Puzzles encountered in atmospheric pressure column simulation
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When performing simulations of atmospheric distillation columns in Pro2, there are several side streams from which products are obtained. During setup, the performance specifications were used to define the requirements for all these side stream products as well as the product at the top of the column; however, no requirements were set for the heavy oil at the bottom of the column (and of course, it’s not possible to set such requirements due to constraints related to the degrees of freedom). It turned out that the TBP5% value of the bottom product did not meet the required level, being 650F, whereas the required value was 695F. To address this issue, I reduced the setting related to vaporization degree: specifically, I set the crude oil feed rate to 23, the stripping steam feed rate to 25 on the last tray, and the extraction point for the last side stream to tray 18. As a result, I reduced the volume ratio of the liquid flow at tray 22 to the crude oil feed rate from 0.03 to 0.02. Yet, this approach did not increase the TBP5% value of the bottom product as I had hoped; instead, it decreased. Why is that? As I understand it, if the degree of vaporization decreases, then the amount of light products flowing to the bottom of the tower (relative to the products at the tower bottom) should decrease; this should allow the TBP5% value of the products at the tower bottom to increase, right? Where did I go wrong in my understanding? 2 To still achieve the aforementioned goal of increasing the TBP5% level of the product at the bottom of the tower, I increased the amount of steam used for stripping at that location, thereby carrying away more light components; this approach did help to increase the TBP5% level at the tower bottom. The second point is mainly to share this with everyone; I also ask everyone to pay close attention to point 1.
The main issue is that the amount of steam used for bottom stripping is too low. :)
I know that increasing the gas at the bottom of the tower can achieve the goal. Regarding point 1, after some thought, I think a possible reason is this: when I reduce the degree of superheating, although the amount of light fraction that flows from the distillation section to the bottom of the tower eventually decreases, at the initial stage of reducing the degree of superheating, the amount of light fraction that flows directly from the feed plate to the bottom of the tower increases, and this effect is greater than the previous one; that’s why such a situation occurs. I wonder what everyone thinks?
With a low degree of superheating, the light oil yield decreases, and the TBP5% value at the bottom of the tower is definitely low. To increase or decrease the degree of superheating, it seems that one needs to increase or decrease the temperature of the oil entering the tower. How is the poster’s approach specifically implemented in pro11? I’m not quite sure. Regarding the second point, the addition of gas vapor reduces the partial pressure of oil and gas; as a result, there are fewer light components at the bottom of the tower, so the TBP value at the bottom must be high. Last edited by *jun on 2009-3-15 15:44.]
"If the degree of vaporization decreases, then the amount of light products that flow to the bottom of the tower (relative to the total products at the bottom) should decrease. The amount of light products flowing to the bottom does decrease, but their composition becomes lighter; it is not possible to determine the ultimate outcome of these changes simply by looking at them superficially； So let’s think about it from another perspective: when specifying the quality of the product to be obtained, an increase in the superheating rate indicates an increase in the ratio of steam entering at the inlet to the amount of feed. With the same quality of product being achieved, an increased superheating rate leads to an increase in the volume of product produced; as a result, more qualified light components are distilled out as product at the bottom of the tower. In PROII, it is possible to specify the superheating ratio directly in the design specifications; this ratio represents the flow rate of liquid on the plate above the feed plate divided by the feed flow rate. The factors that directly control the superheating ratio are likely to be the feed temperature and the amount or temperature of the stripping steam. This post was last edited by LTY on 2009-3-15 19:09.]
“In PROII, it is possible to specify the superheating ratio directly in the design specifications; this ratio represents the flow rate of liquid on the plate above the feed plate divided by the feed flow rate. The factors that directly control the superheating ratio are likely to be the feed temperature and the amount or temperature of the stripping steam. ” I think it’s like this: I set the degree of superheating just as you said. After reducing the degree of superheating, (1) although the amount of superheated vapor decreases and thus the amount of light components that reach the bottom of the tower is reduced, (2) the amount of light components that come from the crude oil and reach the bottom of the tower increases at first ； Finally, the effect of changes in vaporization degree on the bottom product of the tower should take into account the effects of these two factors together. Therefore, the result I obtained – \"decrease in vaporization degree and increase in light components at the bottom of the tower\" – is due to the dominant effect of point (2). That is what you mean by “it’s impossible to judge directly from the surface,” right? Additionally, in the example of an atmospheric pressure column that comes with pro2, I saw a heater installed on the feed plate to adjust the degree of superheating; I’m wondering how this is handled in actual production? In practice, is the degree of vaporization adjusted outside the tower in a heater, or inside the tower, for example by installing a heater at the feed plate? Please give me some advice. This post was last edited by huanglei on 2009-3-16 10:45]
As the degree of vaporization decreases, it is equivalent, from a certain perspective, to a decrease in the reflux ratio; as a result, the separation efficiency declines. Therefore, the content of light components at the bottom of the tower increases
There are specific requirements for the setting of the vaporization amount: 2% to 4%
1. As the vaporization rate decreases, the liquid phase ratio of the oil-liquid mixture coming from the self-rotating oil line increases. As a result, the amount of internal (or external) reflux below the lowest side line is reduced, which means that there is insufficient pressure on the liquid phase; consequently, the initial boiling point of the crude oil decreases. This is in line with general distillation theory. 2. Increasing the amount of stripping steam reduces the partial pressure of light hydrocarbons in the feed entering the tower; this allows for an increase in the recovery rate while effectively controlling the initial boiling point of the atmospheric residue. 3. Of course, if possible, the trays commonly used in the stripping section can be replaced with packing, thereby increasing the number of theoretical plates and operational flexibility. This approach also takes into account the large variations in the gas load in the stripping section, and it enables effective control of the initial boiling point of the crude bottom oil. This post was last edited by kaitian007 on 2009-3-17 12:37.]
Reply to post by CCaniggia on floor 7: Very good understanding. Also, to the original poster: For the simulation of atmospheric pressure columns, do you define the product quality directly? The fraction with a temperature of less than 350° at the bottom of the column should account for no more than 5%; the degree of over-vaporization can be controlled by adding a heater to the feed plate, but the heat generated by this heater still needs to be returned to the heating furnace. It’s also possible to use a controller to adjust the outlet temperature of the heating furnace in order to achieve an over-vaporization rate of 2–4%
What I did was carry out a preliminary design of the tower, rather than optimizing it; therefore, I simply defined the products that were to be obtained at the top of the tower and along the side streams.
You can use product quality as the model specifications. The 95% point of the AGO should be set based on the capabilities of each vacuum tower section; that is, it should be optimized taking into account the limitations imposed by the vacuum heater outlet temperature, the vacuum level, and the capacity of the vacuum pump. The crude oil treatment tower needs to be optimized alongside the vacuum towers, and this optimization should be completed through proper heat integration with the heat exchanger network, either using the pinch method or, more rigorously, by employing mixed integer linear programming. This is just a suggestion