Off-gas analysis

Off-gas analysis is widely used in many industrial fermentation plants to determine the cellular activity of growing cultures by monitoring respiration. One can measure oxygen uptake and CO2 production rates and thus measure metabolic activity/9 In addition, off-gas analysis is also used for monitoring other volatiles, the synthesis of which are strongly dependent on cultivation conditions 10 and product formation. 11 Off-gas estimation and control therefore serves as an indirect method for process analysis and control. 

Off-gas analysis is routinely carried out either by mass spectrometry or by gas chromatography (GC), in contrast with traditional infrared and paramagnetic off-gas analyzers which are maintenance intensive and have slow response times and significant calibration drifts. 12  

Chromatography is an equally reliable and relatively inexpensive technique for analysis of both volatiles (gas chromatography (GC)) and nonvolatiles (liquid chromatography (LC)). Although somewhat slower (2-3 min analysis time after each injection) the availability of ultrafast capillary GC and ultrafast HPLC (high-performance liquid chromatography) renders this technique equally competitive.(14) With 

The use of off-gas analysis, however, is a relatively expensive proposition as a means ofmonitoring/controlling extracellular parameters. It is also an indirect method and is generally used to obtain information about the synthesis of products that cannot be analyzed directly. It is possible, however, to use inexpensive phase fluorimetry-based pCD and pCCh sensors to obtain the same information. 

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Chemical Reaction Measurements. Experimental studies of incineration kinetics have been described (37—39), where the waste species is generally introduced as a gas in a large excess of oxidant so that the oxidant concentration is constant, and the heat of reaction is negligible compared to the heat flux required to maintain the reacting mixture at temperature. The reaction is conducted in an externally heated reactor so that the temperature can be controlled to a known value and both oxidant concentration and temperature can be easily varied. The experimental reactor is generally a long tube of small diameter so that the residence time is well defined and axial dispersion may be neglected as a source of variation. Off-gas analysis is used to track both the disappearance of the feed material and the appearance and disappearance of any products of incomplete combustion. 

There are several methods to monitor the off-gas analysis. Online gas chromatography is commonly used. The daily operation for inlet and outlet gases is balanced to project growth in the bioprocess. High operating cost is the disadvantage of the online system. 

Additionally, there is a growing use of off-gas analysis for measuring oxygen uptake, CO2 evolution and respiratory quotient measurement. 9, 641 However, this is not likely to find widespread use despite its usefulness unless the cost involved can be lowered. 

S. J. Coppellaand P. Dhuljati, Low costcomputercoupled fermentor off-gas analysis viaquadrupole mass spectrometer, Biotechnol. Bioeng. 29, 679-689 (1987). 

Off-Gas Analysis. Gas samples are initially cleaned of particulates and dried to 2% moisture before analysis. Carbon monoxide and carbon dioxide are measured continuously using a Horiba Mexa-300 CO analyzer and a Horiba Mexa-200 CO2 analyzer. Syringe samples are taken downstream of the CO2 analyzer for gas chromatographic analysis, A room temperature molecular sieve 13X column is used to analyze for carbon monoxide, oxygen, and nitrogen, A Poropak Q column at 130°C is used to analyze for carbon dioxide, methane, ethane, ethylene, sulfur dioxide, and hydrogen sulfide. 

Process mass spectrometry (MS) is a very powerful technique for process monitoring and control, providing a unique combination of speed, selectivity, dynamic range, accuracy, precision and flexibility. The technique has become a standard for gas-phase analysis in several industrial applications, including steel manufacturing, fermentation off-gas analysis, and the production of ethylene oxide and ammonia. Among its attributes ... 

A method of off-gas analysis and validation will therefore have to be developed before EDS tests are conducted. Because the Demo II gas samples could not be certified to be free of agent, they could not be taken off-site for complete analysis. 

Accurate modeling of microbial solubilization of lignocellulose will be dependent on knowledge of the dynamics of microbial cell concentration over the course of bioconversion. While measurement of cell concentrations distinct from the concentration of substrate is trivial for soluble substrates, it is a substantial and not-yet-resolved challenge for fermentation of particulate substrates based on plant cell walls. Cell measurement has been approached on the basis of elemental composition (pellet nitrogen, [25]), concentration of cellular macromolecules (total protein [26] or DNA via quantitative PCR [27]), and estimated by indirect methods, such as off-gas analysis [25] and detection of enzymes (ELISA assays [28]). Future efforts using quantitative proteomics approaches also hold promise. 

TGA-FTIR and Headspace GC-MS testing of PPgMAH shows that these materials are much more thermally stable compared with PPgAA. Figure 3.6 shows a TGA curve for PPgMAH. The onset temperature is 451°C, which is similar to virgin polypropylene. Off gas analysis shows evidence of carbon dioxide, carbon monoxide, maleic anhydride, and water. 

During the production, measurements of pH, dissolved oxygen (pOj), temperature, pressure, air flow, off-gas analysis, and batch weight are collected by the process computer system (Figure 27.3) and the process engineers can use these measurements to evaluate the development of the fermentation and interfere if something unexpected should occur. 

The effect of gas leaks on cell tests may be separated into two components a loss of driving potential (Emf) caused by O2 entering to convert H2 to H2O, and a volumetric loss of fuel affecting the fuel utilisation, caused by a pressure gradient versus the ambient. A gas leak may be before, in, or after the cell in a test set-up fuel line. Leaks before the cell affect the intended fuel composition and quantity. However, a true average fuel composition may be obtained from the cell OCV. Leaks after the cell are of no importance to the ceil test, unless off-gas analysis is carried out. Leaks in the cell or in seals cause inhomogeneous gas composition over the cell, even at OCV. 

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