Sulfur recovery
Under a sulfur recovery refers to several industrial processes by which sulfur is of, for example, arises in the course of desulphurisation, returned to his elemental basic form. At the same time, a clean gas is produced that can be emitted in accordance with legal requirements.
Sulfur deposits
Sulfur occurs in various forms on earth. Although most of the sulfur is found in coal (around 80%), most of the sulfur is obtained from the desulfurization of natural gas and petroleum .
Coal is mainly burned, with the sulfur oxidizing to SO 2 . To protect the environment, flue gas desulphurisation processes are used, with which the sulfur is converted into gypsum . A conversion to elemental sulfur is usually economically unprofitable.
Low-sulfur natural gases are cleaned of hydrogen sulfide (H 2 S) with the help of zinc oxide (ZnO) , whereby no elemental sulfur is produced, but the zinc oxide is converted into zinc sulfide (ZnS).
Only the H 2 S produced during coal gasification is converted into elemental sulfur via sulfur recovery. Otherwise, about 97% of the sulfur comes from natural gas and oil.
Sulfur recovery process
Before a sulfur recovery process can be used, the sulfur compounds must be processed from the respective products. How this is done depends on the physical state of the product and the sulfur concentration.
Most of the sulfur found in natural gas is present as hydrogen sulfide (H 2 S). This H 2 S is either concentrated in absorption systems or, in a low concentration, fed directly to the sulfur recovery systems. Unlike the sulfur in petroleum, which is also partially organically bound.
Crude oil desulfurization
When distilling crude oil , the sulfur remains in almost all fractions. Because the limit values for sulfur in the fuels have been successively reduced, especially in the course of the connections around the acid rain and the influence on the performance of exhaust gas cleaning systems, all sulfur compounds must be removed according to the current state of the art. While the sulfur in the low boilers is present directly as H 2 S and can be removed with the help of absorption processes, the sulfur from the middle distillates has to be converted to H 2 S in the so-called hydrodesulphurisation plants. Here all sulfur compounds are converted to hydrogen sulfide with the help of hydrogen.
The sulfur compounds from the high boilers are usually released in the process steps in which they are converted to low boilers (for example in hydrocrackers ). This not only results in gases with a high H 2 S concentration, but also water from various scrubbers that contain ammonium sulfate in addition to H 2 S. This water is processed in sour water strippers , whereby the gases produced there are also fed to the sulfur recovery systems.
Usually, methods are then used which are suitable for processing gases with high H 2 S concentrations.
Gas desulfurization
There are various procedural possibilities how the H 2 S can be removed from the gases. This is mainly dependent on the concentration with which the H 2 S is present.
The resulting hydrogen sulfide must be concentrated for further processing.
Procedure for low flow rates or low H 2 S concentrations
With low H 2 S concentrations or small amounts of gas, the H 2 S can be converted directly to sulfur. Possible methods are the LO-CAT, Crystasulf or Sulfint HP. What all processes have in common is that they cannot produce sulfur in a high degree of purity, as required by further industry (at least> 99.5%, in some cases purities of 99.9% are required). In addition to the low investment costs, the advantages of these processes are their robustness against hydrocarbons compared to processes that are suitable for processing process streams with high H 2 S concentrations.
In some cases, these processes are installed as a tail gas cleaning process step behind a Claus plant.
LO-CAT procedure
The LO-CAT process is particularly suitable for low flow rates (less than one ton per day). In the standard configuration of the process, the H 2 S is absorbed in an absorber on a catalytically active iron chelate solution and oxidized to solid sulfur. The cleaned gas can be processed further and the reduced catalytic solution is fed to an oxidation plant. After regeneration, this solution can be used again in the absorber.
Crystasulf procedure
The Crystasulf process is used in medium-sized plants with a processing capacity of 0.1 to 25 tons per day. The special feature of this process is that the sulfur is first bound in a non-aqueous solution and then crystallized and filtered in a later process step at lower temperatures.
Sulfint-HP process
The Sulfint-HP process is suitable for low flow rates at high process pressure. Here the H 2 S is converted with the help of an aqueous catalytically active iron chelate solution. The sulfur remains in the aqueous solution and is separated out with the help of a high pressure filter. At the moment, however, only one pilot plant for this process has been in operation.
Biological desulphurisation processes
There is practically only one biological desulfurization process that is currently used in two slightly different configurations. The heart of both the Shell-Paques and Thioapaq processes is the bioreactor . Before this, the gases are freed from H 2 S in a washer and the solvent is fed to the reactor. Under aerobic conditions, bacteria oxidize the sulfides to sulfur. The sulfur is then sedimented (Shell-Paques) or removed in a centrifuge (Thiopaque).
Process for high flow rates or high H 2 S concentrations
With larger plant capacities and / or high H 2 S concentrations, the Claus process is almost exclusively used nowadays . Plant sizes from around 50 to 4000 tons per day are possible. There are also a number of variations of the Claus process (such as Superclaus or Euroclaus ) and other processes not listed here (such as Sulfreen ).
The Claus process essentially consists of two stages. In the first stage, the gases are fed to a combustion chamber in which part of the H 2 S is oxidized with atmospheric oxygen. In the subsequent two- or three-stage catalytic reactors, the sulfur oxides react with the remaining H 2 S to form elemental sulfur.
This has a purity of 99.9% and only needs to be freed from the residual H 2 S in a degassing process. This sulfur is therefore not a waste , but a sales product. In Germany's largest natural gas production area south of Oldenburg, gas with up to 35% hydrogen sulfide (so-called sour gas) is produced. In the natural gas processing plant in Großenkneten , 800,000 t of sulfur are extracted from 6 billion m³ of raw gas per year. By the end of 2014, the plant in Voigtei had produced 300,000 t per year. In this way, the entire German demand for sulfur could be covered.
End gas cleaning
The gases that come from the desulphurisation systems usually contain H 2 S concentrations that are above the legal requirements of the TA-Luft (Technical Instructions for Keeping the Air Clean). In normal operation, Claus exhaust gases contain around 250–500 mg / m³ H 2 S. This makes post-treatment of the Claus exhaust gases necessary. Different procedures are available for this. In addition to the already mentioned desulphurisation processes for low H 2 S concentrations, the SCOT process and its modifications are the most common.
The intention to develop a SCOT plant was to increase the sulfur recovery rate of a Claus plant from about 92-97% to a value of over 99.8%. A SCOT system consists of two sections: a reaction and an absorption part.
SCOT exhaust gases can still contain up to 500 mg / m³ H 2 S, normally around 50–200 mg / m³. This concentration is also too high, so that another post-treatment system is used.
Exhaust gas cleaning
Before the exhaust gases from a sulfur recovery system can finally be emitted via a chimney, they must finally be fed to an afterburning facility in order to comply with the emission limits according to TA Luft. Exhaust gas cleaning systems for sulfur recovery are a special type of exhaust gas cleaning system.
A distinction is made between thermal and catalytic afterburning. Both post-combustion processes have in common that residual traces of H 2 S are converted into SO 2 . It should be noted that both the limit for H 2 S and the limit for SO 2 are observed.
Thermal afterburning
In thermal post-combustion , natural gas (also refinery gas in refineries) is used to generate a high temperature at which the H 2 S is oxidized to SO 2 with the help of (air) oxygen .
Catalytic post-combustion
In catalytic post-combustion , a highly selective catalyst is used, which oxidizes the sulfur compound to SO 2 in the presence of atmospheric oxygen .
The advantage of catalytic afterburning lies in its higher energy efficiency, since the processes run at significantly lower temperatures. A disadvantage to be mentioned is that in the event of improper operation of upstream systems, foreign substances (such as hydrocarbons ) can no longer be intercepted here.
Sulfur degassing
The sulfur produced from a sulfur recovery plant still contains a large proportion of bound H 2 S. There are numerous methods of how this H 2 S can be removed. Because most of the H 2 S is physically (and not chemically) bound, the H 2 S can be stripped out by moving the liquid sulfur with blown air .
In the Shell sulfur degassing process, air is introduced directly into the sulfur via two or three columns of bubbles . The SNEA Aquisulf process works with the aid of a spray, in which sulfur is first sprayed through fine nozzles and then almost completely degassed with the help of a catalyst. The Exxon sulfur degassing process works exclusively with a catalyst that is placed directly in the sulfur storage tank. All sulfur degassing processes have in common that they lower the H 2 S concentration in liquid sulfur to below 10 mg / m³.
Combinations of different procedures
A sulfur recovery system always consists of several sub-systems that are combined with one another depending on the different requirements. The decisive factor here is the H 2 S content in the raw gas, the by-products in the raw gas (for example CO, CO 2 , HCN, HC) and the amount of raw gas produced. At the same time, the legal requirements for the clean gas and the sulfur emission level must be taken into account. The clean gas always has to be treated, often several times, in order to comply with the legal requirements.
The illustration shows a common configuration for a sulfur recovery plant in a petroleum refinery.
Individual evidence
- ^ Winnacker-Küchler: Chemical technology: processes and products. Volume 3: Inorganic Basic Materials, Intermediate Products. Wiley-VCH Verlag GmbH & Co. KGaA, 2005. ISBN 3-527-30768-0 .
- ↑ Hardison, LC; Ramshaw, DE: H 2 S to S: Process improvement. Hydrocarbon Processing, Vol. 71, Jan. 1992, pp. 89-90.
- ↑ Crystatech ( Memento of 28 January 2011 at the Internet Archive ). Website of the license holder of the procedure. Retrieved October 13, 2010.
- ^ Pierre-Yves Le Strat, Mathilde Cot, Jean-Pierre Ballaguet, Jean-Louis Ambrosino, Christian Streicher, Jean-Paul Cousin: New redox process successful in high-pressure gas streams . In: Oil and Gas Journal . tape 99 , no. 48 , November 2001, p. 46-54 ( online ).
- ↑ Cameron Cline, Alie Hoksberg, Ray Abry, Albert Janssen: Biological process for H 2 S removal from gas streams. The Shell-Paque / THIOPAQ gas desulfurization process. Laurence Reid Gas Conditioning Conference, Feb. 2003 (PDF; 648 kB).
- ↑ The processing of natural gas. (PDF; 816 kB) Exxon Mobil Production Deutschland GmbH, January 2009, accessed on September 11, 2012 .
- ^ Jacobs Comprimo Sulfur Solutions ( Memento from April 26, 2010 in the Internet Archive ). Retrieved October 13, 2010.
- ↑ Kohl, A .; Nielsen, R .: Gas Purification. Gulf Pub Co, 1997. ISBN 0-88415-220-0 .
- ↑ Overview of key processes that are sold under license from Shell Global Solutions. Retrieved October 14, 2010.
- ^ Rhineland refinery of Shell Deutschland Oil GmbH.
- ^ Refineries of BP Gelsenkirchen .