MDI Emissions Reporting Guidelines for the ... - Polyurethanes

17.0 Spandex
MDI Emissions Reporting Guidelines for the Polyurethanes Industry
One of the unique ways polyol and diisocyanate are used is in the manufacture of spandex.
Polyol and a mixture of MDI/PMDI are reacted with amines to form a segmented
polyurethane. Using a special technique, a final product is obtained from a “dry-spinning”
operation. To control emissions, a scrubber system is installed to control emissions of silicon
oils, various pre-polymers and other VOCs.
Process Description
A reactor system is charged with dimethylacetamide (DMAC), amines, methylenebis (phenyl
isocyanate) (MDI) and polymeric diphenylmethane diisocyanate (PMDI) mixture, and silicon
oils. Reactor is heated to 150 o
F and a blended polyol is added over a period of two hours.
The reaction forms a segmented polyurethane which, through a “dry-spinning” process,
producers a spandex material. The reactor is equipped with a water scrubber to remove
VOCs including any unreacted MDI/PMDI. The scrubber has an exhaust blower and operates
at a temperature between 20 – 40 o
C. The blower operates at 8000 scfm. MDI is charged in
2% excess (total charge of MDI per batch is 600 lbs.). Two batches are run at the same time.
The plant operates 24 hours a day, 365 days a year. The facility process 5.7 million pounds
of MDI.
Calculating Stack Emissions
The stack emissions are based upon the excess amount of MDI charged per batch and the
efficiency of the scrubber. Most water scrubbers are 99% efficient in hydrolyzing MDI. We
will assume that the unreacted MDI will hydrolyze except 1%. The remaining unreacted MDI
will volatilize according to the scrubber temperature.
The amount of MDI emitted to the atmosphere from the scrubber can be determined from
the following equation:
Where:
L s = V air * (1/359) * (273.15/T s ) * (VP MDI /760) * M w
Vair = the annual volume of displaced air through the scrubber in ft3 /yr
Ts = the maximum scrubber temperature
VPMDI = the vapor pressure of MDI at maximum scrubber operating temperature K
Mw = the molecular weight of MDI (250.26)
359 = the molar volume of an ideal gas in ft3 /lb-mole @ 0 o
C and 1-atmosphere
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