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Apr-2020

Pre-reformer catalyst in a hydrogen plant

Good operation following recommended practice with technical support highlight the successful performance of a pre-reformer catalyst.

K R RAMAKUMAR, Johnson Matthey
SULTAN ORMAN and ISMAIL BAHADIR KARA, Tüpras

Viewed : 1483


Article Summary

Pre-reforming is the process by which methane and heavier hydrocarbons are steam reformed and the products of the heavier hydrocarbon reforming are methanated. The adiabatic pre-reformer is usually positioned upstream of the main steam reformer and uses a catalyst with high nickel content.

Three reactions occur in the pre-reformer and each of them will reach equilibrium:

Steam reforming reaction
CH4 + H2O ⇌ CO + 3H2 (endothermic)
CnHm + nH2O → nCO + (n+m/2)H2
(endothermic)

Water gas shift reaction
CO + H2O ⇌ H2 + CO2 (exothermic)

Methanation
CO + 3H2 ⇌ CH4 + H2O (exothermic)

The overall reaction is endothermic for light (natural gas) feeds while for LPG there is an exotherm. For naphtha, there is an endotherm followed by an exotherm. Overall, for heavier feeds the reaction is exothermic.

The benefits of having a pre- reformer are:
• Flexibility to operate with different feeds, especially in refineries where it is important to use the most available and economic feed which might vary at different times of the year
• When the plant is designed to operate at a very low steam to carbon ratio, such as HyCO units where the main objective is to have more CO (and also a specific ratio of H2/CO) in the reformed gas
• Having a pre-reformer can lead to lower capex due to the smaller size of the steam reformer
• Having a pre-reformer can also lead to lower opex due to less fuel consumption in the steam reformer and less steam requirement
• A pre-reformer is also an excellent revamp option for increased hydrogen production as it would typically add about 10-15% additional capacity
• The overall excess steam production (steam export) with the pre-reformer in the flowsheet will be less
• The pre-reformer also facilitates operating the reformer at a higher inlet temperature without any preheat coil cracking issues as the higher hydrocarbons are already converted to methane and hydrogen
• A pre-reformer also acts as a poison buffer, protecting the downstream reformer catalyst in case of any poison slip across the purification section

The hydrogen plant in Tüpraş’s residue upgradation project block, PLT-147, supplies hydrogen for the integrated hydroprocessing unit in Izmit refinery. The block consists of vacuum distillation, hydrotreater, and hydrocracker units.

PLT-147 has a design capacity of 160 kNm3/h and mainly processes natural gas along with hydrogen rich gas from the CCR unit. The unit also processes naphtha and LPG when there is a shortage of natural gas, especially during the winter. Natural gas contains up to 3.5 vppm H2S and 7.5 vppm of organic sulphur.

The unit consists of a purification section (hydrodesulphurisation, H2S absorption, and ultra-purification), pre-reformer, steam reformer, medium temperature shift converter, and a 14-bed PSA system for hydrogen purification.

The feed gas first goes through the hydrodesulphurisation reactor. This has a Ni-Mo catalyst installed which does all the functions of HDS (see Figure 1). The temperature is normally maintained in the range 360-380°C. The HDS reactor also has a small layer of chloride guard beneath the Ni-Mo catalyst to absorb HCl. Following this are two H2S absorbers (ZnO absorbers) which are normally in lead-lag arrangement. The ZnO absorbs the bulk of H2S and reduces its level to less than 0.1 vppm. Below each ZnO, there is a small layer of ultra-purification (UP) catalyst which polishes the H2S to very low vppb levels. The ultra- purification catalyst is a Cu-Zn formulation. The functions of different sections in the purification system are highlighted in Figure 1.

A simplified sketch of the purification system in PLT-147 is shown in Figure 2.

Understanding the mechanism of pre-reformer catalyst deactivation
Deactivation by sulphur poisoning
Even though the ZnO absorbent can remove H2S to below 0.1 vppm, the pre-reformer life in most natural gas feed cases is determined by the gradual sulphur poisoning rate. This is where ultra-purification can help to improve the pre-reformer run length.

First, let us understand the normal performance of pre-reformer catalyst. The typical reaction temperature profile across the pre- reformer from start of run (SOR) to end of run (EOR) for the natural gas feed case is shown in Figure 3.

The temperature profiles 1 and 4 in Figure 3 correspond to the typical profiles at SOR and EOR respectively. Gradual deactivation of the catalyst due to poisoning sets in from the top. As the catalyst ages, the bed depth at which the reforming reaction begins also increases. Deactivation of the pre-reformer can be monitored by drawing a Z-90 plot. A Z-90 plot is a method to monitor and trend the temperature profile movement throughout the pre-reformer catalyst bed, whereby the 90% point of total endotherm is plotted. Typically, the Z-90 over time will increase owing to gradual deactivation of the bed from top. Figure 4 depicts the gradual increase in Z-90 for the bed profile represented in Figure 3.

Deactivation by sintering
Sintering is deactivation due to gradual ageing of the catalyst whereby small crystallites of active metal increase in size, thus reducing the effective surface area and therefore relative activity. The effect of sintering can be known by monitoring the bed temperature profile. The slope of the bed temperature profile is slightly different for catalyst subjected to sintering (see Figure 5). This difference in bed profile assumes that the plant is at the same rate.

Pre-reformer performance in PLT-147
The PLT-147 hydrogen manufacturing unit of the residue upgradation project has a design capacity of 160 kNm3/h and was commissioned in 2015. The unit supplies H2 for the integrated hydrocracker and diesel hydrotreater units and is designed to operate with flexible feeds ranging from heavy naphtha to natural gas. The plant runs close to design feed rate most of the time.

PLT-147’s pre-reformer catalyst is a pre-reduced, high-Ni based catalyst and is operated with the objective of minimising C2+ slip. The pre-reformer catalyst was loaded in February 2015. There are about 40 temperature measurement points, of three sets, within the catalyst bed; these are placed equally round the circumference of the reactor shell. The three thermowell sets are installed vertically down the reactor length.

With a steam to carbon ratio at the inlet of the pre-reformer of normally less than 2.5 mol/mol, the catalyst has been performing exceptionally well for more than four years from the time it was taken in-line. The bed temperature profile has not exhibited any significant change since SOR, which is evident from a very low Z-90 point.


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