y6  Fat  Decomposition  and  Glycerin. 
When  a  vacuum  of  28  in.  is  obtained,  32  in.  of  fat,  about  9100  lbs. 
are  drawn  into  the  still  and  the  temperature  gradually  raised  to 
4750  F.  on  the  still  and  575 0  F.  on  the  superheater.  The  super- 
heated steam  is  then  turned  into  the  still  through  the  perforated 
cross  and  collection  of  distilled  fatty  acid  begins.  This  is  continued, 
slowly  feeding  fat  into  the  still  until  155  in.  has  been  fed  out  of 
the  dry  boxes,  approximately  34,000  lbs.  The  stillman  keeps  close 
watch  of  measurement  of  the  distilled  product  because  he  only 
obtains  82  per  cent,  of  what  he  feeds  in,  the  residue  gradually  becom- 
ing bulky.  When  the  "  charge  "  has  been  fed  in,  the  temperature 
has  been  raised  gradually  to  525 0  F.  on  the  still  and  6500  F.  on  the 
superheater  and  the  time  has  been  about  three  days  of  twenty-four 
hours.  The  distillation  is  continued  until  fat  begins  to  color,  when 
the  fires  are  drawn,  superheater  shut  off,  and  temperature  on  the 
still  allowed  to  drop  below  5000  F.  Then  the  tar  is  pumped  into 
a  tar  still.  This  tar  seems  to  have  a  "  flashing  point "  of  5000  F. 
in  contact  with  cold  air.  The  safe  point  to  pump  is  475 0  F. ;  it 
has  been  pumped  at  495 0  F.  but  only  once,  and  when  the  wreck 
was  cleaned  up  the  yield  of  tar  was  not  quite  as  large  as  it  should 
have  been.  The  tar  still  is  the  same  as  the  fat  still,  except  it  is  not 
operated  under  a  vacuum.  The  remaining  grease  is  blown  out  of 
the  tar  with  superheated  steam,  the  tar  allowed  to  cool,  run  off  and 
barrelled.  In  the  meantime,  the  fatty  acid  still  is  again  started. 
The  process  described  applies  only  to  the  lower  grade  of  fat  or 
Class  No.  3.  Classes  No.  1  and  No.  2  do  not  go  to  the  distillation 
plant.  The  Twitchell  tank  for  handling  these  goods  is  the  same  as  for 
Class  No.  3  with  the  addition  of  being  fitted  with  a  steam  jet  imme- 
diately under  the  cover  and  above  the  surface  of  the  fat.  The 
method  followed  for,  say,  prime  tallow  applies  to  all  other  fats  in 
Classes  No.  1  and  No.  2. 
The  tallow  is  "  steamed  out "  of  barrels  into  the  acid  boil  tank 
and  5  lbs.  of  sulphuric  acid  to  each  barrel  of  tallow  added  and 
boiled  for  two  hours,  then  allowed  to  settle  overnight.  Run  water 
off  and  drop  to  Twitchell  tank.  Assuming  the  Twitchell  tank  is 
clean,  add  about  Yz  as  much  water  as  there  is  tallow  and  %  of 
1  per  cent,  of  the  weight  of  tallow  of  reagent  and  boil  with  open 
coil  until  test  shows  90  per  cent,  free  fatty  acid,  shut  off  open  coil 
and  turn  steam  through  jet  and  keep  it  going  all  during  the  settling 
process.  Air  coming  in  contact  with  the  fat  at  this  stage  would 
discolor  it.  When  the  sweet  water  has  settled,  run  off,  put  in  more 
fresh  water  and  give  second  boil  until  test  shows  96  per  cent,  free 
