Draw rate

The film tube is collapsed within a V-shaped frame of rollers and is nipped at the end of the frame to trap the air within the bubble. The nip roUs also draw the film away from the die. The draw rate is controlled to balance the physical properties with the transverse properties achieved by the blow draw ratio. The tube may be wound as such or may be sHt and wound as a single-film layer onto one or more roUs. The tube may also be direcdy processed into bags. The blown film method is used principally to produce polyethylene film. It has occasionally been used for polypropylene, poly(ethylene terephthalate), vinyls, nylon, and other polymers. 

There is a natural draw rate for a rotating disk that depends on the rotation rate. Both the radial velocity and the circumferential velocity vanish outside the viscous boundary layer. The only parameter in the equations is the Prandtl number in the energy equation. Clearly, there is a very large effect of Prandtl number on the temperature profile and heat transfer at the surface. For constant properties, however, the energy-equation solution does not affect the velocity distributions. For problems including chemistry and complex transport, there is still a natural draw rate for a given rotation rate. However, the actual inlet velocity depends on the particular flow circumstances—there is no universal correlation. 

A rotating disk acts as a pump, demanding a certain flow rate for a given rotation rate. With a semi-infinite space above the rotating surface, the draw rate as required by the similarity solution is established naturally as... 

Leveling agent for acid dyeing of nylon, draw rate with excellent migration. 

If the draw is markedly less in rate than the internal liquid flow, it can be drawn under flow control from an internal or external sump, and arranged so that the tower internal reflux flows through the sump. It is a good idea, nonetheless, to have a seal baffle or a seal pan set up so that if the side draw rate is set higher than the internal reflux flow, the downcomer will not become unsealed. 

If the draw stream rate is to be at or near the total internal liquid flow rate, it is best to provide an internal or external sump, with the draw rate controlled by the liquid level. The residual internal reflux can be a controlled flow to the tray below the draw. 

Yield control is the primary control of product properties, accomplished by adjusting the product draw rates. These adjustments must be made in a fashion to maintain overall material balance, i.e., if operation is steady, at least two rates must be adjusted together to maintain steady state. The separation between side draws is sometimes defined as cut point. This term has had several different definitions a preferred definition is the atmospheric boiling point of the component equally divided between adjacent product draw streams. 

In the experiments of O Connor et al., monodisperse PFPE Z and Zdol, which were fractionated via supercritical fluid extraction in CO2, were dip-coated onto the surface of silicon wafers as shown in Fig. 2A. Film thickness was controlled by altering the PFPE concentration and draw rate. An SME apparatus (Fig. 2B)... 

The liquid flow below the liquid draw tray is reduced by an amount equal to the side draw rate, and if the side draw is taken at tray j, Lj = Lj - Sf. Under the present simplifying assumptions, the vapor flow does not change at the liquid draw tray. Therefore, the L/V ratio below the liquid draw is less than the ratio above it, and the slope of the operating line below the draw is reduced. 

The next section operating line has a smaller slope since the vapor flow is reduced above the draw tray j by the vapor draw rate SJ and the liquid flow is assumed constant. The operating line slope is determined such that the number of stepped off stages matches the number of stages between the vapor draw and the side reboiler. 

Calculate the distillate and bottoms rates consistent with the given feed rates, side draw rate and the product compositions Xp = 0.94, Xj = 0.74, and Xg = 0.03. 

With a fixed configuration and reflux ratio, each product composition may be controlled independently by varying its draw rate. The overhead contains primarily ethane, while heavier components are refluxed. Ethane is the lightest component therefore, any impurities in this product are heavier components. Lowering the overhead rate should improve its purity by decreasing the concentration of heavier components. The obvious penalty is a lower ethane recovery in this product. Since the fractionation is fixed, the overhead rate must be determined to achieve some balance between purity and recovery. Table 9.14 indicates an ethane recovery in... 

An intermediate product such as the upper side draw, which is mostly propane, may contain impurities from components both lighter and heavier than the main component. The upper side draw is at the same time the bottom product of the top column section and the top product of the second section. The fractionation in the upper section determines to what extent ethane is stripped off from the propane product, and the fractionation in the second section determines to what extent butane is removed from the propane product. Again, in this situation since the number of stages in each section and the reflux ratio are all fixed, the fractionation is fixed. The propane recovery and purity depend mostly on its flow rate and on the flow rates of the adjacent products above and below it. If the propane product contains too much ethane, its flow rate should be cut back and the overhead rate increased. If the propane product contains too much butane, its flow rate should be cut back and the lower side draw rate increased. Table 9.14 summarizes the purities of components in different products at different flow rates. The recoveries can also be calculated from Table 9.14. The dependence of the other products compositions on their rates may be analyzed in a similar manner. 

It is required to determine the best draw tray for the side stream and the effect of the draw rate on the separation. 

The acetone concentration peaks at tray 19, or one stage above the reboiler. This would be a logical place to put the side draw. As the side draw rate is increased, its acetone concentration goes down since more and more water gets drawn with the acetone. The variation in the acetone concentration with the side draw rate is shown in Figure 9.8. 

FIGURE 9.8 Effect of side draw rate on acetone concentration (Example 9.7). 

The heat duties Qj and the side draw rates, L and Vf, are assumed fixed. If they... 

Figure 6. Variation in ulHnutte elongation with oxygen uptake by original and remolded films of the linear polyethyl in Figure 3. Draw rate was two in./min.

Values of the tensile strength and the E-moduli were obtained within the ranges from 2.3 up to 9 GPa, at partly quite low spinning and drawing rates of only a few meters per minute. 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

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