Side stream monitoring

The exposure of sensors in a by-pass stream (which can be valved off), is an alternative way of collecting monitoring data although correlation is required between the main-stream and the by-pass The use of a side-stream taken either side of a choke in the main-stream can provide a useful monitoring point. Traps where product streams can be condensed can offer alternative sampling systems. 

Ultraviolet (UV) spectroscopy, mass spectrometry (MS), refractive index (RI) detection, and electrochemical detection (ECD) are common online monitoring techniques for analytical chromatography. UV and RI are regularly used for monitoring preparative operations as well. To employ MS or ECD in a high-flow scheme, usually a side stream must be diverted from the main eluate line via a flow splitter so that what passes through the detector has a flow rate that is acceptable for an analytical-scale system. 

Fryer and Pritchard [1989] emphasise the need to monitor fouling in a food process stream in order that the optimum time for cleaning may be determined so that for instance, a soft otherwise easy to remove deposit, is not converted to a hard intractable form by the action of heat. These authors describe two possible fouling monitors that may be used in a "side stream" fi-om the main process stream. A more detailed discussion of fouling monitors is to be found in Chapter 17. 

The sensor is applied in the sidestream, with a requirement to separate water to prevent unnoticed shifts of the measured value due to aspirated water. The main advantage is the possibility of an easy in-line combination with IR optical sensors which allow the essential anesthetic gases to be measured with one monitor in the sidestream. The fast response of this arrangement cannot be used to calculate derived parameters as the application in the side-stream generates undefined and changing delay times. 

Often the physical state of the sample (gas, liquid, or solid) is dictated by the physical principle upon which the measurement is based, and a phase conversion may be required. Some instruments (for example, gas chromatographs and infrared analyzers) utilize either gas or liquid samples the appropriate phase may be chosen to optimize any of the instrumental parameters discussed earlier. If the measurement is temperature sensitive and the sample temperature may vary, temperature compensation or temperature control must be provided. Temperature compensation is usually supplied as part of the measuring system the sample temperature is continuously monitored and the measurement signal electrically corrected to offset the temperature effects. When temperature control is required, a batch sample or a sample side-stream is used, and the sample temperature is adjusted prior to measurement. This procedure adds dead-time to the measurement because the sample has to be in the temperature converter long enough to come to thermal equilibrium. 

Phattaranawik J, Fane AG, Pasquier ACS. Studies of air slug distributions and preliminary membrane fouling by optical monitoring in a side-stream membrane module. Sep. Sci. Technol. 2009 44 3793-3813. 

In future, it will be indispensable to create databases for agricultural feedstocks and their side streams to document the range of variability in composition and quality. Such databases can be obtained through a long-term monitoring and documentation of raw materials from different origins. Furthermore, it is desirable if the bioprocess itself is not sensitive against a certain variabihty of the feedstock quality. 

The wash oil that is used for de-entrainment is also important for improving the separation between the bottom side stream and the resid stream. Separation is enhanced by condensation of the overflash by vaporizing the bottom sidestream. The amount of overflash that is required is affected by packing type, depth and source of the wash oil. Overflash flow rates should be carefully monitored to make sure that there is no degradation in the bottom side stream fractionation, that there is no increase in pitch entrainment to the heavy solvent neutral stream and there are no major increases in coking in the wash bed zone. 

Side stream monitoring encompasses the temporary use of a by-pass spool or off-take from the main process stream being monitored to provide supplementary information to that produced by the online monitoring devices. Side stream monitoring may be used to examine the short-term effect of chemical additives or process changes upon the electrochemistry of the plant-material-process-fluid system. 

The side stream equipment should comprise one or more probe or coupon holder, and flow control and metering devices. It should be used to supplement on-line corrosion monitoring methods and should not be used in isolation, unless on-line methods are not practical. 

Two different types of dynamic test have been devised to exploit this possibility. The first and more easily interpretable, used by Gibilaro et al [62] and by Dogu and Smith [63], employs a cell geometrically similar to the Wicke-Kallenbach apparatus, with a flow of carrier gas past each face of the porous septum. A sharp pulse of tracer is injected into the carrier stream on one side, and the response of the gas stream composition on the other side is then monitored as a function of time. Interpretation is based on the first two moments of the measured response curve, and Gibilaro et al refer explicitly to a model of the medium with a blmodal pore... 

This technique is based upon the detection of corrosion products, in the form of dissolved metal ions, in the process stream. A thin layer of radioactive material is created on the process side of an item of plant. As corrosion occurs, radioactive isotopes of the elements in the construction material of the plant pass into the process stream and are detected. The rate of metal loss is quantified and local rates of corrosion are inferred. This monitoring technique is not yet in widespread use but it has been proven in several industries. 

The reduction of the sample was made at 2250 K In a flowing stream of hydrogen carrier gas ( SO cm /mln). The total pressure of the carrier gas was approximately 1 atm. The water vapor produced during the reduction was swept by the carrier gas Into an electrolytic-type (P2O5) moisture monitor and a continuous recorder trace of the water concentration as a function of time was obtained. A typical plot of the moisture content of the carrier gas as a function of time Is shown In Figure 3. The region on the left side of this figure where the moisture content... 

Fire monitors should be provided and positioned so that at least two monitor streams can reach each process area, preferably from opposite sides. Often, fire monitors are mounted directly on hydrants. Care should be taken to select and position fire monitors to ensure that their streams can reach elevated equipment. The local fire main s residual pressure under flowing conditions should be determined to ensure it is adequate to deliver effective water streams capable of reaching all elevated equipment. While monitors need to be close enough such that their stream reaches elevated equipment, if they are too close it may not be possible to physically access the monitor to direct its stream due to radiant heat. In these cases, some form of personnel shielding or remote operation is required. 

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