Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
Instrumentation - Acid Concentration
January 4, 2017
The on-line continuous measurement of sulphuric acid/oleum is based on the fact that a physical property will change as the concentration changes. Properties such as conductivity, refractive index and sound velocity are typically used to measure sulphuric acid/oleum concentrations. The recommended methods of measurement over the whole range of sulphuric acid and oleum are given below.
Electrical conductivity is a property of a fluid that can be used to determine its concentration. The graph below shows the conductivity (micromhos) at 25ºC for several different fluids, including sulphuric acid from 0 to 100%wt. The vertical axis ranges from 0 to 900,000 micromhos.
For sulphuric acid, the conductivity varies such that for a given conductivity there could be more than one concentration. This makes conductivity unsuitable for measurement over the full range of acid concentration (i.e. 0 to 100% H2SO4). However, conductivity can be used where the concentration will vary over a small range and where the range will not cross over one of the peaks or valleys of the conductivity curve.
The refraction of light through sulphuric acid will depend on the temperature and concentration. This property can be used to determine the concentration of sulphuric acid.
Energy radiated from a LED passes through the prism surface to be reflected off a mirror to the prism-to-process interface. The light reaching this interface intersects the same interface over a series of angles chosen to include the critical angle (Ic) for the process being measured. Light intersecting the interface at an angle greater than the critical angle is refracted into the solution. Light intersecting the interface at less than the critical angle is reflected up to a mirror and out of the prism up to the charge coupled device (COD) linear array to be scanned.
The image seen by the CCD is composed of simple light and dark periods. As the refractive index of the process changes, the change in critical angle, changes the ratio of light to dark periods as seen by the CCD. This time period change is further enhanced and displayed as a reading in refractive index.
The velocity of sound in sulphuric acid/oleum can be used to determine the acid concentration. The first graph illustrates the variation of sound velocity with acid concentration (0 to 100% H2SO4) and second graph illustrates the same relationship except for oleum.
The nature of sound velocity in sulphuric acid makes this property ideal for determining acid concentrations in the range of 60 to 100% H2SO4. Calibration of the instrument is done by comparison with acid of known concentration.
The velocity of sound in sulphuric acid/oleum is affected by the presence of bubbles and cleanliness of the acid. The sample must be drawn off in such a manner as to avoid entrainment of bubbles and foreign matter which will affect the accuracy of the measurement.
The recommended methods of measurement over the whole range of sulphuric acid and oleum are given in the following table.
|95-100%||-||Conductivity, Sound Velocity|
|Conductivity and Sound Velocity||-||SensoTech GmbH||LiquiSonic||Accuracy: ± 0.05 wt%
Mtg Flg: 51191/EP129A (2")
Temperature compensation required
|Rosemount||Model 222 Toroidal Conductivity||Maximum
Maximum Pressure: 125 psig
Connection: 1" or 2" - 150 lb Flange
Temperature compensation required
|Refraction||-||The Electron-Machine Corp.||
Accuracy: ± 1% of span or 0.000075 Refractive Index (RI) whichever is
Compact Process Refractometer PR-43-GP,
Refinery Probe Process Refractometer PR-23-RP.
Range: Measurement range covers 0-100 % by weight (Refractive Index nD 1.3200 – 1.5300).
Remarks: Reliable and accurate product based on digital sensing technology; even large amounts of suspended solids, bubbles or color changes do not affect the measurement or the accuracy. The accuracy is ±0.1 % b.w. and the measurement is automatically temperature compensated. The technology allows configuring low and high concentration alarms. Intrinsically safe and hazardous area certification is provided for hazardous areas. Special wetted parts materials such as AISI 904L stainless steel, Alloy 20, Hastelloy C-276, Nickel 200 and Titanium ASTM B348.
Process temperature range: -20°C - 150°C / -4°F – 302°F.
|Model 86 Sonic
0.1wt% for 90-100%