Naphtha Hydro-treating (NHT) Plant | Energy Efficiency Guidelines in Refinery


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Naphtha Hydro-treating (NHT) Plant – Hydrotreating process is one of the most mature process technologies in the oil refining processes. In the fifties of the last century, companies started to licensing Hydrotreating processes under names such as “Unifining”; “Unionfining”, and other processes. Since then the process has gone through many changes.

Hydroteating processes differ depending upon feedstock available and type of catalyst used. Hydrotreating can be used to improve the burning quality of kerosene, for instance.
Lube-oil catalytic hydrotreating is also conducted with hydrogen to improve product quality.

Naphtha Hydro-treating (NHT) Plant | Energy Efficiency Guidelines in Refinery

Naphtha Hydro-treating (NHT) Plant | Energy Efficiency Guidelines in Refinery

The objectives in lube hydrotreating include saturation of olefins and improvements in color, odor of the oil. Hydrotreating can also use to improve the quality of pyrolysis gasoline (color, odor, gum-forming tendency), which is a by-product in the ethylene plants.

Hydrotreating process removes undesired materials from petroleum distillates (Naphtha) by selectively having these materials react with hydrogen in a catalyst bed at elevated temperature. These undesired materials include sulfur; nitrogen; olefins and aromatics. Since Naphtha is generally further processed in catalytic reforming units, such as continuous catalyst circulation reactors, CCR unit, that needs certain Naphtha feedstock specification to avoid degradation in the catalyst performance and the life of the downstream unit besides the new stringent environmental laws worldwide, the Hydrotreating of Naphtha and other heavy distillates has become the norm in both old and new refineries.

The chemistry behind the Hydrotreating process can be divided into a number of reaction categories, called hydro-desulfurization; hydro-denitrification; saturation of olefins and saturation of aromatics. For each of these reactions, hydrogen is used to achieve the desired quality of the petroleum fraction.

Desulfurization is by far the most common of the Hydrotreating reactions. Sulfur content and form in hydrocarbons come in number of forms. The degree at which sulfur can be removed from hydrocarbon can vary from petroleum distillate to another. In Naphtha, sulfur removal can reach to near complete. The reaction rates for different forms of sulfur vary too. For instance, the Thiophenol reaction that results in benzene and H2S is very rapid. In any case, the desulfurization reaction results in the production of hydrogen sulfide (H2S) in the reactor and to complete the desulfurization process, such H2S must be removed in downstream fractionation unit.

Naphtha Hydro-treating (NHT) Plant | Energy Efficiency Guidelines in Refinery

Hydro-treating Process OverviewThe main use of the Hydrotreating process in Naphtha applications is in the preparation of feed stocks for the Naphtha reforming unit. The Naphtha Hydrotreating (NHT) process reduces the sulfur and nitrogen in the feedstock to the downstream catalytic reforming process to less than 0.5 wt ppm and the metals to non-detectable levels.

As mentioned above, the purpose of the Hydrotreating process that includes reaction
section, stripping section and Naphtha splitting section is to eliminate impurities, mainly
sulfur, Nitrogen and Arsenic, which affect the performance and life of the downstream
reforming unit with less energy consumption, less emissions and less capital investment.
In the reaction section of this process, full range naphtha is mixed with hydrocracked
naphtha from the Hydrocracking unit and with raffinate stream from the Aromatics
extraction unit, if any, in the Naphtha reactor feed surge drum. The mixture is then pumped to the reaction section where it gets mixed with hydrogen stream. This hydrogen stream is a combined process’s recycle stream and a make up one.

The mixture is heated up to the reaction temperature via process to process heat exchanger(s) and a fired heater. The reaction section is using a vapor phase reactor where the hydrogenation, desulphurization, denitrogenation reactions are taking place on a hydrotreatment catalyst. The reactor effluent is cooled down to separate the gases from the liquid phase in a two or three phase separator where most of the gas is used as a recycle and sent back to the reactor feed via a compressor. The rest, small part, of these gases is purged to fuel gas system to control the reaction section pressure.

The Hydrotreating stripping section purpose is to receiving the hydrocarbon liquid phase from the reaction section separation drum in a stripper after preheating the feed against the stripper bottom stream. Stripper reboiling is conducted by a fired heater and circulating pump. Stripper bottom product is then sent to Naphtha splitter column. Stripper overhead product is partially condensed, normally using air cooler, and sent to the reflux drum. Liquid phase from such drum is used as a reflux to the stripper column. The vapor phase from the reflux drum is sent to the fuel gas system. The Naphtha splitter section purpose is to receiving, in the Naphtha splitter column, the stabilized Naphtha coming from stripper bottom product. Reboiling of the Naphtha splitter is normally conducted using fired heater or MP steam reboiler.

The Naphtha splitter overhead is fully condensed in air cooler and sent to reflux drum where part is used as a reflux to the Naphtha splitter column and the rest is normally sent to the Isomerization unit. Heavy Naphtha from the splitter bottom is pumped and sent to the reforming unit downstream and/or storage tanks after being cooled against the Naphtha splitter column feed in a process-process heat exchanger, air cooler and water cooler.

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