GB2482698A

GB2482698A - Acid concentration measurement

Info

Publication number: GB2482698A
Application number: GB201013451A
Authority: GB
Inventors: Simon Patrick Hunter Wheeler
Original assignee: Mobrey Ltd
Current assignee: Rosemount Measurement Ltd
Priority date: 2010-08-11
Filing date: 2010-08-11
Publication date: 2012-02-15
Anticipated expiration: 2030-08-11
Also published as: GB2482698B; GB201013451D0

Abstract

A method of determining the concentration of a fluid in which the relationship between density and concentration of the fluid passes through either a maximum or minimum value, referred to as the turning point, before respectively either decreasing or increasing again. The method involves determining the viscosity of the fluid and relating this to concentration. The viscosity measurement is taken to establish on which side of the turning point the concentration measure occurs. Preferably the density of the fluid is also determined to improve the accuracy of the determined concentration value. The method can be applied to the measurement of sulphuric acid solution concentrations, which exhibit a monotonic relationship between viscosity and concentration at particular temperatures, and the measurements of density and viscosity can be performed using a single vibrating fork meter.

Description

ACID CONCENTRATION MEASUREMENT

Field of the Invention

This invention relates to acid concentration measurement and in particular, though not necessarily solely, to the measurement of sulphuric acid concentration in water at levels of greater than 94%.

Background to the invention

Conventionally, there is a simple monotonic relationship between acid concentration and density. This is not so with concentrations of sulphuric acid in water above about 94%. As can be seen from Figure 1, at concentrations above 94% the rate of increase in density, relative to concentration, starts declining and, at concentrations of about 97%, the density actually decreases with increasing concentration. As a result, for sulphuric acid at concentrations above 94%, the measurement of density alone, or combined density and temperature, is not sufficient to determine a unique value of concentration.

It is an object of the invention to provide a method which will address the problems set forth above; or which will at least provide a novel and useful alternative.

Summary of the Invention

Accordingly the invention provides a method of determining the concentration of a fluid having a turning point, as herein defined, in the relationship between density and concentration, said method being characterized in that it further includes measuring viscosity.

Preferably said method is applied to the measurement of sulphuric acid concentration.

Preferably said method comprises measuring density and viscosity using a single vibrating fork meter, said meter being selectively operable to give outputs of both density and viscosity.

Many variations in the way the present invention can be performed will present themselves to those skilled in the art. The description which follows is intended as an illustration only of one means of performing the invention and the lack of description of variants or equivalents should not be regarded as limiting. Wherever possible, a description of a specific element should be deemed to include any and all equivalents thereof whether in existence now or in the future.

Brief Description of the Drawings

A working embodiment of vibrating element apparatus embodying the invention will now be described with reference to the accompanying drawings in which: Figure 1: shows a plot of density against concentration for concentrated sulphuric acid; and Figure 2: shows a plot of viscosity against concentration for concentrated sulphuric acid.

Detailed Description of Working Embodiment

This invention describes a method for determining the concentration of fluids having a turning point. That is to say a fluid in which the relationship between density and concentration does not follow a straight line but, as illustrated in Figure 1, passes through a high point and then falls. Similarly this invention could be applied to a fluid in which the relationship between density and concentration falls to a low point, and then rises. It will be appreciated that such fluids present a problem in that a single measure of density could represent two different values of concentration.

As will be apparent from this disclosure, sulphuric acid is one fluid which exhibits a turning point but the invention is equally applicable to all fluids which exhibit such behaviour.

As can be seen from Figure 1, at concentrations of greater than about 94%, a single measure of sulphuric acid density could indicate two different acid concentrations.

Turning now to Figure 2, it will be seen that at sulphuric acid concentrations above about 94% and at given temperatures, i.e. in the region where the density has a turning point, there is a monotonic relationship between viscosity and concentration.

Thus, by measuring viscosity and temperature, it is possible to determine whether the concentration is above or below the value where the density has a turning point.

Then, by measuring density, it is possible to determine the value of the concentration.

It can be seen that, in principle, measurement of the viscosity and temperature would be sufficient to determine the concentration. However the sensitivity of the relative changes of density and viscosity with concentration, and the accuracy of such measurements, make it preferable to measure density in order to determine the concentration.

Density and viscosity may be conveniently measured using a single instrument which is selectively operable to indicate either density or viscosity. One example of such an instrument is a type 7826 vibrating form meter manufactured and sold by Emerson Process Management (www2.emersonprocess.com) The viscosity measure may be displayed or may be used to control an internal switch' that defines on which side of the turning point the concentration measurement lies.

The accurate performance of the invention relies on the fluid being measured behaving as a Newtonian fluid. In the case of sulphuric acid, this requires the absence of hydrogen suiphide (H2S) together with a line pressure of less than about 5 bar. If the pressure is elevated, the liquid's rheology becomes that of a Bingham fluid which could still be measured using a vibrating fork viscosity meter, however a simple offset on the measured viscosity would need to be applied. Should H2S be present, then sulphuric acid becomes non-Newtonian and any fork viscosity measurement would be inaccurate.

It will thus be appreciated that the invention, at least in the case of the embodiment described above, provides a method of accurately determining, in-line, concentrations of sulphuric acid at high concentration levels, using readily available measuring apparatus.

Claims

Claims 1. A method of determining the concentration of a fluid having a turning point, as herein defined, in the relationship between density and concentration, said method being characterized in that it further includes measuring viscosity.
2. A method as claimed in claim 1 when applied to the measurement of sulphuric acid concentration.
3. A method as claimed in claim 1 or claim 2 comprising measuring density and viscosity using a single vibrating fork meter, said meter being selectively operable to give outputs of both density and viscosity.