PTQ Q3 2024 Issue

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

T he 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 (H 2) . 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 hydro- carbon 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 recov - ery, steam generation, steam conditioning, shift conver- sion, process gas heat recovery, process gas cooling, and H 2 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 H 2 production might contain poisonous contaminants that affect the utilised catalysts. Hence, it is necessary to ini- tially 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 tem- perature 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 con- version 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 forma- tion/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 CO 2 ). The moles of car- bon are counted as atoms. Thus, one mole of ethane repre- sents two moles of carbon. • The steam-to-feed ratio: defined as the weight flow of steam to the weight flow of total hydrocarbon feed (includ - ing hydrogen). This mixture is superheated and fed to the reforming sec- tion. The two major reactions that take place in the reform - ing section are:

C n H m + n H₂O ⇔ n CO + (n + m/2) H₂ - Q

(1.1) (1.2)

CO + H₂O ⇔ CO₂ + H₂ + Q

Reaction 1.1 represents the reforming reaction, in which the hydrocarbons are reformed to a mixture of H₂ and car - bon monoxide (CO). During the reforming stage, all higher hydrocarbons (C₂+) are converted to CH₄, CO, and H₂. The installation of a pre-reformer allows more feed flexibility while operating at a higher reformer inlet temperature since there are no C₂+ hydrocarbons The conversion of CH₄ is limited by the equilibrium of the reaction. This equilibrium is favoured by high outlet tem- peratures, 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 H 2 . 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 ⇔ C + CO₂ CO reduction: CO + H₂ ⇔ C + H₂O Methane cracking: CH₄ ⇔ C + 2H₂

(1.3) (1.4) (1.5)

These reactions are suppressed by applying an excess

PTQ Q3 2024