Many refineries have been in operation for decades. Over this time, operating parameters may have shifted resulting in operation outside the original design of equipment. Reviewing current design criteria against current operation is a recommended action for all refiners. This article will explain the current state-of-the-art criteria for process design and metallurgy selection.
At the outset, let us look into some key parameters and operational considerations – which go into designing the combined feed exchanger for a low severity Naphtha Hydrotreating (NHT) unit, from a process licensor’s perspective. The NHT unit prepares feedstock for downstream reforming or isomerization units that have sensitive noble metal catalyst systems.
The schematic diagram in figure 1 shows that naphtha feedstock is brought into the unit, then mixed with hydrogen rich recycle gas. The combined feed is heated first by recovering heat from the reactor effluent in a Combined Feed Exchanger (CFE), and then heated to reaction temperature in a fired charge heater.
Reactor and the Flow Scheme
The reactor contains a cobalt-molybdenum or nickel-molybdenum catalyst system that converts the contaminants of noble metal catalyst systems, such as sulphur, nitrogen, oxygenates, and halides via hydrogenolysis reactions to hydrogen sulphide, ammonia, water and hydrogen halides – so they can be removed from the hydrocarbon stream. Metals in the naphtha are removed by adsorption onto the catalyst and low levels of olefins or trace diolefins are saturated. Hydrotreating reactions are exothermic with olefin saturation having the highest heat of reaction.
The reactor effluent is cooled in the CFE. Wash water is injected directly at the downstream of the CFE. The reactor effluent is further cooled in the products condenser – and the three phases in the stream are separated in the product separator. Makeup hydrogen is introduced into the reactor circuit on pressure control.
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