This application claims benefit of United States Provisional Patent Application No. 61/220,733, filed 26 June 2009.

The present invention relates to a process for removing organic amines and organic acids from a wastewater stream from an MDA or PMDA manufacturing facility.

Methylene dianiline (MDA) and polymeric methylene dianiline (PMDA) are prepared at large scale by condensing aniline with formaldehyde in the presence of an acid catalyst. The product is almost always a mixture of two-ring structures (MDA) and structures that have three or more rings (PMDA). The condensation reaction produces a crude product stream that contains MDA, PMDA, some unreacted aniline and sometimes small quantities of other amines such as cyclohexyl amine. Most commercial sources of the aniline raw material contain small quantities of phenol, which passes through the system and is present in the crude product stream. The crude product stream also contains salts which result from the neutralization of the acid catalyst.

The salts are removed from the crude product stream. A typical way of accomplishing this is to extract the crude product stream with water, which can be slightly alkaline in order to ensure complete neutralization of the acid catalyst. This produces a wastewater stream which contains essentially all of the salts that were contained in the crude MDA/PMDA product stream. The wastewater stream will also contain small quantities of phenol as well as some amount of amines (including MDA, PMDA and aniline). It is often necessary to remove the phenol or both the phenol and the amines from the wastewater stream to very low levels before the water can be reused or discharged into the environment.

It is difficult to remove phenol to very low levels in a cost-effective manner, and more difficult to remove both types of contaminants (i.e., the phenol and other acids, plus the organic amines) to very low levels cost-effectively. Various methods are available, which can cost-effectively remove these contaminants to the 50-150 ppm level. Examples of those methods include chemical oxidation (including treatment with hydrogen peroxide and iron (II)), catalyzed ozonation, phenol stripping, solvent extraction, thermolysis and biological treatments of various types. However, these methods cannot, at a reasonable cost, remove phenol and organic amines to concentrations to 1 part per million below. To do so using these methods would incur high capital and/or operating costs to operate at large scale or at commercially feasible operating rates. Often, these methods simply transfer the phenols and/or organic amines to another medium, which itself represents a disposal problem.

A process which can cost- effectively remove both organic acids such as phenol and organic amines to below 1 ppm levels in the wastewater is desired.

This invention is a process for removing phenolic compounds from a wastewater stream from a manufacturing facility that condenses aniline with formaldehyde, comprising a) contacting the wastewater stream with an activated carbon bed while the pH of the wastewater stream is 5 or less, whereby at least one phenolic compound is removed from the wastewater stream.

In a preferred process, both phenolic compounds and organic amines are removed from a wastewater stream from a manufacturing facility that condenses aniline with formaldehyde. The preferred process comprises a) contacting the wastewater stream with an activated carbon bed while the pH of the wastewater stream is 5 or less, whereby at least one phenolic compound is removed from the wastewater stream and b), before or after step a), contacting the wastewater stream with a second activated carbon bed while the pH of the wastewater stream is 8 or higher, whereby at least one organic amine is removed from the wastewater stream.

The process of the invention offers several advantages. Surprisingly, the process is capable of removing has the advantages of being effective at removing phenolic compounds or both organic amines and phenolic compounds to low levels, with low operating and capital costs. The process can remove both phenols and organic amines to levels of 1 part per million or lower in the wastewater stream, rapidly and at commercially reasonable costs and operating rates. Furthermore, the removed phenols and organic amines are captured on a solid material, from which they can be thermally destroyed with recovery of the adsorbent. Therefore, the process does not simply transfer the adsorbed phenol and organic amines to another stream which only represents another disposal problem. After the adsorbed materials are thermally destroyed, the adsorbent can be recycled and reused in the process (or for other adsorption processes). The ability to recycle and reuse much of the adsorbent results in low raw materials costs. Similarly, other operating costs tend to be low. The needed equipment tends to be of uncomplicated design and for the most part can be made using commonly available, inexpensive metals.

-9.- The wastewater stream is an aqueous stream from a manufacturing facility that condenses aniline with formaldehyde (which may be used as formalin). Such manufacturing facilities commonly produce methylene dianiline (MDA) and polymethylene poly anilines (PMDA), which are useful chemical intermediates for preparing a number of products, notably methylene diphenylene diisocyanate (MDI) and polymethylene polyphenylene polyisocyanates (PMDI) for polyurethanes manufacturing. The crude condensation products, MDA and/or PMDA, contain residual salts which are the by-products of the condensation reaction after neutralization of the reaction mixture. These salts are removed by an aqueous extraction method, whereby the crude MDA or PMDA is washed with an aqueous stream such that water-soluble species migrate to the aqueous phase. The aqueous stream is usually adjusted to a basic pH to neutralize residual acids in the crude MDA or PMDA.

The wastewater stream that is produced in this manner is typically a wastewater stream that may contain 3% or more weight dissolved inorganic salts. Sodium chloride typically is by far the predominant inorganic salt, although other inorganic salts may be present. For example, other alkali metal and alkaline earth salts may be dissolved in the wastewater stream, and the wastewater stream may contain small amounts of other metal ions, such as iron. The inorganic salt loading may be 5% or more, or 8% or more, up to about 28% by weight.

The wastewater stream also includes one or more phenolic compounds. Phenol is generally the predominant phenolic compound, although other phenolic compounds may be present in small quantities. Phenol for the most part is brought in as an impurity in the aniline starting material. Phenol passes through the manufacturing process without reacting and exits at least partially via the wastewater stream.

In addition, the wastewater stream may contain other organic acids. These organic acids are weak to moderately strong acids which may have, for example a pK _a of from about 5 to about 11, preferably from 7 to 11. pK _a is used in its normal sense for purposes of this invention, meaning the negative log of the dissociation constant Ka in water at 25°C. K ₃ equals [A ] [H ⁺ ]/[AH], where [AH] denotes the equilibrium concentration of the acid, [A ] indicates the equilibrium concentration of conjugate base of the acid and [H ⁺ ] indicates the equilibrium concentration of hydrogen ions. The organic acid may contain, for example, one or more carboxylic acids. Depending on the pH of the wastewater solution, carboxylic acid and phenolic groups will be more or less in the ionic or salt form, i.e., -COO M ⁺ in the case of a carboxylic acid and Ar-O M ⁺ in the case of a phenolic compound, where M represents a cation, Ar represents an aromatic ring structure and the carbonyl carbon of each carboxylate group or phenolic oxygen, as the case may be, is bonded to a carbon atom in an aromatic ring.

In addition to the inorganic salts and phenolic compound (or other organic acids), the wastewater may contain one or more organic amines. The organic amines preferably are weak to moderately strong bases which may have, for example, a pKb of from about 5 to 11. pKb is used in its normal sense for purposes of this invention, meaning the negative log of the dissociation constant Kb in water at 25°C. Kb equals [OH ] [BH ⁺ ]/[B], where [B] denotes the equilibrium concentration of the base, [OH ] indicates the equilibrium concentration of hydroxyl ions and [BH ⁺ ] indicates the equilibrium concentration of the conjugate acid of the base. The organic amine contains at least one primary, secondary or tertiary amine group, and may contain multiple such groups. The organic amine may include an aromatic amine compound, in which the nitrogen atom(s) of the amine group or groups are bonded to a carbon atom of an aromatic ring. The organic amine may be one or more aromatic primary amine compounds, which can be represented as Ar-(NHa) _x , where Ar represents an aromatic ring structure, x is at least 1, and the amine nitrogen atom(s) are each bonded directly to a carbon atom in an aromatic ring of the Ar group, x may be any larger number, but typically is from 1 to 5, more typically from 1 to 2, and can be exactly 1. In most cases, the predominant organic amines are aniline, MDA and PMDA, although other aliphatic amines and aromatic amines may be present.

The wastewater stream may contain from 1 to 1000 ppm of phenolic compound(s) and/or other organic acid combined and from 0 to 10,000 ppm of organic amine compounds, prior to treatment in accordance with the invention. The wastewater stream more typically will contain from at least 20 ppm each of phenol and organic amines, prior to treatment. The wastewater stream in most cases will contain up to 500 ppm of each of phenol and organic amines, prior to treatment. More typical levels are from 5 to 100 ppm of each of phenol and organic amines. Wastewater streams containing higher concentrations of phenolic compounds, other organic acids and/or organic amines may, if desired, be pretreated to reduce the concentrations of those impurities into the aforementioned ranges. High organic concentrations in the wastewater stream can lead to short bed-life during the adsorption steps. In addition, other processes such as thermal stripping may be more economical for removing the bulk of the organic amines and/or phenolic compounds when the wastewater stream initially contains those materials in larger concentrations.

In this invention, the wastewater stream is contacted with activated carbon while the stream is at a low pH, in order to remove phenolic compounds and optionally other organic acids. In preferred embodiments, the wastewater stream is contacted at least twice with activated carbon, at least once while at a low pH to remove phenolic compounds and optionally other organic acids, and at least once while at a high pH to remove organic amines. In the preferred embodiments, the order in which the wastewater stream is contacted with the beds is not considered to be critical. Therefore, in certain embodiments, the wastewater stream is first contacted with the activated carbon while at the low pH, followed by contacting it with activated carbon while at the higher pH. However, it is possible to reverse the order of the steps and perform the low pH step of the process last. In that case, it may be necessary to adjust the pH of the treated wastewater after the low pH step is completed, to make the wastewater less acidic so it can be handled by metal equipment in downstream operations.

The "low pH" portion of the process is conducted with the pH of the wastewater stream within the range of 1 to 5, preferably from 1 to 4 and more preferably from 2 to 4. The pH during the "low pH" portion of the process should be such that at least 99% of the phenolic compounds, and other organic acids that may be present, are in the molecular (i.e., acid) form, and no more than 1% of the organic acid is in the form of the ionic (or conjugate base) form.

In the preferred process, step a) (the "low pH" portion of the process) is conducted prior to step b) (the "high pH" portion of the process), and the pH of the wastewater stream is adjusted between steps a) and b). This approach is preferred, because if the treated wastewater is highly acidic, it is usually necessary to raise its pH to avoid corroding metal equipment that may exist in downstream operations. If the low pH step is performed last, it may be necessary to raise the pH of the wastewater stream after the low pH step is completed, so the treated wastewater can be sent to downstream operations. This introduces an additional process step with associated equipment and operating costs that are preferably avoided. On the other hand, if the high pH portion of the process is conducted last, it may be unnecessary to re-adjust the pH of the wastewater stream before it is sent on to downstream operations, and those additional costs are not incurred. Therefore, in the preferred embodiments, the pH of the wastewater stream is adjusted, if necessary, into the range of pH 1 to 5, preferably pH 1 to 4 and more preferably pH 2 to 4, and then contacted with activated carbon while the wastewater stream is at the low pH. The pH of the wastewater stream can be adjusted into this range by adding an organic or inorganic acid to the wastewater stream. The organic or inorganic acid should be a strong acid that has a pKa of 1 or less in water at 25°C. Inorganic acids are preferred to avoid increasing the total organics loading in the wastewater stream. Inexpensive inorganic acids such as sulfuric acid and hydrochloric acid are entirely suitable. A concentrated hydrochloric acid is preferred.

Once the wastewater stream is adjusted into the low pH range, it is contacted with activated carbon.

The activated carbon is a highly porous, high surface area particulate carbon material. It is typically prepared by carbonizing a precursor material such as charcoal, nut flour and the like and activating the carbonized material by exposure to a gaseous or liquid oxidant at a temperature from 500 to 1200 ⁰ C. The activated carbon may be acid washed if desired but that is not necessary and non-acid washed activated carbon can be used equally well. The activated carbon may have a surface area of from 500 m ² /g up to 1500 m ² /g or more. The ash content of the activated carbon should be no greater than 15%, preferably no greater than 10% by weight. The activated carbon preferably has an iodine number of from 500 to 1200 mg/g, and a density of at least 0.35 and more preferably at least 0.50 g/cc. The activated carbon is in the form of a particulate.

The activated carbon may be a so-called powdered type, which typically will pass through an 80-mesh (0.177 mm screen). Granulated activated carbon products, which are retained on a 50 mesh (0.297 mm) screen, are also useful. Granulated products having 8x20, 8x30, 8x40 and 20x40 or similar size designations are suitable. Commercially available activated carbon products that are suitable include DSR-A 8x40, GPG-LF 12x40 and F22 AWD-LF 12x40 activated carbon from Calgon Corporation, LQ- 830 and LQ- 1240 activated carbon from Carbochem Corporation, and various activated carbon products that are available from Carbon Activated Corp. (Compton, CA), Silicon Products Associates, Prominent System, Inc., and Ecologix Environmental Systems.

The activated carbon is generally maintained in the form of one or more beds, in which the activated carbon is supported on a grate or screen. The wastewater stream is contacted with the activated carbon by flowing the stream through the supported bed or beds. The temperature of the wastewater stream during this step can be any temperature above the freezing temperature and below the boiling temperature of the wastewater stream. From 0 to 100 ⁰ C is suitable, 20 to 70 ⁰ C is preferred and 30 to 50 ⁰ C is particularly useful.

The size of the bed and the flow rate of the wastewater stream through the bed are selected together to provide enough residence time to remove the phenolic compounds and preferably other organic acids from the wastewater stream. A typical amount of activated carbon is from 0.05 to 50 kg per 1000 liters of wastewater stream. A preferred amount is from 0.1 to 1 kg per 1000 liters of wastewater stream. The amount of phenolic compounds in the wastewater solution is preferably reduced to below 1 ppm by weight, preferably below 0.1 ppm per weight. The amount of other organic acids is also preferably reduced to like levels. Some organic amines may be removed during this step as well.

As mentioned, the wastewater solution may be contacted with multiple activated carbon beds during this step (or two or more times with a single bed), if desired or necessary to reduce the amount of phenolic compounds and other organic acids to the desired level.

After phenolic compounds and other organic acids are removed in this manner, the pH of the wastewater stream is adjusted upwardly to at least 8, preferably into the range of from 8 to 13 and more preferably from 9 to 12, if step b) of the preferred process is then to be conducted. The pH adjustment is most easily accomplished through the addition of a strong inorganic or organic base. Inexpensive inorganic bases such as alkali metal hydroxides are entirely suitable and are preferred on the basis of cost. Once the pH adjustment is made, the wastewater stream is contacted again with activated carbon to remove organic amine(s) from the stream. Suitable conditions and activated carbon materials for this contacting step are generally as described before with respect to the low pH contacting step. A typical amount of activated carbon is from 0.05 to 50 kg per 1000 liters of wastewater stream. A preferred amount is from 0.1 to 1 kg per 1000 liters of wastewater stream. The volume of the activated carbon and the flow rates are selected together to provide enough residence time to remove organic amines from the wastewater stream. The activated carbon used in this step should not be the same bed as was used to remove the organic acid(s) from the wastewater stream, unless the activated carbon in that bed is replaced or regenerated beforehand. After this treatment step, the wastewater stream should contain no more than 1 ppm by weight, preferably no more than 0.3 ppm by weight of organic amines after this contacting step.

The treated wastewater water can then be sent to further downstream processing, discharged if in compliance with applicable water discharge standards, or recycled into some upstream process step. It may be necessary or desirable to again adjust the pH of the treated wastewater stream before discharging or sending it to subsequent processing. Subsequent downstream processing may include, for example, further purification steps to remove other impurities.

As already mentioned, steps a) and b) of the preferred process may be performed in the reverse order. In such embodiments, the starting wastewater solution is brought if necessary to the high pH and contacted with activated carbon as described before to remove organic amines. The pH of the wastewater stream is then adjusted down to the low pH range, after which the wastewater stream is contacted again with activated carbon to remove phenolic compounds and other organic acid(s) as may be present. Suitable conditions and methods for performing those steps are as described before.

The spent activated carbon, containing the adsorbed phenolic compounds, other organic acid(s) (if any) or organic amine(s), as the case may be, can be thermally regenerated in known manner to burn out the adsorbed organics. Once regenerated, the activated carbon can be re-used in either treatment step of the invention.

The following examples are provided to illustrate the invention, but not to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated.

Example 1

A wastewater stream having a density of 1064.4 g/L is obtained from a wastewater scrubbing unit of a PMDA manufacturing facility. The wastewater contains small amounts of aniline, methylene dianiline, polymethylene polyanilines and phenol. The wastewater is spiked with phenol to a phenol concentration of 105.8 mg of phenol/L of solution. The pH is adjusted to 4.0 with 36% hydrochloric acid and the temperature of the solution is brought to 40 ⁰ C. 5.01 grams of Calgon DSR-A 8x40 activated carbon are added to 100 mL of the wastewater solution in a flask, and the resulting mixture is stirred at 40 ⁰ C for 24 hours. After this treatment step, the phenol concentration in the wastewater solution is measured to be 0.3 mg/L, or about 0.3 ppm. The remaining phenol is adsorbed onto the activated carbon. Phenol loading on the activated carbon is about 21 g of phenol per gram of activated carbon. When the experiment is repeated using 10.2 grams of activated carbon, the phenol concentration in the wastewater stream is reduced to below detectable levels (i.e., less than 0.1 ppm of phenol). The phenol loading on the activated carbon in this case is about 10.3 g of phenol per gram of activated carbon.

To remove the organic amines from the wastewater solution, the pH of the wastewater solution is adjusted to 11-12 by adding 50% caustic. The solution is contacted in series with two beds of Calgon DSR-A 8x40 activated carbon, arranged in series, at a temperature of 40 ⁰ C. Residence time in the first bed is approximately 10-15 minutes; the first bed is sized to operate for about 14 days continuous operation before requiring regeneration. The second bed functions as a guard bed. The aniline concentration in the wastewater solution is reduced to less than 0.3 ppm and the methylene dianiline concentration is reduced to below detectable levels.