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404590000   Summary
   Chapter CH=40459
   Type    TY=404590
   Unit    UN=404590000

Chapter Title: CARBON--ACTIVATED CARBON

Author: BAKER FREDERICK S., MILLER CHARLES E., REPIK ALBERT J., TOLLES E.
         DONALD
Institution: Westvaco Corporation, Charleston Research Center
Source: Encyclopedia of Chemical Technology, 4th Edition, Volume 4, Pages
      1015-1037
Number of Sections = 31   Tables = 6    Descriptors= 56   References = 117

Abstract:
   Carbon,  Activated Carbon. Activated carbon is an amorphous solid with a
   highly developed internal pore structure that will adsorb molecules from
   both the liquid and gas phases. The degree and selectivity of adsorption
   depend  primarily  on the surface area and pore size distribution of the
   carbon.  Activated  carbon  is manufactured from materials such as coal,
   wood,  and  coconut  shell,  and  is produced in granular, powdered, and
   shaped  or  pelletized  forms.  Coal  and  coconut  shell  are generally
   thermally  activated  with  steam  at high temperatures, whereas wood is
   activated  chemically  at  moderate  temperatures.  The world production
   capacity  of activated carbon in 1990 was estimated to be 375,000 metric
   tons.  U.S. production, which represented 40% of world capacity in 1990,
   is  expected  to  increase  at  an  annual rate of 5.5% during the early
   1990s.  The  largest  producers of activated carbon in the United States
   are Calgon, American-Norit, and Westvaco. The price of most products was
   0.70  to  5.50  $/kg  in 1990. Activated carbon can be recycled, but the
   economics   depend   on   the  specific  application  and  the  industry
   regeneration   capacity.   Significant   liquid-phase   applications  of
   activated   carbon   include  drinking  water  purification,  wastewater
   treatment,  sweetener  decolorization, and food and chemical processing.
   Gas-phase  applications  include  solvent  recovery,  gasoline  emission
   control  in  vehicles,  and  industrial  off-gas  treatment. Vol. 4, pp.
   1015-1037, 116 refs. to January 1991.

Section Headings:
   Untitled   
   Manufacture and Processing   
   Chemical Activation Processes   
   Forms of Activated Carbon Products   
   Specifications   
   Analytical Test Procedures and Standards   
   Liquid-Phase Applications   
   Groundwater Remediation   
   Sweetener Decolorization   
   Food, Beverage, and Cooking Oil   
   Mining   
   Gas-Phase Applications   
   Gasoline Emission Control   
   Control by Chemical Reaction   
   Process Stream Separations   
   Catalysis   
   Table 2. World**a Production Capacity, Estimated 1990
                
   Table 4. Source References for Activated Carbon Test Procedures and
                Standards  
   Table 6. Gas-Phase Activated Carbon Uses**a  
   Solvent-Recovery Systems   
   Economic Aspects   
   Applications   

Figure Titles:
   Figure 1. Solvent flow in generalized solvent recovery system.
   Figure 2. Recovery of mixed solvents in fabric coating.
   Figure 3. Recovery of acetone from cellulose acetate spinning using
                water scrubbing.
   Figure 4. Recovery of diluent and clathrating solution in solvent
                refining of oil (1-2).
   Figure 5. Recovery of supercritical CO2 solvent in secondary and
                tertiary petroleum recovery (3-5).
   Figure 6. Recovery of methyl ethyl ketone in azeotropic process for
                producing toluene.
   Figure 7. Recovery of mixed solvents in antibiotic manufacture.
   Figure 8. Recovery of hexane in vegetable oil extraction.
   Figure 9. Recovery of furfural in butadiene purification.
   Figure 10. Recovery of perchloroethylene in dry cleaning.

Descriptors:
   Absorption towers, in solvent recovery, #21:369
   Absorption, in solvent recovery, #21:369
   Acetic acid [64-19-7], recovery in acetate mfg, #21:375
   Acetone [67-64-1], explosive limits in air, #21:370
   Acetone [67-64-1], recovery from cellulose acetate, #21:359
   Adsorption, in solvent recovery, #21:370
   Antibiotics manufacture, solvent recovery system, #21:362
   Azeotropic distillation, in solvent recovery, #21:372
   Benzene [71-43-2], explosive limits in air, #21:370
   Butadiene [106-99-0], furfural recovery system, #21:364
   Butanol [71-36-3], explosive limits in air, #21:370
   Carbon dioxide [124-38-9], supercritical, recovery system, #21:361
   Carbon tetrachloride [56-23-5], explosive limits in air, #21:370
   Carbon, activated, in solvent recovery, #21:370
   Cellulose acetate [9004-35-7], acetone recovery from, #21:357
   Cellulose acetate spinning, solvent-recovery system, #21:357
   Condensers, shell-and-tube, #21:368
   Condensers, vapor-through-tube, #21:369
   Cyclohexane [110-82-7], recovery in Pe mfg, #21:375
   Distillation, fractional, of solvents, #21:367
   Dry cleaning, solvent recovery system, #21:365
   Dryers, for solvent recovery, #21:368
   Ethanol [64-17-5], explosive limits in air, #21:370
   Ethyl acetate [141-78-6], explosive limits in air, #21:370
   Ethyl ether [60-29-7], explosive limits in air, #21:370
   Evaporators, in solvent recovery, #21:366
   Extractors, for solvent removal, #21:366
   Fabric coating, solvent-recovery system, #21:358
   Filtration, in solvent recovery, #21:360
   Fractional distillation. See Distillation., #21:368
   Hexane [110-54-3], recovery from oil extraction, #21:363
   Lubricating oils, solvent refining of, #21:362
   Methanol [67-56-1], explosive limits in air, #21:370
   Methyl ethyl ketone [78-93-3], explosive limits in air, #21:370
   Methyl ethyl ketone [78-93-3], recovery in toluene process, #21:361
   Methylene chloride [75-09-2], vaporization of, #21:367
   n-Butyl acetate [123-86-4], explosive limits in air, #21:370
   n-Decane [124-18-5], explosive limits in air, #21:370
   n-Hexane [110-54-3], explosive limits in air, #21:370
   Oil-well products, separation of, #21:361
   Penicillin [1406-05-6], from fermentation broth, #21:366
   Perchloroethylene [127-18-4], recovery in dry cleaning, #21:365
   Petroleum, product separations, #21:357
   Polyethylene [9002-88-4], production, cyclohexane recovery, #21:375
   Propanol [71-23-8], explosive limits in air, #21:370
   Safety, in solvent recovery, #21:374
   Solvent oil refining, diluent recovery, #21:360
   Solvent purification (See also Solvent recovery.), #21:355
   Solvent recovery, #21:355
   Solvent-recovery systems. See Solvent recovery., #21:355
   Solvents (See also Solvent recovery.), #21:355
   Solvents, explosive limits and vapor pressures, table, #21:370
   Soybeans, oil extraction, #21:375
   Stills, for dry cleaning, #21:367
   Supercritical extraction, CO2 recovery in, #21:361
   Supercritical fluids, in enhanced oil recovery, #21:367
   Tetrachloroethylene [127-18-4], explosive limits in air, #21:370
   Toluene [108-88-3], explosive limits in air, #21:370
   Toluene [108-88-3], MEK recovery in production, #21:361
   Trichloroethylene [79-01-6], explosive limits in air, #21:370
   Vegetable oil, extraction of, #21:372
   Vegetable oils, hexane recovery system, #21:363

CAS Registry Nos:
   56-23-5;  60-29-7;  64-17-5;  64-19-7;  67-56-1;  67-64-1;  71-23-8;
   71-36-3;  71-43-2;  75-09-2;  78-93-3;  79-01-6;  106-99-0;  108-88-3;
   110-54-3;  110-82-7;  123-86-4;  124-18-5;  124-38-9;  127-18-4;
   141-78-6;  1406-05-6;  9002-88-4;  9004-35-7


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321201000   Text
   Chapter CH=32120
   Type    TY=321201
   Unit    UN=321201000

Chapter Title: Solvent Recovery
Section Heading: Untitled

Text:
   The recovery of valuable solvents is an economically rewarding practice.
Ever  since  the  surge  in  solvent  manufacturing  and  solvent-utilizing
facilities  began  around 1930, processes employing solvents have generally
included  solvent-recovery  systems  as part of the initial installation in
order  to  increase  profitability.  However, in some highly innovative and
extremely profitable processes the inclusion of solvent recovery would have
reduced  the  overall  profitability,  and solvent recovery facilities were
generally  added after competition reduced profits to a normal level. There
are   also  operations  that  use  an  inexpensive  solvent  in  quantities
sufficiently  small  in  which  a  solvent-recovery  system  would  not  be
economically justified.
   The  environmental  and  safety hazards associated with the discharge of
organic  vapors  and  liquids  to the air or to sewers was recognized early
with  the  result  that even marginally profitable solvent-recovery systems
were  often  installed  to  comply with industry standards or with state or
local  regulations.  However,  the establishment of the EPA in 1970 and the
ensuing   air-   and   water-pollution   legislation   have  brought  these
considerations  more  sharply  into  focus (see Air pollution; Water, water
pollution). The EPA has the responsibility to survey current and achievable
industrial  practices,  and  to  establish  limitations on the discharge of
polluting  emissions  to air and effluents to waters. These limitations are
to  become  more  stringent  in  a  series  of  steps aimed at an expressed
national  goal  of  zero  pollution.  For  new  installations  the  EPA  is
authorized  to  establish  guidelines  for pollution-reducing equipment and
control  systems  to  be  used either at the end of a process or within the
process (see also Regulatory agencies).

   It   is  difficult  to  speculate  on  the  ultimate  impact  that  such
far-reaching  powers  are  likely to have on the design of solvent-recovery
systems.   Certainly   formulation,   revision,   and  enforcement  of  the
regulations  are  subjected to a variety of disparate influences, including
those  of  large  and  small  operating  companies,  industry associations,
environmental  activist  groups, equipment manufacturers, agency personnel,
cooperating  state  and  local  authorities,  and  the  courts.  Assuming a
reasonable  resolution  of  all  these  influences,  it  is  expected  that
solvent-recovery  designs  will  continue  to  be  chosen by process design
engineers   on   the   basis   of  economic  evaluation  of  a  variety  of
possibilities.  However,  it will no longer be possible to discharge large,
or  even relatively small, amounts of solvents to the atmosphere, municipal
sewers,  streams,  lakes,  or  oceans.  Inclusion  of solvent recovery in a
process  is  indicated  as long as the costs and credits associated with it
are  more favorable than the often considerable cost of eliminating solvent
pollution at the end of the process.

   The  solvent  that has seen the greatest recent increase in recovery and
reuse  is, of course, water. However, because of its special importance and
unique  purification  technology,  this  subject  is covered elsewhere (see
Water,  water  reuse).  This  article deals with the recovery of nonaqueous
solvents  employed in the following industrial processes; the formation and
drying  of  synthetic  fibers  and  films,  plastics  and  rubber products,
smokeless  powder,  impregnated  fabrics, adhesives, printing inks, paints,
lacquers,  enamels, and other deposited coatings; the solvent extraction of
oils  and fats from vegetable and animal products, of fossil fuel materials
from  coal,  shale,  tar, and oil sands, of metallic compounds from treated
ores,  and  other solid-liquid extractions; the solvent-refining of mineral
and   vegetable  oils,  other  liquid-liquid  extractions,  extractive  and
azeotropic  distillations,  and gas-absorption processes; the degreasing of
fabricated   parts,   dry  cleaning,  and  other  washing  operations;  and
polymerizations  and  other chemical reactions carried out in solution, and
precipitation and recrystallization from solution.

   In  most  of these applications the recovered solvent is recycled. There
are, however, a number of applications where solvent separation takes place
at  a  different  place  and  time than solvent addition. In such cases the
recovered  solvent  is  reused  in  a  different  and usually a lower value
application.

   In  a  number  of  applications,  organic  liquids  that are potentially
solvents  are  purified and separated from processes in which they were not
used  primarily as solvents. Except for the use to which the organic liquid
is  put,  such  operations  may  resemble solvent-recovery systems in every
respect.  Among  such  closely  related  applications  are  the recovery of
natural  gasoline  and light hydrocarbons from natural and casing-head gas;
vapor-recovery  operations  in petroleum refining; recovery of ethanol from
fermentation  gases; recovery of organic liquids from wood distillation and
coal-tar  distillation  operations;  and  recovery of by-products or unused
reactants  from chemical synthesis (see Gasoline; Petroleum; Ethanol; Coal;
Tar and pitch).

   The  solvent  flow  in  a  typical recovery system is given in Figure 1.
Fresh and recovered solvents join together and enter an industrial process.
In  the separations section the products are separated from recycle streams
and  a  stream  containing  the  bulk of the solvent for recovery. The last
stream is recycled after one or more purification steps in which solvent is
separated  from the other gas, liquid, or solid components. Solvent that is
not  recycled  leaves the system as a component of the product, by-product,
or  waste  streams.  If  emissions, spills, and solvent leaving in degraded
form are included, it is obvious from Figure 1 that solvent make-up must be
equal  to  solvent loss. Thus, good solvent recovery means low solvent use,
and therefore low activity in the manufacture of chemicals specifically for
solvent applications.


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   Chapter CH=32120
   Type    TY=321201
   Unit    UN=321201001

Chapter Title: Solvent Recovery
Section Heading: Solvent-Recovery Systems

Text:
   Solvent-recovery  operations may differ greatly in scale, and the way in
which they interface with the main process. This is well illustrated by the
series  of  flow  diagrams shown below adapted from actual installations or
commercial  designs.  Adsorption  (see Adsorptive separation), eg, plays an
important  part  in  Figure 2, absorption (qv) in Figure 3, crystallization
(qv)  in  Figure  4,  vaporization  and  recompression  in  Figure  5,  and
liquid-liquid  extraction  (qv)  in  Figure  6.  Figure  7 is exclusively a
distillation (qv) process, the system in Figure 8 uses vacuum, and those in
Figure 5 and 9 operate to a large extent under pressure. A batch process is
illustrated  in Figure 10, semicontinuous processes in Figures 2 and 9, and
truly  continuous  processes in the other figures. Filtration, drying (qv),
refrigeration  (qv),  decantation,  and  evaporation  (qv)  are utilized in
varying   degrees   and   modifications   in  the  different  applications.
Differences  in  design  are  often  called  for  in  similar  or identical
applications.  Thus,  Figures 2 through 10 are not standard designs for the
industries in question, nor do they necessarily represent best practice for
the  particular  application  indicated.  For example, the water-absorption
system  shown  in  Figure  3  for recovering acetone from cellulose acetate
spinning  is  economical  only  when  a  large  supply  of cooling water is
available.  Where water temperatures are high, an adsorption system similar
to that shown in Figure 2 might be suitable for the same application.

   The  principal  similarity  which  exists among solvent-recovery systems
lies  in  the approach that must be taken in order to ensure a satisfactory
design.  The  same  techniques;  ie,  absorption,  adsorption,  extraction,
filtration,  distillation, and condensation. The same factors must be taken
into  account,  ie,  volatility,  solubility, thermal stability, corrosion,
purity  requirements,  capacity,  steam  and  water conditions, safety, and
economics.   In   general,   each   unit  must  be  individually  designed.
Standardization  is  desirable only where all factors are substantially the
same.

   Solvent-recovery units may be classified according to the method used to
make the initial separation between solvent and product streams as shown in
Figure  1.  This  separation  may  be  made  by  any  one of the following:
mechanical  separation,  such  as filtering, settling, draining, decanting,
centrifuging,  or  pressing;  extraction,  using another liquid to wash out
solvent  or  product;  evaporation;  fractional  distillation;  drying  the
product  in  the absence of air or gas; or drying in the presence of air or
inert gas followed by condensation, absorption, or adsorption.

   Although  a  solvent-recovery  system  may  be classified by the initial
step,  subsequent  steps may be of a different type. Thus in Figure 10, the
mechanical separation of garments from solvent is followed by drying in the
presence  of  air.  In  Figure  2, drying the product in air is followed by
adsorption   of   solvent,   evaporation,  condensation,  decantation,  and
distillation.

   It  may also be noted that some flow diagrams show more than one type of
solvent-recovery  system.  Thus, in Figure 9 the separation of the furfural
solvent  from  butadiene  and its separation from polymer may be considered
different   units.  The  same  may  be  said  for  separating  hexane  from
solvent-oil  mixtures  (miscella) and from spent flakes (marc) in Figure 8.
In  Figure  4  the  recoveries  of  volatile  methylene  chloride  and of a
nonvolatile   urea   solution   are   accomplished  by  entirely  different
techniques.


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321201002   Text
   Chapter CH=32120
   Type    TY=321201
   Unit    UN=321201002

Chapter Title: Solvent Recovery
      Text continues in 321201003
Section Heading: Solvent-Recovery Techniques

Text:
   Mechanical  Separation.  Draining  of  liquids  from  solids is a common
operation  in  solvent processes. The solids are usually retained either by
stationary or moving screens, perforated plates, baskets, belts, or chains.
Sometimes  agitation by shaking, tumbling, or rotation promotes separation.
An  example of draining is shown in Figure 8, where the conveyor that moves
the  extracted  flakes  from  the  extractor to the dryer allows solvent to
drain  back  into  the extractor. Another example is the draining of liquid
from fabricated parts in solvent-degreasing operations.

   Filtration  through  screens  or  cloths  is  indicated where solids are
present  in  small  particle  sizes.  Batch,  semicontinuous, or continuous
filters  using  either  pressure or vacuum are used for separating solvents
from  solids.  For reasons of economy and safety, filter closures should be
pressure  tight  or at least fumeproof. Presses or open filters are usually
avoided.

   Continuous  pressure-tight  filters  are  used  for  solvent refining of
lubricating  oils. The operation of the filter shown in Figure 4 is typical
of  such  equipment.  The solids are first filtered and carried on a moving
screen  or  drum  to a section where they are washed with solvent. They are
moved  to sections where liquid is drawn out and the dry solids dumped into
a  screw  conveyor.  The  filters  are under vacuum in order to conduct any
leakage in the rotary joints inward.

   Equipment  of  this  type  is  generally  expensive  and  its use can be
justified  only where the throughput is relatively large. Batch filtration,
on  the other hand, may also be expensive because of the labor required for
removing  the  solids.  An  exception  is the separation of dirt, fines, or
impurities  present  only  in  small  quantities. In such cases, disposable
filter  mediums  are  often used, and the filter is made sufficiently large
that  only  occasional  cleaning  is  required.  Filtration  is  frequently
complicated  by  the  presence  of  polymer-forming, sticky, or very finely
divided  materials. Pretreatment with heat or chemicals and the addition of
filter aids or surface-active agents may solve these problems.

   Settling  followed by decantation or drawing off of the separated phases
is  an  obvious  and  frequently  easy  way to separate immiscible liquids,
solids,  and  gases.  Separation  drums of nominal cross-section often give
fluid  velocities  that are far below the free settling values, and achieve
essentially complete separation of the fluid phases with or without the use
of   baffle,   centrifugal-flow,  or  packed-type  entrainment  separators.
However,  solutions  of some materials or synthetic polymers, often contain
traces  of  surface-active  compounds that produce relatively stable foams,
emulsions,  or solids suspension. This urdesirable surface activity must be
counteracted.   The  appropriate  technique  may  be  very  specific  to  a
particular  solution and much ingenuity and experimentation is often needed
to  devise  the  right treatment. The mixture of residual petroleum, carbon
dioxide,  water,  and  fine  sediment  produced  from  the well in Figure 5
requires  the  expenditure  of  a  significant  amount of heat, energy, and
chemicals for the necessary phase separations.

   Centrifugal   filters,   perforate-basket  centrifuges,  and  solid-bowl
centrifuges  use centrifugal force to increase the efficiency of filtering,
draining,   and   settling   operations,   respectively   (see  Centrifugal
separation).  Examples may be found in the extraction step in dry cleaning,
separating  crystals  from  solvent  solutions,  and  the  clarification of
extracts.  Pressing  between rolls, inside membranes, or in screw expellers
also  promotes  separation  of  solvent  from  fibrous  or flaky materials.
Magnetic or electrostatic separators may be required in some cases.

   The  removal  of  dust,  lint,  or  mist  from  air streams prevents the
following  of  adsorption beds. Air filters are used except where the gases
contain   adhesive  or  varnish-forming  materials.  In  the  latter  case,
entrainment separators or slat-packed towers with water-washed surfaces are
employed  (see  Fig.  2).  Such  an  arrangement  may  also serve to remove
corrosive impurities from air.

   Extraction.  Solvent and product are sometimes separated by washing with
water  or  another solvent (6-7). In Figure 6, the solvent is washed out of
the  nonaromatic hydrocarbon product with water. In other cases the product
is  washed  out  of  the  solvent  with  an  aqueous  solution. Centrifugal
contactors, mixer-settlers, sieve-plates, and baffled or packed towers with
or   without  agitation  are  some  of  the  types  of  equipment  used  in
liquid-liquid  extraction.  If  the liquids are clean and of low viscosity,
extractors   of  reasonable  size  give  effective  countercurrent  contact
equivalent  to  ten or more theoretical stages. However, if the liquids are
viscous  or  turbid, or if they tend to form emulsions, very low extraction
efficiencies may be expected. Sometimes the solvent must be washed out of a
solid  product  in  which  case  various  types  of washers or solid-liquid
extractors are used.

   Extraction  is  sometimes  accompanied  by a chemical reaction between a
component  of  the  washing solution and the impurity (or product) material
being  extracted.  In other cases, the extraction step may be preceded by a
chemical  reaction  in which the material to be extracted is converted to a
more  easily  extractable  compound.  For  example,  in  the  production of
penicillin from dilute fermentation broths, a change in pH fromm 7.5 to 2.5
converts  the  penicillin from a preferentially water-soluble compound to a
preferentially  organic-soluble  one  (see  Antibiotics, BETA -lactams). By
alternating  changing  pH  and  extracting  from  one  phase  to the other,
hundredfold  increases  in  concentration  are  possible  with only minimal
expenditures  of  energy. Similar recovery processes may be expected in the
currently evolving biosyntheses.

   


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321201003   Text
   Chapter CH=32120
   Type    TY=321201
   Unit    UN=321201002

Chapter Title: Solvent Recovery
      Text continued from 321201002
      Text continues in 321201004
Section Heading: Solvent-Recovery Techniques  (continued)

Text:
   Chemical-reaction    techniques    are    particularly    prominent   in
solvent-recovery systems associated with the extraction of transition metal
compounds  from  ores (8) (see also Extractive metallurgy). These compounds
are  made preferentially soluble in organic solvents by reacting the cation
with an organic complexing agent. Changes in oxidation-reduction potential,
pH,  or  anionic  environment  converts the compound to a less organophilic
form,  thus  reversing  the  direction  of  extraction and regenerating the
solvent. Extraction with chemical reaction is performed in the same type of
equipment   as   extraction  without  chemical  reaction,  but  the  design
calculations are more complex.
   Evaporation.  Solvents  are  often  recovered  by simple evaporation and
condensation.  In drycleaning, for example, grease and dirt must be removed
from  the  solvent  (see Fig. 10). Package stills, complete with condenser,
feed  preheater, and semiautomatic controls are available for this purpose.
These  units  are  inexpensive  and they are often used in industries other
than  dry cleaning. For high-boiling Stoddard solvent, the operation may be
under   vacuum,  for  lower-boiling  chlorinated  solvents  at  atmospheric
pressure.  Feed  may be either batch or continuous. Heating is usually with
indirect steam, but provision for gas or electricity is made in some units.
Direct steam may be added for the final removal of solvent from the sludge.

   A natural-circulation evaporator is shown in Figure 8. It is followed by
a  heater  which  is  also an evaporator, but of the once-through type. The
latter  has  a  lower  boiling  point at the inlet and a higher temperature
driving  force  than the former. Other types of evaporators may, of course,
be  used.  Multiple-effect  evaporation  and  its  counterpart, vapor-reuse
distillation,  are  growing in popularity because they effectively conserve
high  cost  energy. Evaporation of solvent at high dilution is particularly
advantageous  in  this  respect because boiling and condensing temperatures
are  close  together,  and  solutions  are  stable  over  a  wide  range of
temperature  and  pressure.  The  double-effect  vaporization  of methylene
chloride  is  shown  in  Figure  4. Condensing vapors from the second, high
pressure, column supply heat to the reboiler of the first column.

   Interest    is    growing   in   solvents   that   are   high   pressure
intermediate-density  supercritical fluids (3-5). The solvent power of such
fluids can be changed by varying the density and less energy is required to
separate  solute  from  solvent  than  with  conventional  vaporization  of
liquids.  These  solvents  are  used  in  secondary  and tertiary petroleum
recovery  (see  Petroleum, chemicals for enhanced recovery). Utilization of
CO2 in such an application is shown in Figure 5. Depending on the pressures
and  temperatures  in the formation, the CO2 may function at various points
as  gas,  liquid,  supercritical  fluid,  solvent  for petroleum fractions,
viscosity  reducer, driving fluid, or a combination of these. Converison to
a  low  density  phase  is  accomplished  in  this case by the reduction in
pressure as the production rises to the surface.

   When  a  volatile  solvent  is  separated from a nonvolatile liquid, the
product  is  saturated  with solvent at the pressure in the evaporator. The
introduction  of  direct  steam into the boiling liquid reduces the solvent
partial  pressure.  However,  steam-stripping  columns, like the one at the
lower  right  in  Figure  9,  are  much  more efficient. The steam not only
supplies  heat  and  dilutes  the  vapor,  but  its  condensate  washes the
solvent-free  polymer  out  of  the  column.  In the vacuum column shown in
Figure  8,  condensation  of  steam  and  consequent emulsification must be
avoided.  Indirect  heat  is  supplied  to the top of the column to prevent
lowering  the  temperature by solvent vaporization. This piece of equipment
is in effect a falling-film evaporator with a steam purge.

   Direct steam is commonly used to steam distill solvent from the surfaces
or  pores  of solids. Since the heat for vaporizing the solvent is provided
by  steam  condensation,  the  separated solid is wet. This moisture can be
removed  by  drying.  Drying  equipment  for evaporating water is generally
simpler   than   that   for  evaporating  solvent.  However,  the  two-step
arrangement consumes more energy.

   Fractional  Distillation. Solvents are commonly separated from products,
water,  or  other  solvents  by fractional distillation. The mixtures to be
separated  are usually nonideal and often azeotropic or partially miscible.
Where  more  than one solvent is used, several different types of compounds
may  be  present,  and  such  mixtures  are often very complex. The mixture
recovered  in  Figure  7,  for example, contains two alcohols, an amine, an
alcohol ether, water, salt, and traces of hydrocarbons.

   Such  complex  mixtures are separated into individual components by long
series  of  distillation  operations. Additional components may be added to
serve  as  entrainers in azeotropic or extractive steps (9) (see Azeotropic
and  extractive distillation). A partial separation of mixed solvents often
suffices  to permit their reuse. In both Figures 2 and 7 additional columns
would be needed for complete separations. In Figure 7, a change in just one
of the solvents might require a complete revision in the distillation-train
arrangement.

   Methods  have  been  developed for estimating multicomponent from bianry
equilibrium  and  designs  of  this  type can be calculated. Sieve-plate or
bubble-cap  columns  are  generally specified, but packed columns and other
designs  are  also  used. Operation is normally continuous, except for very
small quantities (see Distillation).

   


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321201004   Text
   Chapter CH=32120
   Type    TY=321201
   Unit    UN=321201002

Chapter Title: Solvent Recovery
      Text starts in 321201002
      Text continued from 321201003
      Text continues in 321201005
Section Heading: Solvent-Recovery Techniques  (continued)

Text:
   Drying  in  the  Absence  of  Air  or  an  Inert  Gas.  Drying solids by
vaporizing  the  solvent  in the absence of circulating air or an inert gas
has  many  advantages  (see  also  Drying).  Explosion hazards are reduced,
mechanical circulation of air is eliminated, and the task of recovering the
vaporized  solvent  is greatly simplified. On the other hand, heat transfer
to  the  solid  may be more difficult, and a higher temperature is required
for vaporization unless reduced pressure is used.

   Heat is usually supplied by surfaces heated with indirect steam or other
media,  or  occasionally radiant lamps. In Figure 8, a rotary-shelf type is
shown.  Agitation  and flow of solids is obtained by rotary rakes that move
the  solids  inwardly and outwardly across successive steam-heated platens.
Steam-jacketed  screw  conveyors  would  accomplish  the  same  result in a
somewhat  different  way.  Vacuum  drum  dryers,  cylinder,  dryers, rotary
dryers,  or  other  types of indirect dryers may also be used, provided the
solid  has  suitable  properties.  Dryers that employ forced circulation of
superheated  solvent  vapors through the solids give good heat transfer and
eliminate contact of solids with moving parts or heated surfaces.

   At  the  point  where  granular  solids  flow from these dryers, solvent
vapors  may  leak  out or air may be drawn in and overload the gas-handling
capacity  of  the  condensing  system. These problems are alleviated if the
pressure  in  the  dryer  can  be  kept  close  to  atmospheric, mechanical
discharge  devices are used, and the solid void volume can be compressed or
displaced with liquid. A purge stream of direct steam flowing back into the
dryer  through  the  voids  reduces  solvent  losses without increasing the
noncondensible  gas  load  (see  Fig.  8).  If  the  dryer  must operate at
significant  pressure  or  vacuum, the solid can be discharged batchwise or
through locks.

   Drying  with  Air  or  an  Inert Gas. Drying in the presence of air or a
mixture  of  nitrogen  and  CO2  (deoxygenated air) permits vaporization of
solvent  at lower temperatures than in the absence of air. In addition, hot
circulated  air  provides  good  heat transfer and does not require contact
with  hot  surfaces.  The  importance  of  tight  closure  is reduced since
additional air may be drawn into the dryer without serious consequence. Hot
air  may be the sole sources of heat, or additional conducting or radiating
surfaces  may be provided inside the apparatus. If combustible solvents are
present,  a  high  rate  of  air circulation keeps their concentration well
below  the  lower  explosive limit. Although flammable mixtures still exist
near  the  drying  surface,  the  danger  of  explosion is greatly reduced.
Operation  may  also  be  above  the  upper  limit.  The  use  of inert-gas
generators  to  remove  oxygen  from the air is good safety practice and is
justified especially where the air stream is recirculated.

   Recovery  of  solvent  from the air may be by adsorption, absorption, or
condensation.  Since  recovery,  particularly  in  the  last case, is never
complete,  the  air  from the recovery system is often recirculated back to
the  dryer  (see  Fig. 3 and 10). Recirculation is not used when the return
duct-work  is  expensive,  the  exhaust  air is relatively solvent-free, or
undesirable impurities are increased (see Fig. 2). Even with recirculation,
a  small  portion  of  air  is  generally  discharged through a blower. The
resulting suction draws air through the dryer openings and prevents solvent
vapors  from  contaminating the surrounding atmosphere. If dust is present,
the  air  may  be  filtered  at  the  dryer  inlet  and  outlet.  A similar
arrangement is used for inert gas.

   Condensation.  Vaporized  solvents  may  be  liquefied either in surface
condensers  or  direct-contact  condensers.  Direct  contact  with water is
largely restricted to solvents with very low water solubilities, but direct
contact  with  cold solvent may be used in any case. Shell-and-tube surface
condensers  are  used  almost exclusively. With vertical vapor-through-tube
condensers,  the  condensed  liquid  and  vent  gas  leave at a temperature
approaching  that  of  the  cooling  water. With other types, separate vent
condensers  are  used or additional passes for vent gas provided inside the
main  condenser.  Extended  surface  designs  are advantageous for lean air
streams  where sensible heat is an appreciable proportion of the total (see
Heat-exchange technology).

   The air or gas leaving a condenser contains solvent to the extent of its
vapor  pressure at the condensing temperature. The vapor-pressure column in
Table  1  , which gives the minimum
number  of mol of water theoretically needed for complete absorption of the
compound  from  one  mol of lean air at 25 DEGREES C and 101.3 kPa (1 atm).
These  values  would  be 1% of those in the third column of Table 1 .

   The  temperature  of  absorption  determines  the  quantity of scrubbing
liquid  required. With rich air streams the heat of absorption is sometimes
removed  by  indirect  cooling  either  inside  the  absorption  column  or
externally  between stages (see Fig. 3). Refrigerated scrubbing liquids are
sometimes used for the same reason (see Absorption).

   Adsorption.  The predominant method for removing solvent vapors from air
streams  is  adsorption  on activated carbon (see Carbon, activated carbon)
(10-12).  As  commonly  practiced, the air is fed alternately to one of two
adsorbing  vessels,  while  the  other is being fed with low-pressure steam
(see Fig. 2).

   The  steaming  operation  causes  the  solvent  to be vaporized, and the
latent  heat  is supplied by a portion of the steam that condenses into the
bed.  With  some  organic solvents this moisture interferes with subsequent
adsorption,  but  with  most  it  does  not. If water can be tolerated, its
vaporization  into  the air stream keeps the temperature down. In any case,
if  there is a net loss of heat around the cycle the water content rises to
a point where it stops adsorption. This water balance is often an important
factor  in  the  design of an adsorption unit. Heating, cooling, or special
drying periods may have to be included in the cycle of operations.

   Although   such   a   system  has  all  the  inherent  disadvantages  of
intermittent  processes,  it offers great advantages for many applications.
Unlike  water  scrubbing,  it  may  be applied to water-insoluble solvents.
Unlike  condensation,  it  may  reduce  the solvent content of the air to a
figure as low as desired.

   The  theoretical steam requirement for carbon adsorption is usually less
than  for  absorption.  Heats  of  adsorption  are high, that is, the vapor
pressure  of  solvent  above  activated  carbon increases more rapidly with
temperature  than  it  does  above  liquid  solutions.  However,  since the
steaming  operation in adsorption is somewhat inefficient, the actual steam
requirement  may  offer no advantage over a favorable scrubbing application
with  water  or  oil. On the other hand, with mixed solvents and with those
not  ideally  suited  to  absorption, water scrubbing may require excessive
quantities  of  steam  for  reasons already noted. Low pressure drop may be
obtained  in  adsorption systems by reducing the linear velocity of the air
and the thickness of the bed. Beds are usually 30-100 cm thick with uniform
pellets  99% would
usually  not increase the other costs significantly and should be sought in
every case.

   Even  with  efficient  recovery,  make-up  solvent  is  always required.
Designers  of  solvent-recovery  equipment  often  claim  that  the solvent
content  of  the  various  waste streams can be held to negligible amounts.
Although  analyses  of  these  streams  usually  corroborate  such  claims,
inventory  balances  general  show  losses  to  be  much  higher than those
calculated. Only by a careful study of spillage, venting, gauging practice,
chemical instability, etc, can this discrepancy be eliminated.

   For large long-lived systems with recoveries approaching 100%, utilities
present  the  largest  cost.  Steam  costs  can  be reduced to as little as
      Solvent         Explosive limits   Explosive limits  Equilibrium mol
                       mol fraction in    mol fraction in   fraction in air
                          air Upper          air Lower      at 25 DEGREES C
> ------------------  ------------------  ----------------  ---------------
>      methanol             0.360              0.073             0.16
>      ethanol              0.190              0.043             0.075
>      propanol             0.135              0.021             0.026
>      butanol              0.112              0.014             0.009
>      acetone              0.128              0.026             0.29
>    methyl ethyl           0.100              0.018             0.13
        ketone
>   ethyl acetate           0.090              0.025             0.12
>  n-butyl acetate          0.076              0.017             0.018
>    ethyl ether            0.480              0.019             0.70
>      benzene              0.071              0.013             0.13
>      toluene              0.071              0.012             0.037
>      n-hexane             0.075              0.011             0.20
>      n-decane             0.054              0.008             0.002
>       carbon                                                   0.14
    tetrachloride
> trichloroethylene                                              0.10
> tetrachlorethylene                                             0.025


Table Data (part 2):
>      Solvent         Minimum absorption requirement at 25 DEGREES C mol
                                          water/mol air
 
> ------------------  -----------------------------------------------------
>      methanol                               0.26
>      ethanol                                0.33
>      propanol                               0.37
>      butanol                                0.44
>      acetone                                 2.1
>    methyl ethyl                              2.3
        ketone
>   ethyl acetate                               8
>  n-butyl acetate                             15
>    ethyl ether                               50
>      benzene                                >100
>      toluene                                >100
>      n-hexane                               >100
>      n-decane                               >100
>       carbon                                >100
    tetrachloride
> trichloroethylene                           >100
> tetrachlorethylene                          >100


 1/5/15
DIALOG(R)File 302:Kirk-Othmer Encycl Chem
 (c) 1995 John Wiley & Sons Inc. All rts. reserv.

321203000   Bibliography
   Chapter CH=32120
   Type    TY=321203
   Unit    UN=321203000

Chapter Title: Solvent Recovery

References = 25

References:
   1. H. L. Hoffman and co-workers, Hydrocarbon Process. Pet. Refiner
          45(9), 220 (1966).
   2. E. J. Fuller in J. J. McKetta, ed., Encyclopedia of Practice and
          Design, Marcel Dekker, Inc., New York, 1979.
   3. C. A. Irani and E. W. Funk, in N. N. Li, ed., Recent Developments in
          Separations Science, Vol. III-A, CRC Press, Cleveland, Ohio,
          1977, pp. 171-193.
   4. L. W. Holm and V. A. Josendal, J. Pet. Technol. 26, 1427 (1974).
   5. T. B. Reid and H. J. Robinson, J. Pet. Technol. 33, 1723 (1981).
   6. R. E. Treybal in R. H. Perry and C. H. Chilton, eds., Chemical
          Engineers' Handbook, 5th ed., McGraw-Hill Book Co., New York,
          1973, Sec. 15.
   7. L. A. Robbins in P. A. Schweitzer, ed., Handbook of Separation
          Techniques for Chemical Engineers, McGraw-Hill Book Co., New
          York, 1979.
   8. G. M. Ritcey and A. W. Ashbrook, Solvent Extraction, Elsevier
          Scientific Publishing Co., Amsterdam, 1979.
   9. B. D. Smith in R. H. Perry and C. H. Chilton, eds., Chemical
          Engineers' Handbook, 5th ed., McGraw-Hill Book Co., New York,
          1973, pp. 13-36-13-47.
   10. T. Vermeulen, G. Klein, and N. K. Hiester, in R. H. Perry and C. H.
          Chilton, eds., Chemical Engineers' Handbook, 5th ed., McGraw-Hill
          Book Co., New York, 1973, Sec. 16.
   11. P. N. Cheremisinoff and F. Ellerbusch, Carbon Adsorption Handbook,
          Ann Arbor Science Publishers, Inc., Ann Arbor, Mich., 1978.
   12. J. W. Drew in P. A. Schweitzer, eds., Handbook of Separation
          Techniques for Chemical Engineers, McGraw-Hill Book Co., New
          York, 1979.
   13. A. A. Naujokas, CEP Technical Manual, Loss Prevention 12, 128
          (1979).
   14. M. J. Chapman and D. L. Field, CEP Technical Manual, Loss Prevention
          12, 136 (1979).
   15. J. R. Berlow, Proposed Effluent Limitations for the Paint
          Formulating Point Source Category, EPA 440/1-79/049-b, U.S.
          Environmental Protection Agency, Washington, D.C., 1979.
   16. National Fire Codes, Vol. 3, National Fire Protection Association,
          Boston, Mass., 1978.
   17. National Fire Codes, Vol. 6, National Fire Protection Association,
          Boston, Mass., 1978.
   18. R. J. Lewis and R. L. Tatken, eds., Registry of Toxic Effects of
          Chemical Substances, U.S. Dept. of Health and Human Services,
          Cincinnati, Ohio, 1980.
   19. D. Rector, SAE Transactions, No. 790370, V. 88, 1979.
   2O. D. Swern, ed., Bailey's Industrial Oil and Fat Products,
          Interscience, Publishers, a division of John Wiley & Sons, Inc.,
          New York, 1964, pp. 704-711.
   21. R. P. Anderson, Chem. Eng. Prog. 71, 72 (Apr. 1975).
   22. R. Katzen, R. Frederickson, and B. F. Brush, Chem. Eng. Prog. 76, 62
          (Feb. 1980).
   23. J. G. Speight, The Chemistry and Technology of Petroleum, Marcel
          Dekker, Inc., New York, 1980.

General References:
   ''Solvent Recovery'' in ECT 1st ed., Vol. 12, pp. 641-654, by C. M.
          Cooper, Michigan State College
   ''Solvent Recovery'' in ECT 2nd ed., Vol. 18, pp. 549-564, by C. M.
          Cooper, Michigan State University.