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An automated spreadsheet method can quickly help minimize fresh hydrogen consumption while maximizing hydrogen recovery and reuse in petroleum refineries and petrochemical complexes
In petroleum refineries and petrochemical complexes, there are many hydrogen consumers, such as hydrotreaters, hydrocrackers, isomerization units and lubricant plants. There are also hydrogen producers, such as hydrogen plants and naphtha reformers. Through a systematic analysis of hydrogen sources and demands (sinks), hydrogen-pinch analysis attempts to minimize the flowrate of fresh hydrogen makeup and off-gas discharge while maximizing the flowrate of hydrogen recovery and reuse, possibly through off-gas purification techniques.
Mass balance
The first step in the hydrogen-pinch analysis is to perform amass balance on hydrogen sources and demands in the hydrogen network. Hydrogen sources include fresh (or makeup) hydrogen and recycle hydrogen streams, outlet streams from hydrogen producers (for example, reformers), product and residue streams from hydrogen purifiers (for example, membrane separation, pressure swing adsorption [PSA], or cryogenic distillation), off-gas streams from high- or low-pressure separators, and off gasses from hydrogen-consuming units (for example, hydrotreaters and hydrocrackers).
Hydrogen demands consist of inlet streams to hydrogen-consuming units, including any exported streams, including streams sent to fuel, and feed streams to hydrogen purifiers. For each stream, the flowrate, pressure and hydrogen purity are specified. Standard volumetric flowrates or molar flowrates must be used.
Various approaches
Simply put, hydrogen pinch is the purity at which the hydrogen network has neither hydrogen surplus nor deficit. The pinch shows the bottleneck for how much hydrogen can be recovered and reused. The traditional approach to hydrogen-pinch analysis [1, 2, 6, 7] does not consider hydrogen pressure, but the analysis does provide a theoretical, minimum fresh-hydrogen requirement and gives significant insights to hydrogen savings and off-gas purification in petroleum refineries. Changes to the real network necessary to achieve this minimum might be as easy as opening and closing some valves, or as daunting as adding a multistage compressor to connect lowpressure sources to high-pressure demands. An intermediate change could be adding cascades between the purge of one unit and the makeup of another [3]. To account for stream pressure in refinery hydrogen management, we can apply mathematical optimization techniques [4, 5, 8].
Early approaches to hydrogen-pinch analysis [1, 2] are graphical and iterative in nature, and require...