Heater tube coking

It is now well established (2-4) that the oxidation of carbon can be catalysed by certain metals and inorganic compounds although information is less comprehensive for water vapour. As a consequence the effectiveness of a number of elements in promoting the oxidation of the radiant heater tube coke in this environment has been surveyed. 

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

Preventing Heater Tube Coking Field Testing Alarms Trips... 

In the refinery the salts deposit in the tubes of exchangers and reduce heat transfer, while in heater tubes, hot spots are created favoring coke formation. 

When overheated, hydrocarbons tend to breakdown, leaving carbon residues (coke). This coke builds up on the inside of the heater tubes, slowing the transfer of heat from the tube walls to the product by restricting the flow of product and acting as an insulator. As the control system attempts to maintain the process outlet temperature at the setpoint, the fuel valves will open and the tubes subjected to an increased heat load. With the diminished ability of this heat to be transferred to the process fluid, the temperature of the tubing will increase. 

In the process (Figure 9-8), the feedstock and recycle hydrogen gas are heated to reactor temperature in separate heaters. A small portion of the recycle gas stream and the required amount of additive are routed through the oil heater to prevent coking in the heater tubes. The outlet streams from both heaters are fed to the bottom of the reactor. 

For fired heaters subject to creep problems, make sure that the tube metal temperature was considered in materials selection, hi the absence of better information, assume the fireside temperature is 100°F (38°C) higher than the process temperature. (If tube-side fouling is anticipated [e.g., coke formation], assume the tube metal temperature is 150°F [85°C] higher than the process temperature.) If necessary, make a note on the template to ensure that creep is accommodated during design of heater tubes, in accordance with API 530 [23]. 

The tube thinning follows the same pattern of flux distribution. Figure 5.4 shows a fired heater tube with severe thinning creep caused by internal coking especially on... 

The types of problems a fired heater or furnace system typically encounters include flame impingement on tubes, coke buildup inside the tubes, hot spots inside the furnace, fuel composition changes, burner flameout, control-valve failure, and feed-pump failure. 

During normal operations, checklists and samples are collected as advanced instrumentation monitors the process. The types of problems a fired heater or furnace system typically encounter include flame impingement on tubes, coke buildup inside the tubes, hot spots inside the furnace, fuel composition changes, burner flameout, control valve failure, and feed-pump failure. Other problems may include incorrect temperature indicator readings, failure of oxygen analyzers, oxygen leaks on the furnace, and the unexpected shutdown of downstream equipment. A fired heater system is designed to run almost continuously, 24 hours a day, 7 days a week. The operational team is in place to ensure that the equipment and systems operate safely, effectively, and produce a quality product that meets or exceeds customer expectations. 

Coking heater tubes are coked. A sudden loss in heater feed will lead to coking of the tubes. If the firing rate is rapidly reduced, severe tube damage may be avoided. One can count on the need to steam-air decoke the tubes after the coker is forced down because of a foamover. 

Alonizing coking heater tubes Teflon-lined pipe 316(L) in vacuum towers... 

Most coking heaters have several thousand pounds per hour of high-pressure steam injected into the heater tubes. The velocity steam should increase the linear velocity of resid in the lubes, shorten oil residence time, and hence reduce the thermal soaking time to which the resid is exposed. This ought to reduce the formation of coke in the heater tubes. In practice, there seems little correlation between the amount of velocity steam used and the rale at which coke builds up in the lubes. 

Newer heaters typically have thin reflective tiles, rather than massive refractory brick walls. Such newer heaters will heat up more rapidly. Also, the process fluid outlet temperature responds more rapidly to changes in the firing rate. This improves the heater outlet temperature control. Perhaps for this reason, it seems that heaters with reflective refractory walls are less subject to process tube coking and shortened heater run lengths. There is also a process, called "alonizing," that increases the reflectivity of older brick refractory heater walls. 

Sodium content. High amount of sodium can increase the rate of coking in the heater tubes. 

Increase in the production of gas and gasoline, less distillate, less deposition of coke in heater tubes... 

Figure 1 shows a schematic (elevation) of the Plastofrost apparatus as modified for the present study. The two main components are the heater and the coking attachment. The heater consists of a nickel-plated copper slab in which four 300 watt cartridge heaters are enclosed. A chromel/alumel thermocouple insulated with ceramic tubing placed 5 mm beneath the top surface of the slab measures the temperature (see Fig. 1). The bead of the TC is at the centre of the slab. Figure 2 is a photograph of the assembled apparatus.

All the obstacles in the path of distillation progress, however, were not equipment fabrication and design problems. It was discovered very early in the running of sour crudes that the shell still corroded severely at the vapor-liquid interface line and in that portion of the shell in contact with vapors. At the same time severe corrosion in pipe stills and tube stills, along with overheating and coking, resulted in expensive equipment failures. These problems started metallurgists on a chain of developments which produced the corrosion- and heat-resistant alloys used in modern oil heaters and the alloy liners used in distillation columns. 

If this problem—the sudden loss of flow, followed by the premature restoration of flow—occurs repeatedly over a period of a few hours, then layers of fouling deposits or coke are accumulated inside the tubes until a heater shutdown becomes unavoidable. This sort of failure is called a stuttering-feed interruption. 

An isothermal reactor concept incorporating a ceramic membrane is more attractive compared to an adiabatic reactor concept from a thermodynamic point of view. In this concept we assumed a reactor with reactor tubes located in a direct-fired heater and operated in a cyclic way to remove coke formed on the catalyst. Parallel bed and heaters have been assumed [35-37]. 

Delayed coking is the only main process in a modern petroleum refinery that is a batch-continuous process. The flow through the tube oven is continuous. The feed stream is switched between two drums. One drum is filling with coke while the other drum is being steam-stripped, cooled, decoked and warmed up (see chapter 6). The overhead vapors from the coke drums flow to a rectification unit. The rectification tower has a reservoir in the bottom where the fresh feed is combined with condensed product vapors (recycle) to make up the feed to the coker heater. 

---