Overview of the case:
Petroleos del Norte (Repsol group) hydrogen network is considered as case study, in order to improve efficiency in the use of hydrogen as raw material. Hydrogen H2 is an expensive utility required in many processes in a refinery, which is gaining increasing importance in the global economic balance; it is distributed by means of a H2 network from producer to consumer plants. High purity H2 is produced in steam reforming furnaces producer plants. Another low purity H2 producer plants (catalytic platforming) exist, not being truly decision variables in operation as H2 is a byproduct of the process. In consumer plants, H2 is mainly used as reactant for desulfurization, de-aromatization and de-nitrification of naphtha and diesel, in the presence of other H2 consuming side reactions. Desulfurization and de-nitrification reactions enable not to generate acid gases (SOx, NOx), thus avoiding atmosphere pollution.
Decisions regarding H2 management are complex as many plants and operating constraints are involved in the network operation with a high degree of interrelation, not only from an optimality viewpoint but also from a practical viewpoint because several operators at different control rooms are typically in charge; thus a quantitative criterion for decision support based on resource efficiency indicators REIs can prove very valuable.
The efficiency of H2 as a material resource is considered separately of the hydrocarbon treatment processes. Optimal management of the H2 network refers to determining the optimal production rates in the producer plants, and the combination of make-up flows from each header providing H2 to each consumer plant, while fulfilling all the operational constraints:
a minimum ratio H2/hydrocarbon must be ensured at reactor inlet;
a certain minimum H2 purity at the high pressure separator gas outlet must be assured for catalyst maintenance reasons;
capacity range for compressors, producer plants and pipes;
operating range and capacity for membranes;
minimum purge to fuel gas to assure controllability in certain headers.
An accurate plant state estimation is a challenging problem due to uncertainty in measurements, mainly caused by compensations in real-time with current molecular weight conditions and systematic and drift errors; therefore a data reconciliation problem is solved by optimization in the first place to achieve a reliable and consistent estimate of the network state, according to a material balance model.
Expected Achievements of the case:
The objective is to be able to use meaningful REIs to improve efficiency in the use of H2 as resource. The most representative is a relative REI compared to a best case, where Minimum Required Material is assigned as the minimum production to fulfil the H2 requirement constraints, obtained by solving an optimization problem with the material balance model of the H2 network. Even though the potential for profit increase is not high, there is margin due to frequent changes in operating conditions and the amounts involved. The prospective result by the end of the project will be a tool where the developed REIs are implemented for on-line decision support.