Automatic Water Filling to Shorten Coking Cycle
Once the switch has been completed and a small flow of steam (4,000-5,000 Ib/hr for a 20-ft diameter drum) has been estabished, the vapor valve on the full drum will be slowly reopened. Meanwhile, the operators should be lining up the cooling water to the coke drum. For the short coke drum cycles to be maintained, it is necessary to skip the standard “big steam” portion of the cycle. Within 30 minutes of the switch, the flow of water to the coke drum should be established. The flow of water must be controlled automatically as follows:
1. The water rate is increased according to a predetermined time vs flow curve.
2. If the pressure of the drum being cooled increases too fast, the flow of water is restricted.
Figure 2-7 shows the required controls. A plastic sheet cut in the shape shown in Figure 2-8 to serve as a cam-type operator suffices to guide the water flow valve. An excessive increase in coke drum pressure delays the progress along this curve. Careful control of the coke drum pressure during quenching is a major factor in preserving drum mechanical integrity at reduced cycles. The automated device described does an effective job in controlling this pressure.
For the first 45 minutes, the flow of water will increase from 25 to 75 gpm. The entire flow will flash to steam and create a substantial back pressure in the coke drum. During this period, the coke-drum overhead should be diverted from the fractionator to the blowdown system.
Note that 1-2 wt% of the coke will be stripped out as gas-oil vapors. Once the flow of water markedly increases, the coke drum is no longer hot enough to turn all the water to steam and the amount of hydrocarbons being removed from the coke is greatly reduced.
The pump that supplies cooling water to the drum should be sized for 1,500 gpm for a 20-ft diameter drum. Coke drums venting to enclosed blowdown systems are not overflowed. A water level is established 10 ft above the coke level in the drum.
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.
Sometimes the radiation level detector (especially when the more sensitive neutron back-scatter devices are used) will falsely indicate that the water level in the coke drum has risen to the correct level. Apparently, the level detector is fooled by the increasing density of steam as the coke cools.
One can ascertain when a coke drum has been drained without adequate water quenching by noting the appearance of the drain water. If 10 minutes after draining, the water comes out steaming hot, and 20 minutes later, steam and water blow out the drain line together, one can be sure that the drum was never fully quenched. The usual reason for this is that the bed of coke was never completely covered by water. The drained water should get steaming hot (i.e. about 160°F) just before draining is completed— not shortly after draining is initiated.
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