CHAPTER 1: CO 2 CAPTURE
4. Experimental Equipment and Techniques
4.1 Introduction to density
CHAPTER FOUR: EXPERIMENTAL EQUIPMENT AND TECHNIQUES
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measurements of liquids have been reported in the literature [210], [209]. There are different types of manual and automated density measuring instruments (Densitometers) used in various process applications which are based on these operating principals. The selection of the densitometer type normally depends on the performance requirement, application, and budget. Although not a complete list, in this section, I focused on some laboratory methods that are most often used in research and quality.
4.1.1.1! Oscillating U-tube
The oscillating U-tube is a technique used for density determination of flowing systems based on an electronic measurement of the oscillation frequency, from which the density value is measured [207]. It usually consists of a mechanical oscillator constructed in the form of a U-tube (oscillating U-tube) with fix mounted ends. The u-shaped tube is vibrated at the resonance frequency of the oscillator. The eigenfrequency of the oscillation of the U-tube is influenced by the sample 's mass and therefore by the density of the sample [211]. The resonance frequency, ω, is inversely proportional to the square root of the total mass of the tube which is the sum of the masses of u- tube and the inner contents of the tube [207]. Typically, the oscillation period,!F, (1 divided by resonance frequency),!measured instead of the frequency. This period can be measured with high resolution in a simple relation to the density of the sample in the oscillator according to the following formula:
!
Where A and B are the relevant constants of each oscillator [212]. Based on equation (4-3), the density is calculated by knowing the period of oscillations of the U-tube and the reference oscillator quantities. Since the quantities of both the tube mass and the tube's inner volume are known, the vibrating tube method enables calculating the density of unknown fluids after performing a proper apparatuses calibration with two fluids (usually water and air) [213].Despite modern devices that indicate digital density values, in older instruments, the density must be calculated from the measured period of oscillations and the calibration constants [209].
„ =3.Fê−)
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4.1.1.2! Pycnometry
Pycnometery is a technique that uses a defined volume filled with a liquid to determine the liquid density and specific gravity by weighing the mass. Density is then calculated using the equation (4-1) and specific gravity is determined by dividing the density of a substance to the density of water at a defined temperature [214],[215]. The instrument based on this technique is called a pycnometer which is a glass or metal container with a close-fitting ground glass stopper having a capillary hole through it so that air bubbles can escape from the equipment [214]. This enables the accurate measurement of the density of a fluid by reference to an appropriate calibration liquid such as water or mercury [215]. Pycnometers are commonly used under ambient conditions for measuring the density and specific gravity of different materials. The density of the sample can be calculated from the difference in weight between the full and empty pycnometer divided by its known volume [214].
4.1.1.3! Hydrostatic methods
The hydrostatic method is based on Archimedes' principle which states the buoyant force experienced by an immersed object is equivalent to the weight of the fluid it displaces [207], [214].
This method covers density measurements using the following techniques:
4.1.1.3.1! Buoyancy
The buoyancy method is often used to determine the density of bodies and liquids. The liquid density can be determined by measuring either the upward buoyant force of the body or measuring the immersion depth. Through this principle, the volume and therefore the density of an irregularly shaped object can also be obtained by calculating its mass in air and its actual mass while immersed in water. Accurate density measurement, therefore, is highly dependent upon precise weight values [209].
4.1.1.3.2! Hydrostatic balance
A hydrostatic balance is a very accurate balance with a sinker of precisely defined volume. The
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liquid sample, afterward, the apparent weight loss of the sinker is determined. Based on the Archimedes principle, the apparent weight loss of the sinker is equal to the weight of the fluid it displaces so that the precise volume and precise weight are known [209],[214].
4.1.1.3.3! Hydrometer
A hydrometer measures directly the density and specific gravity of liquids from the immersion depth. Hydrometers operate based on the Archimedes Principle or the principle of flotation There are different types of hydrometers; however, the most common form includes a sealed, long- necked glass cylinder with a bulb filled with a metal weight at one end and scale going up at upper part. Based on the depth of hydrometer flotation into the liquid, the density and specific gravity can be read from the scale on the neck. Hence, a hydrometer sinks deeper into a low-density liquid than into that of high density[209],[214].
4.1.1.3.4! Hydrostatic pressure (pressure sensor method)
Hydrostatic pressure is the pressure exerted by the weight of the liquid above a measurement point of the static liquid. By measuring the hydrostatic pressure using a pressure sensor, the density can be calculated from the following very general formula:
ρ = P hg
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where
p = pressure in fluid (Pa)
ρ = density of the liquid (kg/m3)
g = gravimetrical acceleration (9.81 m/s2)
h = height between the liquid level and pressure sensor (m)
For closed systems, two pressure sensors are placed at the height of Δh from each other. In such systems, the liquid density can be determined by measuring the vertical distance between two pressure sensors, Δh, and the pressure difference, Δp [207].