Spectroscopy approach to methanol detection in waste fat methyl esters.

Second-generation biodiesel manufactured from waste cooking oils (WCO) and inedible animal fats (AF) are one of the alternatives to the first generation (1G) vegetable oil-based biodiesel. In this study, a quality control method is proposed to evaluate methanol content in waste fat methyl esters and is based on near infrared spectroscopy (NIR) combined with multivariate analysis. More specifically, calibration models are constructed using partial least squares regression (PLS) for the prediction of methanol content in rapeseed oil methyl ester (ROME), waste cooking oil methyl ester (WCOME), chicken fat methyl ester (CFME) and pork fat methyl ester (PFME) by Vis-NIR spectrometer. The calibration models are based on the absorbance spectra and computed data from five wavelength regions of 400-2170 nm, 780-2170 nm, 1400-2170 nm, 1400-1600 nm and 1970-2170 nm. For the cases with the highest prediction ability obtained in this study, the coefficient of determination of the model's goodness-of-fit for methanol concentrations range 0-5% (v/v) was R2 > 0.990, and for concentrations 0-1% (v/v) was R2 > 0.994, indicating the spectroscopic approach effectiveness in methanol content detection relevant to the biofuel quality assessment. A pseudo-univariate limits of detection (LODpu) and quantification (LOQpu) as well as ratio of performance to deviation (RPD) were used to confirm the validity and to evaluate the practical applicability of developed models. In addition, the obtained results indicate the possibility of developing a transmission sensor for online monitoring of the production process and the quality of biofuel.

The influence of finite aperture and frequency response of ultrasonic hydrophone probes on the determination of acoustic output.

The influence of finite aperture and frequency response of piezoelectric ultrasonic hydrophone probes on the Thermal and Mechanical Indices was investigated using a comprehensive acoustic wave propagation model. The experimental verification of the model was obtained using a commercially available, 8 MHz, dynamically focused linear array and a single element, 5 MHz, focused rectangular source. The pressure-time waveforms were recorded using piezoelectric polymer hydrophone probes of different active element diameters and bandwidths. The nominal diameters of the probes ranged from 50 to 500 microm and their usable bandwidths varied between 55 and 100 MHz. The Pulse Intensity Integral (PII), used to calculate the Thermal Index (TI), was found to increase with increasing bandwidth and decreasing effective aperture of the probes. The Mechanical Index (MI), another safety indicator, was also affected, but to a lesser extent. The corrections needed were predicted using the model and successfully reduced the discrepancy as large as 30% in the determination of PII. The results of this work indicate that by accounting for hydrophones' finite aperture and correcting the value of PII, all intensities derived from the PII can be corrected for spatial averaging error. The results also point out that a caution should be exercised when comparing acoustic output data. In particular, hydrophone's frequency characteristics of the effective diameter and sensitivity are needed to correctly determine the MI, TI, and the total acoustic output power produced by an imaging transducer.

Hydrophones' effective diameter measurements as a quasi-continuous function of frequency.

The spatial averaging effect is strongly dependent on the active aperture of the hydrophone probes used to measure ultrasound fields. An experimental method was developed to determine the effective diameter of the probes as a quasi-continuous function of frequency. The implementation of the method utilizes the time delay spectrometry (TDS) technique and a set of focused acoustic sources. The use of focused sources ensured plane wave conditions for the whole frequency range and TDS eliminated all the reflections from the water tank boundaries. This approach allows effective diameter of circular aperture hydrophones to be determined as a quasi-continuous function of frequency up to 40 MHz. The measurements were performed for both needles and membrane designs having nominal diameters ranging from 50 to 500 microm. The results were successfully employed in the development of spatial averaging correction algorithms. Current efforts are being focused on extension of the frequency range up to 60 MHz by using a novel measurement technique termed time gating frequency analysis.