Issue 31/2015 - Industrial application of liquefied gas deep desulfurization technology

With the intensification of environmental pollution and the enhancement of people's awareness of environmental protection, liquefied gas as an important refining product and chemical raw material, reducing its sulfur content has become an urgent problem
to be solved in refining production.
FCC liquefied gas and delayed coking liquefied gas produced by refiners are important raw materials
for the production of polypropylene and MTBE.
In recent years, due to the upgrading of gasoline quality, the sulfur content of MTBE has been reduced, and the sulfur content in liquefied gas has been reduced to less
than 20mg/m3.
The traditional liquefied gas demercaptan process can no longer meet the new production requirements
due to low desulfurization rate, high energy consumption and environmental pollution.
In this context, a variety of new demercaptan catalysts, equipment and processes have emerged
.

The liquefied gas demercaptan plant of Shengli Oilfield Petrochemical General Plant was put into operation in May 2001, with a design scale of 160,000 tons, using the liquefied gas demercaptan process
of Jinling Petrochemical.
After the device is put into use, the total sulfur of liquefied gas after demobilization is reduced to <100mg/m3<b17>.
In September 2009, after transformation, the secondary fiber membrane alkaline washing and the primary fiber membrane water washing process were added, and the total sulfur of liquefied gas after removal can be controlled below 30mg/m3, but the total sulfur content of propylene in downstream gas separation device products is often higher than 5mg/m3, and the total sulfur content of downstream MTBE plant products is higher than 80mg/m3, which affects the production of
propylene products and downstream MTBE plants.
In July 2013, the desulfurization system of the heavy urging device was reconstructed, and the liquefied gas desulfurization part adopted the patented technology of "deep desulfurization of liquefied gas" of Hebei Jingjian Technology Co.
, Ltd.
, and the scale was expanded to 240,000 tons
.
After the implementation of the project, the total sulfur content of the refined liquefied gas was effectively reduced to meet the production needs
of the downstream plant.

First, the principle and characteristics of demercaptan technology

1.
Analysis of the principle and process of
liquefied gas desulfurization At present, liquefied gas desulfurization and refining includes two parts: hydrogen desulfide and demercaptan, and hydrogen desulfide generally adopts alcoholamine wet desulfurization, and the process is mature; The vast majority of demercaptans are oxidized by Merox extraction
.

The traditional liquefied gas demercaptan technology is affected by product quality indicators, and the total sulfur removal depth after refining is not high
.
Through analysis, it is found that in the traditional liquefied gas demercaptan process, there are almost no disulfides in the sulfide in the liquefied gas before demercaptan, while the sulfides in the liquefied gas after demercaptan are mainly
disulfides.
This result indicates that this part of the disulfide is oxidized by mercaptans during the
demercaptan process.
From the process analysis of liquefied gas demercaptan, there are two ways for disulfide to liquefied gas after entering demercaptan: first, the disulfide formed during the regeneration of lye is not removed from the lye in time, and the disulfide is pumped into the liquefied gas after circulating to the extraction stage with the lye; Second, the traditional process dissolves the regeneration catalyst in the lye to form a chemical base, and the agent alkali is recycled between extraction and regeneration, and under the action of the regeneration catalyst, the sodium mercaptan generated in the extraction section is oxidized to disulfide by dissolved oxygen, and the sulfide is returned to the liquefied gas in the
form of disulfide.

2.
The main technology used in the transformation This transformation adopts the deep desulfurization patented technology of Hebei Exquisite Technology Co.
, Ltd.
, which mainly transforms the original process from three aspects: First, the functional strengthening additive technology
is adopted, which improves the depth of extraction and regeneration reactions, and at the same time improves the removal efficiency of carbonyl sulfur in the demercaptan process; Second, the three-phase mixed oxidation regeneration technology is adopted to transfer the disulfide generated during the regeneration of the lye to the back-pumping oil in time, which not only greatly reduces the content of disulfide in the regenerated lye, but also improves the driving force of the regeneration reaction process, improves the equilibrium depth of the regeneration reaction, and prolongs the service life of the lye; The third is to use fixed-bed regeneration catalyst technology to fix the oxidation catalyst in the regeneration tower, avoiding the oxidation of sodium mercaptan to disulfide
after the catalyst is brought to the extraction section.

Second, deep desulfurization process

The raw material liquefied gas first enters the pre-alkali washing sedimentation tank (V-502) for self-circulating pre-alkali washing
.
Pre-alkali washing qualified liquefied gas and lean extraction solvent, into the first stage extraction contactor for reaction, after completing the first stage extraction reaction, into the first stage extraction sedimentation tank (D-431) sedimentation separation, sodium thiol-rich extraction solvent from the bottom of the first stage extraction sedimentation tank (D-431), through the boundary position control to the regeneration process; The liquefied gas is pressed out from the top into the secondary extraction contactor, fully contacted with the regeneration lean extraction solvent, and enters the secondary extraction and sedimentation tank (D-432) after secondary extraction, and the extraction solvent rich in sodium mercaptan is pressed out from the bottom of the secondary extraction and sedimentation tank (D-432), and the regeneration process
is controlled by the boundary position.
After demercaptan desorption, the liquefied gas is pressed out from the top of the secondary extraction sedimentation tank (D-432), and enters the washing water contactor (FFC-433) and then enters the washing settling tank (D433) outlet device
.
Figure 1 is the flow chart
of liquefied gas refining.

The rich solvent from D-431 and D432 and the oxidation wind and back-pumping oil from the system are pre-mixed by the static mixer (MI-501A/B), enter the lower part of the extractant regeneration tower (T-501), and then regenerate through the packing section in the tower, and the sodium mercaptan carried by the extractant is oxidized to disulfide and dissolved in the
back-pumping oil 。 The extractant and reverse extraction oil are pressed out from the top of the tower into the three-phase separation tank (V501) for separation, and the exhaust gas is controlled to the exhaust gas system by the pressure control of the top of the tower; The regenerated lean solvent enters the extractant deoxidation tower (T-502) for sedimentation filtration and is recycled by pump P-409A/B; The backpumping oil is pumped from the bottom of V501 by the backpumping pump (P-502A/B), part of which is recycled to the regeneration tower static mixer MI-501A/B, and part of which is recycled to the backpumping oil washing tank (V502), and the backpumping oil after washing is pressed out from the top of V502 and sent to the catalytic fractionation column reflux tank (D201).

Figure 2 shows a solvent regeneration flow chart
.

Third, industrial applications

The liquefied gas desulfurization plant was successfully started on November 10, 2013, with a liquefied gas treatment capacity of 20t/h and a load of 90%.

The device has been running smoothly since it was put into use, and the total sulfur content after liquefied gas is stable below 15mg/m3
.
Through irregular analysis of the regenerated lye, it is found that the disulfide sulfur and mercaptan sulfur contents in the regenerated lye are less than 50×10-6, and the NaOH concentration is stable at about 10% for a long time, indicating that the three-phase mixing regeneration effect is good and the service life of the lye is extended
.
In addition, alkali consumption and alkali residue are significantly reduced, and the amount of washing water is also greatly reduced compared with the traditional process, among which the unit consumption of extracted functional agent is 0.
05kg/tLPG, the unit consumption of alkali (30%) is 0.
5kg/tLPG, and the single consumption of demineralized water is 1.
5kg/tLPG
.

IV.
Conclusion

By comparing the production situation and quality data before and after the new technology is put into use, it is found that the device not only greatly improves the desulfurization efficiency, but also reflects many excellent performance
.

1.
Before the new technology is significantly reduced
after deprivation, the total sulfur content of liquefied gas after demobilization is mostly between 20~30mg/m3, and the average value is 23mg/m3
.
After the new technology is put into use, combined with the adjustment of operating parameters, except for abnormal operation conditions such as abnormal circulation, untimely adjustment and interruption of anti-pumping oil, the total sulfur content after deregulation is significantly reduced, controlled below 20mg/m3, with an average of 11.
3mg/m3, and the desulfurization rate reaches more than
98%.
Before the new technology, the total sulfur content in propylene is often between 5~10mg/m3, and after the new technology is used, the total sulfur content in propylene is reduced to between
0~5mg/m3.

2.
The solvent regeneration effect is greatly improved
before the new technology is used, and the disulfide generated during the regeneration of the agent alkali cannot be effectively separated from the lye, resulting in a high content of sodium mercaptan and disulfide in the circulating agent alkali, which affects the accumulation of circulation and causes the effect of demercaptan to not be fully manifested
.
After the new technology is put into use, 4 tons of deodorizing refined liquid are added to 40 tons of lye, and the back-extraction system is added, which greatly improves the activity and regeneration ability of the solvent, the alkali concentration is maintained at about 10% for a long time, and the use cycle of lye is extended to more than
two months.

3.
Before the significant
transformation of the energy-saving and consumption reduction effect, because the alkali concentration of the system drops rapidly, at least 20 tons of 30% sodium hydroxide lye solution are consumed every month, and the alkali is changed according to the alkali concentration reduced to 5%, and the monthly slag discharge is more than
50 tons.
After the use of this special technology, the monthly consumption of 30% of the lye does not exceed 10 tons, saving more than
50% of the new alkali.
When the circulating alkali concentration is reduced to 5%, it can also meet the requirements of demercaptan, reduce the monthly slag discharge to less than 30 tons, and reduce the load and difficulty of subsequent alkali residue treatment; Compared with before the use, the cost per ton of liquefied gas treatment has been greatly reduced, and multiple benefits
of energy saving, emission reduction and consumption reduction have been achieved.

4.
Safe and stable
operation Since the new technology was put into use, the device has run smoothly, the manual operation workload has been reduced, and the total sulfur content of liquefied gas has been low after demercaptan, and there is no adverse impact
on the subsequent production equipment and the environment.