chemical (FC) may be added and further oil/water separation effected by a flotation device. The resulting stream can be monitored and, if it meets the applicable standards, it can be released overboard (waste W). If the sole failure to meet standards is the acidity of the stream, caustic N may be added by injection and the stream thereby neutralized may be released overboard. If desired, the stream may be collected and pH monitored periodically in a small surge vessel SV for periodic release overboard (W). For details of this acidification and separation technique, resort may be made to U.S. Patent No. 4,818,410. At times, nevertheless, the resulting stream from this pretreatment might still not meet the relevant environmental standards, particularly if the standards become more stringent than currently in force. In some cases, the failure may be remedied by treatment with an ion exchange resin or activated charcoal (identified in Figure 1 as "3"), and the so treated stream released overboard (W). In many cases, however, such treatment would not be sufficient and, instead of ion exchange or charcoal treatment, the stream may be treated in accordance with this invention, particularly in cases in which the stream contains significant amounts of higher molecular weight NHOMs. Thus a water stream and/or a mixed water/oil stream, whether pre-treated by the above-described or another technique or a production fluid that has not been pre-treated, is treated according to the present method by subjecting it to ultrafiltration followed by reverse osmosis. The supply stream to an ultrafiltration module or unit is designated in the drawing as "A" and the ultrafiltration module or unit is labelled "1". The ultrafiltration module employs at least one water-wet ultrafiltration membrane. A water-wet membrane is one that by its chemical make-up remains water-wet (that is, in contact with water) as opposed to oil-wet (i.e., oil-coated). Such membranes are sold, for example, by Bend Research, Inc. The ultrafiltration module is made up of hollow fibres and is analogous to large tube membranes, with a reverse osmosis membrane or skin on the inside of a porous support tube, but with a smaller diameter (0.025 inches versus 0.5 to 1 inch; i.e. 0.06 cm versus 1.27 to 2.54 cm) and a much tighter packing density inside a module, resulting in a much larger flow due to the larger membrane area. In conventional (fine) hollow fibre technology, the membrane is supported on the outside of the smaller tubes (0.002 inch; i.e. 0.005 cm) packing inside a pressure vessel and the feed water passes from the outside of the fibres to the centre. By contrast, in the ultrafiltration modules utilized in the process of this invention, the fibres themselves, with a burst strength of 56 to 77 bar (800 to 1,100 psi), act as the pressure vessel, allowing them to be packed in readily available PVC pipes. The hollow fibres of the ultrafiltration unit may be, for example, of 0.026 cm inside diameter, 0.062 cm outside diameter, 0.018 cm wall thickness and about 0.0018 cm estimated reverse osmosis skin thickness. The close packing of the layers of spiral wound cartridges and of conventional hollow fibres make them more prone to solids and oil fouling, and to short-circuiting and dead spots. The relatively large size of the hollow fibres (0.0254 cm diameter or greater) of the ultrafiltration module in the embodiment of the present invention shown in the drawing requires removal of suspended solids down to the 50 to 100 micron range prior to the reverse osmosis system. The ultrafiltration membranes may be formed of polyamide treated with urethane. In laboratory tests, exemplary suitable membranes have been found in tests to reject nearly all of the stearic acid, about three-quarters of the 2-thylhexanoic acid and about half the hexanoic acid, but virtually none of the acetic acid from water containing 500 ppm (by weight) free and emulsified oil without fouling, with a reject stream rate of about 10% of the inlet stream. In the process of this invention, the ultrafiltration step may be accomplished by a single module or by a plurality of modules placed either in parallel or in series. For example, four 1.86m 2 (20 ft 2 ) modules followed by three 0.93m 2 (10 ft 2 ) modules arranged in series may be used to treat about 3.8 to 10.2 litres (1.0 to about 2.7 U.S. gallons) per minute of water containing about 200 to 275 parts per million (by weight) free and emulsified oil. In addition, a pre-filter, such as a self-cleaning Cuno filter, may be included before the ultrafiltration module to prolong the life of the module. The water-wet ultrafiltration removes residual free oil and solids from the water stream. The reject stream therefore contains enriched oil, while the oil-depleted stream that passes through (the permeate) is relatively oil-free water, plus water soluble organics of concern, if any. This oil-depleted stream, which is about 80 to 90% of the flow rate of the feed stream, is then routed to a reverse osmosis membrane 2. The reverse osmosis unit preferably of the spiral wound variety. For example, for a flow rate compatible with four 1.86m 2 (20 ft 2 ) ultrafiltration modules, the unit may comprise, for example, three 0.03 m 2 (10 ft 2 ) Film-Tec spiral wound modules in series, selected according to the degree of purity desired. For instance, Film-Tec modules NF-30, NF-70 and FT-30 are designed to rejection 30%, 70% and 99.6% sodium chloride, respectively. Higher purity, however, comes at the expense of decreased flow rate. It has been found that the higher molecular weight water soluble organics, including non-hydrocarbon organic material, such as those molecules of molecular weight greater than about 125 do not pass through the reverse osmosis membrane. Thus, the water stream B emerging from the membrane (the permeate) contains greatly reduced concentrations of non-hydrocarbon organic materials and other organics (perhaps on the order of three to eight ppm total oil or grease) and, if it now meets the relevant standards, it may be released overboard (W). If desired, it may be further treated with an ion exchange resin or activated charcoal before release overboard. The reject stream (R1) from the water-wet ultrafiltration, can be acidified (for example, at point PR1) and while nominally an oil-rich stream, might still be of satisfactorily low oil concentration and, if so, may be routed (by conduit R) to the reverse osmosis membrane 2. Otherwise, it may be treated with an ion exchange resin or activated charcoal 3 (and pH adjusted, if needed) and released overboard (W) if it meets standards or it may be recycled back to the beginning of the treatment process as shown in the drawing or to a point after pre-treatment, if any. The reject stream R2 from the reverse osmosis membrane 2, which might be about 10% to about 25% of the original flow rate of oil-containing water to be treated, likewise may or may not be acidified (for example, at point PR2) and treated with the ion exchange resin or activated charcoal 3 (and pH adjusted) and released overboard (W), or acidified and recycled. The two reject streams (R1 from the module 1 and R2 from the membrane 2) may be combined to form a single stream C to be treated with the ion exchange resin or activated charcoal 3 and released overboard (W) or to be recycled. If stream C is recycled, it may be treated with a hydrocarbon/crude oil conductor 4, retained in an optional surge tank 5 to permit control of feed rates, and if it is to be returned to be pre-treated in accordance with the method of U.S. Patent No. 4,818,410, it may be acidified further, and routed to the desired return point. The invention is illustrated by the following examples. Example 1 Oil-containing water was fed through a pre-filter (a self-cleaning Cuno filter) to a first stage pump and pumped to a first bank of water-wet ultrafiltration membranes comprising three 0.93 m 2 (10 ft 2 ) modules and four 1.86m 2 (20 ft 2 ) modules provided by Bend Research, Inc. The reject stream containing enriched concentrations of oils, solids and organics was emitted at a flow rater 10-25% that of the feed. The permeate from the first stage, which was relatively free of emulsified oil, was collected in a tank to feed a high pressure second stage consisting of three spiral wound modules from Film-Tec (an NF-30 filter in series with an NF-70 filter and an FT-30 filter). The feed, first stage permeate and second stage permeate were tested at start-up and two hours later. The results are shown in Table I on the following page: Oil/Grease (mg/l) Sample Non-Acidified Acidified TSS (mg/l) TDS (mg/l) Start-up: Feed ― ― 218 29,428 First Stage Permeate ― ― 140 27,730 Second Stage Permeate ― ― 38 30,152 2 Hours Later: Feed 275 290 96 28,230 First Stage Permeate 60 86 54 27,793 Second Stage Permeate 50 56 0 95 Example 2 The apparatus of Example 1 was used to treat another water source, except that in the first stage, one 1.86 m 2 (20 ft 2 ) module was missing and one 0.93 m 2 (10 ft 2 ) module appeared to have ruptured (perhaps in transit). Thus, the membrane area was reduced from 10.2 m 2 (110 ft 2 ) to 7.43 m 2 (80 ft 2 ), thereby reducing the flow through the first stage by 27%. The following results shown in Table 2 were obtained from measurements of the various fees and permeates taken upon reaching steady state conditions: Oil/Grease (mg/l) Sample Non-Acidified Acidified Feed to Cuno Filter 195 30 Feed to First Stage 210 30 First Stage Permeate 140 10 Feed to NF-30 Filter 165 10 NF-30 Filter Permeate 150 5 Feed to NF-70 Filter 155 5 NF-70 Filter Permeate 40 5 Feed to FT-30 Filter 155 5 FT-30 Filter Permeate 5 5 Example 3 The apparatus of Example 1 was run for various periods over four days under a variety of conditions (pressures, flow rates, recycle rates, recovery rates, etc.). The results are shown in Tables 3-6, on the following pages: The first stage removed an average of 76% of the oil over four days, sometimes exceeding 80-85% removal at lower permeate flow rates, and consistently reduced the free oil to 6 ppm or less. Moreover, the ultrafiltration of the first stage even removed some of the water soluble organics (WSO's). Except for a couple of questionable samples, the ultrafiltration membranes averaged 66% WSO removal, occasionally removing up to 79% of the WSO's. The second stage reverse osmosis membranes also achieved significant WSO reduction. The BF-30 filter, rated at 30% NaCl rejection, averaged 80% WSO removal over the brief period it was employed. The NF-70 filter, rated at 70% NaCl rejection, rejected an average of 96% of the WSO's, reducing the levels to as low as 4 ppm from 129 ppm. The first FT-30 filter, rated at 99.6% NaCl rejection, averaged 93% WSO removal even including the samples taken on the first day, when the module was probably pushed beyond its practical limits. In view of the above, it will be seen that the several advantages of the invention are achieved and other advantageous results attained.

Claims: A method for removal of free oil and water soluble organics from oil process water, the method being a continuous-flow process comprising the steps of: a. removing free oil from the oil process water by continuous-flow, ultrafiltration (1) employing at least one water-wet membrane to produce a water-rich filtrate stream; and b. removing water soluble organies from the water-rich filtrate stream by passage of the stream through a reverse osmosis membrane (2). A method as set forth in claim 1, further comprising the step of recycling a reject stream from the reverse osmosis membrane or the ultrafiltration to be retreated by ultrafiltration. A method as set forth in any preceding claim wherein the ultrafiltration is a single pass ultrafiltration. A method as set forth in any preceding claim, further comprising, prior to step a, the step of: pre-treating the oil process water to reduce the content of water soluble organics in the oil process water to produce pre-treated oil process water. A method as set forth in claim 4 wherein the oil process water is pre-treated by incorporating in the oil process water a strong acid to adjust the pH within the range of about 2 to about 6, thereafter or simultaneously making intimate contact between the acidified water and oil with the result that the content of water soluble- organics in the acidified water is substantially reduced by being transferred from the water to the oil, and separating the oil and the water, the resulting water being the pre-treated oil process water.

Inventors: BELLOS THOMAS J, KEATING RIHCARD D, BELLOS, THOMAS J., KEATING, RIHCARD D.
Applicants: BAKER HUGHES INC, BAKER HUGHES INCORPORATED
Application year: 1993
Main classification: C02F1
IPC codes: B01D61, C02F1