Flow time

This equation is the basis for viscosity determination by measuring flow times through a capillary. It can also be used to describe a single liquid at two different temperatures, as required for Eq. (4.63). Combining Eqs. (4.63) and (4.64) yields... 

The viscosity ratio or relative viscosity, Tj p is the ratio of the viscosity of the polymer solution to the viscosity of the pure solvent. In capillary viscometer measurements, the relative viscosity (dimensionless) is the ratio of the flow time for the solution t to the flow time for the solvent /q (Table 2). The specific (sp) viscosity (dimensionless) is also defined in Table 2, as is the viscosity number or reduced (red) viscosity, which has the units of cubic meters per kilogram (m /kg) or deciUters per gram (dL/g). The logarithmic viscosity number or inherent (inh) viscosity likewise has the units m /kg or dL/g. For Tj g and Tj p, the concentration of polymer, is expressed in convenient units, traditionally g/100 cm but kg/m in SI units. The viscosity number and logarithmic viscosity number vary with concentration, but each can be extrapolated (Fig. 9) to zero concentration to give the limiting viscosity number (intrinsic viscosity) (Table 2). 

Absolute viscosities are difficult to measure with capillary viscometers, but viscosities relative to some standard fluid of known viscosity, such as water, are readily determined. The viscometer is caHbrated with the reference fluid, and viscosities of other fluids relative to the reference sample are determined from their flow times. 

The basic design is that of the Ostwald viscometer a U-tube with two reservoir bulbs separated by a capillary, as shown in Figure 24a. The Hquid is added to the viscometer, pulled into the upper reservoir by suction, and then allowed to drain by gravity back into the lower reservoir. The time that it takes for the Hquid to pass between two etched marks, one above and one below the upper reservoir, is a measure of the viscosity. In U-tube viscometers, the effective pressure head and therefore the flow time depend on the volume of Hquid in the instmment. Hence, the conditions must be the same for each measurement. 

The Ubbelohde viscometer is shown in Figure 24c. It is particularly useful for measurements at several different concentrations, as flow times are not a function of volume, and therefore dilutions can be made in the viscometer. Modifications include the Caimon-Ubbelohde, semimicro, and dilution viscometers. The Ubbelohde viscometer is also called a suspended-level viscometer because the Hquid emerging from the lower end of the capillary flows down only the walls of the reservoir directly below it. Therefore, the lower Hquid level always coincides with the lower end of the capillary, and the volume initially added to the instmment need not be precisely measured. This also eliminates the temperature correction for glass expansion necessary for Cannon-Fen ske viscometers. 

The two steps in the removal of a particle from the Hquid phase by the filter medium are the transport of the suspended particle to the surface of the medium and interaction with the surface to form a bond strong enough to withstand the hydraulic stresses imposed on it by the passage of water over the surface. The transport step is influenced by such physical factors as concentration of the suspension, medium particle size, medium particle-size distribution, temperature, flow rate, and flow time. These parameters have been considered in various empirical relationships that help predict filter performance based on physical factors only (8,9). Attention has also been placed on the interaction between the particles and the filter surface. The mechanisms postulated are based on adsorption (qv) or specific chemical interactions (10). 

YFrC r = sum of feed flows times respec tive unit costs OC = operating costs/time... 

There are many types of sen.sors used to feed-back the process operating conditions to the switching logistics of an inverter unit. They can be in terms of temperature, pressure, volume, flow, time or any activity on which depends the accuracy and quality of the process. Direct sensing devices used commonly for the control of a drive and used frequently in the following text are speed. sensors, as noted below. 

The inner balance accounts for the chemical changes over the W kg catalyst by expressing the difference between the large flow times the small concentration change from in to out over the catalyst bed. 

The outer balance gives the overall change between the outside boundaries of the RR system. The chemical change that occurred over the W kg catalyst is now expressed as the difference between the small flow times the large concentration change between in and out of the RR system. 

There is no entirely satisfactory way of measuring flow. In the BS 2782 flow cup test an amount of moulding powder is added to the mould to provide between 2 and 2.5 g of flash. The press is closed at a fixed initial rate and at a fixed temperature and pressure. The time between the onset of recorded pressure and the cessation of flash (i.e. the time at which the mould has closed) is noted. This time is thus the time required to move a given mass of material a fixed distance and is thus a measure of viscosity. It is not a measure of the time available for flow. This property, or rather the more important length of flow or extent of flow, must be measured by some other device such as the flow disc or by the Rossi-Peakes flow test, neither of which are entirely satisfactory. Cup flow times are normally of the order of 10-25 seconds if measured by the BS specification. Moulding powders are frequently classified as being of stiff flow if the cup flow time exceeds 20 seconds, medium flow for times of 13-19 seconds and soft flow or free flow if under 12 seconds. 

Table 17.2 shows good agreement between the retention times from the TSK PEO standards analyzed in groups of two or three and the retention times of the TSK PEO standards analyzed individually. ASTM-D5296 requires that for standards to be run as a group, the molecular weight must differ by a factor of 10 (11). The results in Table 17.2 showed that a difference in molecular weight by a factor of 6 is adequate to obtain consistent flow times for standards for the modem linear columns. 

The difference between the two instruments is the inside diameter (ID) of the outlet flow tube. The SUV uses a 0.176 centimeter ID and the SFV u.ses a 0.315 centimeter ID. The SFV is used for samples that have a flowing time greater than 600 seconds. For most conventional gas oils, the flowing time is short enough that the Universal Visco meter is frequently used. The tube dimensions in the two procedures... 

Extended pot life Void-free uniform, long flow time in mold before gel. Large complex moldings. 

The counterflow configuration has been extensively utilized to provide benchmark experimental data for the study of stretched flame phenomena and the modeling of turbulent flames through the concept of laminar flamelets. Global flame properties of a fuel/oxidizer mixture obtained using this configuration, such as laminar flame speed and extinction stretch rate, have also been widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism. In particular, extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame time and a characteristic flow time. Furthermore, the study of extinction phenomena is of fundamental and practical importance in the field of combustion, and is closely related to the areas of safety, fire suppression, and control of combustion processes. 

Calculations. For determination of the intrinsic viscosity [ti] the prepared pectins were solved in an 0.1 M phosphate buffer with pH 6.0. The relative viscosity was determined by a glass. Ubbelhode viscometer at 25 0.1 °C. The flow time of solvent (L) was 81.8 seconds. At least six pectin solutions with different concentrations were measured in a way that their flow times (ts) comply the order 1.2to[Pg.528]

Pectin lyase (PNL) activity was measured spectrophotometrically by the increase in absorbance at 235 nm of the 4,5-unsaturated reaction products. Reaction mixtures containing 0.25 ml of culture filtrate, 0.25 ml of distilled water and 2.0 ml of 0.24% pectin from apple (Fluka) in 0.05M tris-HCl buffer (pH 8.0) with ImM CaCl2, were incubated at 37 C for 10 minutes. One unit of enzyme is defined as the amount of enzyme which forms Ipmol of 4,5-unsaturated product per minute under the conditions of the assay. The molar extinction coefficients of the unsaturated products is 5550 M cm [25]. Also viscosity measurements were made using Cannon-Fenske viscometers or Ostwald micro-viscosimeter, at 37°C. Reaction mixtures consisted of enzyme solution and 0.75% pectin in 0.05 M tris-HCl buffer (pH 8.0) with 0.5 mM CaCl2. One unit is defined as the amount of enzyme required to change the inverse specific viscosity by 0.001 min under the conditions of reaction. Specific viscosity (n p) is (t/to)-l, where t is the flow time (sec) of the reaction mixture and t is the flow time of the buffer. The inverse pecific viscosity (n p ) is proportional to the incubation time and the amount of enzyme used [26]. Units of enzyme activity were determined for 10 min of reaction. 

The time required for a batch of material to pass through the entire process is called the residence time or flow time tm for product i. The residence time is defined by ... 

This example models the dynamic behaviour of an non-ideal isothermal tubular reactor in order to predict the variation of concentration, with respect to both axial distance along the reactor and flow time. Non-ideal flow in the reactor is represented by the axial dispersion flow model. The analysis is based on a simple, isothermal first-order reaction. 

Discounted cash flow (time value of money)... 

Utility Modules. These perform "housekeeping" operations on time series (eg. multiply a concentration time series by a flow time series, to get a "load" time series). 

MFI is reported as the number of grams of polymer/10 min of flow time. The flow rate will depend on the polymer and the conditions used lower MW polymers will tend to flow through the die faster than higher MW polymers. 

Thioureas mainly find use for the vulcanisation of CR, epichlorohydrin (ECO) and some ethylene propylene diene terpolymer (EPDM) compounds. They show high crosslinking activity, with usually adequate compound flow time before onset of the crosslinking. In EPDMs, the thioureas are used as activators for low activity third monomer types and, in the presence of calcium oxide desiccants, in free state vulcanisation of extrudates, etc. The use of thioureas can overcome the retardation caused by the desiccant. In this case some care must be taken otherwise overcompensation may occur. Thioureas are not used in food product applications and are a known health hazard, particularly for pregnant women. 

Note that the lubrication effect due to particle collisions in liquid is significant. The liquid layer dynamics pertaining to the lubrication effect was examined by Zenit and Hunt (1999). Zhang et al. (1999) used a Lattice-Boltzmann (LB) simulation to account for a close-range particle collision effect and developed a correction factor for the drag force for close-range collisions, or the lubrication effect. Such a term has been incorporated in a 2-D simulation based on the VOF method (Li et al., 1999). Equation (36) does not consider the lubrication effect. Clearly, this is a crude assumption. However, in the three-phase flow simulation, this study is intended to simulate only the dilute solids suspension condition (ep = 0.42-3.4%) with the bubble flow time of less than 1 s starting when bubbles are introduced to the solids suspension at a prescribed ep. 

In many applications, due to the large value of k, the first reaction is essentially instantaneous compared to the characteristic flow time scales. Thus, if the transport equation is used to solve for Y, the chemical-source term iS) will make the CFD code converge slowly. To avoid this problem, Y can be written in terms of by setting the corresponding reaction-rate expression (S ) equal to zero as follows ... 

Pilnik and coworkers163 used the value of specific viscosity rjs expressed as i)s = (t — tb)ltb, where t is the flow time of the reaction mixture, and tb, the flow time of the buffer solution. The activity is determined graphically, by plotting reciprocal specific viscosities against reaction time. The slopes of the straight lines resulting provide the values of the activity. 

Flow (rate) No flow High flow Low flow Reverse flow Time Too long Too short Too late Too soon... 

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