GB2045945A - Measurement of water content of liquids using microwaves - Google Patents

Abstract

A method of determining the water content of liquids having a water: liquid loss factor ratio of at least 80, comprises monitoring the rise in temperature of the water-liquid mixture upon exposure to microwave radiation, which rise in temperature is proportional to the water content of the mixture. The method is particularly suitable for determining the water content of crude oils. The sample passing through a lagged glass tube 6 is exposed to microwaves in cavity F and the temperature of the sample is measured at the inlet E and outlet H by thermocouples. <IMAGE>

Description

SPECIFICATION Measurement of water content of liquids using microwaves The present invention relates to a method of determining the water content of liquids in admixture therewith by monitoring the temperature rise of the admixture upon exposure to microwave radiations.

Various methods have been used to determine the water content of liquids contaminated therewith. Of these, the most widely used are those involving distillation (mainly for miscible mixtures), centrifugation (mainly for substantially immiscible mixtures) and optical methods (for dispersions and emulsions). These techniques taken singly cannot be applied to the entire range of mixtures extending from the completely miscible to the totally immiscible. In addition, the distillation and centrifugation methods are not easily adapted to continuous or on-line analysis. Moreover, the techniques of distillation and centrifugation do not give accurate, reliable or reproducible results with relatively viscous mixtures and optical methods fail if the mixture is substantially opaque to optical transmission.One such mixture which is both viscous and opaque but yet is contaminated with water is crude oil. It is important to determine accurately the water content of crude oil for commercial and fiscal transactions so that correction is made for the proportion of entrained water which may be carried with the liquid through the metering system.

The methods used hitherto also require relatively sophisticated and expensive equipment even though they are of limited applicability.

It has now been found that the problems presented by these known methods can be minimised by monitoring the rise in temperature of the mixture upon exposure to microwave radiation and that in the case of certain liquid-water mixtures the rise in temperature is proportional to the water content of the mixture.

Accordingly, the present invention is a method of determining the water content of liquids having a water; liquid loss factor ratio of at least 80, which comprises monitoring the rise in temperature of the water-liquid mixture upon exposure to microwave radiation.

The dielectric loss factor of a liquid (which may be water) at a given temperature is the product of the dielectric constant of the liquid and the dielectric loss tangent of the liquid at that temperature. For example, in the case of water, the dielectric loss factor of 12.0 at 25"C is derived from the product of its dielectric constant (76.7 at 25 C) and its dielectric loss tangent (0.157 at 25or). On this basis, the dielectric loss factor ratio of water:crude oil is usually greater than 1000 for most crudes.

The ratio of the dielectric loss factor of water to that of the liquid is preferably at least 100.

The method of the present invention may be carried out to determine the water content of mixtures whether in the homogeneous or the heterogeneous phase. That is, the water may be dissolved in the liquid or dispersed within the body of the liquid.

The method of the present invention is suitably carried out by passing a constant flow of the liquid mixture through a microwave field. This may be done by providing a microwave cavity with a tubular member and passing the mixture through the tubular member suitably with the help of a conventional metering pump. The tubular member is preferably of a material which has a negligible dielectric loss factor, very low thermal conductivity and a low thermal mass. A tubular member satisfying these characteristics ensures that the temperature rise in the mixture is an accurate representation of the water content and ensures that heat losses from the mixture to the surrounding atmosphere are minimised. Examples of materials with such physical characteristics are porcelain, flint glass and plastics.

The tubular member may be of any shape but is suitably coil shaped to enable it to occupy the maximum amount of the space available in the cavity.

This design enables uniform saturation of the mixtures with microwaves and also minimises concentration of the unused microwaves at certain points within the cavity which may result in overheating.

The microwave cavity may be any of the conventionally available types provided that it is adapted to receive the tubular member and allows passage of the mixture through the tubular member. The temperature of the mixture entering the cavity is initially measured, the mixture then irradiated by microwaves when passing through the cavity and the temperature of the sample emerging from the cavity measured again. The rise in temperature as a result of irradiation by microwaves will be proportional to the percentage of water in the mixture.

The temperature measurements are suitably carried out by means of thermocouples or thermistors positioned at appropriate points. When the mixture is homogeneous it may be passed directly through the tubular member within the microwave cavity.

When the mixture is heterogeneous such as for instance crude oil containing water, it is preferable to mix the two phases as thoroughly as possible in order to disperse the water uniformly within the main body of the liquid, in this case oil, before entering the cavity. Such thorough mixing ensures that the temperature rise of the mixture as a whole is uniform and therefore gives more accurate results.

In a preferred embodiment, the mixture is preheate to a given constant temperature, depending upon the liquid in the mixture, and the air in the microwave cavity heated to the same temperature; the tubular member within the cavity is lagged to minimise heat loss from the mixture; and the cavity may carry a ballast load of water or other suitable material to prevent damage to the microwave generator (magnetron) by reflective overload.

The method of the present invention is further specifically described with reference to the accompanying drawings.

Input sample (e.g. a mixture of oil and water) is passed through a strainer (A) to remove entrained solids, then through a pressure differential pump (B) running at about 50 or 100 mls/minutes. An overpressure relief valve (C) is provided downstream of the pump.

The sample mixture line then passes through a small constant temperature controlled bath (D) (at about 20-30 C) and then past the first temperature measurement point (E) which is a thermocouple.

The next stage is the microwave oven cavity (F).

The tubular member (G) through here is of glass and lagged. At several points along its length orifices or restrictions are included to ensure mixing of oil and water and hence good transfer of heat from the water to the body of the liquid. The diameter of a tubular member is about a" to 3/e" internally. The air in the oven cavity is force-circulated and temperature-controlled at substantially the same temperature as the sample conditioning bath (D). On emerging from the oven cavity, the sample passes the outlet temperature sensor (H) and then to drain.

In the signal processing unit (J), differential temperature is measured and converted to give an output proportional to percent water. If a wide range of water contents isto be covered, mid pointtemperature is measured by sensor (K): in the event of this rising above a preset level the signal processing unit would halve the microwave power (to prevent sample boiling) and apply compensation to its temperature-differential processing to allow for the new power input. The rise in temperature is proportional to the water content of the mixture.

The principle behind the method of the present invention is further illustrated with reference to the following Example in which known amounts of water was added to a sample of nominally dry crude oil and the resultant rise in temperature was meas ured.

Example Batch Caiibration Tests were carried out on sam ples of nominally dry Forties crude oil in a 500 watt microwave oven. 100 ml samples of the oil was placed in 125 ml glass jars. A known quality of water was introduced by syringe into each jar. Although the jars were shaken, most if not all - of the water settled to the bottom almost immediately.

Temperatures were measured before heating. The jars containing the sample and water were then heated in the oven for3 minutes. During heating the sides of the jars were lagged with 75 mm resinbonded glass wool, and the lagging was arranged to support the jars with a 1" airspace beneath them. A ballast load of 200 ml of cold water was used in the oven for each heating cycle to protect the magnetron from reflective overload. Only one sample was heated at a time. After heating, each sample was stirred (except that containing 5.0% water, which was shaken) in an attempt to obtain temperature homogeneity and the temperature was measured.

Temperatures were read on a mercury-in-glass thermometer, with the bulb about halfway up the homogenised oil layer. The results obtained were: %water TempRise"F 0.50 21 0.75 24 1.0 27 2.0 32 3.0 34.5 5.0 +60.5 wSampleshaken before heating 7.5 63.0 10.0 70-77.5 approx.

15.0 112-130 approx. (water boiled, some loss of sample).

The above results show that with increasing amount of water there is a corresponding rise in temperature. This can be used to plat a calibration curve and the latter can therefore be used for example, to compute the amount of water present in any given sample of Forties crude oil. Similar calibration curves can be drawn for other liquids containing water.

Claims (7)

1. A method of determining the water content of liquids having a water:liquid dielectric loss factor ratio of at least 80, which comprises monitoring the rise in temperature of the water-liquid mixture upon exposure to microwave radiation.

2. A method according to claim 1 wherein the water:liquid dielectric loss factor ratio is at least 100.

3. A method according to claim 1 or 2 wherein the water-liquid mixture is exposed to microwave radiation by passing the mixture through a tubular member in a microwave cavity.

4. A method according to claim 3 wherein the tubular member is coil shaped.

5. A method according to claim 3 or 4 wherein the rise in temperature of the mixture exposed to microwave radiation is monitored by measuring the temperature of the mixture (a) before entering the cavity and (b) upon emergence from the cavity.

6. A method according to any one of the preceding claims wherein the temperature measurements are carried out by means of thermocouples.

7. A method according to any one of the preceding claims wherein the mixture comprises crude oil and water.