Plant Construction & Process Technology

Visualization Techniques in Process Engineering

Excerpts from Ullmann’s Encyclopaedia of Industrial Chemistry

08.06.2015 -

The development of new processes and equipment in the chemical and related industries requires the measurement and visualization of parameters such as composition, temperature, velocity, phase state and distribution, density, and pressure in cross sections or volumetric areas. Their measurement should be nonintrusive.

By conventional local intrusive or nonintrusive measurement techniques, physical properties are measured at certain locations as a function of time. To obtain field visualization by cross section or volume of an apparatus, numerous measurements must be taken by placing the measurement probe at different locations. Because of the rather extended time interval for this kind of measurement, only time-averaged information is gathered, especially from non-stationary flows. To overcome these intrinsic measurement problems, nonintrusive measurement and visualization techniques provide high time and spatial resolution over a complete cross section of an apparatus without interfering with the process itself.

Optical Visualization Techniques

Numerous optical techniques have been used to visualize two- and three-dimensional fields of different parameters such as velocity, density, concentration, temperature, etc. They are nonintrusive and reproduce the measured field parameters with high spatial and time resolution. Their applications require optical accessibility of the measurement object. Video or high-speed camera observations require visible light.

The properties of visible light that are used for measurement and visualization purposes are the intensity, the frequency, and the phase shift of coherent monochromatic light, as well as the velocity of light in media with varying optical density (refractive index).

In Raman spectroscopy, measurements are taken from a plane, which is illuminated by monochromatic light. Electrons hit by a photon are lifted to a higher energy level. When they return to their original energy level, a photon of different energy with a shifted wavelength is emitted. Different types of scattering (e.g., Stokes and anti-Stokes Raman scattering, Rayleigh scattering) are observed depending on this shift. Their analysis leads to the measured temperature or concentration fields.

Infrared light due to thermal radiation is used for measurement and visualization techniques like thermography and pyrometry. Depending  on its temperature, each surface has a certain radiant emittance, which is measured by radiation detectors. For recalculation of the temperature, the surface properties of the examined material, the Kirchhoff radiation laws, and the dependence of the radiation on the emission angle must be known. Several measurement systems are used for the two-dimensional imaging of temperature fields.

Holographic measurement techniques lead to three-dimensional information related to the observed volumetric field. The volume is trans-illuminated by monochromatic light, which is scattered and reflected by the measurement object onto a photographic plate. At the same time, parts of the monochromatic light are led directly from the light source to the photographic plate. The two light beams with passway differences smaller than the coherence length, called the object and reference beams, interfere and form a stable interference pattern on the photographic plate. When the plate is trans-illuminated by the reference beam, the measured object is reconstructed three-dimensionally and can be viewed from different angles.

By taking holograms of an object from two directions, its position can be assessed by cutting the two holograms into slices and comparing them to each other. This kind of technique provides high spatial and temporal information on the process and has been successfully employed for measuring particle sizes and velocities in sprays and bubbly flows. To measure the particle velocity, two holograms are taken at short intervals. By comparing the two holograms, the paths of the particles and their velocity vectors can be measured.

Real-time interferometry is applied for continuous measurements of variations in temperature or concentration profiles.

The hologram of the cross section of a wind tunnel is recorded by the first object beam, which is transmitted through the test section and interferes with the reference beam. The latter is directed past the test section onto a photographic plate. After processing of the photographic plate it must be repositioned. If the reference beam passes the hologram plate, it is diffracted at the microscopic interference pattern, and the first object beam is reconstructed. When the concentration field changes because of injection of carbon dioxide, the refractive index varies. As a result the optical path length of the second object beam differs from that of the first object beam, which is recorded on the hologram plate. Because of the phase shift, a fringe pattern is deflected, which enables visualization of the concentration field in a cross-sectional area of the wind tunnel.

In a similar manner the temperature profiles caused by dissipative heating in viscous polymer solutions are reconstructed to an accuracy of 0.01°C near a rotating stirrer.

Particle image velocimetry (PIV) has become a valuable tool for measurement of two-dimensional velocity fields in flow fields of different types. From these measurements further information on vorticity, path lines and Reynolds stresses are derived. For imaging, the measurement volume is illuminated with a laser sheet for a short time. The illuminated sheet is taken by a CCD camera and digitally stored. This procedure is repeated within time intervals to obtain information on the velocity field. By processing the stored images (in the simplest way, by cross-correlation of two images) the velocity field in the illuminated part of the flow field is calculated. For application of PIV in two-phase flow, special algorithms for the detection of gaseous or liquid particles as well as for the calculation of the flow-field are available.

Laser-induced fluorescence (LIF) methods are based on laser excitation of molecules followed by their natural fluorescence. For this purpose the molecule of interest absorbs one photon of the incident laser light and is shifted to an upper energy state. The exited molecule drops back to a stable energy level, and the emitted radiation is characteristic of the concentration and temperature of the observed species of molecule. The exciting photon energy must be selected to be equal to the difference of two energy levels of the molecule. Since the energy differences are specific for each species of molecule, the frequency of the laser must be chosen in accordance with the molecule of interest.

Two-color laser-induced fluorescence is applied for visualization and measurement of the progress in mixing on macro- and microscale simultaneously. This is done by injecting a mixture of two fluorescent dyes into the mixing vessel. Both are excitable at the same wavelength, and their emission characteristics are distinguishable. For mixing purposes the inert dye serves as a tracer for macromixing. The other dye undergoes a chemical reaction that alters its fluorescent behavior. Since the chemical reaction requires mixing on the molecular scale, the reacting dye visualizes the micromixing indirectly. This is quantitatively done by means of the degree of deviation, derived from the ratio of the local concentration of the reacting dye and the concentration of the inert dye. The latter equals the concentration the reacting dye would have locally if the reaction had not taken place.

Tomographic Visualization Techniques

Tomography has become increasingly popular in the chemical and process engineering industry. It is a nonintrusive visualization technique for two- and three-dimensional fields of density, concentration, temperature, velocity or local void fractions in cross sections of different kinds of equipment. Since the measured fields are often of non-stationary or even of transient character, tomographic measurement techniques should provide high spatial and temporal resolution.

The term tomography originates from the Greek tomos (slice) and graph (image). The two-dimensional images visualize the areal or volumetric profiles of a physical property. For medical applications this might be the density or the water fraction related to the texture of a tissue. In process and chemical engineering the applications of tomographic measurement techniques are, in comparison to the medical sector, still in rapid development for applications in research and production.

Tomographic measurements have two major advantages: First, the measurements are conducted without interfering with the object. Second, the target quantities in the measurement plane are generated by mathematically derived reconstruction techniques from simultaneously taken integral measurements of local values along defined passways.