Sulfur recovery from lean H2S gas

Sulfur is recovered from H2S in so-called Claus plants. There poisonous H2S is oxidized by air to elemental, harmless sulfur. The most simple version of a Claus plant is one that contains just a direct oxidation reactor. The process can be applied to lean acid gas streams with up to appr 20 % H2S as emerges e,g, from gas sweetening in natural gas plants or in the chemical industry. As the process contains no burner it can also be applied for desulfurization of fuel gases containing besides H2S also hydrocarbons, as CH4. A typical process flow diagram represents Fig. 1.

Main topics of this process are:

* catalytic gas phase reaction, i.e. no burner applied
* no consumption of chemicals or scrubbing media
* no waste, product is valuable sulfur of high quality
* safe operation, proven in various installations
* sulfur recovery rate typically in the range of 90% up to 98%, depending on feed gas composition

Fig 1 : Typical process flow diagram of a sulfur recovery plant with cooled reactors, DEGsulf-DO Blue background: optional tailgas treatment
Fig. 2: Direct oxidation plant for lean H 2 S direct oxidation sulfur recovery

Description

The acid gas is preheated to the required reaction temperature of the catalyst in use and mixed with air. Passing through the upper uncooled catalyst section, the reaction 2 H2S + O2 -> 2/x Sx + 2 H2O starts and is performed to completion. Side reactions lead to SO2 production, which is then reacted according to the standard Claus reaction 2 H2S + SO2 -> 3/x Sx + 2 H2O. The Claus reaction is strongly temperature dependent and therefore reaction conditions are optimized by means of internal temperature control of the reactor. It is achieved by submerged heat exchanger surface in the catalyst bed, where evaporating boiler feed water or thermo-oil are used to remove reaction heat from the system.

After passing through the reactor the gas saturated with sulfur is cooled, the sulfur condensed and obtained in liquid form at 125 to 140 °C. It is sent to the storage pit. The process may be enhanced by using a recycle blower as shown in the flow scheme. The gas recycle system serves for temperature moderation and thus recovery increase. The figure also shows the optional incinerator, in this case a catalytic version. But the more conventional thermal incinerator may be applied as well.

Fig.3: Sulfur recovery plant at NYNÄS refinery

Operating experience

So far appr 10 plants using the DEGsulf process have been built, treating H2S gases of 1.5% up to 18% H2S and feed gas quantities between 100 and 12 000 m³/h. All the plants run totally automatic. No unplanned outages have been reported which were related to the process. As the process is so simple it is easy to operate and as it contains so few items the risk of failure is low. This leads to high availability. In one plant, however, the reactors’ heat exchanger had to be replaced after appr 8 years of operation due to corrosion by the boiler feed water. Chlorine ions had accumulated in the cooling cycle and attacked the heat exchanger material due to sub-standard de-ionization of the boiler feed water.

A number of these plants show substantial fluctuations of the H 2 S feed content. Nevertheless they are easy to operate as the air required is automatically adjusted to the H 2 S content of the feed. The temperature of the reactor outlet is automatically kept constant within a very narrow range since any heat exchanger reacts to increased D t with increased heat removal. Therefore a Thermoplate reactor is self-controlling to a major extent. The sulfur recovery rate is also rather constant as it depends primarily on the H 2 S/air ratio and the outlet temperature of the reactor. This allows to operate the units unmanned, with one control routine per shift recommended.

Sulfur recovery from H2S rich gas for up to 99.85% recovery rate

In larger plants with higher H 2 S feeds nowadays sulfur recovery rates of more than 99% are very often required by the authorities. When the guaranteed sulfur recovery efficiency is up to appr. 99%, SubDewPoint processes as CBA or the direct oxidation process SUPERCLAUS may be considered. Very often, however, the sulfur recovery efficiency required (to be guaranteed) is 99.5+%. For these higher values a very attractive technological solution is DEGsulf-SDP which allows sulfur recovery rates up to 99.85%. This high value cannot be reached by conventional SubDewPoint processes, nor by SUPERCLAUS. The process principle of DEGsulf-SDP is shown in Fig 4. It results when combining two Thermoplate reactors in series downstream of a conventional Claus furnace.

Fig 4 : Process principle of the DEGsulf-SubDewPoint process

In the DEGsulf–SDP process the two catalytic reactors of the Claus plant are replaced by reactors with Thermoplates for internal heat transfer. The first reactor is operated as are conventional reactors, i.e. at 260°C to 300°C at the outlet. The sulfur converted is recovered in the downstream sulfur condenser. The 2 nd reactor then is operated at its outlet in the temperature range 100°C to 130°C, i.e. below the sulfur dew point or even below the sulfur solidification point. This low temperature shifts the Claus equilibrium towards more sulfur formation so that the sulfur recovery rate becomes as high as 99.85%. Further enhancing high recovery is the efficient adsorption of the sulfur vapor at the low temperature.

When the cold reactor is charged with sulfur it is switched by 2 four-way valves into the position of the former 1 st reactor where it is regenerated at the hot temperature. The former 1 st reactor is cooled down to the cool operating temperature of the 2 nd reactor. The pressure drop of the reactors is designed very similar to the conventional adiabatic reactors, i.e. appr 50 mbar. It does not increase due to sulfur loading. However, after switch-over a lot of sulfur is melted from the former cold and now hot reactor and that leads to a slight, though only temporary increase of the pressure drop of this reactor and the downstream sulfur condenser.

The process has been applied in two plants so far and proved to be easy to operate and due to its simplicity very reliable. In the 1 st generation of the process coiled heat exchangers were applied. They proved out very well. But they are very expensive. In the 2 nd generation of this process the reactors are equipped with Thermoplate exchangers.

DEGsulf-SDP Operating Experience

The sulfur recovery rate of 99.85% was measured in the Swedish NYNÄS refinery just before a turn-around. The SRU there is a 25 t/d plant which is fed by a 90%+ H 2 S acid gas that also contains some ammonia. The refiners agreed to testing the maximum possible recovery rate. For the purpose the 2 nd reactor was cooled from the normal 125°C even further down to 108°C. While at 125°C sulfur is still liquid, at 108°C it is solidified. That prohibits to apply such low temperature in normal SubDewPoint processes since any heat exchanger cooling the process gas to this temperature would soon be blocked by solid sulfur. In DEGsulf-SDP, however, the solid sulfur is deposited in the catalyst and does not influence the gas flow. The a.m. positive effects of the low reaction temperature resulted in the very high recovery rate. Please note that this was with aged catalyst that had been in operation for appr 3 years.

The plant’s sulfur recovery rate required by the authorities was only 99.1%, while the operating company required 99.4%. For this value the unit was designed with an outlet temperature of the 2 nd reactor of 125°C. In that operation mode it safely met the required sulfur recovery rate. In the optimized mode, i.e. with the outlet temperature lowered to 108°C it reached the peak value of better than 99.8%. This coincided precisely with the calculated value.

Since the DEGsulf process is so simple it proved to be very reliable. A DEGsulf plant consists of only few items in comparison to conventional processes of similar efficiency. Any item that is not there cannot fail. Availability proved to be between 99.5% and 100% over the years in the Swedish refinery’s DEGsulf-SDP. Claus plant outages invariably mean high emissions. As there were almost no outages the emissions were very low. Nominal and actual emissions were almost identical.

High reliability also means low repair and maintenance cost. The plant is easy to operate. Therefore personnel requirement is only appr 0.2 man/shift for field and control room.

DEGsulf-SDP shows a number of favorable features different from other sulfur recovery processes. The first is that the sulfur recovery rate remains virtually constant independent of the load. As shown in Fig. 4 between appr 20% and 80% load the sulfur recovery rate remained at 99.5%. This behaviour is much different from conventional Claus plants where low load typically means a drop in recovery rate. Again these data were measured in the Swedish refinery’s SRU.

Fig. 4: Sulfur recovery rate versus load as measured in NYNÄS refinerye

Another feature is that ammonium salts have very little influence on DEGsulf-SDP. The reason is that these salts form in the cold parts of a Claus plant. In DEGsulf that is primarily in the cold 2 nd reactor. However, deposits formed there are removed during regeneration as most of the ammonium salts are unstable at high temperature. This behaviour expected and found in the Swedish refinery again is a lot different from other SDP processes which usually are rather sensitive to ammonium salt deposits.

Retrofitting a conventional SRU to DEGsulf-SDP

In view of current stringent environmental protection regulations often conventional Claus plants have to be upgraded. The conventional way of doing that is to add a tailgas treatment downstream. However that means substantial investment and additional pressure drop which often is not available. An alternative option is to retrofit a Claus plant by replacing two reactors by Thermoplate reactors including their steam condensers and the two 4-way valves for switch-over. This actually has been done in the NYNÄS refinery.

Summary

More than 700 Thermoplate heat exchangers have been built so far, ranging from a few m² to several thousand m² per piece. In the application in reactors Thermoplate exchangers are an efficient tool to control the temperature in the optimal range for catalytic reactions.

Applied for sulfur recovery the respective process is called DEGsulf.
Two options were desribed:

* a 1-stage direct oxidation plant for lean H2S gas of up to 18% H2S feeds,
* a 2-reactor process which allows up to 99.85% sulfur recovery rate from rich H2S feeds

The DEGsulf family of processes employs Thermoplate heat exchangers in the 2 nd generation of internally cooled catalytic reactors. The 1-stage process is used for lean H2S gases where the H 2 S content of the feed may even fluctuate widely without negative influence on the process operability. Sulfur recovery rates or appr 90% can be achieved, depending on the feed gas composition. For rich H2S gases DEGsulf-SSP is to be applied. The process principle of DEGsulf-SSP was used in two plants so far and shows sulfur recovery rates of up to 99.85%. Due to its simplicity the process is very reliable, easy to operate, cheap in maintenance and low-cost both in capital and operation. It combines sulfur recovery and tail gas treatment in a simple, but high-performance set-up.