Archive for June, 2013

Dirty Naphtha

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Naphtha products may suddenly assume a yellowish cast and then return to a normal water-white condition. If this proves to be a recurring problem, the difficulty is probably water in the top reflux. First, obtain a sample of the reflux naphtha. Does it contain free water? If so, manually drain the water from the reflux …

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Light-Naphtha End Point

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Operators may attempt to maximize heavy naphtha production at the expense of light naphtha by increasing the fractionator top reflux rate, which drops the tower top temperature. The water vapor in the overhead hydrocar­bon vapors begins condensing at its dew point. If the tower top temperature is too low, water will condense on the top …

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Raise Pumparound To Save Energy

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Raise Pumparound To Save Energy

Operators are usually more interested in making on-spec products than in saving energy. In the field one will often find pumparound rates cut back to 50% of their proper level. Try increasing pumparound circulation until the product separation is adversely affected. Perhaps an end-pont spec can be extended. Up to the point at which the …

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Improper Heat Balance

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Improper Heat Balance

Hot vapors, flowing up the fractionator from the flash zone, are partially condensed by contact with cooler pumparound liquid. The heat absorbed by the pumparound stream is used to preheat crude. As the pumparound circulation rate is increased, both heat removal from the fractionator and from crude preheat increase. This saves furnace fuel. Higher pumparound …

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Upset Tray Decks

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Although trays can corrode through, a more common cause of damage is unit upsets. A high liquid level, above the flash zone, will cause the trays to be bumped by the up-flowing vapors. Slugs of water can dislodge tray decks when the water suddenly flashes. It seems that many tray upsets occur during short unit …

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Decreased Fractionation

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Accounting for a loss in fractionation is a common troubleshooting assignment. For example, crude unit operators find that they can no longer meet furnace oil end-point specs unless they sacrifice furnace oil yield. On one unit, furnace oil production had dropped from 7,000 B/SD to 4,000 B/SD. Possible explanations for this type of problem are …

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Foamovers

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Foamovers

A large carry-over of coke or partially coked resid from the coke drum is called a foamover. Preventing such foamovers is vital to continued opera­tion of a delayed coker. Depending on the volume of material carried over, the effect on unit operability will range from bad to disastrous. Several problems are initiated by foamovers. Coke …

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Coking Cycle

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While the continuous aspect of delayed coking is straightforward, prob­lems arise in the batchwise filling and emptying of coke drums. The table below summarizes a typical cycle. While one drum of the pair is filling, the other drum is either steaming, quenching with water, hydraulically decoking, or warming with hot vapors. The most commonly encountered …

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Delayed Coker Process

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Delayed Coker Process

Coking is an old process but one that is becoming more important as the quality of the world’s crude supply deteriorates. As the sulfur, metals, and conradson carbon contents of crudes increase, coking the bottom of the barrel is looking better to many refiners. The greater part of the barrel of resid produced from a …

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Dead Time in Process Control

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The time interval (Td) in which no response of the system is detected following a small (usually 0.25% – 5%) step input. It is measured from the time the step input is initiated to the first detectable response of the system being tested. Dead Time can apply to a valve assembly or to the entire …

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