Jun-2021

Tail gas hydrotreating in a high H2S gas plant

A tail gas hydrotreating catalyst delivers stable sulphur recovery operations, including low emission levels and energy savings, with a low rate of deactivation.

ZHANG XIAOGANG,WEI JINGHUI and ZHANGWENBIN
Sinopec

Article Summary

In the development of global oil and natural gas resources, the Claus sulphur recovery process has become an important part of natural gas and refinery gas purification.¹ Because the total sulphur conversion rate of the Claus unit is limited by chemical balance, the sulphur recovery rate of the conventional Claus process for two-stage catalytic conversion is 90-95%, and that for three-stage conversion can only reach 95-98%. The Chinese environmental standard GB16297-1996 Integrated Emission Standard of Air Pollutants strictly specifies the emission concentration and total emission amount of SO2, according to which the corresponding total sulphur recovery rate of the sulphur recovery unit must exceed 99.8%. This can only be achieved through the use of a tail gas treatment unit.² In tail gas treatment, the SCOT process has been widely used due to its strong adaptability to Claus sulphur recovery units and high degree of purification. The hydrotreating catalyst used is the core of the SCOT process.³

The Puguang Natural Gas Purification Plant of Sinopec Zhongyuan oilfield is the biggest ultra-high H2S content natural gas purification plant in Asia and is also an important part of the Sichuan-East natural gas transmission project. Based on a Black & Veatch design package, with Sinopec Engineering Corporation as the EPC contractor, and with a design scale of 120×108 m3/y, it consists of six complexes. Each complex includes two desulphurisation units, one dehydration unit, one sulphur recovery unit, two tail gas treatment units, and one acid stripping unit.

This article introduces the long-cycling commercial application of the tail gas treatment catalyst 234 developed by Shell Catalysts & Technologies in the Puguang Natural Gas Purification Plant.

Process flow of tail gas treatment units
There are 12 trains of sulphur recovery units in the Puguang Natural Gas Purification Plant. The design scale of each train is 200 kt/y, the operating flexibility is 30-130%, and the annual operating time is 8000h. In combination with the SCOT tail gas treatment unit, the sulphur recovery rate can reach more than 99.8%. The design SO2 content in the flue gas is less than 400 mg/m3.

The tail gas treatment section uses the hydrotreating reduction absorption process to address sulphur emissions. Tail gas from the sulphur recovery unit enters the hydrotreating feed combustion furnace, mixes with the high temperature flue gas generated by combustion in the hydrotreating feed combustion furnace, and is then sent to the SCOT reactor, also known as the hydrotreating reactor. The heated gas meets the surface area of the hydrotreating catalyst, the SO2 and elemental sulphur present in the gas react with the reducing gas (hydrogen), and the carbonyl sulphide (COS) and carbon disulphide (CS2) undergo hydrolysis reaction in the hydrotreating reactor. Tail gas leaving the hydrotreating reactor is cooled by the quench tower, then enters the tail gas absorption tower. H2S present in the tail gas is absorbed by the MDEA solvent, and the tail gas at the top of the tail gas absorption tower enters the incinerator. The tail gas is heated to 650°C by heat generated from the combustion of natural gas. The remaining low concentrations of H2S and COS in the tail gas are combusted in the furnace and converted to SO2. Other combustibles such as hydrocarbons, hydrogen, and CO are also burned off at the same time. The high temperature flue gas leaving the furnace enters the incinerator waste heat boiler for heat recovery, generating medium pressure superheated steam, and the flue gas is discharged through the chimney.

The tail gas treatment unit has the following process characteristics. A hydrotreating feed combustion furnace is used for a secondary oxidation reaction to provide a heat source and a reducing gas required by the hydrotreating reaction. The semi-rich amine liquid of the absorption tower is further used as a semi-lean liquid for the natural gas desulphurisation unit to save the steam energy consumption of solvent regeneration throughout the plant and, for full utilisation of the waste heat of the unit, a hydrotreating reactor outlet cooler is arranged to generate waste heat of the low pressure steam recovery unit. The tail gas incineration section is provided with a high pressure steam superheater; the waste heat generated by the incinerator is recovered, and the high pressure steam generated by the unit is superheated and sent to the system pipe network (see Figure 1).

Plant production
Catalyst and main reactions
The Shell C&T 234 catalyst exhibits strong hydrotreating performance, with alumina as the carrier, and cobalt and molybdenum as active components. The main reactions of sulphur-recovered tail gas under the action of hydrotreating catalyst4 are:

SO2 + 3H2 ’ H2S + 2H2O              [1]

S + H2 ’ H2S                                  [2]

CS2 + 3H2 ’ CH3SH + H2S           [3]

CO + H2O ’ CO2 + H2                   [4]

COS + H2O ↔ CO2 + H2S            [5]
 
CS2  + H2O ↔ COS + H2S            [6]

where Equation 1 is the main reaction which is rapid and complete. Equations 5 and 6 are hydrolysis reactions and are limited by thermodynamic equilibrium.

Catalyst loading and presulphiding
The Puguang Natural Gas Purification Plant carried out catalyst loading. Two layers of supporting ceramic balls were placed at the bottom of the reactor. The catalyst bed was loaded with 234 catalyst to a height of 1050 mm. On top of the catalyst bed a thin layer of ceramic balls with a diameter of 13 mm was loaded (see Figure 2).

The fresh catalyst contains active metal components in the oxidation state, and the catalyst needs to be sulphided for maximum hydrotreating activity and stability, then the operating mode needs to be transferred to applicable commercial operating conditions.
Acid gas containing H2S was used for catalyst sulphiding. The temperature of the catalyst bed in the hydrotreating reactor was raised to 240°C at the final sulphiding stage. Holding the temperature at 240°C until the H2S content at the inlet and outlet of the reactor showed the same value, the sulphiding process was completed after conditioning the catalyst for four hours. The sulphiding procedure for 234 was strictly followed to ensure that the catalyst would reach its optimum performance level.