MXPA99007906A - Method and device for determining the stability of a water-hydrocarbon emulsion - Google Patents

Abstract

The invention concerns a method and device for determining the temperature stability of a water-hydrocarbon emulsion capable of phase separation by monitoring the weight variations of a gravimetric sensor part of which is immersed in said mixture. The method consists in a first step of cooling or heating the emulsion to a predetermined temperature;and a second step during which the emulsion is maintained at this temperature, the time-dependent variation curve of said weight in time enabling the determination of the solid mass collected and the separation speed into two phases by determining the slope of this curve, the stability of the emulsion being obtained by comparison with reference emulsions known to be stable.

Description

PROCESS AND DEVICE FOR DETERMINING THE STABILITY OF EMULSION OF WATER AND HYDROCARBONS The present invention relates to a process for determining the stability of a water and hydrocarbon emulsion. This process can be used to determine the stability of an emulsion of water and hydrocarbons which is stable at room temperature, generally useful as a fuel, which, under the influence of a variation in temperature - cooling or heating - is capable of dividing in two or more liquid and / or solid phases due to the separation or crystallization of the water, followed or preceded by the sedimentation of the paraffins in the hydrocarbon matrix. In the following, the term "emulsion" or "emulsion of water and hydrocarbons" will denote, without preference, an emulsion of an aqueous dispersed phase in hydrocarbons and their possible additives constituting the continuous phase, or alternatively an emulsion of dispersed hydrocarbons in a phase watery It is well known that the presence of a small fraction of water dispersed in a hydrocarbon improves the combustion quality of this hydrocarbon and substantially reduces the amount of harmful emissions, incombustible and nitrogen oxides, the vization of water, resulting in a reduction of the temperature in the combustion chamber. Unfortunately, the immiscibility of the two fluids substantially limits the use of this property to their implementation in burners that prepare the emulsion at the site. Attempts to produce fuels and combustion alcohols consisting of an emulsion by the addition of surfactants to the mixture failed because they were not stable enough for industrial application. Recent research has provided the formula for new fuels whose stability is such that their industrial exploitation seems possible. (See Patent Application WO 97/34969 of March 17, 1997). This industrial application requires the development of a reliable process to control the stability of the emulsions thus manufactured, this process being reliable both with time and under the influence of temperature. The problem is difficult because of the complex phenomenon which takes place in a medium, which is heterogeneous in nature, in particular when subjected to temperature variations. The reason for this is that crude or refined hydrocarbons contain a larger or smaller proportion of ---- paraffins, which are soluble "under hot conditions" but which, under the influence of a decrease in temperature, can crystallize and then settle and therefore give rise to dysfunctions in storage or during use. The stability of the emulsion is sensitive to temperature both under hot conditions, since an increase in temperature promotes the separation phenomenon, as well as under cold conditions, and in which case the crystallization of the free water accelerates the separation process. In this way, the possibility of providing conditions under which a liquid emulsion, which is initially stable at room temperature, can be separated into at least two phases under the influence of time and / or temperature is a considerable asset for the optimal use of this emulsion. The emulsion can be prepared with any hydrocarbon, such as alcohols, gas oils, domestic fuel oils or heavy fuel oils, these fuels possibly containing various additives or components known to those skilled in the art, such as oxygenated compounds (alcohols, ethers or methyl esters of the vegetable oil) . The same types of problems arise for all products, particularly products containing paraffins, for which filtration, pumping and blocking problems are observed, particularly in engines and industrial and domestic heating systems. By analogy, reference will be made to summer or winter emulsion formulas, as common terminology, for domestic fuel oils, summer fuel oil and winter fuel oil according to current specifications. The additives of surfactants that facilitate the formation of the emulsion and ensure its stability are added to the mixture of water and hydrocarbons to avoid the appearance of the separation phenomenon. To avoid the crystallization and after the sedimentation of the paraffins, during the use under cold conditions, the additives whose action retards the addition of the crystals, prevents their development, keeps them in suspension or prevents their sedimentation are added to the emulsions that already contain your own additives. It is therefore important to measure the impact of these additives on the phenomenon of phase separation of an emulsion. There are several methods to measure the characteristics of appearance and separation of a solid phase in liquid. A first method is based on measuring the weight of solids, such as paraffins in gas oils which have crystallized at a certain temperature.

These paraffins are extracted from the hydrocarbon by centrifugation (Patent EP-0,355,053 A2) or by aggregation in a gravity settler (Patent of the United States 4,357,244). These tests make possible only the determination of the total amount of paraffins that have crystallized and which can settle.

They provide a measure of excessive sedimentation. A second type of test simulates real-time sedimentation in small tanks (standard NF M 07-085) in which hydrocarbons are stored at a low temperature for 24 or 48 hours.

The person performing the experiment then visually evaluates the appearance and volume of each phase, in particular the position of the interface between the two phases. These tests give an approximate qualitative measure of sedimentation. There are also optical methods to measure the appearance characteristics of two immiscible liquid or solid liquid phases. Reference may be made to Patent FR 2,577,319 which is directed to the determination of the turbidity point of gas oils and Patent FR 2,681,428 which is directed to the separation of two liquids (measurement of the aniline point of hydrocarbons). These methods have disadvantages and inconveniences: • They are prolonged, since they usually last 24 or 48 hours. • They are not reliable, since they depend only on the subjectivity of the observer. • More especially, however, they do not make it possible to measure the quantities of the separate phases, or to know the speed of separation of the phases, or even to explain and quantify the successive states through which the liquid passes when the temperature changes. The process for determining the stability of an emulsion of water and hydrocarbons by thermogravimetric analysis, which is the subject of the invention, solves the problem of the quantitative measurement of the separation of immiscible liquid or solid phases using a liquid which has been homogeneous fact. The subject of the present invention is a process for determining the stability of an emulsion of water and hydrocarbons subject to exhibit phase separation, characterized in that • In a first step, by subjecting the emulsion to a suitable heat treatment, it is brought to a The predetermined test temperature and the variation in the apparent weight P of the gravimetric detector, a portion of which is immersed in the emulsion, is continuously measured by thermogravimetry, after • In a second step, the emulsion is maintained at this temperature while continuously measuring the variation in the apparent weight of the detector by thermogravimetry and the curve of the variation of this weight is recorded simultaneously and then • The mass of the phase "separated collection, on the one hand, and the separation speed of the phases corresponding to the inclination of the curve, mainly the speed measured at the breaking point corresponding to a substantial and continuous increase in the apparent weight P at the start of the second step , on the other hand, it is determined from the curve, and • The stability of the emulsion is deduced by comparison with known reference emulsions, whose stability over time has been corroborated by permanent stability tests. here the constant temperature at which you want to measure the stability of the emulsion, but also, for the conduct under conditions s cold, the temperature at which the separation is visible, that is, detectable by the naked eye or by infrared light as described in patents FR 2,577,319 and FR 2,681,428. The process of the invention will be carried out according to two main variants depending on whether it is directed to the stability at a predetermined temperature above the temperature of the crystallization of the water or, exceptionally, of certain heavy paraffins (conduct under hot conditions), or the stability at a predetermined temperature which is below the crystallization temperature of at least one of the ingredients (behavior under cold conditions). Two profiles of the curves for the variation in apparent weight of the detector as a function of time and temperature show substantial differences in the duration of the various steps if the stability of the emulsion is monitored under hot or cold conditions. The reason for this is that the stability under hot conditions leads to a first step whose duration is associated with the difference in temperature between the temperature of the prepared emulsion, that is, generally close to the ambient temperature and the constant temperature of the test. If the test is performed at room temperature, this duration can be zero. If the test temperature is higher than the initial temperature of the emulsion, the emulsion will have to heat up. On the other hand, the second step, which is complete when the change in weight becomes zero (that is, when the phases are completely separated), can be very long, especially if a particularly stable emulsion is tested. In this case, the separation speed will be the predominant factor that will be taken into account. In order to assess the behavior of the temperature of the emulsions, the predetermined test temperature is between 10 and 70 ° C, the emulsion being brought to this temperature at a rate of heating or cooling, from room temperature, generally from between 0.05 and 10 ° C / minute. The determination of the stability of an emulsion at low temperature consists of monitoring the crystallization and sedimentation of water, on the one hand, and paraffins, on the other hand, in an emulsion. In a first embodiment, the first step consists of the gradual reduction of the temperature at a rate generally between 0.05 and 10 ° C / minute between the crystallization temperatures of the water and the paraffins, continuously recording at the same time the variation in the apparent weight of the detector. This weight decreases because of the increase in the density of the emulsion. During the second step, the change in the apparent weight of the detector is recorded, while maintaining the constant temperature. This weight remains substantially constant up to the point of crystallization of one or other of the two phases depending on whether the crystallization temperature of the water is lower or higher than the temperature of the paraffins. In a second embodiment, the first step consists of the gradual reduction of the temperature at a rate generally between 0.05 and 10 ° C / minute at a predetermined temperature which is lower than the crystallization temperatures of the paraffins and water, but greater than the flow temperature of the hydrocarbon-based mixture. The advantages of the process, which is the subject of the invention, are the precision, reliability and reproducibility of the results obtained, both to evaluate the separation speed of the phases, and to measure the weight variations of the separated phases. The subject of the present invention is also a device for measuring the separation of an emulsion in several liquid and / or solid phases, comprising a thermogravimetric balance equipped with a gravimetric detector, whose portion immersed in a tank containing the emulsion is a crucible, the tank being connected to a cooling circuit, the device being characterized in that the crucible is free, preferably coaxial with the tank whose cylindrical cross section is such that the ratio of the largest diameter of the crucible to the diameter of the tank is between 0.1 and 0.9. The crucible has a cylindrical shape comprising a base and edges, whose height does not exceed the level of the liquid in the tank. The height of the edges is between 0.5 mm and 30 mm and generally equal to 5 mm. The characteristics of the present device will be more apparent in the revision of Figures IA, IB and IC and the description thereof. The device shown in Figure IA comprises a thermogravimetric balance (of the SETARAM type), a tank containing the macroscopically homogeneous liquid mixture to be studied, a temperature control device (not shown in the diagram) to cool or heat the tank and a computer system (not represented in the diagram) to record and process the data.

The ace of the balance carries, suspended on the left arm in the diagram, a crucible submerged in the tank containing the mixture. The tank has a jacket and allows the temperature of the mixture to be modified, by means of a heating or cooling circuit, not represented in the diagram. The crucible has a cylindrical shape, like the tank, and comprises a base and edges. A standard optical and magnetic system, combined with the balance, allows variations in the weight of the crucible to be measured and recorded. Figures IB and IC show the details of the crucible. Figures 2 through 5 show, in the form of curves, the results of the measurements obtained in several examples of phase separation. The features and advantages of the process of the present invention will emerge more clearly upon reading the examples for carrying out the process which are provided below, in a non-limiting manner, with reference to Figures 2 through 5. EXAMPLE 1 The present Example describes the use of the process of the invention to determine the stability under cold conditions of a water and diesel oil emulsion of the winter formula. The emulsion is brought to a temperature below the crystallization temperatures of the water and the paraffins, but, needless to say, above the point of flow and the crystallization and sedimentation of the water, on the one hand, and of the paraffins, on the other hand , they are supsed. The process is carried out as follows: A model B60 or TGA92 of thermogravimetric balance, with electromagnetic compensation, sold by SETARAM is used. The crucible is a container of 20 mm in diameter with edges of 5 mm in height. It is placed in a cylinder of 30 mm in diameter and 100 mm in height containing the diesel to be tested. The crucible is submerged in the tank 33 mm below the surface of the diesel. The temperature of the emulsion is then reduced to -7.5 ° C, at a rate of 0.7 ° C per minute, at which temperature the formation of crystals is visible and the tank is then kept at this temperature for 18 hours. The variations in the relative weight of the crucible during the reduction of the temperature and during the stable temperature are recorded. In this way a decrease in the relative weight of the crucible is obsd, due only to the variations in the density of the emulsion, which increases as the temperature decreases, followed by an increase in the relative weight, due to the water and paraffin sediment in the crucible. The increase Gp in the weight of the crucible due to the paraffins and / or the sedimented water is obtained by subtracting, at each instant, the relative weight Ps of the device measured at the beginning of the second step (constant temperature), of the weight P of the detector measured in the time t: Gp = P-Ps. In this way the total weight increase is the difference in the apparent weight of the device between the end and the start of the second step. The curve recorded is the curve of Figure 2: • The first portion of the curve is explained by an apparent loss of relative weight, due to the increase in the density of diesel during temperature reduction (OA). • Afterwards a waiting time is obsd, in which the relative weight increases only a little (AB) due to the sedimentation of the paraffins alone, the water presenting a phenomenon of excessive cooling. • The third portion (BC), reflects a rapid increase in relative weight, corresponding to the weight of the paraffins and water being deposited on the surface of the crucible during the stationary phase in which the temperature is maintained at -7.5 ° C. { ± 0.2 ° C).

• In the fourth portion (CD), the curve shows a break D, from which point the increase in relative weight is due solely to the sedimentation of the paraffins. Since the water has settled completely, the visual obstion made at that moment shows phase separation. • An extension of the test would show on the curve, in addition to point D, a break from which point the increase in relative weight is zero. Therefore, there is no other sedimentation in the crucible, the relative weight measured remaining substantially constant. In practice, this is not useful since the precipitation of the free water establishes the instability of the emulsion. The sedimentation curve in Figure 2 makes it possible to define three characteristics: 1 / The increase in relative weight (Gp) in milligrams, which represents the total amount of water and paraffin sedimented during the experiment. 2 / The sedimentation rate (V) of the emulsion in mg per hour, established from the inclination of the curve in its portion BC at its inflection point. This rate of sedimentation makes it possible therefore to compare different emulsions. 3 / The waiting time in hours, which represents the time corresponding to the constant stage AB of the curve, during which the emulsion remains stable at the test temperature. In this case of the emulsion tested with the curve in Figur --- 2, it can be seen that the waiting time is 8 hours. EXAMPLE 2 In this Example, another emulsion of winter formula is analyzed which comprises paraffins which are crystallized at a lower temperature, by the supply of additives with specific behavior under cold conditions, the first step being stopped at -8.5 ° C before of the crystallization of the paraffins, that is, above the turbidity point. Only the water crystallizes. The record of the change in weight gave the curve in Figure 3. After the decrease in the relative weight of the crucible (OA), a waiting period (AB) can be seen due to the excessive cooling of the water and finally an increase of the rapid weight (BC) due to the crystallization of water. In addition to C, the weight gain becomes 0, since all the water has been crystallized. The temperature profile of the medium is characteristic of the phenomenon with the maximum value due to the exothermic crystallization of the water. Therefore, the device makes it possible to accurately distinguish each of the thermal events experienced by the water and diesel oil emulsion and the qualification of the separation level by measuring the sedimentation rates and the apparent weight increase. EXAMPLE 3 This Example illustrates the determination, according to the process of the invention, of the ambient temperature stability (20 ° C) of two emulsions EMU01 and EMU02 obtained by mixing 13% by weight of water in a gas oil of type EN590, containing specific additives for the maintenance of emulsions and in which the particle size distributions of the water droplets in the gas oil are very different. • Emulsion EMU01 has a monodisperse particle size distribution with droplet diameters centered around 1 μm (see photo 1). • Emulsion EMU02 has a smaller, highly dispersed aqueous phase referred to as a polydispersed phase, with droplet diameters ranging from 0.1 μm to 50 μm (see photo 2). The record of apparent weight gain allowed the curves given in Figure 4 to be established. In this case, the constant temperature (20 ° C) is greater than the crystallization temperature of water and paraffins. A linear change of the increase in relative weight over time is obtained. The two main parameters (V and Gp) are notably higher for the EMU02 emulsion, which is thus less stable than the EMU01 emulsion. Since the sample EMU01 shows a more homogeneous dispersion of water in the continuous phase, it will have little tendency to regulate and settle. This example clearly shows that it is possible, by means of the process according to the invention, to establish a quantitative scale of the stability of the emulsions made by comparison with an approved reference and at a certain temperature. In addition, particle size analysis through image processing provides only a local analysis, which, although possibly being analyzable in statistical form, is long and complex, while the process according to the invention allows a global analysis in all the volume of the sample, regardless of the temperature, without having to dilute the sample. EXAMPLE 4 This Example has the purpose of showing that the process according to the invention makes possible the approval and optimization of an industrial emulsion manufacturing process. Specifically, several emulsions were prepared in industrial form in a closed circuit plant comprising an emulsion mill and a sample outlet to allow samples to be taken after a certain number of recycling operations or runs. The stability of these samples at room temperature is measured with the process according to the invention under the conditions described in Example 3 and is compared with the stability of an EMU REFERENCE emulsion containing 13% water, mentioned hereinafter in the present as the REFERENCE, whose stability was verified during a long period. This REFERENCE was prepared in the laboratory ---- in order to obtain a size distribution of monodisperse particles of water droplets centered around 1 μm, this distribution having been analyzed by electron microscopy and image processing. A certain amount of diesel EN590 and 13% by weight of water, in relation to the amount of diesel, the mixture being emulsified (EMU03), are introduced to the closed circuit of the industrial plant. Samples were taken after 4 and 7 recirculation runs of the mixture. The emulsions were analyzed by the process according to the invention. The apparent weight increases and sedimentation rates were measured after 1 hour (Vi) and 6 hours (V2): these data are collected in Table 1. TABLE 1 It can be clearly seen that it is possible to control the stability of an emulsion in order to modify the manufacturing process to reach the reference level. EXAMPLE 5 In this Example, the stability of two emulsions EMU04 (summer formula) and EMU05 (winter formula) was studied as a function of the predetermined temperatures (between 40 ° C and -8 ° C), working as described in Examples 1 and 3 for the increase or decrease of temperature during the first step of the process of the invention. The results obtained are collected in table 2 TABLE 2 = crystallization of the aqueous phase It can be seen that the emulsion stability increases as the temperature decreases, to the point at which it breaks due to the crystallization of the water and the paraffins. EXAMPLE 6 In this Example, the stability of the emulsion at a temperature of 70 ° C is studied. During the first step of the process, the temperature of the emulsion is gradually increased, at a speed of 1 ° C / minute, until reaching the constant stage of 70 ° C. The measurements made continuously on the apparent weight of the crucible allow the curve in Figure 5 to be established. First, during the first step, a substantial increase in weight is observed due essentially to a decrease in the density of the hydrocarbon fraction of the emulsion. Then, a non-linear change (AB uniformly accelerated) is observed in the relative weight, due to the separation of the water present in the composition of the emulsion, up to the point of the total separation of phases.

Claims (8)

1. Process to determine the stability of an emulsion of water and hydrocarbons subject to phase separation, characterized in that • In a first step, subjecting the emulsion to a suitable thermal treatment, it is brought to a predetermined test temperature and the variation in the apparent weight P of the gravimetric detector, a portion of which is submerged in the emulsion, is continuously measured by thermogravimetry, then • In a second step, the emulsion is maintained at this temperature while continuously measuring the variation in the apparent weight of the detector by thermogravimetry and the curve of the variation of this weight is recorded simultaneously and then • The mass of the phase separated collection, on the one hand, and the separation speed of the phases corresponding to the inclination of the curve, mainly the speed measured at the breaking point corresponding to a substantial and continuous increase in the apparent weight P at the beginning of the second step, on the other hand, it is determined from the curve, and • The stability of the emulsion is deduced by comparison with known reference emulsions, whose stability over time has been corroborated by permanent stability tests.

2. Process according to claim 1, characterized in that the predetermined test temperature is between 10 and 70 ° C and that the emulsion is brought to this temperature at a rate of heating or cooling, from room temperature, generally between 0.05 and 10 ° C / minutes. Process according to claim 1, characterized in that the predetermined test temperature is between the crystallization temperature of the water and the paraffins, the first being higher than the second, or vice versa, and in which the emulsion is carried at this temperature by accelerated cooling at a rate generally between 0.05 and 10 ° C / minutes. Process according to claim 1, characterized in that the predetermined test temperature is lower than the crystallization temperatures of the paraffins and of the aqueous phase, but higher than the point of flow of the mixture based on hydrocarbons and in that the emulsion is brought to this temperature at a cooling rate between 0.05 and 10 ° C / minutes. 5. Device for thermogravimetric separation in two liquid phases, an aqueous phase and a hydrocarbon-based phase, of an emulsion, comprising a thermogravimetric balance equipped with a gravimetric detector, part of which is immersed in a tank containing the emulsion. crucible, the tank being connected to a cooling circuit, characterized in that the crucible is free, preferably coaxial with the tank whose cylindrical cross-section is such that in the ratio of the largest diameter of the crucible to the diameter of the tank is between 0.1 and 0.9. Device according to claim 6, characterized in the crucible has a cylindrical shape comprising a base and edges, whose height is between 5 mm and 30 mm and generally equal to 5 mm. 7. Application of the process according to claims 1 to 5, to the measurement of the effectiveness of additives that promote the maintenance of the homogeneity of a liquid emulsion. 8. The application of the process according to claims 1 to 5, to the measurement of the effectiveness of a process to make an emulsified fuel.