PTQ Q3 2026 Issue

Relative rate of coking

Bulk uid vs lm temperature

30.00

360. 340. 320. 300. 380. 400. 420. 440. 460. 480.

1 wt% steam 0.5 wt% steam 1.5 wt% steam Dry vacuum

25.00

20.00

15.00

1 wt% bulk 1 wt% lm

Dry-bulk Dry-lm

10.00

5.00

5 0 Tube number from shock to radiant outlet 15 20 10

5 0 Tube number from shock towards radiant outlet 0.00 15 10

Figure 4 Bulk fluid temperature versus film temperature

Figure 3 Relative rate of coking

The most important point to observe in Figure 4 is that the film temperature for the 1.0 wt% steam injection case is similar to that for the dry vacuum case. However, the gap between the bulk fluid temperature (effectively the heater coil outlet temperature) and the film temperature is much smaller in the steam injection case than in the dry vacuum case. From a process perspective, this means the refiner has the flexibility to target a higher COT with steam injection than in the dry vacuum case, without exceeding the max - imum allowable film temperature limit. Higher COT trans - lates into higher distillate yield, a sought-after aspect of many vacuum units. Considering all these bespoke observations, do many vacuum heaters have multiple steam injection points? Which one should be used, and how much steam should be injected at each point? Location of steam injection Evaluating ‘where’ to inject steam will help the end user reap the maximum benefits from the investment made in this important utility. For this, three scenarios have been analysed again. First, complete steam is injected at the inlet of the convection section itself. Second, complete steam is injected at the convection to radiant crossover section. Third, 50% steam is injected at the crossover and 50% at the inlet of the 8 inch tube, where the tube velocity is reduced for the first time within the radiant section. In all three scenarios, the rate of steam injection has been kept constant. In addition to the three scenarios, an ‘aber - ration’ scenario is simulated, wherein it is assumed that the steam injection nozzle at the crossover location is choked due to coking and the complete steam flow is directed to the 8 inch coil. However, the refinery operator is unaware of this, since the common flowmeter still indicates the requisite amount of steam being injected into the RCO. The impact of these cases is shown in Figure 5 for the res- idence time inside the heater and in the radiant section. As can be seen, the residence time of RCO in the radiant section is minimum in the case of steam injection in the convection inlet coil or for the crossover location. Thus, understanda - bly, the rate of coking is minimum when velocity steam is injected at either the convection inlet or crossover location. Here, the typical assumption is that the major coking forma - tion starts in the radiant only.

is increased from 0.5 to 1.0 wt%. However, there is little significant change when the rate is further increased from 1.0 to 1.5 wt%. Higher steam injection rates can also affect the downstream column operation and the vacuum system. Hence, care must be taken while evaluating this aspect. Now that the degree of vaporisation is established, the next topic of interest is the film temperature shown in Figure 2, which clearly indicates that the film temperature for the 0.5 wt% case is higher than that of the 1.0 wt% case. Also, as expected, since the vaporisation impact evens out when moving from 1.0 to 1.5 wt%, film temperature impact also evens out. However, one intriguing observation at this point is the similarity in film temperature between the optimised 1.0 wt% steam injection case and the dry vacuum case. If that is the case, how is steam injection beneficial? Readers may wish to frame their opinion while progressing to the next section. It is prudent at this juncture to analyse the last section on the rate of steam injection (i.e., the relative coking rate), as shown in Figure 3 , where the relative rate of coking plot provides a qualitative insight into the impact of vaporisa - tion and film temperature, as explained in earlier sections. The repercussions of the actions initiated in Figure 1 are felt in Figure 2 and finally verified in Figure 3. As shown, the relative rate of coking is appreciably lower throughout the high-temperature and high-flux radiant section zone for both the 1.0 wt% and 1.5 wt% cases, as compared to the 0.5 wt% case. However, the rate starts to increase towards the end as the tube size expands from 6 to 8 inches, then to 10 inches, which reduces the heat transfer coefficient in these larger tubes and increases the coking rate. A partial answer to the earlier observation on the film temperature for dry vacuum units can be inferred from this plot. The rate of coking in the plot with dry vacuum is substan - tially higher than in the plots with steam injection. Although the rate of coking is a relative parameter and cannot be directly correlated with absolute figures, tripling the cok - ing rate for the dry vacuum case as compared to the 1.0 wt% case directionally indicates that the run length can be reduced by a third for the dry vacuum case as compared to that with 1.0 wt% steam injection.

PTQ Q3 2026