Handbook of Industrial Drying

CONCLUSION

tion/dryi ng of polyest er chips from initial moisture content of about 0.3 to 0.5% (wb) to under 50 ppm . Aside from low average moisture con tent it is also necessa ry to ensure uniform distribut ion of mois ture, especi ally for certa in prod ucts, e.g. , producti on of

It is clear from the discus sion in this ch apter
tthhiant
films . The unifor mity con straint is less severe if

numerous dr yer types can be used for drying of pothlye- chips are to be used to make PET bottles. Figure mers. Rotating doubl e-cone vacuum dryers, e.g., can41.15 shows schema tics of the cryst allizat ion/drying
be used up to 380C0 and 0.1 torr a bsolute pressure fosrteps involv ed. Genera lly, it is a two -step process . The process ing PET, PBT, and liquid-cry stal polyme rms.aterial is heat- sensi tive. The init ial cryst allizat ion/ The combinat ion of high tempe rature and low prdesr-ying is fast er than the drying step at low mois ture sure assists semicryst alline an d nonc rystalline pollye-vels. A two-stage dryer is indicated and is com-
mers to cryst allize and align to increa se the strenmg othnly used. It is possible to use different dryer types
of the polyme r. Polyest er, nylon, fluorop lastics, afnodr each stage as shown in Figure 41.16. A single polyuret hane can also be dried in rotating- conderyer type (e.g., column or packed bed dryer with

Wet chips
<50 ppm Moisture

Final Dryer
l Vacuum tumbler


Direct
l Fluid bed
l Vibro-fluid bed


Indirect
l Paddle dryer

l Pulsed fluid bed
l Vortex (spouted) bed
l Column dryer (with mixer)



Continuous
l Vacuum tumbler
Direct
Indirect

l Column dryer with internal tube
l Multistage fluid bed
l Paddle dryer

FIGURE 41.15 Schematic diagram of crystallization/drying steps in the production of polyester chips.

the chips moving downw ard slowly unde r gravity) flioswin g powder . With pr oper choice of atomi zer, cheaper a nd hen ce recomm ended for the lowe r qsuparla-y chamber de sign, gas tempe ratur e, and flow

ity grade but a more exp ensive fluid bed followe drabtey it is possibl e to ‘‘eng ineer’’ powder s of desired
another fluid bed or co lumn dryer may be ne ededpafortricle size and size dist ribution. Tabl e 41.2 shows
the higher quality g rade. Note that numerou s a lhteorw- the ch oice of the atomizer affe cts chamber
natives are possibl e in each case. It is also impor dtaensitgn, size, as well as energy con sumption of atom-
to ope rate the dryers at the correct con ditions of izgaatsion and particle size dist ribution . The newly devel- flow rate, tempe ratur e, and hum idity. Dehu midifioepded tw o-fluid sonic nozzles appear to be especi ally air is needed to achieve low final moisture content satitnractiv e choices when ne arly mon odisper se powder s accordance with the equili brium moisture isot hermnseed to be produ ced from relative ly moderat e viscos -
of the prod uct. ity feeds (e.g., under 2 50 cp) at capacit ies up to 80 t/h
Ano ther example of dryer selec tion is related btyo using mult iple nozzles. M ore exampl es may be
the choice of a suitab le atomizer for a spray dryfoeru.nd in Kudra and Mujumdar [21].
A spray dryer is indica ted when a pumpab le slurr yN, ew drye rs are be ing developed con tinuous ly as a solution , or suspensi on is to be reduced to a frreeseu-lt of industrial demand s. Over 2 50 U.S. patent s

e.g., for magnetic tape e.g., for speciality fiber,
film
e.g., for PET bottles, staple fiber, etc.

FIGURE 41.16 Possible dryer types for drying of polyester chips.

TABLE 41.2
Spray Drying of Emulsion-PVC. Effect of Selection of Atomizer on Spray Dryer Performance: A Comparison between Different Atomizers


Parameter Rotary Disk Two-Fluid (Sonic) Two-Fluid (Standard)

Dryer geometry Conical/cylindrical