Prolonging the operating cycle

Processing coker naphtha streams is a common challenge experienced by numerous refiners worldwide. Coker naphtha streams are highly olefinic and typically contain large amounts of silicon: properties that create specific issues for naphtha hydrotreaters.

PetroChina Liaohe Petrochemical Company’s 150-kilotonne-a-year coker naphtha hydrotreater in Panjin, China, suffered serious coking during operation, especially at the top of the first reactor, which incurred frequent shutdowns. Skimming the catalyst on the first reactor’s upper bed was required four times in as many years, which caused lost production and impacted on operational costs. When the company wanted to revamp the unit and expand its capacity to 200 kilotonnes a year, it became economically imperative that the coking issue be resolved.

Liaohe operations and engineering personnel, and refining and hydroprocessing experts from Criterion Catalysts & Technologies (Criterion) and Shell Global Solutions formed a joint team.

Their analysis found the coking in the first reactor to be the result of the high levels of dienes in the coker gasoline. These speed up the oxidation of oil products. The coker gasoline also contains high levels of oxygen, sulphur and nitrogen, which cause soft coke to be generated. During operations, this deposits on the heater and the heater tube and changes to hard coke. When this coke is carried to the top of the first reactor, it causes coking that triggers a pressure drop across the reactor.

The analysis found that improving the catalyst loading scheme and the coke and scale tolerance of the upper catalyst bed would help to prolong the unit’s operating cycle.

To enhance the coke tolerance of the catalyst bed, a sophisticated grading loading scheme was designed for the upper bed of the first reactor. This comprises several types of guard catalyst whose particle size gradually decreases from the top to the bottom of the reactor.

In addition, the catalysts’ activity progressively increases from the top to the bottom of the reactor. This enables the highly reactive dienes to contact the low-activity guard catalyst at the top of the reactor, which decreases the reaction intensity and reduces coking at the top of the reactor.

There was also a strong focus on creating a silicon trap. Silicon is a common contaminant in various process streams that poisons the catalyst and reduces its activity. Criterion’s MaxTrap[Si] desilicification guard catalyst, which is designed to maximise silicon removal while maintaining olefin saturation capacity, was used to combat this in high-temperature areas.

The main refining catalyst is Criterion’s DN-200 nickel–molybdenum catalyst. This trilobal alumina extrudate is designed for enhanced hydrodesulphurisation and hydrodenitrogenation performance in naphtha hydrotreaters.

Because DN-200 catalyst is presulphided, it unlocked benefits for Liaohe. For instance, it eliminated the need for sulphur-injection-related facilities, reduced the required investment and simplified and expedited start-up.

In addition, the operating results demonstrated robust performance, according to You Feng, Engineer, Hydrotreating Unit, PetroChina Liaohe Petrochemical Company. He says, “DN-200 catalyst has demonstrated high hydrodesulphurisation and hydrodenitrogenation performance, and good olefins saturation at comparatively low operating temperatures.”

“During the first six months of operation, the rate of pressure drop across the catalyst bed was slow, which indicates that the graded-bed solution is effective,” adds Liu Yu, Engineer, Technical Department, PetroChina Liaohe Petrochemical Company. “This is important because it shows that the project has met our objectives: it will help to prolong the unit's operating cycle."