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Sep-2006

Main fractionator water wash systems

When properly designed and operated, main fractionator wash water systems can remove salt with little upset

Christopher F Dean, Saudi Aramco
Scott W Golden, Process Consulting Services
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Article Summary
Main (fractionator) column water wash systems are sometimes needed when producing heavy naphtha product to ensure the FCC feed rate and gasoline quality can be met throughout the run. Heavy naphtha product draws are used to segregate the high sulphur portion of the gasoline or to reduce the liquid load through the gas plant. This is becoming more prevalent due to the production of propylene for petrochemical feedstocks. Consequences include a lower main column overhead temperature, higher wet gas rate and reduced gas plant propylene recovery. Lowering the overhead temperature can lead to ammonium chloride deposits on the trays or in the packing. These salts must periodically be removed; otherwise, FCC capacity will be reduced. 

Heavy naphtha product draws and pumparounds
Figures 1 and 2 show two heavy naphtha product draw systems. The product in Figure 1 is withdrawn from the same location as the pumparound, which is the most common arrangement. Pumparound temperature is set by the heavy naphtha product rate. As the overhead product naphtha rate decreases and the heavy naphtha product draw increases, the draw temperature drops. When cold-feed preheat or gas plant C3/C4 splitter and stripper reboiler services use the heavy naphtha pumparound heat, a reduction in the draw temperature can create problems in each of these systems. Existing exchanger surface areas or pump capacity are often insufficient to meet the required duties. It is common during revamps to have to add pump capacity and increase the exchanger surface area due to higher product draw rates.

However, in many cases, modifying the flow scheme eliminates or minimises the pumparound system investment. Heavy naphtha product can be withdrawn above the pumparound (Figure 2), allowing existing equipment to be reused without modifications. This permits a higher product draw rate without lowering the pumparound draw temperature. The internal liquid rate above the pumparound is sometimes less than the targeted heavy naphtha product yield. In these instances, it is necessary to use a dual draw system, with the majority of product yielded above the pumparound and a smaller amount from the pumparound draw tray. This maximises the pumparound temperature.

Salt formation
Localised temperatures, HCl and ammonia concentrations, and water dewpoint all contribute to salt formation. Ammonium chloride salts are deposited after condensed water has absorbed the ammonia and HCl, and subsequently the water vapourises. Water forms when localised temperatures are below the water dewpoint, which depends on the quantity of water present in the column overhead vapour and operating pressure. High concentrations of water reduce the dewpoint temperature. Water content of the overhead vapour depends on the feed nozzle atomising steam, reactor stripping steam and main column steam, with rates varying dramatically between units. 

Column overhead vapour HCl and ammonia concentrations play a central role. FCC feeds containing high chlorides are mainly unhydrotreated atmospheric and vacuum residue containing residual inorganic salts (MgCl, CaCl and NaCl) from the crude unit desalters, and purchased gas oil contaminated with seawater. As overhead vapour HCl and ammonia concentrations increase, salt forms at higher temperatures. Severely hydrotreated feeds contain small amounts of nitrogen compounds and almost no chlorides. Therefore, overhead vapour ammonia and HCl concentrations are low. High chloride feeds generate large amounts of HCl, so the column overhead temperature may need to be as high as 265°F to avoid salt formation, whereas low chloride feeds can operate as low as 235°F without rapid salt formation.

Reflux temperature and rate influence the rate of salt deposition because they set localised temperatures. Reflux temperatures vary from 75–120°F, depending on ambient conditions and the condenser system design. Reflux rate depends on column heat balance. Overhead vapour temperature may be 240–300°F, yet the liquid temperature leaving the top tray may be only 150-180°F. High rates and cold reflux reduce the local temperature. Recognising the difference between localised liquid temperatures on the top trays and measured vapour temperatures is essential to understanding the mechanism of salt formation.

Heat is exchanged when cold reflux and hot vapour mix on the tray. Figure 3 depicts the change in liquid temperature across a one-pass tray. In this case, reflux enters at 90°F, heat (and mass) is exchanged between the vapour and liquid, and liquid leaves at 160°F. A portion of the vapour entering the top tray is condensed. Localised temperatures below the water dewpoint contribute to salt formation because the water absorbs HCl and ammonia from the vapour. Water eventually vapourises because temperatures increase as liquid flows down the column, and salt is deposited.

Column overhead temperature
Column overhead temperature is set by operating pressure, the amount of water and overhead naphtha product endpoint. A question often asked is “What is the maximum percentage of FCC gasoline that can be withdrawn as heavy naphtha?” The answer depends on the rate of salt deposition that is tolerable. If the rate of salt laydown is high, there needs to be an effective way to remove it; otherwise, the tray will plug, eventually causing flooding.

Refiners yielding 20% or more of the gasoline as heavy naphtha will likely have high rates of salt deposition unless the feed has no chlorides. Those main columns that have operated problem-free for years at 215°F or lower have well-designed water draw trays and good operating procedures. One refiner has been operating at below 190°F for several years (Figure 4), but continuously withdraws water during normal operation. Hence, salts never build up on the trays. Yet, it is essential that all free water be withdrawn; otherwise, water containing HCl and ammonia will flow down the tower. This is difficult to remove when it deposits below the water draw tray.  
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