THIS MONTH'S PUZZLER
We're experiencing quality problems that we've never had before with delayed coker (DC) feed that we supply to another refinery. The feed, which is 100% residue hydro-conversion residue, takes about a day to travel via rail tank cars from our site to the other refinery. The problem is that the material pops and bubbles (anywhere in the range of 80° to 200°C) during the ignition temperature tests upon arrival — and so is rejected and sent back. We sample the DC feed before dispatch, taking material from the top of the rail tank cars, and don't observe any popping and bubbling during our ignition temperature test. We haven't changed the test methodology or the tank-car loading and unloading procedures, and we haven't cleaned the tanks recently. We've investigated and excluded the presence of water in the DC feed. Strangely, material from only some tank cars, not all, shows problems. The tank cars in question have not been used for transferring lighter materials in the past. What is causing this problem?
TACKLE TWO ISSUES
There are two possible root causes: poor operation of the coker and poor maintenance. Unfortunately, you'll have to address both to assure that you get rid of the bubbling.
Let's consider the operational changes first. For our purposes, a coker unit consists of a furnace, two duplex coker drums, a fractionating column with side-draws and a second stripping column for purifying gas oil; coker naphtha and gas are produced as the lights while light gas oil comes off the stripper. Heavy gas oil is pumped through the furnace and then to the bottom of the active coker drum; steam is injected into the heavy gas oil going through the furnace before it enters the coker drums — the heavy gas oil is recycled to extinction. Gas oil is returned from the top of the drum to the bottom sump of the fractionating column. Fresh feed is fed to the lower part of the fractionating column.
A high furnace outlet temperature increases the production of gas and naphtha-cracking while decreasing the yield of light gas oil. These gases could become trapped in the heavy oil and in the coker drum. Another effect is coking in the furnace tubes. If pushed too far, the furnace tubes could become fouled way too soon. Your operators may be sacrificing product quality by impulsively cranking up the furnace to compensate for the higher resistance from coke buildup. A higher temperature gradient across the tubes results in greater stressing of the tubes. So, where do the lights go if not out the fractionator? They're trapped in the coker drum.
One option is to increase the furnace flow velocity, thereby raising flow out of the coker drum. Because furnace heat transfer is almost entirely by radiation, this would decrease thermal stress on the furnace tubes while possibly forcing lights out of the coker.
Another option with a similar effect is to boost the fractionating operating pressure. This has its own set of rules. Increasing the pressure will mean more load on the furnace and condenser, probably more gas flow and, perhaps, less feed to the unit. But it also will mean less wear on the furnace tubes.
Lastly, it might be worth decreasing cooling in the stripping column. Cooled gas oil is refluxed into the fractionator above the feed tray to isolate the feed from the product stream and raise the initial boiling point of the product gas oil. As long as steam flow to the bottom of the stripper can be increased to heat the gas oil and separate lights, it may be worth forcing the lights out of the fractionator by using the stripper.
Now, let's consider when the coke drum becomes filled with coke and must be decoked for re-use. Usually, decoking is done with 2,000–4,500-psig water jetted into the drum to break coke loose. Normally, coke from a delayed coker appears as spongy chunks that are six inches or smaller. If larger chunks are forming, they could contain pockets of volatile compounds, including water. Take samples and evaluate directly from the source but be careful. Coking still is one of the most potentially lethal operations in oil refining.