Proper Design and Operation of NHT CFE Equipment Ensures Economy

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The gas from the product separator is recycled back to the reactor. An equilibrium level of hydrogen sulphide builds up with the recycle gas scheme. The hydrotreated naphtha from the separator is sent to a stripping column to remove hydrogen sulphide, water and light ends before being fed to a downstream unit. Sour water containing ammonia, hydrogen chloride and equilibrium hydrogen sulphide is removed from the boot of the separator for further processing.

The naphtha feedstock to a low severity NHT unit would contain less than ~20 per cent cracked stock. Moderate operating conditions are employed with pressure set in the range of 285-800 psig [20-55 kg/cm2(g)].

Hydrogen circulation is set to achieve a target partial pressure to ensure the desired conversion of the defined contaminants and undesirable components. Temperature is increased in the range of 600-650°F [315-345°C] over the life of the catalyst. The temperature increase across the reactor bed is less than 50°F.

The discussion above shows that an important part of the NHT unit design is - to understand the source of the naphtha feed and makeup hydrogen, to be able to adequately quantify the expected contaminant levels, as well as trace elements that may not set the severity of the unit – but will impact the design. Some particular considerations for CFE design will now be discussed.

CFE Design Considerations

Care must be taken in how the naphtha feed is led to the unit. Sending naphtha with olefinic or diolefinic compounds through storage tanks, even if gas blanketed, would greatly increase fouling in the reactor circuit, including the combined feed exchanger. This operating scenario should be avoided. In units – where the naphtha feed is transported to the refinery, oxygen contamination would most likely have occurred during transportation.

To minimize fouling from oxygen contamination in that situation – an oxygen stripper should be considered to remove free O2 and break down any peroxides formed. Determining the chloride level in the naphtha and makeup gas is important to CFE design. Because the unit is not water free, corrosion could be an issue on both sides of the CFE. A second concern is that the hydrochloric acid and ammonia conversion products react to form corrosive and fouling ammonium chloride salts, which precipitate out as the reactor effluent cools. As discussed above, water washing downstream of the CFE is used to solubilize these components. Because salt formation is a function of contaminant level and operating conditions, continuous wash water downstream of the CFE ensures that if the contaminant levels in the naphtha or makeup gas fluctuate, the equipment in the circuit is always protected.

During CFE design, an additional analysis is performed to determine which exchanger salts could precipitate out based on the operating conditions. An intermittent wash water injection point is included upstream of the bundle, where the salts are likely to form to allow flushing if thermal or hydraulic performance decreases. Since some liquid water is necessary to ensure proper flushing, the intermittent water washing point should not be set farther upstream than required. If detectable levels of chlorides are expected in the feed, the metallurgy selection of the unit is affected. If cost of exotic metallurgy is an issue, limiting the number of affected CFE shells could also be considered.

Evaluating Ease of CFE O&M

For NHT CFE service, horizontal shell and tube exchangers tend to be specified since they are cost effective and easily cleanable. Hydrotreating reactions are not pressure sensitive enough to benefit from the low pressure drop, high heat recovery but higher cost of vertical or welded plate style exchangers. Also, vertical and welded plate exchangers can be difficult to clean if gums or salts must be removed. The choice of whether to place the feed on the shell or tube side of a bundle is not always straightforward. Economic considerations show – higher pressure feed is placed on the shell side and higher temperature reactor effluent is placed on the tube side.

Although, it is usually more economical to put the process fluid with the highest temperature on the tube side, another consideration for the CFE is the ability to flush out ammonium chloride salts.

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