JP3154162B2 - Distillation tower with tray - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a distillation column having a plate with high self-control.

[Conventional technology]

It is well known that petroleum refining, petrochemical, general chemical industries, and the like use a distillation column having a plate (hereinafter referred to as a plate distillation column) as a means for separating a plurality of components having different vapor pressures. According to FIG. 5, the shelf 2 of the tray distillation column 1
Will be described. The shelf 2 is provided with a weir 4 so that the liquid stays on the shelf, and the liquid overflowing from the weir 4 flows down from the downcomer pipe 6 to the lower shelf 2. In addition, the vapor from the lower shelf 2 (which contains more low-boiling components) rises in the space between the shelves 2 and rises slightly upward from the weir 4 into a nozzle-like rising pipe 8 protruding above the shelf 2. Then, it enters the bubble bell 10 and rises in the liquid from the slot 12 as a bubble. During this time, the liquid and the vapor come into contact with each other to perform a distillation operation. Then, all the vapor from the top stage is condensed by a condenser, a part of the vapor is refluxed to the tray distillation column 1, the remainder is taken out as a distillate, and the liquid at the bottom is circulated with the reboiler 38. To heat and take out the canned liquid to the outside.

The specifications of the tray distillation column 1 are designed in consideration of so-called tray dynamics such as pressure loss, flooding, entrainment, and liquid dispersion so that stable operation can be obtained under the required operating conditions. You.

[Problems to be solved by the invention]

By the way, during the distillation, when disturbance such as fluctuating the control conditions is performed, or when an operation such as changing the set liquid composition of the detection shelf is performed, the quality of the distillate and the canned liquid is disturbed, and until the quality becomes stable. However, there is a problem that poor quality products are leaked. Therefore, conventionally, in order to improve such characteristics,
Various control means such as a decoupler method and a pairing method have been developed.

FIG. 6 shows a control system of a tray distillation column of a non-interference control system by a decoupler method. In the figure, a portion 12 surrounded by a dotted line indicates a dynamic characteristic of the distillation column itself, a suffix 1 indicates a column top, a suffix 2 indicates a column bottom, r indicates a composition set value, and c indicates a liquid set value. Represents the composition. That is, c ₁ (S) represents the top composition, and c ₂ (S) represents the bottom composition. m ₁ (S) is a process variable for controlling the top composition, for example, the reflux amount, and m ₂ (S) is a process variable for controlling the bottom composition, for example, the steam flow rate supplied to the reboiler. K ₁₁ g ₁₁ (S) is the transfer function of the diagonal element loop 14 of the top composition, K ₂₁ g ₂₁ (S) is the transfer function of the non-diagonal element loop 16 of the bottom composition, K ₂₂ g ₂₂ (S) represents the transfer function of the diagonal element loop 18 of the bottom composition, and K ₁₂ g ₁₂ (S) represents the transfer function of the off-diagonal element loop 20 of the top composition.

In FIG. 6, the top composition c ₁ (S) from the top is fed back through the feedback loop 22 and compared with the top composition set value r ₁ (S). ₁ Change (S). As a result, a desired change in c ₁ (S) is generated at the top according to the transfer function K ₁₁ g ₁₁ (S), and a non-diagonal element loop is formed at the bottom.
16 through disrupt c ₂ (S) in accordance with the transfer function K ₂₁ g ₂₁ (S).

Therefore, until this change occurs,
Even if the state of r ₂ (S) = c ₂ (S) is maintained, the disturbance is fed back through the control loop 24.
Thus, the reboiler feed steam volume m ₂ (S) is modified to cancel the disturbance and match r ₂ (S), the result being that the transfer function K ₁₂ g through the off-diagonal element loop 20
₁₂ An effect corresponding to (S) appears at the top of the tower. As described above, between the top and the bottom, a change in the composition of the top (or the bottom) results in a change in the composition of the vapor and the overflow, and the bottom and the top each other. There is a problem that the change is amplified due to the influence. If the effect of the off-diagonal element loops 16 and 20 is large, the self-controllability of the tray distillation column is reduced.

The decoupling method uses this off-diagonal element loop 16,2
This is a control method in which the above-mentioned problem is solved by canceling the influence of 0 by a control operation. The control is, for example, By adding the control operation obtained in the above to the normal feedback control operation, respectively, the non-diagonal element loops 16, 20
Can negate the effects of However, this decoupler method has a disadvantage that it cannot be performed without knowing the transfer function accurately. Therefore, in actual application, some means for ensuring certainty is required.

The pairing method using the relative gain as an index is the sixth method.
The search for a combination of the control variable and the manipulated variable in which the process gain of the off-diagonal element loops 16 and 20 in the figure is small, that is, pairing such that K ₂₁ * K ₁₂ ≪K ₁₁ * K ₂₂ is performed, Diagonal element loop 16,2
A combination of a control variable and an operation variable that minimizes the effect of 0 is obtained. However, this method has the disadvantage that it is necessary to obtain each process gain K and that the dynamic characteristics of the process are not taken into account, and in the same way as described above, means for ensuring reliability is required. .

The present invention focuses on the time characteristic of a transfer function, changes the liquid hold-up, enhances self-control, and suppresses the interference between the top composition and the bottom composition. It is intended to provide.

[Means for solving the problem]

In order to achieve the above object, the configuration of the distillation column having the shelf plate of the present invention includes a detection stage for detecting the distillate composition and a detection stage for detecting the composition of the bottom liquid, each of which is disposed between the detection stages. The liquid hold-up amount of the shelf is larger than the hold-up amount of the other shelves.

As means for changing the liquid hold-up amount,
For example, it can be performed by changing the height of the weir provided on the shelf, or increasing the liquid depth on the shelf.

A detection stage for detecting the distillate compositions, the liquid hold-up volume Q _D of the respective shelves which are arranged sandwiched between the detection stage for detecting the cans out solution composition, other shelves liquid hold-up volume Q _S The value to be increased is not particularly limited, but generally, Q _D
It is preferred that / Q _{S be} 1.1 to 3.0. When Q _D / Q _S is less than 1.1, the difference from Q _D / Q _S = 1 (conventional value) is small, and a sufficient effect cannot be obtained, and when Q _D / Q _S exceeds 3.0, stable. A problem arises in securing operations.

The configuration in which the liquid hold-up amount of each shelf sandwiched between the upper and lower detection stages is larger than the liquid hold-up amount of the other shelves is a disturbance, that is, the composition of the overflow liquid flowing down from the upper stage and the lower portion. The vapor composition rising from the stage acts to improve the self-controllability, which further reduces the effect on the liquid composition of the shelf. Further, the configuration in which the liquid hold-up amount of the upper detection stage and the shelf above it, and the liquid hold-up amount of the lower detection stage and the shelf below it is relatively smaller than the liquid hold-up amount of the shelf sandwiched therebetween. It acts so as to be sensitive to a situation change, that is, a change due to a disturbance, a change in a set value, or the like, and to reflect the change in the control operation.

The present invention can be applied to multicomponent distillation as well as binary distillation.

Control factors for controlling the quality of the distillate and the canned liquid are not particularly limited, and temperature, viscosity, specific gravity, analytical values, and the like can be appropriately used as in the related art.

〔Example〕

Hereinafter, the distillation column having the plate tray of the present invention will be specifically described by Examples 1 and 2, with reference to the attached drawings.

Referring to FIG. 1, a tray distillation column 1 of Examples 1 and 2 will be described. The tray distillation column 1 shown in FIG.
Figure in illustrates the respective shelves 2 ₁ to 2 _14, when collectively referred to are simply indicated by 2), the raw material liquid is supplied to the middle of the solution feed stage 2 ₆ from the raw material supply pipe 26, the fourth stage a shelf 2 ₁₁ of the shelf 2 ₄ and 11 stage using the composition detection stage. The distillate consisting of low boiling components, liquefied vapor on the shelf 2 ₁ uppermost in the condenser 28, temporarily stored in reflux tank 30, and flows back to the shelf 2 ₁ part through the reflux pipe 32 The other is taken out from the distilling tube 34 as an effluent. In addition, the canned liquid comprising the high-boiling components is taken out from a canned pipe 36 provided at the bottom of the tower, and the liquid at the bottom is heated by a reboiler 38 as a heat source.

In the present embodiment, detected by the temperature detection shelf 2 _4, 2 ₁₁ liquid composition detection for quality control, the control of the reflux liquid amount is controlled by the control valve 40, also the heating of the reboiler 38 by the control valve 42 The amount of steam for use was controlled. The supply amount of the undiluted solution is controlled by a control valve 44, and the distillate flow rate and the canned liquid flow rate are controlled by a control valve 46, respectively. The QC in Fig. 1
Denotes a liquid composition detector (in this embodiment, a temperature detector), FC denotes a flow rate controller, and LC denotes a liquid level controller.

The weir heights of the tray distillation columns 1 of Examples 1 and 2 were set to the values shown in Table 1, respectively, and the same was used for all others. A comparative example was performed in the same manner as in Examples 1 and 2, except for the tables.

When the weirs shown in Table 1 were used, the liquid hold-up amount of each shelf was the value shown in Table 2.

The control uses the PI control method, and the tuning of the parameter is performed by the Biggest Log Modulus Tuning method (In
d.Eng.Chem.Res., Vol.27, No.6, 1988, page.969).

FIG. 2 shows the results obtained by actually measuring changes in the top composition and the bottom composition with respect to time when the set value of the top composition was changed, and obtaining the dynamic characteristics of the tray distillation column of this example. (Top composition) and FIG. 3 (column bottom composition). The horizontal axis in FIGS. 2 and 3 is the time axis, and the vertical axis represents the low boiling point component compositions at the top and bottom of the column, respectively.

In Fig. 2, the top composition change when the initial set value is suddenly changed from 0.800 to 0.900 (curve a in the figure) (Fig. 2)
Example 1 shows the temporal variation between the change in the bottom composition and the change in the bottom composition (FIG. 3).
(Curve △), Example 2 (curve □) and Comparative Example (curve）)
ISE (integrate)
l of absolute value of error), IAE (integral of
squared error) method and ITAE (integral of time mul)
Table 3 shows the results of the evaluation by the tiplied by absolute value of error method.

In FIG. 2, when the set value is changed as shown by a curve a, the composition of the top changes in such a way as to follow this in any of Examples 1, 2 and the comparative example, and once the value exceeds the set value. After that, it changes by drawing a curve that converges to the set value. In this case, in the comparative example, the rise was faster, but all converged to the set value after approximately 7 minutes, and no substantial difference was observed.

On the other hand, the bottom composition is affected as shown in FIG. The magnitude of the change decreases in the order of Comparative Example, Example 1, and Example 2, and it can be seen that the influence of the liquid hold-up amount is clearly large. This result is clearly shown in the evaluation results shown in Table 2.

As is well known, the dynamic characteristic function term g of the transfer function can be expressed by the following equation.

Here, the suffixes i and j represent 1 or 2, τ _Pij represents a time constant, and t _dij represents a _dead time.

When determining the relationship between the dynamic characteristics function g ₁₂ to the time t constant tau _P12 as a parameter when a result of FIG. 4 is obtained.

That is, the dynamic characteristic function g ₁₂ is close to 1 over time, it approaches the time the more constant tau _P12 large slow.
Characteristics of FIG. 4, the same results can be obtained for the dynamic characteristic function g _21. That is, as shown in Table 2, by making the liquid hold-up amount larger than the weir height of the other trays, that is, the liquid hold-up amount, the time constant of the off-diagonal element loop is relatively reduced. Since the time constant of the diagonal element loop of the top and bottom composition becomes small, the influence of the change of the top composition hardly appears on the bottom composition, and the composition detection performance is improved and the control operation is improved. It can be understood that the relative speed can be increased, and as the time elapses and the gain increases, the tower top composition converges to the set value, so that substantial interference is eliminated.

Although the present invention has been described above with reference to the case where the composition at the top of the column is varied, similar results can be obtained when the composition of the column at the bottom varies.

〔The invention's effect〕

As described above, in the distillation column having the shelf plate of the present invention, the liquid hold-up amount of each intermediate shelf sandwiched between the upper and lower detection stages is made larger than the liquid hold-up amount of the other shelves. The composition change sensitivity of the middle shelf is moderated, and the composition change of the top and bottom is relatively accelerated by the composition change of the middle shelf to improve the composition detection sensitivity. Or tower bottom) changes in the composition of the tower bottom (or top)
, That is, the self-controllability of a tray distillation column can be improved. Therefore, disturbance due to disturbance, change in set value, or the like is suppressed.

[Brief description of the drawings]

FIG. 1 is a control system diagram of a distillation column having a plate tray according to an embodiment of the present invention, and FIGS. 2 and 3 are graph diagrams for explaining the effects of the embodiment shown in FIG. 1 in comparison with a conventional example. FIG. 4 is a graph for explaining the change over time of the transfer function of the off-diagonal element loop of the embodiment of FIG. 1, FIG. 5 is a partial sectional view of a conventional distillation column having a plate, and FIG. FIG. 2 is a control system diagram based on the decoupler method. Distillation column having 1 ...... trays 2,2 ₂₁ to ₁₄ ...... trays, 2 _4, 2
₁₀ Detection stage, 4 Weir.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B01D 3/00-3/32 B01D 3/42

Claims (3)

(57) [Claims] 1. A liquid holding-up amount of each shelf disposed between a detection stage for detecting a distillate composition and a detection stage for detecting a canned-liquid composition, and a hold-up amount for each of the other shelves. A distillation column having a tray larger than the volume. 2. The liquid hold-up amount of each shelf interposed between a detection stage for detecting a distillate composition and a detection stage for detecting a cantilever composition, and the hold-up amount of each shelf other than the above. The distillation column having a tray according to claim 1, wherein the value is 1.1 to 3.0 times the amount. 3. The distillation column according to claim 1, wherein a height of the weir provided on the shelf is increased as a means for increasing the liquid hold-up amount.