Reference flows

Unit process Dust emission per unit process reference flow, O (kg/ton or kg/MJ) Unit process reference flow per product system reference flow, F (ton/ton or MJ/ton) Dust emission per product system reference flow, D x F (kg/ton steel-sheet)... 

For each unit process, a reference flow may be defined, and the inputs and outputs to the unit process calculated in relation to the reference flow. For instance, the reference flow for mining of iron ore is the mass of iron ore mined per year, and the emissions to the air may be expressed as kg dust per metric ton of ore. 

Reference flow and the functional unit are defined for the entire product system, and the elementary flows are calculated in relation to these. The flow figures are normally aggregated, and the total flow of each substance recorded and used for impact assessment. 

Refractive index detectors. These bulk property detectors are based on the change of refractive index of the eluant from the column with respect to pure mobile phase. Although they are widely used, the refractive index detectors suffer from several disadvantages — lack of high sensitivity, lack of suitability for gradient elution, and the need for strict temperature control ( + 0.001 °C) to operate at their highest sensitivity. A pulseless pump, or a reciprocating pump equipped with a pulse dampener, must also be employed. The effect of these limitations may to some extent be overcome by the use of differential systems in which the column eluant is compared with a reference flow of pure mobile phase. The two chief types of RI detector are as follows. 

As shown in Fig. 14,4 reference flows (known flow rates of nitrogen, or on occasion, helium) are introduced into the cathode and anode chambers, where they mix with the hydrogen and fluorine. 

Over the range of 100 to 600 mA cm"2, the current efficiency for the production of hydrogen was 100%, with a standard deviation of 2%. Over the range of 100 to 600 mA cm-2, the current efficiency for the production of fluorine was 100% with a standard deviation of 2%. Essentially the same results were obtained with helium reference flows. 

HF is supplied from a 100-Jb cylinder on an electronic scale which has a 10-g resolution. The cylinder is heated to 37°C with a band heater with two integral temperature sensors, one for control and an independent one for shutdown in case of overheating. From the cylinder, the HF passes through a throttling valve, a backup shutdown valve, and a control valve. A continuous nitrogen flow (actually the cathode reference flow) was added downstream of the control valve. 

The classic Lockhart- Martinelli (1949) method is based on the two-phase multiplier defined previously for either liquid-only (Lm) or gas-only (Gm) reference flows, i.e.,... 

It is necessary to estimate / for the two-phase flow. Using method 3 outlined above, it is appropriate to use the liquid as the reference flow because the quality is low (0.0196). The pressure gradient for the whole flow as liquid is... 

For a gas-liquid two-phase flow there are four possible reference flows ... 

When the reference flow is the whole of the two-phase flow as liquid, then the two-phase frictional pressure gradient is given by... 

When the reference flow is only the liquid in the two-phase flow, the equations are slightly different because the liquid flow rate and not the total flow rate must be used for the reference. 

The notation used here and in Section 7.7 is standard in the literature on this subject and originates in the pioneering work of Martinelli and co-workers. There are two aspects of the notation that may lead to confusion and error. First, note that LO and GO do not denote liquid only and gas only reference flows, as might be expected. On the contrary, they denote flows in which the whole of the flow rate is liquid or gas. It may help to remember them as liquid overall and gas overall . The second point to note is that 2 denotes the two-phase multiplier. Correlations may present values of but it must be remembered that this is the square root of the two-phase multiplier. 

If the only liquid reference flow had been used the frictional term in equation 7.81 would be 2fL(l — w)2G2VL2Lldi. 

Lockhart and Martinelli (1949) used only liquid and only gas reference flows and, having derived equations for the frictional pressure gradient in the two-phase flow in terms of shape factors and equivalent diameters of the portions of the pipe through which the phases are assumed to flow, argued that the two-phase multipliers and 4>g could be uniquely correlated against the ratio X2 of the pressure gradients of the two reference flows ... 

It was assumed that four flow regimes could occur depending on whether each phase was in turbulent or laminar (viscous) flow. Their empirical correlation is shown in Figure 7.13. The second and third subscripts denote the type of flow of the liquid and gas respectively. Note that and X are the square roots of the two-phase multiplier and the ratio of reference flow pressure gradients. 

In order to determine whether each phase is in laminar or turbulent flow, Lockhart and Martinelli suggested tentatively that the Reynolds number for the appropriate reference flow should be greater than 2000 for turbulent flow and less than 1000 for laminar flow. At intermediate values the flow was thought to be transitional. 

When a change of phase occurs, as in boiling, it is necessary to use the wholly liquid reference flow (an only liquid basis would change as the liquid flow rate decreases during boiling). At low pressures, the results of the Lockhart-Martinelli correlation can be used for the frictional component of the pressure gradient but it is necessary to convert the only liquid basis used in the earlier correlation to the wholly liquid basis. It is assumed that the frictional pressure gradients for the two reference flows are related by the expression... 

The flow sensitivity indicates that while a detector may be accurately balanced in terms of resistance and voltage, the reference flow only reduces the flow sensitivity of the analytical column flow by a factor of three to four. Much of the drift due to flow changes comes from the flow controllers. One type commonly used today has been found to have a mass flowrate proportional to absolute temperature. Flow induced noise, however, can come from column temperature fluctuations. Even if the front of the column is fed from a perfect flow source, a temperature change in the column will lead to a viscosity change in the carrier gas. Since the gas is compressible, a transient flow change occurs in the detector. Needless to say, a fluctuation in column temperature also leads to a fluctuation in the bleed level, which affects the recorder baseline by a much more direct process. 

Instead of setting the reference flow equal to the flow in the analytical column, it can be optimized for minimum drift during temperature-programming (a) Set the flows equal. (b) Make a temperature program run, observing the drift on a recorder,. 

Hold at the maximum temperature, (c) Adjust the reference flow until the recorder pen returns to its value at the beginning of the run. (d) For a more accurate adjustment, repeat steps (b) and (c). ... 

To derive dimensionless equations, we introduce the reference flow rates Oref (Eq. (12.12)) and define the following dimensionless parameters ... 

Two specialized detectors have been developed with specific applications for SEC. The first is the laser light-scattering detector, which has been around for almost 20 years but now has new electronics and computer data acquisition capabilities. Substitution of bulky He-Ne gas lasers with small, inexpensive diode lasers has greatly reduced the size and cost of laser light-scattering detectors, and the development of reference flow viscometers has provided similar size and cost advantages for viscometer detectors. 

The detector consisted of a Wheatstone network of capillary tubes that were drilled out of a high conductivity copper block and was fairly compact. The reference flow of mobile phase and the eluent from the column entered at two opposing junctions of the bridge arms (the center of tube (C)) such that the eluent was contained in one vertical arm (C) and the pure mobile phase in a parallel vertical arms (A) and (B). The increase in pressure at the base of tube (C) due to the presence of solute in (C) applied a pressure to the bottom of tube (A). This caused a flow of gas through the anemometer from tube (A) to tube (B) providing an output that was fed to a recording milliammeter. Subsequently all flows exited from the top and bottom of tube (C). 

The density bridge was situated in an appropriate thermostat and a reference flow of solvent and the column flow connected to the... 

Figure 6.2 Block diagram of a UV-visible absorbance detector. Light from a source, S, is focused into a flow cell, C, through which the HPLC eluent is passed, and the transmitted light intensity is measured at a photodetector, D. A wavelength selection element, W, is placed after the source. A second light path is produced by the beamsplitter, BS, passing through a reference flow cell, RC, to the reference photodetector, RD. L are lenses M is a mirror.

Figure 6.3 Block diagram of a diode-array UV-visible absorbance detector. S, light source BS, beamsplitter L, lenses C, flow cell M, mirrors G, diffraction grating PDAl, PDA2, photodiode arrays RC. reference flow cell.

A functional unit is not just a quantity of material. Practitioners may compare, for example, alternative types of packaging on the functional unit basis of Im of packed and delivered product. The comparison is then in terms of the service that the packaging provides. The quantity of packaging material required to provide this functional unit, termed the reference flow, will vary in the studies depending on the packaging option selected (e.g., paper, plastic, metal, and composite). 

As indicated in the reference flow sheet for oxide fuels (Fig. 1) the processing of the fuel constituents requires a reduction step. Pyrochemical reduction of UO2 >(14) Pu02, (15) and... 

Where R2 is known as the Two-Phase Multiplier its value depends on which single-phase flow is chosen as the reference flow. 

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