PRODUCTS

Field of the Invention

The present invention relates to an energy-efficient and environmentally friendly process for the pyrolysis of sulfur-containing organic materials. More particularly the invention relates to processes for the recycling of sulfur-containing products, such as vulcanized polymers. A particularly interesting process of this type is the recycling of used tires by a process that does not release sulfur containing compounds into the atmosphere.

Background of the Invention

Many methods for the conversion of discarded tires are known in the art, to produce useful products such as fuels. Also known in the art are methods for cleaning obtained products from polluting compounds. However, process known in the art suffer from a variety of drawbacks, such as they release polluting gaseous sulfur compounds into the atmosphere or require expensive purification steps to remove them prior to the release of exhaust gases. Other processes end with other liquid or gaseous polluting discharges or require cleaning steps that are not economically efficient; furthermore, prior art processes often fail to exploit the pyrolysis products in an efficient manner.

For instance, US 4,240,587 discloses a process, in which the tires are first cryogenically fragmented and then pyrolysed at 450-500°C in a rotary inclined cylinder, which is heated indirectly. The remaining pyrolysis oil may either be further utilized as an initial component for manufacturing chemical compounds, such as lubricants, or it may be used as a fuel propellant. The remaining pyrolysis gas is utilized for the direct operation of a gas turbine. However, all the suggested products of this invention contain sulfur and therefore their combustion products cause sulfur dioxide pollution of the atmosphere.

An illustrative cleaning process is found in US 4,806,232, which describes a method for the desulphurization of sulfur-containing heavy fuels or used oil, in which the fuels are mixed with solid basic additive (preferably lime) and metal in finely divided form (preferably iron powder). The mixture obtained is injected into the pyrolysis and the sulfur is absorbed or chemically bonded to the basic additive and separated. The permanent gas formed in the pyrolysis and the simultaneously formed condensate may be directly fired as low-sulfur fuel. This method is expensive and cumbersome to perform and, therefore, has not found broad application in the art.

It is therefore clear that it would be highly desirable to provide a simple process by which pyrolysis products can be obtained, which are free from harmful sulfur product or could be used as fuels without release into the environment of harmful sulfur- containing products.

It is therefore an object of the present invention to provide a process that overcomes the drawbacks of the prior art. It is another object of the invention to provide an environmentally-friendly process for the recycling of sulfur-containing high- molecular and other organic materials. It is yet another object of the invention to provide an efficient recycling process that maximizes the utilization of the products of pyrolysis.

Other objects and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention

In an aspect of the present invention there is provided a process for the recovery of sulfur from the products resulting from the pyrolysis of sulfur-containing high- molecular weight organic material, comprising the steps of: a) carrying out the combustion of liquid pyrolysis products, thereby to obtain sulfur dioxide; b) reacting hydrogen sulfide recovered from gases generated in the pyrolysis process with said sulfur dioxide; and c) reacting hydrogen sulfide recovered from generator gases, if any, with said sulfur dioxide; thereby to obtain elemental sulfur, gaseous products essentially free from hydrogen sulfide and exhaust gases essentially free from sulfur dioxide.

According to one embodiment of the invention sulfur dioxide is contained in diesel exhaust gases formed in a diesel electro-generator.

The invention can be usefully exploited in a process for the treatment and utilization the products of the pyrolysis of sulfur-containing high molecular weight organic - A -

materials, to generate electric power without environmental contamination and without the formation of non-usable by-products. The process includes the gasification of the solid organic materials and the use of the produced hot gas, together with pyrolysis gas, as a direct heat carrier for a pyrolytic reactor, and the subsequent use of both gas mixtures, after cleaning from hydrogen sulfide, for electric power generation.

In another embodiment of the invention the processing of hydrogen sulfide produced resulting from processes pyrolysis and gasification accordingly the known Claus process is not needed for the thermal stage of the process for obtaining sulfur dioxide, as it is done usually in the Claus process (and as described e.g. in: Ulmann's Encyclopedia of Industrial Chemistry, 2003, Sixth, Completely Revised Edition, volume 34, pp. 605 - 627). Thus, such very complicated thermal stage is eliminated.

In the process of the present invention only the second, catalytic stage, of the Claus process:

2H ₂ S + SO ₂ → 3 S + 2H ₂ O is in principle used. This reaction runs over a catalyst: activated alumina at 240 - 330°C in different steps of the process. The stream of hydrogen sulfide interacts with exhaust gases from the liquid product combustion, which contain sulfur dioxide. For the complete purification of the exhaust gases from H ₂ S residue, the third sub-stage is used for oxidizing H ₂ S directly over a suitable catalyst according to the reaction:

2H ₂ S + O ₂ → 2S + 2H ₂ O. In the final stage the exhaust gases undergo treatment by a suitable sorbent. In an embodiment of the present invention the total physical heat of the exhaust gases, those containing sulfur dioxide, and those that do not contain sulfur dioxide (formed in a clean gaseous fuel combustion) is enough for the said total catalytic process, i.e., an additional heat source is not required for heating the reaction mixtures after their cooling at each stage of the process for sulfur vapors condensation. This obviates also the need for equipment for heating the reaction mixture, which is a prerequisite in the classical Claus method.

In another aspect the invention encompasses an efficient electrical power production process, wherein:

(a) the fuels are gaseous and liquid products of the organic material (e.g., discarded tire shreds) pyrolysis, and the gaseous product of the pyrolysis solid product gasification;

(b) the liquid product obtained from a pyrolysis step is used directly for power production;

(c) the solid carbonized product is gasified in a gas generator resulting in gaseous fuel (generator gas) containing hydrogen sulfide. The hot generator gas, which is partially cooled by mixing with cool gas of the total process, is directed into the pyrolysis reactor as a heat carrier; d) the mixture of gases, formed through the pyrolysis of raw material, and said gaseous heat carrier outgoing from the pyrolysis reactor together with the formed vapors, after cooling and separation from condensed liquid product, undergoes purification from hydrogen sulfide by known methods, e.g., by the monoethanolamine process. After this step two streams are obtained: the separated hydrogen sulfide stream and the mixed cleaned gas that is used as a fuel for electric power generation;

(e) the exhaust gases resulting from the combustion of the liquid pyrolysis products, which contain sulfur dioxide, are interacted with the said separated hydrogen sulfide stream, resulting in sulfur-free final exhaust gases, and sulfur as a recycled product.

Brief Description of the Drawings

Fig. 1 is a schematic illustration of a total pyrolysis process according to one embodiment of the present invention; and

Fig. 2 is a detailed illustration of the sulfur recycling process according to one embodiment of the process of the invention.

Detailed Description of the Invention

The process that is schematically illustrated in Figs. 1 and 2 can be carried out using a variety of different equipment and arrangements and, as will be apparent to the skilled person, is not limited to any particular arrangement of equipment and process steps, as long as the composition resulting from the process at its outlets is essentially comparable to the one that will be described in greater detail hereinafter.

Without departing from the generality of the above, one exemplary process will be described in greater detail, with reference to Fig. 1 , which is a schematic flow sheet of a pyrolytic process for discarded tires, provided for the purpose of illustration. The discarded tires are pyrolysed in a pyrolytic reactor after having been shredded into large pieces of size 250-300mm, which are fed through a feeding system schematically indicated at numeral 1. The pyrolysis reactor can be of any suitable type and is therefore not described herein, for the sake of brevity. In the reactor the tire pieces are heated up to a typical temperature of 480-500°C, e.g., by a gaseous heat- carrier indicated at 2. The heat-carrier is preferably - but not limitatively - a gas 10 produced by the gasification of the solid carbonized product 7 and partly cooled by mixing in a mixing chamber, using cooled and cleaned final gases, up to 650 - 700 ⁰ C.

In the example of Fig. 1 the hot heat-carrier passing through the pyrolysis reactor is partially cooled by its contact with the tire pieces and is mixed in the reactor with the gases and vapors formed during tire shreds pyrolysis. The combined stream leaves the reactor at numeral 3 and is cleaned from dust in a dedicated separator 4, for example a Vortex system, manufactured by Vortex Co., Israel. Then the vapor-gas stream 4 is directed to a system for cooling and vapor condensation, which includes two stages. In the first stage the vapor-gas stream is cooled in an air cooler by means of air stream 8, directed from a source of air, typically up to about 13O ⁰ C. Here part of the vapors is condensed. The cooler provides also flushing of the vapor-gaseous stream from the remainder of dust, by part of condensate formed in the process. The hot air 13 leaving the cooler is directed to the gas generator, where its heat is utilized, thus utilizing part of the heat of the vapor-gaseous pyrolysis products and of the gasifying gases. The formed liquid and the stream of gas and of non-condensed vapors 14 from the air cooler enter the gas-liquid separator, after which the first liquid product 15 is directed for treatment to a centrifuge, where the liquid is cleaned from the last dust, and is directed to its oil collector. The gas and non-condensed vapors 16 enter the second stage cooler-condenser, where it is cooled to about 15-2O ⁰ C. The condensed second liquid and the gas come into a separator (see 17) and after their separation are directed as follows: the gas 22 - to cleaning from hydrogen sulfide and the liquid product is directed to its collector or is mixed with heavier liquid 19. The produced oil is ready to be used as a fuel for diesel or other electro-generators, accordingly the requirements for their type of engine. This fuel combustion, as previously mentioned, is accompanied in the prior art by the emission of sulfur dioxide in the exhaust gases. The sulfur recovery process according to the invention solves the environmental problems associated with the use of said liquid fuels, as further discussed below.

The cooled gas separated from the oil, as hereinbefore mentioned, enters at numeral 22 into a system where it is cleaned from hydrogen sulfide by monoethanolamine. The clean gas can be combusted without damage to the environment and is used as a gaseous fuel for electric power generation by means of electro-generators. Illustrative examples of such generators are, for example, those manufactured by GE Jenbacher GMBH & CO OHG (Austria) with gas engines from 342kWe up to 3,119kWe. Part of the cleaned gas is directed to a mixing chamber for partially cooling the hot gas stream 10 from the gas generator and preparing the heat carrier 11 for the pyrolysis reactor. Another part of the cooled and cleaned gas 25 acts as a fuel in a burner for a heat exchanger, which is used to control the final temperature of the gaseous heat carrier 2 that is fed to the pyrolytic reactor. The recovered hydrogen sulfide 24 is reacted with exhaust gases 21 obtained from the combustion of the liquid fuel obtained from the pyrolysis. The reaction runs as a modified Claus catalytic reaction, as already discussed above. The resulting exhaust gases are sulfur dioxide-free and are non-polluting.

The sulfur that is formed in the reaction between hydrogen sulfide and sulfur dioxide (from the exhaust gases), is collected and is a marketable product.

The solid pyrolysis product formed during the pyrolysis of discarded tire shreds, is evacuated at 6 from the reactor; it comprises solid carbonized material and steel cord in the form of wire. The carbonized material is fragile and when treated in a suitable crusher, such as a hammer crusher, it can be readily reduced in size and thereafter separated from the cord by sieving or by electromagnetic separation. The crushed carbonized solid product separated from cord steel is directed at 7 to rising- or fluidized bed gasification and the cord steel can be recycled.

The solid carbonized product gasification can be carried out as a steam-air (respectively 9-13) process without the introduction of external heat. Resulting from such process, taking place at temperatures around 1,000° C, a semi-water generator gas is formed. The generator gas 10 undergoes cleaning from dust in a cyclone system known per se in the art and therefore not described herein in detail. Further, the generator gas is partial cooled by its mixing with cool gas 25 so that the temperature of the mixture is decreased to 650 - 700°C. The mixture of gases so obtained is a gaseous heat carrier 2, which can be used to heat the pieces feedstock in the pyrolytic reactor.

The above description, as stated, is not intended to limit the invention in any way but it does provide a more comprehensive understanding of a practical scenario in which the invention can be advantageously implemented. Of course, the skilled person will be able to devise alternative scenarios and setups in which the invention can be advantageously carried out.

Reverting now to Fig. 2, the process of the present invention for the sulfur regeneration is shown in detail. The regeneration of sulfur comprises: a) The recovery of hydrogen sulfide contained in gaseous products resulting from the pyrolysis of sulfur-containing organic material, as well as in gases resulting from gasification of solid pyrolysis product; b) The recovery of sulfur contained in sulfur-organic compounds present in liquid products.

According to the present invention hydrogen sulfide 24 (Fig.l) recovered from pyrolysis and generator gases interacts with sulfur dioxide, which is contained in exhaust gas 21 (Fig.l) formed in an electro-generator during the combustion of liquid pyrolysis products. The reaction is a modified Claus catalytic reaction that runs through three sub-stages. The molar ratio H ₂ S to SO ₂ in the reaction mixture entering the first sub-stage is up to 3.7. The operating temperature in this sub-stage is preferably about 320°C. This temperature promotes the hydrolysis of COS and CS ₂ , which can be formed during the liquid fuel combustion. Activated alumina, which is the known Claus catalyst, is used. With this catalyst the H ₂ S oxidation by oxygen present in the exhaust gases (in a small amount) is minimal. The gaseous reaction mixture after cooling, and the formed sulfur condensation, are fed to the second sub- stage which is previously heated in heater 210; the reaction runs at 200-220°C over a mixture of alumina and titanium dioxide catalysts where the reaction between residual hydrogen sulfide and sulfur dioxide reaches completion. The processing exhaust gas still contains residues of H ₂ S. In order to remove said H ₂ S the residual hydrogen sulfide is directly oxidized into sulfur over the appropriate catalyst. This can be done by means known in the art; for instance, US 5,262,135 discloses a complete stage of the Claus process as contacting the tail gas preliminary admixed with oxygen and heated preferably up to 220°C in a fixed bed wit a catalyst comprising at least 80% by weight TiO ₂ and containing of an impregnate selected from the group consisting of nickel, iron and cobalt. In this case air is introduced into the processed stream and hydrogen sulfide is oxidized accordingly the reaction

2H ₂ S + O ₂ → 2S + 2H ₂ O

After the said reaction the stream is cooled to obtain sulfur condensation. In one embodiment of the invention it is preferred to organize the oxidizing stage using the known Superclaus catalyst - iron and chromium oxides supported by α-alumina or silica (described in Ulmann's Encyclopedia of Industrial Chemistry referred to above) or using the catalyst disclosed in US 6,506,356, that do not depend on the presence of steam. In principle, according to WO 1987/002653, a practically complete after-treatment is achieved in the last stage by passing the gases through the solid metal oxide sorbent, e.g. zinc oxide, combined with a porous carrier material and iron, cobalt and nickel oxides with further regeneration of the sorbent. According to the present invention the temperature (usually about 450°C) and heat content of both exhaust gases (21 and 26, Fig.l) is sufficient for carrying out all stages of the modified Claus catalytic reaction. This obviates the need for additional heat and heating equipment for in each sub-stage of the process, which is a prerequisite in the classical Claus method.

Process Description

The process flow diagram for sulfur recovery according to one embodiment of the invention is shown in Fig. 2. The exhaust gases 202 and 202.1 (21, 26 in Fig.l) from the electro-generators enter the heater 210 (Fig.2) of the second sub-stage of the process. Here they heat the reaction mixture 209, formed after cooling the stream in the first sub-stage by cooler 207, from 140 ⁰ C up to 200-220°C. Then the partially cooled exhaust gas 202 (21 in Fig.l) containing sulfur dioxide is fed to the mixing and heating into a chamber 203, where it mixes with the hydrogen sulfide stream and thus a hot reaction mixture is formed for the first sub-stage of the catalytic reaction of hydrogen sulfide with sulfur dioxide contained in the exhaust gas; if it is necessary to raise the temperature of the reaction mixture, the clean exhaust gas 202.1, which is free of sulfur dioxide, goes through a heat exchanger of the chamber 203 and heats the said reaction mixture. The reaction mixture 204 enters reactor 205, where the Claus catalytic reaction takes place over an activated alumina catalyst. According to one alternative embodiment of the invention instead of a fixed bed reactor-converter a rotating horizontal reactor- converter is used. The rotating reactor is equipped with a horizontal shaft provided with mixing blades. At low speed of rotation (0.2 - 3 rpm) sufficient mixing is achieved while avoiding catalyst abrasion and improving the contact between gaseous reaction mixture and the catalyst surface. Application of modern catalysts in the form of balls with a diameter 4.8mm and more having enough high strength showed a slight dust formation in an actual test. The feeding of the gaseous reaction mixture is carried out through a distribution manifold installed outside the reactor at its bottom. The gases leave through the outlet pipe in the upper part of the reactor and further pass through the said Vortex chamber for cleaning from the said slight amount of dust. A periodic withdrawal of part of the spent catalyst for regeneration is performed simultaneously with its completion without stopping the reactor by means of charging and discharging lock chambers preventing leaking the gases into the atmosphere.

After leaving the reactor, stream 206 containing the formed sulfur is cooled down to 140°C in cooler 207, the condensed sulfur is separated from the gas stream in liquid state and is removed at 208 from the cooler. The reaction mixture is fed (at 209) to the second sub-stage of the catalytic reaction. Here the mixture is heated again in the heater 210 by exhaust gas streams 202 and 202.1, as previously discussed, and enters the reactor 212 (at 211) for the second sub-stage of the catalytic reaction over alumina and titanium dioxide catalysts. Then the mixture 213 leaves the reactor 212 and is cooled in cooler 214 down to the temperature required for sulfur condensation (140°C), separated and removed (215). Furthermore air is injected into the reaction mixture and the mixture is heated again up to 220° C in a heater 217 by means of the exhaust gas 202.1 leaving the first sub-stage heat exchanger (placed in chamber 203). The heated reaction mixture is fed to reactor 219 for the direct oxidation of the tail hydrogen sulfide with sulfur formation, over the Superclaus catalyst or over the catalyst containing oxides of vanadium, titanium, and of element selected from group of Fe, Mn ₅ Cr, Ni, Sb and Bi (see US 6,506,356).

For reliability in the complete purification the processed exhaust gases 202 from hydrogen sulfide they are passed through a sorbent consisting of activated carbon, particularly the granular non-impregnated GC Sulfursorb Plus for H ₂ S Treatment or the Spectrum XB- 17 (50/50 blend of activated carbon with granular media impregnated with potassium permanganate) which are produced in General Carbon Corp., USA.

AQMD (Air Quality Management District) publishes results of Carbon Scrubber Hydrogen Sulfide Removal Performance (2006 year), wherein when the inlet H ₂ S concentration in air is 10-20ppm the outlet concentration can be between 0.01-0. lppm ( the allowable concentration is lppm). This demonstrates the possibility of complete v after-purification of exhaust gases from H ₂ S.

Example

Checking the interaction of exhaust gases, resulting from the combustion of pyrolysis liquid fuel in a diesel engine and containing sulfur dioxide, with the stream of hydrogen sulfide. The liquid fuel was produced from the pyrolysis of discarded tire shreds in an experimental, 7 liter reactor. The final pyrolysis temperature averaged 493 °C. The average yields of pyrolysis products are, in mass %; gas - 10.9; liquid - 44.4; solid - 44.7 (including steal cord wire).

The liquid product density is 0.890; the sulfur content 0.95%.

The system for the sulfur regeneration testing included equipment for carrying out three sub-stages of catalytic reaction: three reactors, two heaters and three coolers and also the exhaust gases source. The exhaust gases containing sulfur dioxide were produced by a motorcar ("Renault") operating on diesel fuel. The exhaust gases were passed through an intermediate box and further transported by a blower into the reactor, where they displaced the air from the box and from the reactor, which was preliminarily filled up to 2/3 of its volume with catalyst. Then the car was refueled by 6kg pyrolytic liquid (6.7 liter) and continued to work (without motion) and burned all the fuel in 134 min. In the inlet tube the exhaust gas was mixed with H ₂ S flow from a balloon, thereby forming the reaction mixture. The rate of the H ₂ S flow was 1.1 liter per minute, which corresponds to the flow of H ₂ S extracted from pyrolysis and gasification gases while 6kg of liquid are produced. The molar ratio between H ₂ S and SO ₂ was 3.7:1, as it is in real conditions in the recycling process for discarded tires shown in Fig.1. The reactor-converter for the Claus catalytic reaction was a horizontal cylindrical vessel of 7 liters volume with side covers that was equipped with a horizontal rotating shaft and with mixing blades. It was also provided with devices for the charging and discharging of the catalyst, as previously discussed and can be heated electrically by means of a spiral located around its outer surface. In the first reactor the activated alumina catalyst was used in the form of balls of 4.8mm diameter. The reaction mixture after the reactor was fed to the cooler and was cooled down to 140°C. The formed sulfur was condensed and fed to the separator (the lower empty part of the cooler). There the vapors and gases were separated from sulfur and were fed to the tube heater that was heated by an electric coil up to 220°C, and was fed to the second preliminary heated reactor, which was similar to those described above. The catalyst mixture used was activated alumina and titanium dioxide. After passing the reactor, the reaction mixture was fed to the second sub-stage cooler and after separation of the formed sulfur it was fed to the third sub-stage. The residual hydrogen sulfide was directly oxidized by injected air over the Superclaus oxidizing catalyst (α-alumina supported iron and chromium oxides) at 220°C. Further the reaction mixture was cooled in the third sub-stage cooler down to 140°C and was separated from the condensed sulfur. For the extraction of any H ₂ S residue the exhaust gas was additionally fed to an absorber filled with activated carbon of type GS Sulfursorb Plus with the addition of Activated Carbon impregnated with soda caustic.

The following experimental results were obtained: Amount of regenerated sulfur: in the first sub-stage - 164.2g in the second stage - 87.Og in the third stage - 18. Ig in the adsorption stage H ₂ S - 0.7g

The total sulfur recovery was 99.7%.

The H ₂ S concentration in the cleaned exhaust gas was 7 ppm, which is less than the Occupational Safety and Health Administration (OSHA, USA) acceptable ceiling limit in the workplace (20ppm). It is also less than the limit set by the National Institute for Occupation Safety and Health (NIOSH, USA), which recommends 10 ppm in the work place.

As discussed above, the total after-purification from H ₂ S could be obtained here using an activated carbon scrubber of greater capacity, where the hydrogen sulfide concentration can be decreased down to O.lppm and less, i.e. less than the allowable concentration in atmospheric air.

All the above description of the process, system and examples has been given for the purpose of illustration and is not intended to limit the invention in any way. Many modifications can be effected to the various process steps, materials and equipment, and many different raw materials may be processed, all without exceeding the scope of the invention.