Biomedical subjects

Carmen Stavarache

Publications and source records attributed to Carmen Stavarache.

7 recordsLinked to original sources

PubMed2007

Ultrasonically driven continuous process for vegetable oil transesterification.

A bench scale continuous process for the manufacture of biodiesel from neat vegetable oils under high power low frequency ultrasonic irradiation was investigated. The experimental studies explored variations in alcohol-oil stoichiometry and type of oil. Important parameters such as residence time and reaction volume were considered. The highest conversion was achieved when short residence time was employed. The transesterification under ultrasonic irradiation is mainly influenced by the residence time in the reactor and alcohol-oil molar ratio.

Esterification↗

PubMed2006

Aspects of ultrasonically assisted transesterification of various vegetable oils with methanol.

The batch transesterification of vegetable oil with methanol, in the presence of potassium hydroxide as catalyst, by means of low frequency ultrasound (40 kHz) was studied with the aim of gaining more knowledge on intimate reaction mechanism. The concentration of fatty acid methyl esters, of mono-, di- and triglycerides of the actual reaction mixture were determined at short reaction time by HPLC. The effect of ultrasounds on the lipids transesterification correlated with triglyceride structures is discussed. It was found that under ultrasonic activation the rate-determining reaction switches from DG-->MG (classical mechanic agitation) to MG+ROH-->Gly+ME (ultrasonically driven transesterification).

Journal Article↗

PubMed2005

Ultrasonic versus silent methylation of vegetable oils.

The profile of fatty acid methyl esters (FAME) of different vegetable oils produced under ultrasonic irradiation and conventional heating were compared. In the presence of potassium hydroxide as catalyst, the distribution of FAME was quite similar for both procedures, while in the case of sodium hydroxide ultrasonic irradiation gave better results. The FAME profile resulted from the reaction catalyzed by sulfuric acid was almost the same with the one resulted from the reaction catalyzed by KOH, while boron trifluoride can give rise to many artifacts, thus is not a reliable catalyst.

Esters↗

PubMed2005

Fatty acids methyl esters from vegetable oil by means of ultrasonic energy.

The transesterification of vegetable oil with short-chain alcohols, in the presence of base-catalyst, by means of low frequency ultrasound (28 and 40 kHz) in order to obtain biodiesel fuel was studied. By using ultrasounds the reaction time is much shorter (10-40 min) than for mechanical stirring. The quantity of required catalyst is 2 or 3 times lower. The molar ratio of alcohol/oil used is only 6:1. Normal chain alcohols react fast, while secondary and tertiary alcohols show some or no conversion after 60 min of reaction. Surprisingly, 40 kHz ultrasounds are much more effective in the reduction of the reaction time (10-20 min). Twenty eight kilohertz give slightly better yields (98-99%), but longer reaction time, while higher frequencies are not useful at all for the transesterification of fatty acids.

Alcohols↗

PubMed2004

Short-time sonolysis of chlorobenzene in the presence of Pd(II) salts and Pd(0).

The effect of PdSO4, PdCl2 and Pd(0) on the degradation of chlorobenzene in aqueous solution within 10 min under 200 kHz ultrasonic irradiation, was investigated. The reaction products remaining in the aqueous mixture were analyzed and quantified. The mechanism of chlorobenzene decomposition having benzene as key intermediate is discussed.

Journal Article↗

PubMed2003

A comparison between the sonochemical and thermal reaction of 5H,5Cl-dibenz[a,d]cycloheptatriene with nitrobenzene.

The reaction of 5H,5Cl-dibenzo[a,d]cycloheptatriene with nitrobenzene was investigated under both thermal (210 degrees C) and ultrasonic (50 degrees C, 40 kHz) activation. The reaction products of both procedures are similar, but their amounts depend on the activation source. To account for the products a common electron transfer reaction is postulated through the dibenzotropylium cation. The differences between the thermal and ultrasonic process is thought to arise from the role of nitrobenzene as oxidant. Under thermal conditions this occurs throughout the reaction mixture but under sonication it occurs only in the cavitation bubble and in its immediate vicinity.

Journal Article↗

PubMed2002

Sonolysis of chlorobenzene in Fenton-type aqueous systems.

The influence of ultrasounds (200 kHz frequency) on the decomposition of chlorobenzene (CB) in a water solution (around 100 ppm concentration) containing iron or palladium sulfates was investigated. The intermediates of the sonolysis were identified, thus allowing a deeper insight into the degradation mechanism. It was established that CB degradation starts by pyrolysis inside the cavitation bubbles. The initial sonolysis product is benzene, formed in a reaction occurring outside the cavitation from phenyl radicals and the hydrogen atoms sonolytically generated from the water. Polyphenols as products of the CB sonochemical degradation are reported for the first time. The palladium salt was found to be a useful and sensitive indicator for differentiating the sites and mechanisms of the product formation. An alternative mechanism for the CB sonolysis is advanced, explaining the formation of phenols, polyphenols, chlorophenols and benzene.

Benzene↗