Holdback

The holdback FI is the fraction of fluid within a vessel of age greater than t, the mean residence time. As a fraction, it is dimensionless. It can be obtained from age-distribution functions (see problem 13-4). 

---

Auxiliary Equipment Elevating conveyors must be equipped with some form of holdback or brake to prevent reversal of travel and subsequent jamming when power is unexpectedly cut off. Ratchet and wedge roUer-type holdbacks are commonly used. Solenoid brakes and spring clutches may also be employed. 

From a glass tip it is impracticable to measure specimens smaller than lOpl. This is so because the holdback of the wetted glass, on the outside, is a significant portion of the volume sanq>led. If the tip is then dipped into the solution and then used as a washout pipet, a serious error would be introduced. 

Carriers frequently are stable isotopes of the radionuclide of interest, but they need not be. Nonisotopic carriers are used in a variety of situations. Scavengers are nonisotopic carriers used in precipitations that carry/incorporate other radionuclides into their precipitates indiscriminately. For example, the precipitation of Fe (OH)3 frequently carries, quantitatively, many other cations that are absorbed on the surface of the gelatinous precipitate. Such scavengers are frequently used in chemical separations by precipitation in which a radionuclide is put in a soluble oxidation state, a scavenging precipitation is used to remove radioactive impurities, and then the nuclide is oxidized/reduced to an oxidation state where it can be precipitated. In such scavenging precipitations, holdback carriers are introduced to dilute the radionuclide atoms by inactive atoms and thus prevent them from being scavenged. 

Fig. 10.9 Forward and reverse screw and kneading element sequences, both of which provide holdback capability.

The SSE is an important and practical LCFR. We discussed the flow fields in SSEs in Section 6.3 and showed that the helical shape of the screw channel induces a cross-channel velocity profile that leads to a rather narrow residence time distribution (RTD) with crosschannel mixing such that a small axial increment that moves down-channel can be viewed as a reasonably mixed differential batch reactor. In addition, this configuration provides self-wiping between barrel and screw flight surfaces, which reduces material holdback to an acceptable minimum, thus rendering it an almost ideal TFR. 

An example of removing multiple interfering elements is strontium purification in the presence of fission products. Ferric ion is added as a holdback carrier for the rare earths (and other radionuclides) and then precipitated as Fe(OH)3, the scavenger that carries these radioactive impurities. This or any other step can be repeated for enhanced removal of impurities. 

Step 13. Dissolve the Y(OH)3 in a minimal amount of concentrated HC1 added dropwise.. Add 15 mL of water. Add 3-5 drops of strontium nitrate solution as holdback carrier, stir, and again precipitate Y(OH)3 with 5 mL of concentrated NH4OH. Centrifuge and discard the supernate. Wash the precipitate with 20 mL water. Centrifuge and discard the wash water. 

Tubular Reactors. The simplest model of a tubular reactor, the plug-flow reactor at steady state is kinetically identical to a batch reactor. The time variable in the batch reactor is transformed into the distance variable by the velocity. An axial temperature gradient can be imposed on the tubular reactor as indicated by Gilles and Schuchmann (22) to obtain the same effects as a temperature program with time in a batch reactor. Even recycle with a plug flow reactor, treated by Kilkson (35) for stepwise addition without termination and condensation, could be duplicated in a batch reactor with holdback between batches. 

The sample is fused in 10 g NaeOg containing V46 tracer, 10 mg V carrier and 10 mg Cu holdback carrier. 

After a second pptn. with Inactive Ge holdback carrier, the arsenous sulfide is Isolated for counting. 

Kugo et al. (K2) omitted the e in their paper, but it is apparent from their F-curves and holdback values that e was incorporated in their calculation of holdback. Their statement that holdback = 1.0 for perfect mixing is incorrect rather holdback = 1.0 for total deadwater (Dl). 

Na and Cl products will dominate the pulse-height gamma-ray spectrum. The radiochemical separation procedure for Ba and Sb from the activated sample as developed by GA is as follows The activated sample is placed in a beaker, along with about ISmi of distd water, 3.3ml of coned HCl, 1 ml of coned HNO3, about lOOmg of NaC (as holdback carrier), and small amts (1 ml each) of carrier solns of Ba, Sb, and Cu (20.0mg each), and the mixt boiled for 10 mins — to dissolve any particles present, and equilibrate any Ba, Sb, and Cu present with their respective nonradioactive carriers. 

Temperatures in the range 15°-35°C have no influence on the delivery, due to the very small differences in expansion coeflflcients between polyethylene and water (Sanz, private communication). These pipets may be used uncalibrated because the imknown sample as well as the standard solution is measured with the same pipet. Owing to the hydro-phobic character of polyethylene the holdback of these pipets is negligible, provided they are emptied slowly. 

The sample—typically about 20 ml water—is distilled for purification. Reagents and holdback carriers may be added to the distilling flask to prevent volatile forms of contaminant radionuclides from being distilled. Initial distillation may be... 

Separation size limitation, safety distance, holdback measures... 

We can use these same mass balance expressions to calculate a term we have previously called a stripping or holdback factor . The holdback factor is the ratio of metal-ion... 

To avoid confusion with the separation factor (Sr ), we will refer to this term as a holdback factor in this discussion. 

Fig. 11. Calculated holdback factor, normalized to Gd=1.0, for (a) lanthanide (solid curves) and Y (dashed curve) complexes with hiba at pH 4.5, and (b) lanthanide (solid curves) and Y (dashed curve) complexes with...

We compared in sect. 3.1 the holdback factors for several carboxylic and aminopolycar-boxylic acids that have been (or could be) used for lanthanide separations. It is clear that among the carboxylic acid eluants, a hydroxide group on the a-carbon atom is necessary for consistent performance across the series. One might expect that the proximity of the OH group creates the possibility of simultaneous coordination of both a carboxylate oxygen and the hydroxide, thus creating a chelate complex of enhanced stability. However... 

The target is dissolved in concentrated HCl containing a small amount of I for "holdback" carrier. After the metal is dissolved any residual oxide or carbide may be dies dived by the addition of a small amount of cone. HNO. It was found that iodide ion was not oxidized during dissolution if present originally in low concentration. 

The target is placed in a flask containing about 50 mg of KI as holdback carrier, attached to the vacuum system, and the apparatus evacuated. H2 is started bubbling through the solution and hot 1 1 sulfuric acid admitted to... 

---