Double draw-off

Fig. 13. Simplified schematic diagram of clear-Hquor advance or double-draw off (DDO).

A novel Double Draw-Off (DDO) ciystallizer has been designed in order to improve the particle size distribution in the precipitation of CaS03 V 20 simulated Flue Gas Desulfurization (FGD) liquor. The effects of DDO ratio and residence time on the mean particle size were studied. Industrial conditions were maintained in all experiments as far as practical. Significant improvement in mean particle size was achieved. The performance of an actual industrial DDO ciystallizer (DuPont) for gypsum ciystallization was reported. 

White, E.T. and A.D. Randolph, "Optimum Fines Size for Classification in Double Draw-Off Crystallizers," Industrial and Engineering Chemistiy Research, No. 3,28. 276 (1989). 

Hulburt, H.M. and D.G. Stephango, "Design Models for Continuous Crystallizers with Double Draw-Off," CEP Symposium Series No. 95,65 50 (1969). 

Clear-liquor advance from what is called a double draw-off crystallizer is simply the removal of mother liquor without simultaneous removal of crystals. The primary action in classified-fines removal is preferential withdrawal from the crystallizer of crystals of a size below some specified value this may be coupled with the dissolution of the crystals removed as fines and the return of the resulting solution to the crystallizer. Classified-product removal is carried out to remove preferentially those crystals of a size larger than some specified value. In the following discussion, the effects of each of these selective removal functions on crystal size distributions will be described in terms of the population density function n. Only the ideal solid-liquid classification devices will be examined. It is convenient in the analyses to define flow rates in terms of clear liquor. Necessarily, then, the population density function is defined on a clear-liquor basis. 

Randolph, A.D., Mukhopadhyay, S., Sutradhar, B.C., and Kendall, R. (1990). Double Draw-Off Crystallizer Major Player in the Acid Rain Game In Crystallization as a Separations Process, (Myerson, A.S., and Toyokura, K., eds.) ACS Symposium Series No. 438, American Chemical Society, Washington. 

The operation of the clear-liquor advance is shown in Figure 64.3a and b. The system in Figure 64.3b can also be called a double draw-off crystallizer. The liquor may be recycled separately resulting in the product stream having a high suspension density. For operation with double draw-off, as the production rate is the same as in the mixed-solution-mixed product removal (MSMPR) crystallizer for a given... 

Clear liquor overflow (CLO), also known as double draw-off (DDO), increases the residence time of crystals in the growth zone and increases the magma density. This mode of operation also allows excess fine crystals to be removed from the system. Most crystallizers suffer from excessive nucleation, and to produce reasonably large crystals the excess fines must be removed. In terms of mass, the small crystals may appear insignificant the cumulative masses of crystals in a mixed suspension varies as the fourth power of their size so, for... 

The double-column system works like the single-column system except for the addition of the rectification section. In the double-column system, entering air is introduced in the middle of the lower column instead of at the top. Part of the liquid nitrogen product stream from the lower column is throttled to the operating pressure of the upper column and sent to the top of the upper column as reflux. The enriched liquid air from the lower reboiler is also throttled and introduced as feed into the middle of the upper column. Depending on the number of plates used, any practical purity level of either or both components may be obtained. When extremely high-purity products are desired, the argon present in the air must be considered as a third component of the mixture and removed in a draw-off stream from the upper column." The operation of such a column can best be shown with the aid of an example. 

It is critical to recognize that (baring a key exception) YOU CAN ONLY DRAW MULTIPLE RESONANCE STRUCTURES OF A MOLECULE IF IT HAS EITHER A +1 OR -1 FORMAL CHARGE, AND AN ADJACENT PI BOND. That is to say, a molecule with all zero formal charges rarely has multiple resonance forms. Many students assume that any molecule containing a double bond can experience resonance stabilization— this is incorrect. A possible source of this misconception is the fact that benzene and other aromatic molecules do have multiple (usually two) resonance forms. Do not let diis exception cloud your comprehension of die rule in ALL CAPS which leads off diis bullet. 

Note The transfer of embryos can be accomplished using a pipette and a plastic tip that has been cut off at the end to prevent drawing embryos through a small opening. Prior wetting of the pipette tip with liquid (heptane here and buffer [see below]) wlU prevent the embryos from sticking to the pipette tip. Heptane and methanol will form a double layer, with embryos resting at the interface. There should be an equal volume (-500 pi) of heptane on the methanol layer. 

---