AU650752B2 - Production of synthesis gas from ash-rich hydrocarbons - Google Patents

Abstract

Synthesis gas produced by partial oxidation (1) of high-ash hydrocarbons with the formation of from about 0.1 to about 0.3% by weight of soot (based on the carbon employed as hydrocarbon) is cooled first in a radiation cooler (2) and then in a convection cooler (3). The crude gas is then scrubbed in a wet-dust separator (6), and the scrubber water after filtration (10), optionally combined with the water phase of the radiation cooler (2), is returned to the gasification plant. Tbe water which has precipitated from the crude gas saturated with water vapour contains only traces of metals and metal compounds and can be fed directly to a water purification plant.

Description

P/00/011 2815/91 Regulaion 3.2(2)

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged:

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Invention Title: PRODUCTION OF SYNTHESIS GAS FROM ASH-RICH HYDROCARBONS S S

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*5 S S. S The following statement Is a full description of this Invention, including the best method of performing It known to us Production of synthesis gas from ash-rich hydrocarbons The present invention relates to a process for producing synthesis gas from hydrocarbons, in particular from ashrich heavy oil.

The conversion of crude oil or crude oil fractions into synthesis gas, i.e. a mixture of hydrogen and carbon monoxide, can take place by various processes which differ, inter alia, in the type of raw materials and oxidizing agents and in the reaction conditions. The partial combustion of crude oil at elevated pressure has found particularly wide application for producing synthesis gas. It is mainly applied in the form of two processes which were developed respectively by the Texaco Comp. together with the Hydrocarbon Res. Inc., and by the 15 Shell Comp, respectively. In both processes a quantity of about 1 to 3 by weight (relative to the cil input) soot is produced as a byproduct under normal operating conditions, i.e. at 1100 to 1500 0 C and 1 to 4 MPa, and is washed out of the gas with water. Soot contents of the 20 order of 1 to 3 usually 1.5 to 2 are necessary S" under the present normal conditions for a good gas yield and the transport of the liquid ash components, especially through heat exchangers. This is because the soot coats the ash and in this way tends to prevent it from depositing on the walls of parts of the plant.

The washwater containing soot can be fed to a conventional water treatment plant which operates independently of the gasification process. More often it is freed from soot in a plant integrated into the gasification process.

In accordance with one proven process the soot is separated with light oil from the water-soot suspension, the light oil-soot slurry is mixed with heavy oil and the light oil is drawn off as the top product in a oil stripper. The bottom product comprising soot and heavy oil is either returned to the ceactor or burnt externally.

2 The washwater is usually recirculated to the wash stage.

A small proportion, originating from the steam supplied to the gasification reactor as a reaction component, is directed as so-called grey water into the waste water.

As a rule heavy fuel oils, such as are used in the Texaco and Shell processes, contain nickel and vanadium as principal ash constituents. In the gasification procedure they are carried away with the raw gas stream, disregarding small quantities which remain in the gasification reactor, and are separated from the gas together with the soot by washing with water.

A large proportion of the vanadium/nickel components therefore appears in the process water, i.e. in the washwater and the grey water, and are therefore directed 15 into the waste water. The remainder is deposited in the reactor, in separators for the liquid slag, or in other parts of the plant.

The known procedures are not free from deficiencies with respect to reliable, environmentally friendly operation S: 20 of the gasification plant.

The consequence of recirculation to the gasification reactor of the soot extracted from the aqueous phase is that the metals contained in the soot also return to the reactor. They lead to disruptive deposits not only 25 there, but also above all in the heat recovery plants which as a rule are arranged as fire tube boilers. In the course of time these block up, necessitating expensive cleaning operations combined with plant shut-downs.

Where the soot/ash ratio is too low, ash deposits are also observed on the thermally highly stressed inlets to the convection boilers where they cause increased turbulence with very severe wall overheating and in this way lead to destruction of the inlets.

When the soot-containing water is introduced into 3 conventional, e.g. biological, treatment plants, which avoids returning the metals to the reactor, unacceptable quantities of the particularly environmentally harmful metals, nickel and vanadium, are released to the open system. Finally, due to the good extractability of vanadium pentoxide, the storage of the combustion ash from the slurries of such plants has to be carried out on special land-fill sites and is therefore burdened by high costs.

The indicated deficiencies are particularly onerous because it must be expected that in the future the situation will become aggravated as a result of the increasing processing of crude oil to gasoline, diesel or naphtha or the use of bitumen which inherently con- 15 tains higher proportions of heavy metals.

The object was therefore to ensure, during the gasification of ash-rich heavy oil, that the metals nickel and vanadium, which are contained in the gasification products as compounds, are separated completely and at 20 the highest possible concentration. In particular, the waste waters leaving the gasification plant should be free from metals.

The invention solves this problem with the aid of a process for producing synthesis gas by partial oxidation 25 of ash-rich hydrocarbons with the formation of soot as a byproduct, wherein the hydrocarbons are oxidized under conditions which lead to the formation of approximately 0. 1 to approximately 0.3 by weight soot relative to the carbon used in the form of hydrocarbons, the raw gas is cooled first in a radiant cooler, then in a convection cooler, and then finally washed with water in a wet separator, the washwater produced, which in particular contains suspended soot and dissolved solid and gaseous substances, is filtered, if necessary after combining it with the aqueous phase from the radiant cooler, and returned to the gasification plant, the raw gas saturated 4 with water vapor is cooled in a heat exchange system and the separated water is directed to a waste water treatment plant unless it is returned to maintain the water level in the plant.

The process according to the invention starts from ashrich hydrocarbons as the raw material for producing synthesis gas. Heavy oil is normally used, i.e. mixtures of liquid hydrocarbons which are produced as high-boiling fractions in the distillation of crude oil. In particular, heavy oils contain compounds of vanadium and nickel as well as of iron, chromium and molybdenum as ash-forming constituents in quantities of about 1 to 2500 ppm metal.

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The partial oxidation of the hydrocarbons or hydrocarbon 15 mixtures takes place by the state of the art process described at the outset. It is an important feature of the new process to guide the oxidation so that only small quantities of soot are formed, namely between 0.1 and a maximum of 0.3 wt. soot relative to the carbon used in 20 the form oi hydrocarbons. The level of the soot fraction in the reaction product is set in a manner known per se through the quantity of oxygen supplied, and furthermore the use of a specially configured burner can be recommended for example, EP-B1-0 095 103).

In addition to small quantities of soot the raw gas leaving the gasification reactor at a temperature of 1300 to 10O 0 C contains metals and metal compounds in liquid form. In accordance with the invention it is first pre-cooled in a radiant cooler to temperatures at which the metal impurities solidify without substantial contact with the cooler wall, i.e. to temperatures of 500 to 1000 0 C. To some extent the solid particles settle in the water sump of the radiant cooler from where they are removed. The pre-cooling of the raw gas in a radiant cooler, which was not previously usual in the gasification of heavy oil, has the advantage that the impurities 5 which are liquid or plastic under the prevailing temperature conditions are not deposited on the heat exchange surfaces of the sensitive convection cooler, a phenomenon which can cause blockages and, as already mentioned, wall overheating and wall destruction, in conjunction with increased turbulence, at the inlets to the convection coolers normally used.

For further cooling to 250 to 300 0 C, preferably 260 to 280 0 C, the raw gas containing the remaining quantities of fine metal particles and soot particles is directed to a convection cooler. Since the impurities carried by the gas have already solidified they do not impair the effectiveness of the convection cooler by blocking the :flow paths and by deposits on the exchange surfaces.

15 The virtually complete separation of the solids takes place by washing the gas with water. This sub-stage of the process can be carried out in a known manner with the aid of state of the art wet separators. Packed towers irrigated with water, which if necessary can also be used 20 in conjunction with Venturi scrubbers, have proved particularly effective. It is necessary to pre-heat the fresh washwater, preferably by heat exchange with the laden washwater. It is advisable to provide the top of l the packed tower with an effective droplet separator.

25 After the gas wash, two streams of material require further, separate, treatment: the aqueous phase laden with solids (washwater) and the solids-free gas phase with a high water vapor content.

The laden washwater, which as already stated can be supplied to the wash device preheated by heat exchange, is stngly heated by the heat transferred from the raw gas, namely the sensible heat and the heat content of the partially condensing water vapor. it is therefore expedient to pass it from the sump of the wash device through one or more heat exchangers and, preferably after 6 cleaning with sump water from the radiant cooler and if necessary after intermediate storage, to supply it to a filtering device. The separation of the solid and aqueous phases takes place here. The solids, containing soot and a high proportion of metal or metal compounds, can be processed further by known metallurgical methods to recover the metals.

The aqueous phase is virtually free from suspended solids. However, it still contains up to 2 mg metal compounds in solution per liter, a quantity which is in general not tolerable for further treatment of the waste water in treatment plants. In accordance with the invention this water fraction produced in the entire :process is therefore returned for cleaning the raw gas.

15 The main quantity is supplied to the wet separator, and a smaller fraction is used for maintaining the water level in the radiant cooler.

The solids-free gas from the water wash is saturated with water vapor. It is cooled in one or more stages to 20 extract the useful heat. In the context of the complete a 4o process this results in a second water fraction which contains metals and metal compounds in quantities 0.1 mg/l, that is in a concentration which lies far below the legally permissible maximum concentration.

This waste water can therefore be supplied directly to a water treatment plant for separation of volatile compounds. These include hydrogen sulfide, hydrogen cyanide and ammonia, which can be present in combined or free form. In accordance with a preferred process the impurities are removed by stripping the aqueous solution with an inert gaseous medium at elevated temperature. It has proved successful to set the pH of the solution, preferably directly after it is produced, to a value of about 3 or less and first to separate hydrogen sulfide and hydrogen cyanide in a first column stripper, and then to raise the pH to about 10 or more and to remove ammonia in a second column stripper. Preferably air, and in 7 particular steam, are used as the inert gaseous media.

One embodiment of the process according to the invention, shown in the accompanying diagram, is explained in detail below.

In a reactor 1 heavy oil is converted by means of water vapor and oxygen to raw synthesis gas (raw gas) which, after precooling in a radiant cooler 2, is passed into a convection cooler 3. The solids separated in the radiant cooler 2 are removed, suspended in water, from the cooler through an outlet gate 4 and transferred to a tank The raw gas which still contains solid particles is washed in a wet separator 6, in the present case a packed wash tower which is fitted with a droplet collector 6a.

The washwater containing suspended solids is directed 15 from the sump of the wash tower via a heat exchanger 7 and a cooler 8 to the tank 5. From there it is passed by means of a pump 9 to a filtering device 10. The filtrate, the aqueous phase freed from solids, is collected in a tank 11ii and returned with the aid of a pump 12 via a line S 20 13 and the heat exchanger 7 preferably to the wet o separator 6, but to a lesser extent to the radiant cooler 2 to maintain the water level. The gas leaving the wet separator 6 passes to the consumer after cooling in a heat exchanger 14 and a cooler 15. The water condensed from the gas is held in intermediate storage in a tank From there a small quantity is returned to the process with the aid of a pump 17 via a line 1.8 and the tank 11 insofar as it is needed for maintaining the water level in the plant, and the majority is directed via a line 19 to a downstream water treatment plant.

Claims (4)

1. A process for producing, synthesis gas by partial oxidation of ash-rich hydrocarbons with the formation of soot as a byproduct, wherein the hydrocarbonu are oxidized under conditions which lead to the formation of -appreimatezly 0.1 to -apprrewimately. 0.3 by weight soot relative to the carbon used in the form of hydrocarbons, the raw gas is cooled first in a radiant cooler, then in a convection cooler, and then finally washed with water in a wet separator, the washwater produced, which in particular contains suspended soot and o•r 0iof ily dissolved solid and gaseous substances, is, filtered, -a&-nscoccy after combining it with the aqueous 15 phase from the radiant cooler, and returned to the gasification plant, the raw gas saturated with water vapor is cooled in a heat exchange system and the separated water is directed to a waste water treat- ment plant unless it is returned to maintain the water level in the plant. ge 4** 6 2. A process as claimed in claim 1, wherein the raw gas is cooled in the radiant cooler to 500 to 1000°C. oe.* 3. A process as claimed in claim 1 or 2, wherein the raw gas is cooled in the convection cooler to 250 to 25 300°C, preferably 260 to 280 0 C.

4. A process as claimed in one or more of claims 1 to 3, wherein the raw gas is washed in packed columns irrigated with water. Process as claimed in claim 4, wherein the washing is carried out with preheated water.

6. Process as claimed in claim 5, wherein the fresh washwater is preheated by heat exchange with laden A

9- 7. Process as claimed in one or more of claims 1 to 6, wherein the solids-free raw gas saturated with water vapor is cooled in a plurality of stages with the recovery of heat. DATED this 25th day of May 1992. HOECHST AKTIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS "THE ATRIUM" 290 BURWOOD ROAD HAWTHORN. VICG. 3122. :*bto 94teS s 09 Frankfurt, 22.05.1991 PAT/rcht-sei Hoe91/Y002 Hoechst Aktienqesellschaft, Frankfurt am Main Abstract Synthesis gas produced by partial oxidation of ash-rich hydrocarbons with the formation of approximately 0.1 to approximately 0.3 by weight soot (relative to the carbon used in the form of hydrocarbons) is cooled first in a radiant cooler and then in a convection cooler. The raw gas is then washed in a wet separator and, after filtration, the washwater is returned to the gasification plant, if necessary together with the aqueous phase from the radiant cooler. The water separated from the raw gas saturated with water vapor contains only traces of metals and metal compounds and can be supplied directly to a S. water treatment plant. 9* 0 9e