(Dinonyl polysulfide) (DNPS)
TU 2492-004-40655797-2016, RPB № 44178696.20.71786
Organic polysulfide (dinonylpolysulfide, DNPS) pilot-produced by (R&D Center) «AhmadullinS» is used to reduce coke deposits in coils and increase the life of the furnace during the pyrolysis of gas and liquid raw materials, it is a replacement for dimethyl disulfide (DMDS).
In the presence of sulfur-containing additives in the production of ethylene, an undesirable coking reaction is blocked.
The main advantages of DNPS:
- more convenient to handle than many other sulfur compounds. This liquid is less volatile, toxic, and explosive than ethyl mercaptan, butyl mercaptan, and dimethyl sulfide (DMS);
- convenient to use:
This is a non-volatile liquid – the temperature of the beginning of decomposition is 160 ° C, the vapor pressure is less than 0.1 mbar;
its flash point is much higher than that of ethyl mercaptan, butyl mercaptan, DMS and DMDS;
it is practically insoluble in water, which is a plus from the point of view of ecology – an accidentally spilled product can be collected from the surface of the earth and water;
3. it is more effective in pipe passivation after coke removal, it has a fairly wide range of decomposition in the presence of steam – from 540 to 825 degrees Celsius, while the interval for decomposition of DMDS in steam is 675-730 degrees Celsius.
Application of DNPS
- increases the mileage of the furnace by an average of 3-4 times (from 700-800 hours to 1500-2400 hours);
reduces the yield of CO2 by an average of 8-10 times (decrease in the concentration of CO2 in pyrogas to 20-100ppm with a slight increase in the formation of H2S);
this increases the yield of ethylene;
the load on the unit for alkaline purification of pyrogas is reduced;
the formation of heavy pyrolysis resin decreases with a simultaneous increase in the yield of LPS.
Currently, in Russia, for the pyrolysis of gas raw materials, mainly imported DMDS is used as an inhibitor of coke formation.
In the pyrolysis of liquid raw materials, sulfur-containing inhibitors of coke formation are not yet used in Russia. This is motivated by the presence of sulfur already in the raw material. However, abroad, sulfur-containing additives are also used in the pyrolysis of liquid raw materials (gasoline fraction, gas oil).
Several pyrolysis plants in Russia use potash as a coking inhibitor. In this case, soft coke is obtained, which is quite easy to gasify. However, the use of K2CO3 practically does not lengthen the furnace run (the experience of Kazanorgsintez, Nizhnekamskneftekhim) and leads to a significant increase in the CO2 content in the pyrogas, which negatively affects the ethylene yield and the alkali consumption in the unit for alkaline pyrogas cleaning from acid impurities.
Basic recommendations for improving the efficiency of coils in pyrolysis furnaces
A clear dosage of DNPS.
Inhibitors of coke formation are very often introduced in excess, which gives a negative effect. The optimal concentration of DNPS as an inhibitor of coke formation in the pyrolysis of ethane, propane, butane is 64-90 ppm S, i.e. 190-270 ppm DNPS in relation to the raw material.
For a more accurate dosage of the inhibitor, it is usually recommended to control the amount (monitoring) of CO at the outlet of the furnace – install a CO analyzer, the optimum is 300-400 ppm, control the supply of sulfur-containing inhibitor depending on the concentration of CO at the outlet.
The lower the amount of CO at the outlet, the lower the coke content in the coils.
С + Н2О = СО + Н2
С + СО2 = 2СО
If it is not possible to measure the amount of CO at the exit from the furnace, use statistics on the length of the furnace run.
Place of entry.
DNPS is recommended to be introduced into dilution steam. Steam is a better carrier and better dispersant of DNPS than hydrocarbon feedstock.
DNPS should be introduced into superheated steam (without condensate), preferably in a horizontal section, at a point after which the condensation process will be excluded.
A sulfur-containing inhibitor should be supplied to each furnace, as
- the amount of sulfur-containing inhibitor substance individually for each furnace (for initially identical furnaces operated under the same conditions – the same time, the dosage may differ by 1.5 times). An overdose of an inhibitor may also increase coking;
- Dosing an inhibitor into a supply line to several furnaces causes an uneven supply of inhibitor to different furnaces – usually the first furnace in the chain receives less inhibitor, the last one more.
Treatment of coils after decoking
The surface of the coils must be regenerated after the oxidative decoking process.
In the absence of H2S: Ni/Cr metals favor oligomerization and coke formation.
In the presence of H2S: Ni/Cr sulfides exhibit reducing properties favoring hydrotreatment and coil cleaning.
For passivation of the coils, after each removal of coke, 200-250 ppm DNPS is introduced into the dilution steam at a coil temperature of 700 degrees Celsius for 2 hours. During passivation, the use of DNPS provides a wide range of decomposition in the presence of steam (abundant decomposition in the range of 540-825 degrees Celsius) and, accordingly, guarantees reliable passivation along the entire length of the coil.
Types of coke and methods of removal
- to remove coke coming from the furnace, it is recommended to install a dry fender or cyclone in front of the distillation column (so-called oil column)
- Oligomers – mainly heavy aromatics and polycyclic aromatics – are removed by filtration, and the dimensions of the filter holes must be very small. Filters should be with two nozzles – one in operation, the other is being cleaned. The firm has such equipment.
- Unsaturated compounds like styrene polymerize to form solids which must then be filtered off. However, the amount of polymers can be reduced by adding a circuit breaker to the system at the level of the oil column circulating pump. In this case, 50-100 ppm methyl mercapto propionate into the column liquid stream may be recommended.
Installation of the DNPS supply system
The container is mounted and equipped with a dosing pump and a flow meter. DNPS is poured into a container from barrels. For pumping from a container, the system must be connected to a nitrogen line. DNPS should not come into contact with air!
Carbon steel pipes can be used, but alloy steel is preferred.
To supply DNPS, injection devices or consumable washers are used.
The temperature of the furnace walls is 1050°C. Raw material temperature ~ 860°С.
The amount of steam in relation to the feedstock ranges from 30% (ethane) to 50-60% (liquid feedstock).