WO1997019729A1

WO1997019729A1 - Method and apparatus for cleaning waste acid

Info

Publication number: WO1997019729A1
Application number: PCT/CA1996/000778
Authority: WO
Inventors: Saleh Witwit; Peter Groeneweg
Original assignee: ICI Canada Inc
Current assignee: PPG Architectural Coatings Canada Inc
Priority date: 1995-11-28
Filing date: 1996-11-27
Publication date: 1997-06-05
Anticipated expiration: 1998-05-28

Abstract

An economical, efficient method for cleaning organics, and particularly nitrated aromatics, from waste acid streams is provided. The method involves the heating of the waste acid under at least its own vapor pressure, optionally in the presence of an oxidizing agent. Also provided is a new continuous reactor useful in this method and for a variety of applications.

Description

METHOD AND APPARATUS FOR CLEANING WASTE ACID

Field of the Invention

The present invention relates to methods and apparatus for cleaning of organic compounds from waste sulfuric acid streams.

Background of the Invention

Waste acid streams contaminated with organic compounds have been a continuous concern to the industry for many years. Because of the undesirable environmental concerns about the presence of organics, the current options for handling waste acid streams are very limited and expensive. Further, low concentration of acids in such waste streams cause difficulties in recycling the same process, and add to the expense of handling and transportation.

Currently, the most common recycling process is the Pauling Process, which uses a stirred vessel reactor at atmospheric pressure and a temperature of about 300°C. In the Pauling Process, the organics are partially destroyed and partially stripped away with the vapor. A disadvantage of this system is that there is a high corrosion factor and a significant amount of maintenance required as a result, thus rendering the operating cost for this system very high.

Other conventional waste acid processes are also quite expensive, and thus undesirable. One such process involves the evaporation of acid by the use of very high temperatures (about 600°C) . Another process achieves destruction of organics using an electrochemical process. However, the specialized electrodes required are costly and the hydrogen generated by the process presents a hazard. Yet another commonly used acid cleaning process, involves solvent extraction, which requires large quantities of solvent, and may result in contaminated and difficult to clean solvent streams. What is needed is a relatively inexpensive, efficient, and environmentally acceptable process for cleaning waste streams.

Summary of the Invention

The present invention provides a method of destroying organics in waste acid by heating the acid under at least its own vapor pressure to a temperature suitable to destroy the organics. The invention also provides a novel continuous reactor, which may be used in a multi-stage process. The waste acid is heated to a temperature of between 100°C and 350°C and subjected to at least its own vapor pressure. The method of the invention may further comprise the step of adding an oxidizing agent to the waste acid. In another aspect, the present invention provides a reactor for use in destroying organics in waste acid under pressure. The reactor is useful in a continuous mode whereby a continuous stream of waste acid flows into the reactor and a continuous flow of purified acid flows out of the reactor.

In such a reactor, the continuous stream of waste acid is pumped into an inlet on a pressurized elongate tube means. The tube means has a pre-determined length and an outer periphery. The inlet is located at one end of the tube means and leads to a remote end with an opening through which the acid stream flows. The waste acid stream flows into an outer pressurized elongate vessel which is spaced from and surrounds the outer periphery of at least a portion of the predetermined length of the tube means. The outer vessel has a closed first section adjacent the remote end of the tube means for receiving the acid stream and for reversely directing the stream toward a second section of the outer vessel spaced along the outer periphery of the tube means. An outlet is formed in the second section of the outer vessel to allow the acid stream to continuously exit the outer vessel. A heating means located external to the outer vessel and located adjacent to the closed first section of the outer vessel is used to heat the waste acid stream to a pre-determined temperature. The organic compounds are destroyed as the acid stream is heated under pressure to the pre-determined temperature. As the heated acid stream flows along the outer periphery of the tube means, the heat from the stream is transferred to the waste acid stream inside the tube means.

Other aspects and advantages of the present invention are described further in the following detailed description of the preferred embodiments thereof.

Brief Description of the Drawings Fig. 1 is a schematic diagram illustrating a reactor for cleaning and concentrating acids according to the present invention.

Fig. 2 is a schematic diagram of a multi-stage process using the reactor.

Detailed Description of the Invention

The present invention provides a method of removing organic compounds from waste acid which is more efficient and less costly than presently known methods. Without wishing to be bound by theory, the inventors have found that the use of pressure provides favorable reaction conditions which destroy the organic contaminants by keeping the reactant together and allowing the organics to be destroyed rather than stripped away. Also provided is a novel reactor useful for performing this acid cleaning method.

Thus, in a first aspect, the invention provides a method of destroying organics in waste acid, permitting the reuse of the acids. As used herein, the term

"destroying" refers to the conversion of contaminating organics to gas or vapor (e.g. , water, carbon dioxide and nitrogen oxide) . The method of the invention is particularly well adapted for use with spent sulfuric acid (H₂S0₄) streams.

The process of the invention involves heating the waste acid to a temperature between about 100°C and about 350°C. Preferably, particularly when the waste acid is sulfuric acid, the temperature is between about 120°C to about 300°C, and more preferably about 150°C to about

280°C. The reaction is not vented, and thus, the acid is subjected to increased pressure from the vapors resulting from the heating step. Thus, the waste acid is subjected to at least its own vapor pressure during the heating.

The vapor pressure of the reaction is dependent upon the weight percentage of sulfuric acid in the acid stream. For example, a concentrated sulfuric acid waste stream may have a vapor pressure as low as about 10 pounds per square inch gauge (psig) , while a stream having a low acid and/or high water content, may have a much higher vapor pressure. Alternatively, one of skill in the art may choose to subject the reaction to a pressure higher than its own vapor pressure. Suitable pressures can be readily selected by one of skill in the art. Generally, a suitable pressure will be in the range between about 10 psig to about 500 psig, with a more preferred range being between about 100 - 200 psig, and most preferably about 100 - 150 psig. Optionally, an oxidizing agent may be added to the waste acid. However, in contrast to known methods which use oxidizing agents, the method of the invention requires the consumption of a smaller quantity of oxidizing agent. For example, in the preferred embodiment in which spent sulfuric acid is being cleaned, the oxidizing agent is nitric acid which is present in an amount of between about 0.1 to about 10%, and preferably between 0.1 to 5% by weight. Other suitable oxidizing agents may be readily selected by one of skill in the art. Unlike prior art methods, there is no loss of the oxidizing agent in the method of the invention. Rather, under pressure the oxidizing agent remains in the acid.

The resulting product, treated (purified) acid, can be directly reused or stored for future use.

Thus, in another aspect, the present invention provides a novel reactor, which is particularly well adapted for use in a continuous process of destroying organic compounds in a waste acid stream. This reactor is not limited to use with the method of the invention, or for use in cleaning sulfuric acid streams. Rather, the reactor of the invention is generally useful where heating under pressure is desirable and/or where it is desirable for the "hot" zone of the reactor to be located remotely from the seal. These features of the reactor of the invention are discussed in more detail below.

With reference to FIG. 1, a continuous supply 10 of waste acid is supplied to reactor 12. After the acid 10 flows through reactor 12 in a continuous process, the acid is purified and exits through pressure control valve 14 to storage or flash evaporation 16.

The reactor 12 utilizes a unique reverse flow operation. The acid 10 enters the reactor 12 at an inlet 18. The inlet 18 is connected to a pressurized elongate tube means 20. As shown in FIG. 1, the tube means 20 is a single cylindrical elongate tube. An alternate embodiment of inlet 18 may be a plurality or manifold of individual elongate cylindrical tubes (not shown) located adjacent one another, which may feed into the same or different locations in the tube means 20. The pressurized elongate tube means extends from the inlet 18 at one end 22 a pre-determined length to a remote end 24. The remote end 24 has an opening 28 through which the acid stream flows.

After the acid flows through opening 28, it is reversely directed along the outer periphery 30 of the tube means 20. To this end, an outer pressurized elongate vessel 32 is spaced from and surrounds the outer periphery 30 along the length of the tube means 20. As shown in FIG. 1, the outer vessel 32 is concentric with the tube means 20. An alternate embodiment which is not shown is to have the outer vessel 32 extend along only a portion of the tube means 20.

The outer vessel 32 has a closed first section 34 which is adjacent the remote end 24 of the tube means 20 for receiving the waste acid stream 10 and for reversely directing the stream 10 along the outer periphery 30 of the tube means 20. The outer vessel 32 has a second section 36 with an outlet 38 for allowing the acid stream 10 to continuously exit the reactor 12. The waste acid stream 10 is heated in a heat reaction zone 40 which combines with the internal pressure of the reactor 12 to destroy the organic compounds in the acid stream. To this end, a heating means 42 is located on an external side of the outer vessel 32 adjacent the closed first section 34. The heating means can include, for example, a heating jacket, half pipes, hot baths such as oil hot baths and sand baths, or the use of microwave frequency radiation.

The reactor 12 of the present invention also provides pre-heating of the waste acid stream 10 before it enters the heat zone 40. This is accomplished by reversely flowing the heated acid stream 10 back along the outer periphery 30 of the tube means 20. As the heated acid stream 10 leaves the heat zone 40 and flows along the second section 36 of the outer vessel 32, the heat of the purified acid stream 10 is transferred to the incoming waste acid stream 10 flowing in tube means 20.

The tube means 20 and outer vessel 32 are filled with a packing 44 to continuously mix the flowing acid stream. Examples of products used as the packing 44 include rasching rings, pall rings, and intalox saddles made from glass or ceramic.

A seal 46 is used to seal the pressurized tube means 20 at the one end 22 and the outer vessel 32 at the second section 36. Since the heating means 42 is located a distance from the seal 46, the seal can advantageously be made out of commonly available materials and need not be capable of withstanding high temperatures. For example, suitable materials for the seal include Teflon^® and the like. In another aspect, the reactor 12 could be used in a multi-stage process, as illustrated in Fig. 2. The number of stages depend on the kind of organics in the acid, the quality of the feed and the quality of the product required. Each stage consists of a pump 48, a reactor 12, and a flash evaporator 50. Each flash evaporator 50 connects to the next stage's pump 50 and to a conduit 52 for flowing the vapor to a condenser (not shown) . As the number of stages increase, the concentration of the acid increases and the pressure required to operate at higher temperatures decreases.

By way of example, and not by way of limitation, the following provides dimensions of an exemplary bench-scale reactor 12 of the present invention. Reactors of the invention can be readily scaled up using proportional measurements by one of skill in the art. Thus, a reactor 12 of the present invention could include an inner tube means 20 with a predetermined length of about 475 mm, an inner diameter of about 10 mm, and an outer diameter of about 16 mm. The tube means 20 can be manufactured from glass or merely lined with glass. The outer vessel 32 can have a length of 480 mm, an inner diameter of about 25 mm and an outer diameter of 35 mm. The outer vessel 32 can also be made of glass or lined with glass. The spacing between the inner diameter of the outer vessel 32 and the outer diameter of the tubing 20 can be approximately 9.5 mm. The tubing 20 is spaced from the outer vessel 32 through the use of several glass rasching rings. The heating means 42 can be a hot oil bath with the oil heated by an electrical heater. The hot oil bath can extend up the second section 36 of the outer vessel 32 approximately 13 mm. The predetermined temperature applied to the acid stream can be in a range of 100°C to 350°C, and more preferably about 150°C to about 300°C. The reactor can be designed to withstand internal pressures up to 500 psig, and preferably 100 - 150 psig at the pre-determined temperature.

These examples illustrate the preferred methods for cleaning waste acids according to the invention. These examples are illustrative only and do not limit the scope of the invention.

Example 1 - Cleaning Waste Acids

The following provides an example illustrating the advantages of the method of the inventor in a continuous pressurized reactor and a batch pressurized stirred reactor.

The following two runs were carried out in a batch pressurized bench scale stirred reactor, in which denitrated TNT waste sulfuric acid was used as feed and nitric acid as oxidizing agent, The following are two examples of these results.

First Example Feed Second Example Feed

Quantity (grams) 569 569

Temperature (°C) 179 179

Pressure (psig) 30 32 Residence Time

(min.) 20 60 Total Organic

Compounds (ppm) 2030 2070

Color yellow yellow

Product Product

Total Organic

Compounds (ppm) 1040 990

Color colorless colorless

The following two runs were carried out using the continuous pressurized reactor made of glass and heating with oil batch, as illustrated in Fig. 2, in which concentrated denitrated TNT sulfuric acid was used as feed, and nitric acid as oxidizing agent. These results are as follows: First Second

Example Feed Example Feed

Flow 9 grams/min) 1.54 1.6 H₂SO_<,% wt. 85 89 HN0₃,% wt. 0.45 0.11

Temperature (°C) 279 270 Pressure (psig) 75 75 Residence Time (min) 9 9 Total Organic Compounds (ppm) 840 75 Color yellow light yellow

Product Product

Total Organic

Compounds (ppm) 320 25 Color colorless colorless

Temperature (°C) 46 41

The ability of the method of the invention, using a pressurized reactor, to destroy organics in waste sulfuric acid was demonstrated in both a batch stirred reaction and continuous reactions under various temperatures, wt% HS0₄ and organics concentration. These four examples show that the total organic compounds using the methods of the invention are reduced and the color is changed to colorless.

Numerous modifications and variations of the present invention are included in the above-identified specification and are expected to be obvious to one of skill in the art. Such modifications and alterations to the compositions and processes of the present invention are believed to be encompassed in the scope of the claims appended hereto.

Claims

What is claimed is:

1. A method for destroying organic compounds in waste acid comprising the steps of providing waste acid containing organic compounds heating the waste acid to a temperature between 100°C and 350°C; wherein when said heating step is performed, the acid is subjected to at least its own vapor pressure, causing destruction of the organic compounds.

2. The method according to claim 1 wherein said organic compounds are nitrated aromatic compounds and said acid is sulfuric acid.

3. The method according to claim 1 wherein said method further comprises the step of adding an oxidizing agent to said waste acid.

4. The method according to claim 3 wherein said oxidizing agent is added in an amount of between about 0.1 to about 10%, by weight, of the waste acid.

5. The method according to claim 3 wherein said oxidizing agent is nitric acid.

6. A method for destroying organic compounds from waste acid comprising the steps of: supplying a continuous stream of the waste acid to an inlet on at least one pressurized elongate tube, said at least one elongate tube having a remote open end; flowing the waste acid through said open end of said at least one pressurized elongate tube into a pressurized outer vessel which is spaced from and surrounds at least a given length of said at least one elongate tube; redirecting the flow of the stream along an outer periphery of said at least one elongate tube with said outer vessel; heating the waste acid to a temperature between

100°C and 350°C as the waste acid flows through said open end to said outer vessel; and flowing the heated acid out of said outer vessel through an outlet located on an upper end of said outer vessel; wherein when said heating step is performed the acid is subjected to at least its own vapor pressure and the organic compounds are destroyed; and wherein as the heated stream flows along said outer periphery of said at least one elongate tube, the waste acid stream in said at least one elongate tube is preheated.

7. A reactor for use in destroying an organic compound contained in a waste acid, comprising: pressurized elongate tube means having a pre-determined length and an outer periphery, said tube means having one end with an inlet for receiving a continuous stream of the waste acid and a remote end with an opening through which the acid stream flows; an outer pressurized elongate vessel spaced from and surrounding said outer periphery of at least a portion of said pre-determined length of said tube means, said outer vessel having a closed first section adjacent said remote end of said tube means for receiving the waste acid stream and for reversely directing the stream toward a second section of said outer vessel along said outer periphery of said tube means, said second section having an outlet for allowing the acid stream to continuously exit said outer vessel; and means external to said outer vessel and located adjacent to said closed first section for heating the waste acid stream to a pre¬ determined temperature as the stream flows from said remote end of said tube means into said closed first section of said outer vessel; whereby the organic compounds are destroyed as the acid stream is heated under pressure to said pre-determined temperature, and whereby the flow of the heated acid stream along said outer periphery of said tube means pre-heats the waste acid stream inside said tube means.

8. The reactor according to claim 7, wherein said tube means is a single cylindrical elongate tube.

9. The reactor according to claim 8, wherein said single elongate tube and said outer vessel are concentric.

10. The reactor according to claim 7, wherein said tube means is a plurality of individual elongate cylindrical tubes located adjacent one another.

11. The reactor according to claim 7, wherein said tube means and said outer vessel are filled with a packing which aids in mixing of the acid stream.

12. The reactor according to claim 11, wherein said packing is selected from the group consisting of rings, balls and saddles.

13. The reactor according to claim 7, wherein said pre-determined temperature is in a range from 100°C to 350°C.

14. The reactor according to claim 13, wherein said heating means is selected from the group consisting of a jacket, a half pipe, and a hot bath.

15. The reactor according to claim 13, wherein said tube means and said outer vessel are designed to withstand internal pressures up to 500 psig at said pre-determined temperature.

16. The reactor according to claim 15, wherein said tube means and said outer vessel are made from glass.

17. The reactor according to claim 15, wherein said tube means and said outer vessel are lined with glass.

18. The reactor according to claim 16, wherein said one end of said tube means and said second section of said outer vessel are sealed with a seal made of Teflon^®.