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Case Studies


Tracking Fouling in an HDS Unit

The Monitor computer program has been used successfully in non-crude applications, notably in feed/effluent exchangers in hydrodesulphurisation units. One UK refinery is running its gasoil HDS unit at 20% above its design capacity, generating a profitability improvement of $10,000/day.

HDS applications present special challenges for fluid properties, phase determination and data reconciliation. In a typical HDS unit, the reactor effluent preheats a diesel feed. The reactor effluent is two phase and the diesel stream may change phase in the exchanger, so both streams must be characterised by assay curves. The hydrogen stream contains significant qualities of methane, ethane, propane, i-butane, n-butane and nitrogen. Databank values are used to predict the mixture properties of this stream.

The mixed hydrogen and reactor effluent is a combination of petroleum components, synthesised by the assay procedure, and databank components.

Monitor thermodynamics use the Grayson Streed method to determine the phase behaviour of mixtures of hydrocarbons and non-condensable gases. The mixing of the hydrogen and reactor effluent is modelled before the combined stream enters the exchanger. In the real plant, the hydrogen and effluent proportions remain constant.

During reconciliation, if one of the feed rates is changed, the other feed must be changed by the same amount. This is achieved by specifying the mixer product rate as a variable in the reconciliation.