Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1220~6
--1--
,
This invention relates to a process for removing
methanol from aqueous formaldehyde solutions by stripping the
solution with an inert gas.
There is also provided a process for the manufacture
of formaldehyde by the oxidative dehydrogenation of methanol in
the presence of a silver catalyst. More particularly, this
aspect of the invention relates to a process for the manufac-
ture of formaldehyde by the oxidative dehydrogenation of methan-
ol in the presence of a silver or copper catalyst in which the
aquèous formaldehyde product is stripped of methanol and water
by means of an inert gas stream.
In the industrial manufacture of formaldehyde from
methanol by dehydrogenating oxidation with air over silver or
copper catalyst in the presence of steam, the formaldehyde is
usually washed out of or scrubbed from the reaction gas with
water. The starting mixture in general contains methanol (cal-
culated as 100% strength) and water in a ratio of from 0.1 to
1.8 moles of water per mole of methanol. On absorption of the
reaction mixture, the steam produced by the reaction and the
steam and methanol left from the starting mixture are con-
densed. The formaldehyde combines with water to give methylene
glycol and higher polyoxymethylene glycols and with residual
methanol to give methyl hemiformal and polyoxymethylene glycol
monomethyl ethers. The higher polyoxymethylene glycols tend to
precipitate from concentrated aqueous formaldehye solutions as
paraform. Hence aqueous formaldehyde has been conveniently
used at a concentration of about 30 to about 37% by weight
since such solutions are stable over extended periods of time
without precipitation of paraformaldehyde and have been conven-
iently used in the manufacture of phenolic and amino resins.
In more recent times, with the development of im-
proved stabilizers to suppress paraformaldehyde formation, high-
er concentrations of aqueous formaldehyde have been accepted
for the handling of formaldehyde and the manufacture of resins
and have provided energy savings since they improve shipping
,~
lZ20~6
and handling efficiency and r~duce the amount of water to be
removed from the resin products. Such concentrated solutions
are generally prepared by distilling the aqueous formaldehyde
solutions formed in absorbers under conditions which allow most
of the unconverted methanol to be removed. The distillation
step requires high reflux ratios and considerable energy input.
One embodiment of the present invention provides an
improved process for removal of methanol from an aqueous formal-
dehyde solution, by stripping the solution with an inert gas.
In contrast with the high reflux ratios required in the removal
of methanol and water by distillation, little or no reflux is
required and considerable improvement in energy efficiency of
the process is realized. By means of the stripping step, a con-
centrated aqueous formaldehyde solution of low methanol content
can be readily obtained and can be used advantageously in the
manufacture of phenolic and amino resins and particularly of
C2 and higher alkylated amino resins. Methanol contents of
less than 2 weight percent are readily obtained.
In accordance with this embodiment, there is provided
a process which comprises stripping methanol from the aqueous
formaldehyde solution with an inert gas stream by counter-
current flow in a stripping column comprising at least about
1.5 theoretical transfer units for methanol stripping. The
stripping temperature and the ratio of stripping gas to aqueous
formaldehyde is selected to provide a concentration of condens-
ible vapors of aqueous formaldehyde in the gas emerging from
the stripping column of no more than about 50 mol percent.
The inert gas stream can be treated to recover most
of the stripped methanol, and any accompanying water and formal-
dehyde and can be recycled to the bottom of the stripper columnto repeat the stripping process.
L~
~2Z0~
The inert stripping gas is any non-condensible gas
which is inert to or non-reactivè with the various components
of the aqueous formaldehyde solution. Preferably, it is a non-
flammable, relatively non-toxic and relatively water-insoluble
gas. Thus it is advantageously selected from the group consist-
ing of nitrogen, helium, neon, argon and mixtures thereof.
The operation of the stripping column is dependent on
several parameters including temperature of the column, ratio
of stripping gas to aqueous formaldehyde solution, the number
of ideal stages or theoretical transfer units in the column,
the residence time of the aqueous formaldehyde solution.
The temperature of the stripping column can be any
temperature from atmospheric temperature up to the temperature
at which the partial pressure of the vaporized aqueous formalde-
hyde under the operating conditions of the column is no morethan about 50 percent of the total gas pressure of the gas leav-
ing the top of the column or in other words the temperature at
which the gas leaving the top of the column contains no more
than about 50 mol percent of vapors of the aqueous formalde-
hyde. Preferably the temperature is selected so that, underthe operating conditions of the column, the gas leaving the top
of the column comprises about 20 to about 40 mol percent of va-
pors of aqueous formaldehyde. Advantageously the column is
operated under conditions such that the temperature of the col-
umn is in the range of about 60 to about 85C. and more prefer-
ably in the range of about 65 to about 80C. Similarly, the
weight ratio of stripping gas to aqueous formaldehyde solution
passed through the column per unit time is selected so that un-
der the operating conditions of the column, the gas mixture
leaving the column contains no more than about 50 mol percent
of vapors of aqueous formaldehyde and preferably contains from
about 20 to about 40 mol percent. In practice, the inert gas
bj,
1220'786
stream in the stripping column is advantageously maintained at
a pressure above atmospheric pressure, preferably in the range
of about l.01 to about 2 atmospheres and the ratio of gas
entering the stripping column to aqueous formaldehyde solution
added to the stripping column is advantageously in the range of
about 0.5 to about 2.5 by weight per unit time and is
preferably in the range of about l.0 to about 2Ø
In order to obtain significant removal of methanol
from the aqueous formaldehyde by means of the stripping column
without excessive removal of formaldehyde, the stripping column
should comprise at least about 1.5 theoretical transfer units
and preferably about 3 or more theoretical transfer units. The
number of theoretical transfer units is determined from the
following relationship:
NTU = Pt b
l/2 (/\ Pt + ~ Pb~
where NTU = number of transfer units in the stripping column
under steady state conditions,
Pt = vapor pressure of methanol in the gas stream
emerging from the top of the stripping column,
Pb = vapor pressure of methanol in the gas stream
entering the bottom of the stripping column,
A Pt Pt~e) Pt
Pt(e) = equilibrium vapor pressure of methanol for
the aqueous formaldehyde/methanol solution at the top of the
column, at the temperature at the top of the column,
~ Pb Pb(e) Pb
~22~7~36
-5-
Pb(e) = equilibrium vapor pressure of methanol for
the aqueous formaldehyde/methanol solution at the bottom of
the column, at the temperature at the bottom of the column.
The equilibrium vapor pressures are determined from
standard vapor liquid equilibrium data, for example they
may be obtained from data stored in the data base sold by
Monsanto under the registered trademark Flowtran.
In aqueous formaldehyde solutions a methanol-methyl
hemiformal-polyoxymethylene glycol monomethyl ether equili-
brium exists and favors the hemiformals. The equilibriumcan be displaced towards methanol by raising the temperature
and by dilution of the formaldehyde solution with water.
Methanol can be readily removed from dilute aqueous formalde-
hyde solutions by fractional distillation. However with con-
centrated solutions of aqueous formaldehyde which are gain-
ing commercial favor, wherein the formaldehyde concentration
is above about 40 weight percent and particularly wherein
the formaldehyde concentration is above about 50 weight
percent, processes to remove methanol and in particular the
stripping process of the present invention to remove methanol
at the relatively low temperatures used-with the purpose of
conserving energy, is more difficult because most of the
methanol is chemically combined as hemiformals at these
temperatures and, although reversal of the ~.ethanol hemiformal
reaction occurs progressively with the removal of methanol
from the solutions, the rate of reversal is rather low.
- Efficient removal of methanol in a stripping column com-
prising conventional packin~ or tray columns would require
an excessively high column. It is therefore advantageous
to increase the residence time of the aqueous formaldehyde
in the stripping column by any convenient means to allow
equilihrium reversal of the methanol hemiformal reaction to
occur. Preferably residence zones are introduced to allow
the aqueous formaldehyde solution to remain quiescent out of
contact with the stripping gas generally for at least about
4 minutes until a significant concentration of free methanol
has been established. The column then becomes a series of
stripping zones separated by residence zones, with the free
methanol being generated by reversal of the methanol hemi-
~ -6-
formal reaction in the residence zones. One way to obtain quies-
cent residence zones is by introduction into the column of a num-
ber of chimney trays which are essentially overflow liquid trays
with gas chimneys to allow the stripping gas ascending to pass by
the aqueous formaldehyde solution held in the chimney trays.
Another way is by means of circulation loops placed at intervals
along the stripping column, the loops being equipped with reser-
voirs of suitable size to isolate the formaldehyde solution from
the gas stream for the desired time. Thus with stripping columns
comprising conventional packing or trays such as sieve trays,
glass trays, bubblecap trays or valve trays to provide intimate
contact between the aqueous formaldehyde solution and the
stripping gas for efficient extraction of methanol by the gas
stream, it is advantageous to include residence zones at inter-
vals in the column to reduce the height of the column requiredfor efficient stripping of methanol. For example a 30 meter
stripping column capable of handling about 5 metric tons of forma-
lin product per hour, provides about four theoretical transfer
units determined by means of the relationship set forth above,
for the stripping of methanol when it is packed with 21 meters of
Pall ring packing divided into 5 zones with each zone separated
with a chimney tray of 10 cm. depth, providing a residence time
of about 6 minutes in each residence tray. Similarly a 30 meter
stripping column containing 45 sieve trays can provide about four
theoretical transfer units when 4 residence trays each providing
a residence time of about 6 minutes are placed at intervals along
the column. Thus by means of the residence zones, transfer units
of height in the range of about 1 to about 10 meters are readily
obtained and allow the desired weight ratio of gas to liquid
passing through the stripping column per unit time to be
achieved.
The inert stripping gas which emerges from the
stripping column contains methanol, formaldehyde and water
vapors. The gas is advantageously treated to remove the condens-
ible components for example by scrubbing the gas with water orwith aqueous formaldehyde.
~220~786
--7--
Optionally, the stripping gas which emerges from the
stripping column can be passed to a partial condenser and the con-
densate can be returned to the top of the stripping column. In
this manner, most of the formaldehyde and water is condensed and
returned as a reflux to the stripping column while the inert gas
stream retains most of the methanol remo~ed from the aqueous for-
maldehyde solution in the stripping column. When the aqueous for-
maldehyde condensate is refluxed to the stripping column in this
manner, the stripping column can advantageously be equipped with
a top stage comprising a contact zone for intimate contact be-
tween the ascending inert gas stream and the descending aqueous
formaldehyde reflux and a residence zone above the entry port for
the initial aqueous formaldehyde solution entering the stripping
column. The inert gas stream emerging from the partial condenser
is then passed through a condenser and a condensate comprising a
substantial amount of methanol is obtained.
In comparison with a fractional distillation column for
removal of methanol from an aqueous formaldehyde solution, the
gas-stripping process of the present invention can reduce the
energy requirement for methanol removal by about 50 percent or
more without sacrifice in separating efficiency. Advantageously,
for a balance in energy savings and separating efficiency, the
stripping column is maintained at a temperature in the range of
about 60 to about 85C, and more preferably in the range of
about 65 to about 80C and the ratio of stripping gas fed into
the bottom of the stripping column to the aqueous formaldehyde
solution fed to the stripping column is in the range of about 0.5
to about 2.5 by weight per unit of time and more preferably in
the range of about 1.0 to about 2Ø
Since the stripping column is operated at a relatively
low temperature and since the reflux, if any, returned to the
D
.
~zz0786
--8--
stripping column is a very minor fraction of the total amount
of aqueous formaldehyde solution in the stripping column, the
heat required to maintain the stripping column at the operating
temperature is substantially less than the heat required for a
distillation column.
The following steps may be carried out to manufacture
an aqueous solution of formaldehyde:
a) oxidatively dehydrogenating methanol with air in
the presence of a silver or copper catalyst and steam at ele-
vated temperature;
b) absorbing the reaction product in an absorption
train comprising one or more absorption stages in series to
form an aqueous formaldehyde solution containing free and com-
bined methanol; and
c) stripping the methanol from the aqueous formalde-
hyde solution according to the present invention.
The solution can be maintained at an elevated tempera-
ture during the stripping operation with heat generated in the
oxidative-dehydrogenation of the methanol starting material.
This process is advantageously carried out con-
tinuously in the following sequence:
1. a mixture of water and methanol vapors is mixed
with air, the mixture is passed over a silver or copper cata-
lyst bed and the methanol is oxidatively dehydrogenated;
2. the reaction product comprising a gaseous mixture
of formaldehyde, steam, residual methanol, nitrogen, carbon
dioxide and hydrogen is cooled and fed to an absorber comprising
a series of stages containing aqueous formaldehyde solution, each
stage being equipped with a circulation loop and a cooling
system. Sufficient water or dilute aqueous formaldehyde solution
is added to the final absorber stage to maintain the desired con-
centration of formaldehyde in the final product, and aqueous for-
~.~
l:~;aO7f36
maldehyde solution is passed through the absorber counter-
currently to the reaction gas mixture to absorb most of the for-
maldehyde, water and methanol from the gas mixture;
3. aqueous formaldehyde solution is fed from the
absorber to a multi-stage stripping column while,
counter-currently to the aqueous formaldehyde flowing in the
multi-stage stripping column, a stream of inert gas is
circulated to strip some of the residual methanol from the
solution and simultaneously some water and formaldehyde;
4. the gas stream which emerges from the stripping
column is treated to recover most of the stripped methanol,
water and formaldehyde and is recycled to the bottom of the
stripper column;
5. the aqueous formaldehyde solution which is drawn
from the bottom of the stripping column constitutes the final
formalin product from the process.
The operation of the stripping column is dependent on
several parameters, including temperature of the column, etc.
as discussed above, and in addition, in this embodiment, the
number of aqueous formaldehyde streams supplied to the
stripping column from the absorber.
In general, the absorber can contain any number of
absorption or scrubber stages for absorption of the reaction
products of the oxidative dehydrogenation of methanol. Conven-
iently from 2 to 4 absorption stages each equipped with a recir-
culation loop and cooling system may be used. Sufficient water
is continuously added to the system to maintain the desired for-
malin product concentration, and the temperatures and concentra-
tions of the aqueous formaldehyde solutions in the stages are
preferably maintained at levels for efficient absorption of for-
maldehyde and methanol from the reaction gas stream without
~:~207B6
--10--
formation of paraform in the stages. Part of the circulating
stream of at least the first absorption stage is passed to the
stripping column. Preferably none of the aqueous formaldehyde
solution passed to the stripping column from the first absorp-
tion stage is returned to the absorber. It is preferably takenoff at the bottom of the stripping column as formalin product.
Preferably, part of the recirculating stream of at least the
first two absorption stages is passed to the stripping column,
the points of entry being intermediate to top and bottom of the
stripping column with the more dilute aqueous formaldehyde solu-
tion entering near the top of the column, and the more concen-
trated solution entering near the bottom while at the same time
the more dilute aqueous formaldehyde solution after descent in
the stripping column is partly drawn off as a side stream above
the entry point for the more concentrated solution and added to
the circulation loop of the prior more concentrated aqueous for-
maldehyde absorption stage to maintain the concentration of for-
maldehyde in that absorption stage at a level to avoid formation
of paraform at the particular temperature of the stage. Where
part of the aqueous formaldehyde solution in an absorption stage
other than the first absorption stage is supplied to the
stripping column, the side stream from the stripping column may
provide the only route whereby formaldehyde solution from that
absorption stage can flow to the prior absorption stage contain-
ing more concentrated aqueous formaldehyde. While the strippercolumn preferably has at least two formaldehyde streams entering
it from the absorber it can have as many entering streams as
there are absorption stages in the absorber, the streams provid-
ing a formaldehyde concentration gradient in the stripping
column.
In this embodiment, the inert stripping gas which
emerges from the stripping column entraining methanol, formal-
dehyde and water also entrains small amounts of hydrogen and
carbon dioxide carried over from the absorption train. The gas
is advantageously treated to remove the condensible components
for example by scrubbing the gas with a cooled recirculated solu-
tion of aqueous formaldehyde to which water is constantly added
and from which a portion is constantly taken off and passed to
the circulation loop of the last absorption stage of the absor-
ber. The amount of water added is adjusted to maintain the
desired concentration of water throughout the entire system and
to provide for the desired concentration of aqueous formaldehyde
product. The treated gas can then be passed through a condenser
to form an aqueous formaldehyde-methanol solution which is rela-
tively rich in methanol and can be advantageously volatilized and
recirculated to the methanol reactor. Because a major portion of
the methanol present in the aqueous formaldehyde solution in the
stripper column can be advantageously removed from the stripping
action, the aqueous formaldehyde-methanol solution recycled to
the reactor is characterized by a methanol to formaldehyde mol
ratio of at least about 0.25 and a ratio of at least about 0.45
can be readily achieved. The mol ratio is generally at least
about 10 times higher than the ratio of the starting solution. A
minor fraction of the inert gas stream emerging from the conden-
ser can advantageously be bled from the system, to avoid buildup
of the hydrogen gas and the bleed can be combined with the off-
gas which emerges from the absorber and can be incinerated. The
remainder of the inert gas stream is passed to the bottom of the
stripping column for recirculation.
Optionally, the inert gas which emerges from the
stripping column can be passed to a partial condenser and the con-
densate can be returned to the top of the stripping column, as
discussed above, with regard to the stripping process of the pre-
sent invention wherein most of the formaldehyde and water is con-
densed and returned as a reflux to the stripping column while the
inert gas stream retains most of the methanol removed from the
aqueous formaldehyde solution in the stripping column. As pre-
viously discussed, the stripping columnn can advantageously be
D
~220'786
-12-
equipped with a top stage comprising a contact zone for intimate
contact between the ascending inert gas stream and the descending
aqueous formaldehyde reflux and a residence zone above the entry
port for the most dilute aqueous formaldehyde solution entering
the stripping column from the absorption train. The inert gas
stream emerging from the partial condenser is then passed through
a condenser and the condensate comprising mostly methanol can be
volatilized and added to the reaction gas stream. The methanol
condensate can advantageously have a methanol to formaldehyde mol
ratio of at least about 0.6 and a ratio of at least about 1.0 can
be readily achieved and the mol ratio of recirculated formalde-
hyde to total methanol in the reaction gas stream can advantage-
ously be less than about 0.035 and indeed less than about 0.03 to
avoid unnecessary recycling of formaldehyde.
The ratio of stripping gas fed into the bottom of the
stripping column to the most concentrated aqueous formaldehyde
solution fed to the lower part of the stripping column from the
absorber is in the range of about 0.5 to about 2.5 by weight per
unit of time and more preferably in the range of about 1.0 to
about 2Ø
In this embodiment, the heat required to maintain the
stripping column at the operating temperature can be readily sup-
plied by the absorption solution from the first stage of the
absorber and no external source of heat may be required.
The process, according to this embodiment, may be
carried out by the following method A (Figure I): Air, methanol
vapor and water vapor are charged continuously through an inlet 1
and fed via line 3 into a reactor 4 equipped with a silver
catalyst bed. After reaction in 4, the reaction mixture passes
via connecting line 5 into the first column 6 of a two stage
absorber. A part of the formaldehyde solution formed in 6 is
passed via lines 7 and 8 to a stripping column 9 comprising
stages 9a, 9b, 9c, 9d and 9e, line 8 entering column 9 at the top
of stage 9c. Concentrated aqueous formaldehyde of low methanol
content is discharged continuously from the stripping column
1220786
-13- C-06-12-1024
through line 10. The off-~as from the first absorption
stage 6 passes via connecting line 11 into the second
absorption stage 12 where it meets a dilute solution of
aqueous formaldehyde and issues at the top of the absorber.
A part of the formaldehyde solution formed in 12 is passed
via lines 13 and 14 to the stripping column 9 entering the
column at the top of stage 9e and flowing down the column
countercurrent to the stripping gas stream. Part of the
formaldehyde solution descending in the stripping column is
drawn off at the bottom of stage 9d and flows via connecting
line 15 to section 21 of the circulation loop 16, 17, 20,
21 of absorption stage 6. Part of product stream 1~ is cir-
culated via lines 18 and 19 through heat exchanger 17 and
is returned to the top of stage 9a of the stripping column
to provide a heat source for the stripping gas entering the
stripping column through line 25 and passing up through
stage 9a. The stripping gas emerges from the top of the
stripping column and passes through line 26 and counter-
currently in scrubber 27 to a circulated dilute aqueous
formaldehyde solution with water make-up added through
line 28. The aqueous solution formed in scrubber 27 passes
through line 29 to section 33 of the circulating loop 30,
31, 32 and 33 of the second absorption stage. The stripping
gas stream passes from the scrubber through line 35 to con-
denser 37 and moves countercurrently to a water stream
added through line 36. The aqueous condensate of methanol,
and formaldehyde passes through line 38 to vaporizer 39 and
is added to the reaction gas stream 3 via line 2. The gas
stream emerging from the condenser passes via lines 40 and
42 through blower 23 and heat exchanger 24 and enters the
bottom of the stripping column 9 through line 25. A partial
bleed of the circulating gas through line 41 to limit the
build-up of hydrogen is compensated with addition of fresh
inert gas through line 22.
Alternatively the process may be carried out by method
B ~Figure 2): air and water vapor through line 101 and
methanol vapor through line 140 are charged continuously
via line 102 to a reactor 103. After reaction in 103, the
reaction mixture passes via connecting line 104 into the
,
12Z0~
-14- C-06-12-1024
first stage 105, of the absorber, then via line 109 to columns
110 and 114 and emerges through line 115 as an off gas com-
prising a major proportion of nitrogen and minor proportions
of hydrogen and carbon dioxide. The off-gas is passed to
an incinerator, The absorption stages 105, 110 and 114 are
equipped with circulating loops 106, 107 and 108, 111, 112
and 113 and 116, 117 and 118 respectively. Sufficient water
is added through line 119 to section 118 of the circulating
loop of absorption stage 114 to maintain a water balance
in the system and to provide formalin product of the desired
concentration, The aqueous formaldehyde solutions in the
absorption stages are circulated to provide flow counter-
current to the reaction product gas stream. The temperatures
and concentrations of the aqueous formaldehyde solutions in
the three absorption stages are maintained at levels to
enhance the absorption of formaldehyde frDm the reaction
gas stream and to avoid the formation of paraform. Portions
of the aqueous formaldehyde stream 106 and 111 are passed
via lines 120 and 125 respectively to stripping column 121
consisting of stages 121a, 121b, 121c, 121d and 121e. Line
120 enters column 121 at the top of stage 121b and line 125
at the top of stage 121d. Part of the solution which descends
through stage 121c is taken off as a side stream and passed
via line 126 to section 108 of the circulating loop of absorp-
tion stage 105. Part of the product solution which emergesfrom the bottom of the stripping column in line122 is circu-
lated ~ia line 123 to heat exchanger 107 in the circulation
loop of absorption stage 105 and is returned via line 124
to the top of stage 121a of the stripping column to provide
a heat source for the stripping gas which enters the column
through line 127 at the foot of stage 121a and ascends
countercurrently to the formaldehyde solution in the column.
The stripping gas emerges from column 121 via line 128 and
is passed to a partial condenser 130 which condenses much
of the formaldehyde a~d water in the gas stream and returns
them via line 129 to the top of stripping column 121. The
gas stream proceeds from the partial condenser via lines 131
and 133 through a condensing system consisting of a precon-
denser 132 and a condenser 134 and emerges virtually free of
)7~;
C-06-12-1024
formaldehyde, methanol and water as stream 141. Partial
bleed of stream 141 through 142 to prevent build-up of hydro-
gen gas in the stripping gas is compensated with a make-up
of fresh inert stripping gas added through line 143. The
stripping gas is then recirculated through line 127. The
condensate obtained ~rom the precondenser and condenser,
comprising mostly water and methanol is collected in dis-
tillate receiver 137 via lines 135 and 136, and is passed
with the main methanol charge added via line 144, through
line 138 to vaporizer 139 and then via lines 140 and 102
to the reactor. Water is added via line 145.
In the embodiments of the invention, the absorber com-
prises at least two stages or columns. In the first stage
or column, the:aqueous formaldehyde solution which circu-
lates in the circulation loop preferably contains from about45 to about 70 weight percent of formaldehyde, from about
1.0 to about 6 weight percent methanol and from about 25
to about 49 weight percent of water; in the circulation
loop of the second stage, the aqueous formaldehyde prefer-
ably contains from about 25 to.about 40 weight percent for-
maldehyde, from about 2.5 to about 7.5 weight percent of
methanol and from about 52.5 to about 67.5 weight percent
water; and in the circulation loop of a third stage, the
aqueous formaldehyde preferably contains from about 10 to
about 20 weight percent formaldehyde, from about 4 to about
15 weight percent methanol and from about 65 to about 84
weight percent water. The absorption is preferably carried
out at temperatures in the range of about 60 to about 90C
in the first absorption stage, about 20 to about 60C in the
second stage and about 10 to about 25C in the third stage.
Advantageously the aqueous formaldehyde solution supplied
to the stripping column from the second absorption stage
and optionally from succeeding absorption stages may be
heated prior to entry into the stripping column to a tempera-
ture within the range of from about 10C below, up to aboutthe temperature of the formaldehyde solution at the bottom
of the stripping column.
The formaldehyde solution manufactured by the process
of the invention, is a disinfectant, tanning agent, reducing
agent and a starting material for the manufacture of organic
lZZ0786
--16--
C-06-12-1024
chemicals and synthetic resins and adhesives,
The invention is further illustrated but is not intended
to be limited by the following examples in which parts and
percentages are by weight unless specified otherwise,
EXAMPLE 1
~ plant comprising a reactor, absorber, stripping
column, scrubber and condenser as described for method A is
employed. The stripper column is 27.7 meters high and com-
prises 21 meters of Pall ring packing in 5 zones, the zones
being separated by chimney trays of 10 cm. depth. Per hour
a gaseous mixture of 7000 parts of methanol, 2795 parts of
water, 708 parts of formaldehyde and 11058 parts of air is
fed continuously to the reactor. Per hour an aqueous for-
maldehyde solution containing 5284 parts of formaldehyde,
171 parts of methanol, 3401 parts of water, 36 parts of
nitrogen, 0.3 parts of hydrogen and 7 parts of carbon
dioxide at a temperature of 86C is introduced continuously
into the stripper column from the circulating loop of the
first absorption stage of the absorber and passed downward
in the stripping column, and an aqueous formaldehyde solu-
tion containing 3179 parts of formaldehyde, 390 parts of
methanol, 5108 parts of water, 68 parts of nitrogen, 0.5
parts of hydrogen and 26 parts of carbon dioxide at a temp-
erature of 65C is introduced continuously into the stripper
column from the circulating loop of the second absorption
stage and passed downward in the stripper column. Per hour,
an aqueous formaldehyde solution containing 1719 parts of
formaldehyde, 87 parts of methanol, 2547 parts of water, 27
parts of nitrogen, 0.1 parts of hydrogen and 6 parts of
carbon dioXide is drawn off from the side of the stripping
column above the entry port for the first absorption solu-
tion and is passed to the first absorption stage. Per hour
11176 parts of stripping gas comprising 10067 parts of nitro-
gen, 9 parts of formaldehyde, 32 parts of methano~ 104 parts
of water, 92 parts of hydrogen and 872 parts of carbon dioxide
is introduced into the bottom of the stripping column and
passed upward~ The temperature of the stripping column is
74C at the bottom and 71C at the top. The gas stream
emerging from the stripper column comprises 1063 parts of
~;~2()7~G
-17- ~ C-06-12-1024
formaldehyde, 404 parts of methan~ 2606 parts of water,
10080 parts of nitrogen, 93 parts of hydrogen and 885 parts
of carbon dioxide. The gas stream is passed to a scrubber
to which 250 parts of water is added continuously per hour.
The aqueous formaldehyde solution formed in the scrubber,
containing 347 parts of formaldehyde, 34 parts of methanol
and 645 parts of water is passed to the second absorption
stage. The gas stream emerging from the scrubber is then
passed through a condenser countercurrent to a downward
flow of circulating aqueous formaldehyde made up with 30 parts
of water per hour and aqueous formaldehyde solution is drawn
off at a rate of 3221 parts per hour comprising 708 parts of
formaldehyde, 337 parts of methan~ 2137 parts of water, 27
parts of nitrogen and 12 parts of carbon dioxide and is re-
cycled to the reaction gas stream. 9332 parts of aqueousformaldehyde solution containing 5690 parts of formaldehyde,
103 parts of methanol and 3462 parts of water is drawn off
from the bottom of the stripping column as product. The
yield is 88% and the con~ersion is 98.5%. The methanol con-
tent is 1.1 percent. The energy requirement for the stripping
column is 1.0 gigajoule per metric ton of product. The net
energy balance for the process is -89 kilojoules per metric
ton of product. The mol ratio of recycled methanol to re-
cycled formaldehyde is 0.45.
EXAMPLE 2
A plant comprising a reactor, absorber, stripping
column, partial condenser and condenser system as set forth
in Figure 2 is used. The stripper column is 27.7 meters
high and comprises 21 meters of Pall ring packing in 5 zones,
the zones being separated by chimney trays of 10 cm. depth.
Per hour, a gaseous mixture of 7001 parts of methanol, 2795
parts of water, 223 parts of formaldehyde, and 11051 parts
of air is fed continuously to the reactor. Per hour an
aqueous formaldehyde solution containing 4532 parts of for-
maldehyde, 163 parts of methanol, 3358 parts of water, 35parts of nitrogen, 0.3 parts of hydrogen and 7 parts of
carbon dioxide at a temperature of 88C is introduced con-
tinuously into the stripper column from the circulating loop
of the first absorption stage of the absorber and passed
D
122o7~6
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downward in the stripping column, and an aqueous formaldehyde
solution containing 2325 parts of formaldehyde, 321 parts
of methanol, 4032 parts of water, 46 parts of nitrogen, 0.4
parts of hydrogen and 14 parts of carbon dioxide at a temp-
erature of 65C is introduced continuously into the strippercolumn from the circulating loop of the second absorption
stage and passed downward in the strip~ngcolumn. Per hour,
an aqueous formaldehyde solution containing 1210 parts of
formaldehyde, 66 parts of methanol, 2043 parts of water, 23
parts of nitrogen, 0.1 parts of hydrogen and 4 parts of car-
bon dioxide is drawn off from the side of the stripping
column above the entry port for the first absorption solution
and is passed to the first absorption stage. Per hour 11551
parts of stripping gas comprising 10521 parts of nitrogen,
0.4 parts of formaldehyde, 24 parts of methan~ 142 parts of
water, 96 parts of hydrogen and 768 parts of carbon dioxide
is introduced into the bottom of the stripping column and
passed upward. The temperature of the stripping column i5
77C at the bottom and 72C at the top. The gas stream
emerging from the stripper column comprises 986 parts of
formaldehyde, 529 parts of methanol, 2764 parts of water,
10519 parts of nitrogen, 96 parts of hydrogen and 775 parts
of carbon dioxide. The gas stream is passed to a partial
condenser and the partial condensate comprising 765 parts
of formaldehyde, 199 parts of methanol and 1885 parts of
water is returned to the top section of the stripping column.
The gas stream emerging from the partial condenser is then
passed through a precondenser and countercurrent in a final
condenser to a circulated aqueous stream made up with 1000
parts of water per hour. The aqueous formaldehyde solution
containing 223 parts of formaldehyde, 306 parts of methanol,
and 1745 parts of water is collected in a distillate receiver
and mixed with 6692 parts of fresh methanol for recycle to
the reaction gas stream. 10245 parts of aqueous formaldehyde
solution containing 5426 parts of formaldehyde, 112 parts
of methanol and 4609 parts of water is drawn off from the
bottom of the stripping column as product. The yield is 87%
and the conversion is 98.4%. The methanol content is 1.1
percent. The energy requirement for the stripping column
~, ~
~.
. 2:Z07~96
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is 0.87 gigajoule per metric ton of product. The net energy
balance for the process is -0,1 megajoule per metric ton of
product. The mol ratio of recycled formaldehyde to total
methanol fed to the reactor is 0.032 and the mol ratio of recycled methanol to recycled formaldehyde is 1.39.
EXAMPT.F~ 8
Example 2 is repeated with a stripping column tempera-
ture of 85C at the bottom and 79C at the top. A 53.5
weight percent formalin solution containing 0.79 percent
methanol is obtained as the product. The yield is 88 and
the conversion is 99%. The energy requirement for the
stripping column is 1.522 gigajoules per metric ton of pro-
duct. The net energy balance for the process is +1204 mega
joules per metric ton of product.
EXAMPLE 4
Example 2 is repeated with a stripping column tempera-
ture of 65C at the bottom and 59C at the top. A 52.9
weight percent formalin solution, containing 1.63 percent
methanol is obtained as the product. The yield is 86% and
the conversion is 98%. The energy requirement for the
stripping column is 0.451 gigajoules per metric ton of
product. The net energy balance for the process is -0.1
megajoules per metric ton of product.
