A characteristic indicator of many technologic processes is medium viscosity or other rheological properties changes during the process.
For example, in the fermentations by the growth of microorganisms the viscosity of cultivation media changes. In the process of lacquer production various resins are synthesized. By the end of the process of synthesis viscosity starts rapid increase. However, in milk production kefir, curdled milk and yoghurt are produced with the fermentation technologies. As the milk mass curdles, its solidity changes which is characterized also with increase of viscosity. In all of the mentioned cases change of the technological medium viscosity “informs” about the end of the process or about the need to perform influence on the process. In addition to the given examples one can find comparatively many examples where the characteristic parameter of a process would be viscosity.
Currently the described processes cannot be yet directly controlled depending on their viscosity because there are no devices with the help of which it would be possible to comparatively easily on-line measures the viscosity in various technologic processes. The given device should be convenient, robust, not complicated, with comparatively small dimensions (at least the transmitter part which is immersed in technologic environment) and with an economic price. The available systems are expensive, complicated and require auxiliary equipment. For this purpose in 2006 in Latvian State Wood chemistry institute a project of European Structure Fund Development (ERAF) ”Indicator of rheological characteristics as a tool for control of biomass and other production processes” was commenced. Now the project is in the final phase and Latvian (P-08-92 from 26.05.2008) and European patents (Priority Nr. 08010576.0/EPO8010576 from 11.06.2008) are pended.
Taking into consideration the above mentioned principles and conditions the viscosity controlling device prototype was developed.
The given device (see block scheme) consists of a transmitter which is made on the basis of a bimorph piezoelectric multi-layer element (1), which is connected to a bimorph element signal processing block (2) and signal generating device (3). The output signal generated by the block (2), then transformed into 4-20 mA or 0-10V signal form, is transferred to the process logical controller PLC or another control device (4). The transmitter is initiated with 30 V impulses which are generated in the signal generating device (3) and from the moment of initiation of signal the damping oscillations in the block (2)are analyzed. PLC or another control device (4) by analyzing the signal of viscosity changes issues control signal to executive mechanisms of production for regulation of technological process in accordance with the given algorithm. The given algorithm foresees actions which are needed when reaching a value of specified for viscosity. The algorithm depends on character of technological process.
Block schema of viscosity control device
Transmitter’s (1) construction is supplemented with elements which significantly increase oscillations amplitude initiated in the bimorph piezoelectric element and the damping time of oscillations, thus increasing sensitivity of the transmitter and thus also the viscosity limits foreseen for control.
Transmitter’s signal in the signal block (2) is processed with analogue to digital converter. Further the amplitude of each oscillation period is defined and on the basis of the given information a logarithmic fluctuation damping decrement d is determined in accordance with the following formula:
- d – logarithmic damping decrement of oscillations
- n – number of oscillations
- xo – reference oscillation range
- xn – n oscillation amplitude
Between the damping decrement and dynamic viscosity there is an increasing functional coherence:
This coherence is determined experimentally, by performing calibration of the device with different medium viscosities. The given calibration function is entered into the block (2). A signal is formed in the output of signal processing block which complies with one of the industrial analogue signal ranges (for example, 4 – 20 mA or 0 – 10 V). Furthermore, the given exit signal is directly proportionate with the dynamic viscosity.
In order to determine correlation function of the signal processing block As a criterion for issue of a message in respect to the actions necessary for completion of fermentation process. the fluctuations initiated by the transmitter were analyzed (1). For this purpose an oscillograph Textronix TDS2024 B was used; information gained by it was analyzed and processed with computer software in graphical and tabular form. Viscosity was modeled by the means of various concentration CMC (carboxymethyl cellulose) solutions. Dynamic viscosity was determined with the help of the viscosimeter SV-10 (A&D Company, Limited). In such manner the damping curves of the transmitter (1) in viscosity range from 1 mPas to 344 mPas with optimal viscosity transmitter (1) construction dimensions were gained:
In order to determine the type of control of partial pressure pO2 of the dissolved oxygen (i.e. with the mixer rotation speed change, oxygen enrichment, substrate adding or otherwise);
As a criterion for issue of a message in respect to the actions necessary for completion of fermentation process. The process control strategies for other processes can be developed based on analysis of on-line viscosity measurements.