DMS Exact-S® FAQs

Which sulfidation agents for ethylene production are commercially available?

The most widely used sulfur chemicals have been dimethyl sulfide (DMS) and dimethyl disulfide (DMDS). Polysulfide oils, mercaptans, and hydrogen sulfide have been used to a lesser extent.

What is the most important feature of a sulfidation agent?

From a performance standpoint, sulfidation agents appear to all work in the same way: they decompose at cracking temperatures to provide sulfur species which interact with the coil surface and free radicals formed during cracking. It could be argued that the most important feature of a sulfidation agent is really cost effectiveness: when adjusted for differences in sulfur content, which sulfur agent is most economical?

Is the sulfur content of a sulfidation agent important?

As all sulfidation agents do basically the same thing, sulfur content of the sulfidation agent is not particularly important. Techology licensors typically prescribe a dosing level of ca. 100 ppm during normal operating conditions. This requires the furnace engineer to simply adjust the flow rate of the sulfidation agent when changing from one having lower percent sulfur level.

As an example, an engineer switching from DMDS (%S 68%) would simply increase the flow of Exact-S (%S 51.6%) such that 100 ppm/w sulfur is fed into the furnace steam header. DMS is more effective than DMDS from a cost standpoint, an important motivating factor for such a switch.

Why is purity of the sulfidation agent important?

Steamcracking operations require a non-stop stream of sulfur chemicals to operate efficiently. The contribution of process contaminants by feedstock, furnace steam, and all additives should be minimized. Specifically, the impurities below can cause significant process problems for the reasons listed:

DMS Exact-S^® carefully controls levels of problematic impurities and is the purest sulfidation agent available.

It is useful to remember that a world-class ethylene facility (1,500,000 MTA capacity) can consume over one million pounds per year of sulfidation agent. Even at low dosage levels, poor quality sulfidation agents can contribute to process contamination problems.

Is the decomposition temperature of sulfidation agents important?

The complete decomposition temperatures of the most popular sulfidation agents – DMS and DMDS – is about 675°C and 575°C, respectively. This is well under standard cracking temperatures, which range from about 825 – 950°C. Both DMS and DMDS are completely decomposed in the furnace.

There is some evidence that DMDS, which is slightly less thermally stable than other sulfidation agents, can actually accelerate coking relative to other sulfidation agents. Presumably this is due to the ability of DMDS to increase rates of noncatalytic coking-  a potential disadvantage.

What are the decomposition products from DMS and DMDS?

The decomposition profiles of sulfidation agents under steam cracking conditions has not been studied in a comprehensive way. An early study (2) lists the decomposition products for DMS as methane, methyl mercaptan, and H2S, and the decomposition products for DMDS as methyl mercaptan, H2S, methane, ethylene, ethane, and ‘heavier sulfur compounds’.

There have been reports that DMDS use can affect quench tower acidity in a way that may be attributable to SOx byproduct formation. If this occurs, it suggests that not all sulfur in the DMDS molecule is used productively; any DMDS sulfur converted to SOx introduces operational costs associated with quench water pH regulation. A recent paper has modeled the decomposition pathway for DMDS, which is indeed complex (3).

Dimethyl Sulfide (DMS) is a cost effective and proven sulfidation agent for steamcracking operations. The high purity of DMS Exact-S^® helps to minimize the overall burden of feedstock impurities entering the furnace.