Catalytic Method for Hydrolyzing Urea

1. Field of the Invention

This invention relates to a method of hydrolyzing urea contained in aquoeus solutions, such as waste-water streams. In particular, the present invention relates to a novel catalytic process for hydrolyzing urea.

2. Description of the Prior Art

Urea is commonly made today by reacting ammonia and carbon dioxide under elevated pressures and temperatures. This reaction also forms water as a by-product. After the reaction is over, the urea is usually separated from the reaction mixture by conventional concentration or crystallization methods. Thus, a urea product, normally in solid form, is recovered. The by-product water is also collected in various waste-water streams. Such streams still may contain a minor amount of urea, urea-related compounds and/or unreacted ammonia and carbon dioxide. Therefore, in nearly all commercial urea processes, these streams, before discharged as effluents, are passed through an ammonia desorber or stripper which recovers unreacted ammonia from the waste-water streams for re-use in the urea process or other chemical processes. However, when these waste-water streams are passed through the ammonia desorber, very little urea contained therein is affected by the stripping operation and, thus, substantially all of the urea in the waste-water streams is disposed of in the effluent streams.

It is now recognized by both the urea industry and the responsible governmental agencies that having urea-containing effluent streams may create possible environmental hazards. Therefore, several processes have been suggested for treating these urea-containing streams. For example, it is known that one of the most effective ways of removing urea from a water stream is simply to hydrolyze it back to ammonia and carbon dioxide at above ambient temperatures (the reverse reaction of the above-described urea synthesis). Some hydrolysis treatments have been disclosed. See U.S. Pat. Nos. 3,826,815, issued to Mavrovic on July 30, 1974 and 3,922,222, issued to Van Moorsel on Nov. 25, 1975. However, these hydrolysis treatments of urea have certain disadvantages. Most importantly, they require the construction of additional equipment for the hydrolysis. Also, this additional equipment must be constructed of explosive stainless steel because the deposition products of urea solutions may cause corrosion of ordinary carbon steel equipment. Still further, hydrolysis processes such as those illustrated in the cited patents require substantial energy costs because large amounts of steam are normally required to heat the urea hydrolysis reaction.

As described in U.S. Pat. No. 4,087,513, which issued on May 2, 1978, the present inventor has found that it would be quite advantageous to utilize the CO.sub.2 recovery streams of adjacent ammonia production plants on hydrolyze the urea contained in waste-water streams formed in the production of urea. Specifically, that invention was found to be superior to the known urea hydrolysis processes because no new equipment was necessary and it was also possible to lower the energy requirements of the ammonia process.

The present invention is directed to a process for hydrolyzing urea in aqueous solutions such as waste-water streams which comprises heating the aqueous solution containing urea in the presence of a catalytic amount of vanadium compound for sufficient time and at sufficient temperatures to effect hydrolysis of at least a portion, preferably, substantially all, of the urea to ammonia and carbon dioxide.

Any aquoeus solution containing a minor percentage of urea (i.e., less than about 50% by weight of the total stream) can be treated by the process of the present invention. Any waste-water streams from any conventional urea production processes are most likely to be treated by the present invention. Yet, other sources including water streams from urea storage and loading facilities (i.e., to collect spillage and leaks) may also be advantageously treated according to the process of the present invention.

The term "urea" as employed in the present specification and claims includes urea-like chemical compounds such as biuret and the like which, when hydrolyzed, forms carbon dioxide and ammonia. Preferably, the percentage of urea in the waste-water stream is from about 0.01% to about 20% by weight of the stream. More preferably, the present process is advantageously used to treat waste-water strams containing from about 0.05% to about 10% by weight of urea.

For illustration, the following briefly describes a urea production process that produces a waste-water stream which may be advantageously treated according to the process of the present invention. First, ammonia and carbon dioxide are reacted together, for example at about 175.degree.-200.degree. C. and about 2500-3000 psig, to form ammonium carbamate ##STR1## which by an equilibrium reaction forms urea and water. These reactions are represented by the following equations: ##STR2## Following substantial completion of these reactions, the gaseous reaction mixture is passed through a multi-stage gas separator to remove large amounts of unreacted ammonia and carbon dioxide gases and lower the pressure of the reaction mixture to about atmospheric pressure. Then, the reaction mixture is passed through a flash tank which is under vacuum. This step effects the removal of large quantities of water vapor. After passing through the flash tank, the resulting reaction mixture, which now usually contains about 70-80% by weight urea and about 30-20% water, is sent through a two-stage evaporator wherein the remaining water is evaporated off and substantially anhydrous urea product (i.e., above 99% by weight urea) is formed. The substantially anhydrous urea product is then granulated or prilled into a solid urea product suitable for sale or use in other chemical processes. The vaporized water from the flash tank and from the evaporator is combined and then condensed to form a waste-water stream. This waste-water stream is preferably passed through an ammonia desorber which removes a large portion of the ammonia without affecting the small amount of urea contained threin. This waste-water stream may be then treated according to the process of the present invention. However, it should be recognized that the present invention is not limited to any particular process by which the waste-water stream is formed. As stated above, the process of the present invention encompasses the concept of hydrolyzing the urea contained in such waste-water streams in the presence of a catalytic amount of one or more vanadium compounds for sufficient time and at sufficient temperatures to hydrolyze substantially all (i.e., more than about 90 percent by weight) of the urea contained therein. This hydrolysis reaction can be represented by the following equation: ##STR3##

A proper catalytic amount of a vanadium compound for this reaction will be different for each application because of the variance of many reaction parameters and other factors. Included among such parameters and factors and the general method and/or apparatus employed for such hydrolysis; the presence or absence of other impurities in the waste-water strams besides urea; the desired rate of hydrolysis; the specific vanadium compound employed and the like. However, for most purposes, the catalytic amount of a vanadium compound employed will be in the range of from about 0.2 percent to about 1.0 percent by weight (expressed as vanadium) of the aqueous solution being treated. Preferably the amount of a vanadium compound will be from about 0.05 percent to about 0.7 percent by weight (expressed as vanadium) of the aqueous solution. Specific vanadium compounds which have demonstrated their effectiveness for the present invention include sodium ortho vandate, sodium meta vanadate, ammonium vanadate, vanadium pentoxide and vanadyl sulphate. See the specific Examples discussed below. Vanadium pentoxide is preferred because this compound has been found suitable for aqueous solutions that are commonly employed in a carbon dioxide recovery system in an ammonia production process. Sodium ortho vanadate is also preferred because it has shown the greatest effectiveness of the vanadium compounds tested for catalyzing this hydrolysis reaction.

The urea-containing aqueous solutions must be treated at a sufficiently high temperature so that substantially all of the urea will be hydrolyzed. Such temperature will vary for each application and will depend on many factors including the desired rate of reaction. Generally, for most applications it is desirable to carry out the urea hydrolysis at a temperature in the range of about 70.degree. C. to about 200.degree. C., more preferably, from about 80.degree. C. to about 180.degree. C., and most preferably, from about 90.degree. C. to about 160.degree. C.

Like the reaction temperature, the reaction time is an important consideration for determining the extent of the urea hydrolysis. However, the optimum time will also vary for each installation and is also dependent on many factors including reaction temperature. Generally, a reaction time of at least 10 minutes, preferably about 15 to about 360 minutes, is desirable for most applications. The present catalytic method for hydrolyzing urea may be carried out in any suitable apparatus. When treating urea-containing waste-water streams from urea production processes, it is particularly advantageous to use a vanadium compound in the normally existing ammonia desorber of most urea production plants or in the CO.sub.2 recovery systems of adjacent ammonia production processes. The additional high cost of constructing new equipment is prevented by utilizing such existing facilities.

After the urea has been hydrolyzed to carbon dioxide and ammonia by the present catalytic process, it is generally advisable to remove the just-formed ammonia from the aqueous solution. Environmental standards normally prohibit the discharge of NH.sub.3 (the same as urea) in effluent waste-water streams. The formed carbon dioxide does not normally represent any significant environmental hazards.

If the catalyzed hydrolysis occurs in the conventional ammonia desorber, then the NH.sub.3 formed from urea will be easily removed with the rest of NH.sub.3 present in the waste-water streams. Thus, additional ammonia may be recovered from the hydrolyzed urea and recycled back to the urea production processes or other chemical processes from the top of the desorber.

Still further, since some aqueous solutions utiized in CO.sub.2 recovery systems in ammonia production processes may contain V.sub.2 O.sub.5 (see U.S. Pat. Nos. 3,851,041 and 3,896,212, issued to Eickmeyer on Nov. 26, 174 and July 22, 1975, respectively), it may in some situations be desirable to instead hydrolyze the urea in these waste-water streams with such V.sub.2 O.sub.5 -containing aqueous solutions. This concept is more fully described in the above-noted U.S. Pat. No. 4,087,513, filed by the present inventor which is incorporated herein by reference in its entirety. Alternatively, when such ammonia and urea production plants are adjacent to each other, it may be desirable to circulate a small portion of said V.sub.2 O.sub.5 -containing aqueous solution utilized in the CO.sub.2 recovery system to the ammonia desorber feed tank or directly to the ammonia desorber itself. In particular, this portion of the catalyst-containing aqueous solution may be fed from the CO.sub.2 recovery system to the top of the ammonia desorber and travel downward therein while contacting the urea-containing aqueous stream in the desorber. Thus, at least a portion of the urea in the waste-water stream will be hydrolyzed in the desorber. Upon reaching the bottom of the desorber, the resulting mixture of the two aqueous solutions will be recycled back to the CO.sub.2 recovery system rather than be immediately discharged as effluent. In the CO.sub.2 recovery system, this mixture of aqueous solution can be re-used as an energy-saving reflux and for the completion of the hydrolysis of the urea contained in the aqueous mixture as indicated in U.S. Pat. No. 4,087,513.

The following example and comparison further illustrate the process of the present invention. All portions and percentages are by weight unless explicitly stated otherwise.

Several aqueous solutions, each containing 20 grams of urea per liter of water and varying amounts of vanadium compounds, were heated in stirred glass laboratory reactors at a gentle reflux for specific amounts of time. The specific compounds, their amounts and times of hydrolysis are given in Table I, below.

An inert gas purge was fed at a controlled rate to sweep any ammonia formed from the glass reactors through a reflux condenser and vapor line and into traps containing 2 percent by weight boric acid. The amount of ammonia produced as the result of the hydrolysis of urea was measured and this value was used to determine the percent of urea hydrolyzed. These latter values are given in Table I, below.

A comparison of the results in Table I demonstrates the value of vanadium compounds as catalysts for promoting urea hydrolysis.