Coke Drum Pressure

The main effects caused by changes in the pressure of the coke drum are the following  

Since refinery economics reqnire operating at minimum coke formation, the new 

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Heater outlet temperature, Coke drum pressure, psig °F 900-950 15-90... 

Characteristics of feedstock quality, recycle ratio, and drum pressure affect the coke yield. Highly aromatic feedstock contains more carbon per feed volume and typically produces a high coke yield. Heavy coker gas oil can be recycled back into the coker feedstock to help improve the coke yield. Also, increasing the coking drum pressure tends to increase the coke yield. Typically, a higher coke yield results in a reduced liquid product yield. 

Three operating control variables in a delayed coker dictate the product quality and yields for a given feedstock. These variables are the heater outlet temperature, coke drum pressure and the ratio of recycle to fresh feed. 

A decrease in pressure has the effect of vaporizing more heavy hydrocarbons. As the production of desirable liquid hydrocarbons is increased at low pressures, the coke yield is correspondingly decreased. Thus, most modern delayed cokers have been designed to operate at a low coke drum pressure. 

To provide an example of how this matrix can be defined. Fig. 4 illustrates some logics applied to a Petrobras DCU module. This illustration presents some of the permissive logics defined for this Unit, i.e. conditions that should be satisfied before the command to open or to close a valve is allowed to be executed. These conditions can be based either on other valves positions or on process parameter values (e.g. a permissive logic where the coke drum pressure must be lower than a pre-determined value to allow a valve to open). 

This all shows the importance of examining the coke drum pressure recorder chart when troubleshooting a foamover. This chart is analogous to the fingerprints left at the scene of a crime. The process engineer should be able to tell from the pressure recorder the portion of the drum cycle that was taking place at a given time. 

The formation of cross-linked aromatic rings can be reduced by lowering the coke drum pressure, raising the coke drum outlet temperature, improving the vacuum tower operation, and by adding steam to the coke drum. 

Other ways to improve needle coke quality include minimizing steam in the healer passes, reducing the gravilily of the recycle gas oil, and raising the coke drum pressure. 

Using automatic water addition and an adequately sized pump permits a proper water level to be reached in two hours. The water in the drum is boiling, so occasional water addition may be required to hold a proper level. The drum s radiation instrumentation is used to follow the water level. The water spreads out from the center of the drum at a rate that has been measured at 8-10 ft/hr. For a 20-ft diameter drum, the water must sit in the drum for an hour before it is drained. For the last 10-20 minutes of this period, the coke drum overhead should be diverted from the enclosed blowdown system to a large-diameter (4 in. for a 20-ft drum) atmospheric exhaust. This will minimize the coke drum pressure when the top head is unbolted. 

The process engineer indicated that the capacity limit on the coker was coke drum pressure. He indicated that he was busy with an economic analysis of the coking unit and couldn t spend much time with me on the problem. He did say that incremental coking capacity at the time was worth 5.17/bbl. He suggested I work with one of the day-shift foremen. 

Before contacting the day-shift foreman, 1 reviewed the unit log sheet (Fig. 3-3). It was apparent from the log sheet that the high coke drum pressure was caused by an excessively high pressure loss upstream of the wet gas compressor, combined with limited capacity of the compressor to handle the uncondensed coker vapors. 

Each 8 psi reduction in coke drum pressure reduces coke yield on feed by 1.0 wt%. 

Pressure surges on switchover or during drum filling may initiate a foam-over. On one unit a faulty combination lower level controller allowed the resid level to rise above the vapor inlet nozzle. As this nozzle was submerged, back pressure against the coke drum increased. After some lime the control room operator noticed the high level. He rapidly pulled the resid level down. The sudden drop in coke drum pressure, as the resid dropped past the vapor inlet nozzle, started a coke drum foamover. 

The proper method to control drum foamovers during a rapid switchover is to control coke drum pressure by pinching back on the outlet vapor valve to maintain the pressure in the full coke drum. While the operator at the switch valve is diverting the feed to the empty drum, he observes the full coke drum pressure. As this pressure falls during the switch, he closes off the vapor outlet valve from the full drum to hold the pressure within 5 psig of normal operating pressure. Using this technique, the switchover can be accomplished in about 20 minutes. 

High coke drum pressure Plugging combination tower bottoms screen Coked heater tubes Coke lay-down on trays Plugged blowdown system... 

Coke yield decreases by reducing coke drum pressure. 

These correlations are based on the following conditions coke drum pressure of 35-45 psig, feed is straight-run residue, final boiling point of gas oil and gasoline of 468°C-496°C and 204°C, respectively. All the correlations proposed by Gary and Handwerk (2001) are function of the CCR of the feed and can be used as first approximation and for preliminary studies. 

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