Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR PRODUCING STABLE, ACIDIC UREA SOLUTIONS
BACKGROUND
Field of the Invention
The invention relates to acidic urea solu~ions and,
in particular, to methods for producing acidic urea solutions
having low equilibrium vapor pressures.
Background of the Art
Urea is widely used in several industries as a
chemical precursor, fertilizer, feed supplement ~or ruminant
mammals and is a component of various mixtures such as dermal
salves and creams. A large volume of urea is employed in the
agricultural industry as a fertilizer and protein nitrogen
source for ruminant mammals, and much of this is used in the
form of aqueous solutions, including dispersions containing
insoluble matter. In many instances, it is preferable to
acidify urea solutions to increase the solubility of
otherwise insoluble materials such as zinc, iron and
manganese nutrients as well as various phosphates employed in
the agricultural and chemical industries. Acidified urea
solutions are also employed to counteract alkalinity of water
supplies with which the solution may be mixed and/or to
counteract alkalinity of alkaline soils prevalent in the
western United States.
A plant nutrient combination ideal for many
applications is a solution containing sufficient available
phosphorus and nitrogen to supplement or completely supply
plant needs. Phosphoric acid is one of the most readily
available and least expensive sources of water-soluble
available phosphateO However, ammonium salts have only
limited solubility in phosphoric acid solutions, and nitrates
are converted, in the presence of phosphoric acid, to nitric
acid which is unacceptable to many manufacturers and users.
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Urea, on the other hand, is both ~table and highly soluble in
phosphoric acid. Thus, aqueous solutions of urea and
phosphoric acid are ideal for many plant nutrient
applications, and concentrated solutions of these two
components are particularly desirable for economic reasons,
e.g. cost of shipment, handling, etc. Both dilute and
concentrated solutions of urea and sulfuric acid have also
found wide acceptance in the agricultural industry as
fertilizers, soil adjuvants and herbicides. Such fertilizers
and their methods of manufacture are disclosed by Donald C.
Young in U.S. Patents 4,445,925, METHODS OF PRODUCING
CONCENTRATED UREA-SULFURIC ACID REACTION PRODUCTS, issued May
1, 1984 and 4,447,253, TOPICAL FERTILIZATION M~THODS AND
CO~OSTIIONS FOR USE T~EREIN, issued May 8, 1984, the
disclosures of which are incorporated herein in their
entireties.
While the benefits of using acidifiea urea
solutions in many chemical and agricultural applications have
been recognized for years, most often it has proven
impossible to obtain such solutions which do not generate
such elevated vapor pressures that they cannot be stored in
closed containers. And, while this problem has not prevented
the use of acidic urea solutions in large volume r industrial
applications, it has proven to be a significant problem in
smaller containers employed in industry and, in particular,
in the home and garden market where inversion of vented
containers is highly probable. In view of this problem, a
significant market in vented containers, designed
specifically for acidic urea solutions, has grown up due to
the demand of the home and garden market. While such
containers do prevent the problems of container expansion and
explosion which occurs with unvented containers, their use
does not prevent spillage, and it raises yet another problem
when surfactant-containing solutions are employed.
Surfactants are often used in urea fertilizers to facilitate
distribution, and they are also employed in various
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industrial applications ~uch as the pxomotion of 2 0 6 7 ~ ~ 2
acid-catalyzed reactions with urea-sulfuric acid compositio~s
containing the monourea adduct of suluric acid as disclosed
by Donald C. Young in U.S. Patent 4,722,986, ACID CATALYSTS
AND METHODS OF USE, issued February 2, 1988, the disclosure
of which is incorporated herein in its entirety. The use of
surfactants in such acidic urea solutions in ventea
containers poses a signiicant problem of leakage and foaming
due to foam formation as the solution off-gases during
s~orage.
For years, it has been beli~ved that the high vapor
pressure characteristic of acidic urea solutions was due to
either chemical or enzymatic hydrolysis during storage,
either of which would be persistent throughout the solution's
storaqe life, and this belief was due primarily to several
observations. First, if unvented containers of acidic urea
solutions stored ~or a period suficient to build significant
pressure, e.g. one day or more, are opened to release the
accumulated gas and reclosed, they will build pressure upon
further storage. Secondly, some investigators reported
limited, infrequent success in reducing pressure buildup by
adding to the acidic urea solutions chemicals which denature
urease enzyme. In addition, it was noted that the pressure
buildup associated with such solutions was due primarily, if
not exclusively, to the accumulation of carbon dioxide, a
urea hydrolysis product. Nevertheless, the problem of
persistent pressure buildup in unvented containers of
acidified urea solutions still remains, and its remedy would
benefit several industries.
SUMMARY OF THE INVENTION
It has now been found that the persistent vapor
pressure buildup over acidic urea solutions is due to the
presence of carbonates, apparently introduced with the urea,
in combination with unusual ~inetic and equilibrium behavior
of carbonate in such solutions. It has also been found that
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the vapor pressure buildup of stored acidic urea solutions
can be alle~iated or elimina~ed by stripping the solutions
under conditions sufficiently severe to reduce the total
carbonate content to a level which, if all present as carbon
dioxide, does not result in an unacceptable equilibrium vapor
pressure even after prolonged storage. This finding, i.e.
that the problem of vapor pressure buildup is associated with
the presence of carbonate, was not expected since carbonates
typically decompose immediately to carbon dioxide upon
acidification, and the resulting carbon dioxide is
immediately evolved from the solution. In fact, rapid carbon
dioxide evolution from basic urea solutions during
neutralization with acid was observed to be so rapid by
Donald C. Young in U.S. Patent 4,345,099, METHOD OF
SELECTIVELY REMOVING BIURET FROM UREA AND COMPOSITIONS FOR
USE THEREIN, issued August 17, 1982, that Young advised the
exercise of caution during acid addition for the reason that,
in that instance, carbonate neutralization caused carbon
dioxide evolution at a rate so rapid that it could produce
foaming and volume expansion. (See, in particular, column 9,
lines 7~17.) While rapid C02 evolution did occur during
acidification of the alkaline urea solutions described in
U.S. Patent 4,3~5,099, it has been found that such rapid
conversion and carbon dioxide evolution from solutions
containing the minor amounts of carbonate present in most
commercial ureas, or a residual minor amount of carbonate
remaining after evolution of the principal amount of carbon
dioxide generated by acid neutralization, does not occur. In
fact, acidified urea solutions can be vented numerous times
and will continue to build pressure in a closed container.
Without intending to be constrained to any
particular theory, it is presently believed that the
relatively severe stripping employed in these methods either
favorably shifts an equilibrium otherwise existing in the
solution and/or increases the rate of carbonate conversion to
carbonic acid and carbon dioxide sufficiently to reduce
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total carbonate content to a level tha~ does not result in
unacceptable equilibrium vapor pressures. For the purposes
of this invention, ~total carbonate content" i5 intended to
include carbonates, bicarbonates, carbonic acid and carbon
dioxide. ~Equilibrium vapor pressure" is intended to mean
the vapor pressure existing over the solution in a closed
container in which 90 percent of the volume is occupied by
the solution after storage for 48 hours. Herein, equilibrium
vapor pressure is expressed and measured at 20 C.
Obviously, the equilibrium vapor pressure of these solutions,
as with all vaporizable solutions, increases as temperature
is increased.
The resulting solutions have siqnificantly improved
storage stability in that they do not develop significant
equilibrium vapor pressures even after prolonged storage.
Thus, they can be stored in closed containers, thereby
avoiding the added cost and the hazards and inconvenience of
storage in vented containers such as spillage, foaming (when
surfactants are present) and contamination.
DETAILED DESCRIPTION
Acidic, aqueous urea solutions having acceptable
equilibrium vapor pressure during storage in closed
containers can be obtained from solutions having a pH of 5.5
or less and a total carbonate content which, if present as
carbon dioxide, would produce an equilibrium vapor pressure
of more than 0.5 psig. at 20C C. by stripping such solutions
under conditions su~ficiently severe to reduce the total
carbonate content to a level which, if present as carbon
dioxide, would produce an equilibrium vapor pressure of 0.5
psig. or less at 20 C. Adequate stripping can be achieved
by sufficiently severe gas stripping and/or mechanical means,
preferably by a combination of the two. The gases used for
this purpose are preferably, relatively insoluble in the
solution and include air, nitrogen, and other chemically
inert gases. The gas can be introduced into the solution by
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any ~uitable means ~uch as sparging into the liquid or
passage upwardly through a packed tower through which the
solution is passed downwardly. Other procedures known in the
art for stripping liquids with gases are also suitable.
Mechanical stripping can be achieved by severe
agitation with either mechanical means such as stirrers or
impellers, by contact with high ~urface area media such as
packed columns, or by rapid gas injection into the liquid
through a gas aistribution means. Combined gas and
mechanical stripping are particulaxly preferred, and this can
be achieved b~ the use of packed columns as described above
or by agitating the solution by mechanical means combined
with sparging a stripping gas into the solution.
The time required for adequate stripping depends,
to a large extent, on the severity of mechanical and/or gas
stripping employed, and should be sufficient to reduce the
total caxbonate content to a level which, if present as
carbon dioxide, would produce an equilibrium vapor pressure
over the solution of 0.5 psig. or less at 20 C. Typically,
stripping will be continued for at least about 1 minute,
generally at least about 2 minutes and can involve about 2
minutes to several hours, e.g. 2 hours or more depending on
the efficiency (severity) of the stripping procedure
involved. The exact time required for a given stripping
procedure to achieve the necessary reduction in total
carbonate content can be xeaailv determined by subjecting
separate samples of the solution to stripping by that
procedure for various lengths of time, e.g. 1 minute, 10
minutes, 1 hour, etc., storing the sample solutions in closed
containers in which the solution occupies 90 percent of the
container volume, and measuring the vapor pressure over the
solution after a selected period of time, e.g. 48 hours, 1
week, or longer period as desired. The total quantity of
solution can then be stripped for the time required to
achieve the desired reduction in equilibrium vapor pressure
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and, if desired, can be 6tored in either closed or open
containers.
The temperature of the solution during stripping is
not critical, and adequate stripping can be achieved at
ambient temperature, e.g. 20 C. However, as disclosed by
Young in U.S. Patent 4,445,925, referred to above,
temperatures of 70 C. ~158 F.) should not be exceeded when
employing relatively concentrated urea-sulfuric acid
compositions having urea/H2SO4 molar ratios above 2, and
temperatures of 80 C. (176 F.) should not be exceeded with
compositions having urea/H2SO4 molar ratios below 2, since
those solutions tend to decompose above those respective
temperatures. Furthermore, stripping can be accelexated at
elevated temperature, i.e. up to 70~ C. Thus, stripping
temperatures axe usually within the range of about 20 to
about 60 C.
The solutions can contain any min~ral or organic
acid capable of obtaining a p~ o about 5.5 or less in the
urea solutions. Illustrative acids include sulfuric, nitric,
phosphoric, hydrochloric, citric, acetic, etc., and
combinations of one or more ~ineral or organic acids can be
employed. The mineral acids are presently preferred, with
sulfuric and phosphoric acids being particularly preferred.
Acid concentration is sufficient to obtain a pH of about 5.5
or less, usually about 5 or less~ and most often about 2 or
less. (Equilibrium vapor pressure buildup with solutions
having pH levels above 5.5 is less evident than with
solutions having pH levels of 5.5 and less.) The acid
concentration required to achieve these p~ levels, of course,
depends upon the relative acid strength of the acid and
usually will be at least about 0.5, typically at least about
l, preferably at least about 5, and most preferably at least
about lO weight percent. Thus, acid concentrations within
the range o about 0.5 weight percent to saturation can be
employed, and these usually correspond to about O.S to 60
weight percent depending on the acid selected, urea
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concentration, and the presence and concentration of other
components.
Urea concentration can also vary considerably and
is usually within the range of about 5 weigh~ per~ent up to
the solubility limit of urea in the solution. ~owever, since
the problem of gradual carbon dioxide evolution and
consequent vapor pressure buildup in closed containers is
more severe in the more concentrated solutions, these methods
are particularly useful for treating solutions containing at
least about 10 weight percent urea, and are even more
beneficial at higher urea concentrations, e.g. about 20
weight percent and above.
The acidic urea solutions to be treated by the
described stripping procedures will generally have an
equilibrium vapor pressure above 0.5 psig. since vapor
pressures of 0.5 psig. or less are acceptable for storage in
many containers. However, even lower vapor pressures of
about 0.1 psig. or less are preferred, while pressures of
about 1 atmosphere absolute or less at 20 C. are most
preferred for storage in either open or closed containers.
Thus, solutions having vapor pressures only slightly above 1
atmosphere absolute at 20 C. can be treated by the de~cribed
methods with resulting improvements in storage stability.
For that matter, solutions containing any amount of residual
carbonate, even those having equilibrium vapor pressures of
about 1 atmosphere or less at 20 C., can be beneficially
treated by these methods since such treatment further reduces
total carbonate content and reduces equilibrium vapor
pressure at temperatures above 20 C. Thus, the stripped
solutions will generally have an equilibrium vapor pressure
of 0.5 psig. or less, preferably about 0.1 psig. or less, and
most preferably about 1 atmosphere absolute or less at 20~ C.
In terms of total carbonate concentration, the most preferred
equilibrium vapor pressures of about 1 atmosphere ~bsolute or
less correspond to total carbonate concentrations equal to or
below the saturation limit of an equivalent amount of carbon
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dioxide in the ~olution at 20 C. and 1 atmosphere pressure
absolute. In o~her words, ~he total carbonate concentration
is most preferably below the level which would generate
sufficient carbon dioxide to exceed the saturation limit in
the solution at 20 C. and 1 atmosphere p~essure absolute,
since that results in the most preferred solutions having
equilibrium vapor pressures of about 1 atmosphere absolute or
less at 20~ C. Since the solubility of the carbon dioxide in
such solutions varies as a function of solution composition,
e.g. concent.ation of urea and identity and concentration of
acid and other components, the tolerable amount of total
carbonate in the finished solutions will vary with
composition. Generally, however, total carbonate content
will be equivalent to about 0.2 weight percent or less,
prefexably about Q.l weight percent or less expressed as
equivalent carbon dioxide. Acceptably low total carbonate
concentrations can be identified by storing samples of the
treated solution in closed containers in which the solution
occupies 90 percent of the container volume for a suitable
period of time, e.g. 48 hours, 1 week or longer, as desired,
at 20 C. (or higher temperature if higher temperatures are
to be employed) and measuring the pressure existing above the
solution. However, total carbonate concentrations which
exceed that acceptable for long~term storage generally can be
identified within 48 hours of storage.
While the problems associated with gradual carbon
dioxide evolution exist in the more dilute solutions
described, they are even more severe with more concentrated
solutions. Accordingly, these methods are useful for the
treatment of solutions in which the urea and acid, in
combination, constitute at least about 5 weight percent of
the solution, and they are even more beneficial when the urea
and acid, in combina~ion, constitute at least about 10,
preferably at least about 20, and most preferably at least
about 50 weight percent of the solution. Typically, the urea
and acid, in combination, will constitute about 5 to about
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80, most often about 10 to about 80 weight percent of the
solution, particularly when sulfuric and/or phosphoric acids
are employed.
The finished solutions can contain one or more of
numerous materials which contribute to the quality of the
solution, such as surfactants, major and minor nutrients,
non-aqueous diluents and solvents other than water, such as
alcohols, ethers, etc. Insolubl~ matter such as protein,
cellulose andtor other polysaccharides or polyesters can also
be present as described in U.S. Patent 4,722,986, referred to
above, and U.S. Patent 4,664,717, METHODS FOR HYDROLYZING
POLYSACC~ARIDES AND COMPOSITIONS USEFUL THEREIN, issued May
12, 1987, the disclosure of which is incorporated herein in
its entirety. Such materials can also be present in the
solutions prior to stripping provided they do not interfere
with the ability of the stripping procedure to reduce
carbonate content to an acceptable level. Materials, such as
surfactants, which cause solution foaming are preferably not
present during stripping for obvious reasons.
The more dilute of the above-described acidic urea
solutions can be prepared by simply adding acid to a urea
solution, or vice versa by any acceptable meansO However,
manufacture of more concentrated solutions, e.g. those in
which the urea and sulfuric acid, in combination, constitute
at least about 30 weight percent of the solution and which
have relatively high acid concentrations, are preferably
manufactured by procedures which provide sufficient cooling
to control the heat evolved by the exothermic reaction that
occurs between urea and some acids, such as sulfuric acid,
and/or the heat of acid dilution. For instance, concentrated
urea-sulfuric acid solutions can be safely and efficiently
prepared by the methods described in U.S. Patent 4,445,925,
referred to above.
The invention is further described by the following
examples which are illustrative of specific modes of
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practicing the invention and are not intended as limiting the
scope of the invention as defined by the appended claims.
Example 1
Approximately 20 tons of an aqueous solution of
urea and sulfuric acid reaction product was produced by the
me~hod described in U.S. Patent 4,445,925, referred to above,
and stored in a vented tank for 1 day. The product contained
49 weight percent equivalent sulfuric acid, 32 weight percent
equivalent urea, and 19 weight percent water and had a pH
below 2. This composition has a urea/H2S04 molar ratio of
1.06, and therefore consists, primarily, of the monourea
adduct of sulfuric acid, i.e. monocarbamide dihydrogen
sulfate. Briefly, the manufacturing procedure involved
simultaneously adding the required amounts of urea, sulfuric
acid and water to a 6500 gallon reactor containing a mixture
of previously reacted material and freshly added reacting
material. A recycle stream was continuously withdrawn from
the reactor and passed through a direct contact, packed bed
air cooler in which the reacting liquid phase of urea,
sulfuric acid, and water, in addition to previously reacted
material, was passed downwardly in direct contact with
upwardly rising air under conditions sufficient to maintain
the temperature of all of the material in the reaction 20ne
at less than 80 C. The cooler comprised a bed of 2-inch
INTERLOX polypropylene saddles approximately 5.7 feet deep
and 10 feet in diameter over which the urea-sulfuric acid
composition was passed downwardly into contact with upwardly
rising air.
Following approximately 1 day of storage,
approximately 10 tons of the composition were returned to the
reactor and recycled through the direct contact air cooler at
a rate corresponding to 26 passes of the total reactor volume
per hour with an air flow rate of approximat~ly ~o,oao cubic
feet per minute. Recirculation through the cooler, which in
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this operation acted as a stripper, was continued for 3.5
hours.
The composition was sampled befoxe and after
stripping, and the sampl~s wexe maintained in gas-tight
pressure vessels. The vapor pressure of each sample was then
monitored with a water manometer for 17 days, and these
results are reported in Table 1.
TABLE 1
Va~or Pressure! cm. H O
Sample Stripping
No. Time, Min. Day 0 Day_~ Day 2 Day 3 Day 17
0 Maa >100 >loob >loob
2 20 0 0 0 0 0
b Observation inadvertently omitted
Significant increases not observed after Day 2, possibly
due to CO2 adsorption into and venting through manometer.
These results demonstrate that material that had been
manufactured in the described reactor-cooler system and
stored in a vented tank for 1 day developed a vapor pressure
of greater than 100 centimeters water (greater than 1.5
psig~) in a closed container within 2 days. ~The first day
observation ~or the unstripped material was inadvertently
omitted.) Thus, the manufacturing method, which also
involves passage of the material through the pacXed bed
cooler, for some reason, i5 not adequate to prevent vapor
pressure buildup in acidic urea solutions. This inadequacy
is due to the conditions maintained during the reaction which
are necessary for production of the urea-sulfuric acid
composition. In contrast, recirculation of the previously
manu~actured composition through the stxipper for 20 minutes
(9 cycles) produced a composition which generated no
observable vapor pressure even after storage for 17 days.
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Example 2
The equilibrium vapor pressure of an aqueous
solution containing 20 weight percent urea and lO weight
percent orthophosphoric acid having a pH below 2 can be
reduced to a level below 0.5 psig. by agitating 10 gallons of
the solution in a 30 gallon vessel with a 4-inch diameter
impeller mixer at 300 xpm for 30 minutes while sparging air
into the bottom of the vessel and below the liquid surface at
a rate of 0.5 standard cubic foot per minute.
Example 3
The equilibrium vapor pressure of an aqueous
solution containing 20 weight percent urea and lO weight
percent acetic acid having a pH below 2 can be reduced to a
level below 0.5 psig. by stripping the solution for 30
minutes by the procedure described in Example 2~
While particular embodiments of this invention have
been described, it will be understood, of course, that the
invention is not limited thereto since many obvious
modifications can be made, and it is intended to include
within this invention any such modifications as will fall
within the spirit and scope of the appended claims~
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