Data-driven approach to steam-to-carbon ratio optimisation for the HGU

Development of advanced analytics-supported models helps identify the optimal S/C ratios for minimising operational costs within the hydrogen generation unit.

Mert Akçin, İbrahim Bayar, Berkay Er, Gizem Kayar Öcal and Muratcan Özpınar
SOCAR Turkey

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Article Summary

The hydrogen generation unit (HGU) is designed to process natural gas (NG) as primary feed and light naphtha as alternate feed in order to produce high- purity hydrogen (H2). The HGU can be divided into two discrete unit sections. The first is referred to as the ‘front-end section’, which includes feed desulphurisation and comprises all necessary process steps to generate a hydrocarbon feed stream suitable in terms of chemical quality, pressure, and temperature for the downstream unit section.

The downstream unit section is commonly referred to as the ‘back-end section’, which includes pre-reforming and reforming, reformer heat supply and flue gas heat recovery, steam generation, steam conditioning, shift conversion, process gas heat recovery, process gas cooling, and H2 purification. The front and back end are divided by the feed valve, where the ratio of steam and hydrocarbon feed is controlled at the back end.

As a process principle to consider, the feedstock used for H2 production might contain poisonous contaminants that affect the utilised catalysts. Hence, it is necessary to initially remove contaminants to an acceptable level to protect catalyst activity. Then, the reforming section is divided into two sections: the pre-reformer and the reformer.

The pre-reformer operates adiabatically, where the temperature results from the equilibrium and enthalpies of the reactions in the reformed feed. For a low molar weight feedstock, such as natural gas, the result is a temperature decline over the pre-reformer. Although the reforming of higher hydrocarbons is endothermic, the low operating temperature of the pre-reformer results in partial conversion of the formed carbon oxides into methane (CH₄), resulting in an overall temperature rise across the reactor.

Reducing refomer load
The installation of a pre-reformer reduces the reformer load and, in addition, the reformer size. It also allows more feed flexibility while operating at a higher reformer inlet temperature since there are no C₂+ hydrocarbons (C₃, C₄,), which can potentially cause fouling due to carbon formation/polymerisation reactions. In the reforming section, the pre-reformer effluent (steam, methane, carbon oxides, hydrogen) is mixed with steam under a fixed ratio in excess of the stoichiometric ratio. The ratio can be defined as:
•    Steam-to-carbon ratio: defined as the ratio of the steam moles to carbon moles (excluding CO2). The moles of carbon are counted as atoms. Thus, one mole of ethane represents two moles of carbon.
•    The steam-to-feed ratio: defined as the weight flow of steam to the weight flow of total hydrocarbon feed (including hydrogen).
This mixture is superheated and fed to the reforming section. The two major reactions that take place in the reforming section are:

CnHm + n H₂O  Û  n CO + (n + m/2) H₂ - Q                (1.1)
CO + H₂O Û CO₂ + H₂ + Q                                          (1.2)

Reaction 1.1 represents the reforming reaction, in which the hydrocarbons are reformed to a mixture of H₂ and carbon monoxide (CO). During the reforming stage, all higher hydrocarbons (C₂+) are converted to CH₄, CO, and H₂. The conversion of CH₄ is limited by the equilibrium of the reaction. This equilibrium is favoured by high outlet temperatures, a high steam-to-feed ratio, and low pressure. Reaction 1.2 represents the CO shift reaction, in which the CO is converted with H₂O to CO2 and H2. The CO shift reaction is favoured by low temperature and high steam-to-feed ratio. Besides these bespoke reactions, there are a number of side reactions which are not desirable:

Boudouard reaction: 2CO Û C + CO₂                   (1.3)
CO reduction: CO + H₂ Û C + H₂O                       (1.4)
Methane cracking: CH₄ Û C + 2H₂                        (1.5)

These reactions are suppressed by applying an excess of steam so that carbon will eventually be removed by the reverse CO reduction reaction (1.4). A low steam-to-carbon ratio can lead to carbon deposition and, thus, long-term catalyst damage. Reformer temperature and steam-to-carbon ratio will affect the final gas composition of the reformer effluent.

Project objectives
The cost optimisation formula for this project incorporates the sum of natural gas and fuel gas consumption costs, deducts the profit from steam production, and divides the result by the hydrogen demand (see Figure 1). To achieve this, the development of analytic models was essential to accurately predict the consumption and production of natural gas, fuel gas, and steam, the key variables influencing the operational cost in the HGU.

A controlled test was initiated within the HGU to elucidate the impact of varying steam-to-carbon (S/C) ratios. The S/C value was systematically altered to several predefined levels over designated periods during this test. The primary objective was to observe the consequent effects on the unit’s performance. Additionally, this exercise was instrumental in generating a clean dataset that would later be utilised in the modelling phase to ensure robustness and accuracy of the predictive analytics.

Model development approach
After completing the necessary tests concerning the S/C ratio, the initial objective was to construct an analytical model capable of forecasting the required natural gas consumption at a given S/C value to meet H2 demand. Subsequent efforts focused on developing analytical models for fuel gas consumption, steam production, and methane slip. In the data analysis phase, the primary focus was on understanding the influence of the S/C parameter on energy consumption and production levels. To capture this relationship, a search for suitable general models commenced, employing tree-based and regression algorithms to ensure a comprehensive understanding of the S/C ratio’s impact (see Figure 2).

Natural gas consumption model
The constructed natural gas consumption model was intended to estimate the consumption of natural gas contingent upon the H2 requirements and the S/C ratio. While the S/C ratio was initially posited as an independent variable in the model, its direct correlation with the target variable could have been more robust in the dataset than theoretically expected. Consequently, the model was refined to reflect the S/C ratio’s effect through the intermediary of methane slip – a variable that showed a more pronounced relationship.

All of the analytical model’s development involved scrutinising data from various periods across 2022 and 2023, with validations of different model iterations ensuring robustness. Key parameters incorporated included the hydrogen requirement, the molecular weight of natural gas, reformer inlet temperature, and S/C ratio. Collaborative reviews with the process department yielded a consensus on the logical coherence of the variables exhibiting high correlations, indicating their relevance and impact within the model framework. The detailed heatmap in Figure 3 visually represents these relationships while maintaining the confidentiality of the specific model dynamics.

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