BACKGROUND OF THE INVENTION

As explained in the scientific article "Design of caustic wash system for light hydrocarbons such as LPG, NLG and Naphtha" published by Tajerian and co-workers, the purification of gaseous streams containing undesired acidic substances in the chemical industry is normally achieved by caustic washing. As the caustic solution is saturated with acidic products, the pH of the solution decreases gradually. When the pH in the solution drops below certain value, the caustic solution is not reactive enough and the solution is substituted by fresh caustic solution. This discarded basic solution is known as spent caustic.

There are different types of spent caustics depending on the types of molecules that are removed from the gaseous streams. An important type of spent caustic produced in the petrochemical industry is sulfidic spent caustic. In this case, carbon dioxide and hydrogen sulfide are removed from gaseous streams that are generated in naphtha and ethane-based crackers. As a consequence, the resulting spent caustic contains high levels of carbonates and sulfide salts.

According to the Lehmann et al. Patent No. WO 2009/035642 Al , typical sulfidic spent caustics produced in refineries present a composition similar to the one shown in table 1.

Due to the high values of pH, COD and TOC and to the presence of dangerous substances in the solution such as sodium sulfide and sodium hydroxide, sulfidic spent caustics cannot be treated in standard wastewater treatment facilities and need to be processed separately. The article "Wet air oxidation: a review of process technologies and aspects in reactor design" by McLurgh et al. describes one of the most common methods currently used for the treatment of sulfidic spent caustic: Wet Air Oxidation (WAO). In WAO, sulfides and organic matter (only partially) are oxidized to obtain less dangerous materials. However, the energy consumption in these types of processes is very high and the resulting products, mainly sulfates and carbon dioxide, find very little applications and are normally considered as waste.

The paper "Treatment of Olefin Plant Spent Caustic by Combination of Neutralization and Fenton Reaction" by Weng and co-workers and the patent No. US 5,368,726 by Masoomian describe the chemical oxidation of spent caustic using alternative oxidizing agents such as hydrogen peroxide or ozone. In these cases, the oxidation reaction tends to be low yielding and excess of reagents is normally required, resulting in prohibitively expensive treatments. In addition to that, the "Ullmann's Encyclopedia of Industrial Chemistry" published by Wiley- VCH, defines hydrogen peroxide as an unstable substance with a long list of acute hazards, whereas Masschelein et al. Patent No. US 6, 193,852 shows how ozone requires a special infrastructure to be generated, posing risk of fire and explosion when heated or in contact with combustible substances.

The papers "Biological treatment of refinery spent caustics under halo-alkaline conditions" by Janssen et al. and "Applications of ozone for industrial wastewater treatment" by Rice describe alternative methods which have been used in the past for the treatment of spent caustic.

All the methods previously discussed require harsh conditions (high pressure and temperature), employ dangerous reagents or require an excess of oxidizing agents in order to achieve the desired results. As a consequence, these methods are poorly cost-effective and/or highly technologically demanding.

In order to solve the problems above mentioned, very little attention has been paid to other chemical transformations different to oxidation which could involve some of the reactive molecules found in sulfidic spent caustics. The chemistry of sulfide and hydroxide salts, for instance, is well known in the literature and it might lead to the development of new convenient methods to treat spent caustic.

Based on this idea, we have explored different routes towards the formation of new products derived from sulfide and hydroxide salts under mild conditions. The results of this work have allowed us to develop a simple and mild process to treat sulfidic spent caustic which is described in the embodiment of the present invention. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. The Applicant makes no admission that any reference constitutes prior art - they are merely assertions by their authors and the Applicant reserves the right to contest the accuracy, pertinence and domain of the cited documents. None of the documents or references constitute an admission that they form part of the common general knowledge in any country.

SUMMARY OF THE INVENTION The invention describes the treatment of su!fidic spent caustic in two steps:

1 . Reaction of sulfidic spent caustic with sulfur.

2. Fractional distillation of light organics and water under reduced pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned paragraphs as well as the following detailed description of the disclosure will be better understood when read in conjunction with the diagrammatic representation. The embodiments illustrated in these figures herein are by way of example and not by way of limitation. These figures are purely diagrammatic. In the drawings:

A schematic block diagram of the treatment process is shown in Fig. 1. The diagram shows all the treatment steps and each of the products obtained during the process.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, certain specific elements are provided in order to understand the various disclosed embodiments. However, a person skilled in the art who read the disclosure, the drawings and the claims will realize that the disclosed embodiments can be practiced with or without one or more of these specific details or with other elements without deviating from the scope of the disclosure.

Reference wil l be now made in detail to the various embodiments of the disclosure. Schematic representation of the invention has been illustrated in the accompanying drawings. Wherever possible, the same reference will be used throughout the description to refer to the same or like embodiments.

Sulfidic spent caustics are highly corrosive basic aqueous solutions that contain a range of hazardous substances such as sulfide and hydroxide sodium salts. In addition, neutralization of sulfidic spent caustics leads to the generation of highly toxic and flammable gases that represent an important risk for plant personnel and the environment.

Among sulfide and hydroxide sodium salts, sodium carbonate and toxic aromatic organic compounds (only in a small percentage) are also found in sulfidic spent caustic. The presence of these substances in spent caustic prevents the treatment of this type of waste in standard wastewater treatment plants.

The disclosed invention represents a safe, economical and straightforward method for the treatment of sulfidic spent caustic produced in the (petro)chemical industry. At the core of the invention is the idea that some reactive hazardous substances present in sulfidic spent caustic can be chemically converted into other useful products which can be isolated. In the embodiment of the invention, sulfidic spent caustic is transformed into clean water and a sodium polysulfide (Na ₂ S _x , where x = 1 - 5) aqueous solution in a two-step process. Both steps of the process are performed under mild conditions of temperature and pressure. The only reagent employed during the process is elemental sulfur, a low-toxic and affordable substance.

In the first step of the process, sulfidic spent caustic is mixed with elemental sulfur. The resulting suspension is gently stirred and heated until all the sulfur is dissolved and a pale red solution is obtained. During this step, sulfide and hydroxide sodium salts present in spent caustic are consumed and two new substances, sodium polysulfide and sodium sulfite, are produced. Sodium carbonate and aromatic organic compounds, also present in spent caustic, remain unreacted during this step. The reaction between sodium sulfide and elemental sulfur is relatively fast and affords sodium polysulfide as the only product (equation 1 ). The reaction between sodium hydroxide and sulfur also affords sodium polysulfide as the main product, but in this case low-toxic sodium sulfite and water are also produced (equation 2).

Na ₂ S 9H ₂ O + (x - l )S = Na ₂ S _x + 9H ₂ O ( 1 )

(2x + 1 )S + 6NaOH = 2Na ₂ S _x + Na ₂ S0 ₃ + 3H ₂ O (2)

The formation of sodium polysulfide from sodium sulfide and sodium hydroxide is described in the paper "Polysulfides" by Vietti et al. In the second step of the process, the pale red solution obtained in the first part is gently heated under reduced pressure and aromatic organic compounds and clean water are separated from the rest of the salts through fractional distillation. After the separation of most of the water and organic compounds, a red solution is obtained as distillation heel. This final solution is composed of a mixture of water, sodium polysulfide, sodium carbonate and sodium sulfite. In the mixture, water and sodium polysulfide represent the major products, while sodium sulfite and sodium carbonate are present in lower concentrations.

Upon completion of the two-step process, each of the three products finally obtained can find useful applications. The major product, clean water, can be employed as industrial water or disposed through evaporation or sewage discharge. The small fraction of aromatic organic compounds obtained during the distillation can be blended with pyrolysis oil and used as secondary fuel. In last place, the resulting sodium polysulfide aqueous solution obtained in step 2 can find applications in mineral processing and in the purification of flue gas.

EXAMPLE 1

In order to prove our invention, synthetic sulfidic spent caustic was used in the first batch of experiments. Sulfidic synthetic spent caustic, containing 2.5% of Na ₂ S, was prepared in the laboratory by dissolving 38.5 g of Na ₂ S.9H ₂ O, 12.5 g of NaOH, 12.5 g of anhydrous Na ₂ CO ₃ and 0.5 mL of toluene (99.5%) in 500 g of ultrapure water. The mass and supplier of each reagent is indicated in table 2.

Step 1 : Treatment of synthetic sulfidic spent caustic with sulfur.

In the first step of the experiment, 550 g of synthetic spent caustic were mixed with 35.89 g of sulfur and the resulting suspension was stirred and heated at 1 15 °C for 2 h in a 1 L two- neck round-bottom flask until all the solid sulfur was dissolved and a pale red solution was obtained. The solution was allowed to cool down to room temperature and the pH and mass of the solution were measured (see results in table 4).

Step 2: Fractional distillation of treated spent caustic under reduced pressure.

Standard vacuum distillation glassware was connected to the reaction flask used during the treatment with sulfur and the content of the flask was distilled under reduced pressure ( 150 mbar, 30 °C), making sure the temperature in the flask (distillation pot) was always kept below 70 °C. Fractions of liquid were successively collected and weighted during the distillation (see results in table 4). At the end of the distillation, the pressure in the flask was further reduced to 30 - 100 mbar in order to distil the maximum amount of water.

Fractions 1 , 3, 5, 7 and 9 were fully characterized in order to determine the amount of organic compounds and sodium salts in the distilled water. pH values measured in all the fractions were close to neutral and no sulfide salts were found present in any of the distilled fractions. COD and TOC values were very low and sodium values (<1 ppm) proved the distilled water to be very soft.

EXAMPLE 2

In order to prove that the effectiveness of the process remained the same when real sulfidic spent caustic was used instead of synthetic spent caustic, the same experiment described in example 1 was carried out using real spent caustic supplied by a well-established generator of this type of waste. The properties and initial composition of the real spent caustic employed in the experiment is shown in table 5.

Step 1 : Reaction between real sulfidic spent caustic and sulfur.

In this experiment, 800 g of real sulfidic spent caustic were mixed with 13 g of sulfur in a 3L three-neck round-bottom flask. The resulting suspension was heated at 1 15 °C for 1.5 hours until all the solid sulfur was dissolved. After this time, a pale red solution was obtained which was allowed to cool to room temperature.

Step 2: Distillation of treated real spent caustic under reduced pressure.

In the second part of the experiment, standard vacuum distillation glassware was connected to the 3 L round-bottom flask containing the pale red solution previously obtained and water and organic compounds were distilled under reduced pressure. Five fractions of colorless liquid were successively collected and weighted during the vacuum distillation. The mass of each liquid fraction and the distillation conditions are shown in table 7.

After the distillation, all the fractions were fully analyzed in order to determine the amount of organic compounds and sodium salts present in the distilled water. The results of these analyses are summarized in table 8.

pH values of all the fractions, except fraction 1 , were close to neutral and practically no sulfide salts were found in the distilled water. COD, TOC and total Na values in fractions 2, 3 and 4 were very low and comparable to the parameters of treated industrial water published in the article "Membrane Bioreactor (MBR) Technology - a Promising Approach for Industrial Water Reuse" by Drioli et al.

Therefore, it needs to be appreciated that the aforementioned description is just an illustration of the principles of the invention. Since numerous modifications, variations, improvements and changes will readily occur to those skilled in the art, it is not considered to l imit the invention to the exact described format of the invention or its manufacturing and accordingly all suitable modifications and equivalents may be falling within the scope of the invention.