METHOD FOR OPERATING A DISTILLATION COLUMN

Abstract

The present invention relates to a method for continuously operating a distillation column, which is designed to separate a mixture S, which contains essentially a substance A and a substance B, which boils significantly higher than substance A. In the method according to the invention, the reflux ratio is changed according to the feed flow and, at the same time, the energy input by means of the heat-transfer medium is changed proactively (so-called feed-forward control) by accounting for the feed flow by means of feed-forward control. At the same time, the bottom temperature is observed and the control structure is changed if the bottom temperature falls too far when the heat-transfer medium is reduced by means of the feed flow.

Claims

1. A method of continuously operating a distillation column set up for separation of a substance mixture S comprising a substance A and a substance B which is higher-boiling than substance A, wherein the distillation column comprises: (I) a column body in a vertical arrangement, the column body comprising a stripping section and a rectifying section arranged above the stripping section, where the stripping section has a temperature measurement device for measuring the stripping section temperature T(AT); (II) a column bottom arranged beneath the stripping section for accommodating a liquid bottom product B1 at a bottom temperature T(B1) up to a bottoms level H(B1); (III) a column top arranged above the rectifying section for accommodating an evaporated top product A1; (IV) a feeding point for the substance mixture S, the feeding point being configured to feed the substance mixture S to the distillation column with a flow rate {dot over (m)}(S); (V) a circulation evaporator configured to heat the column bottom by indirect heating of a first portion B11 of the liquid bottom product B1, wherein the circulation evaporator is supplied with a heat carrier medium W with a mass flow rate {dot over (m)}(W) and the circulation evaporator has a heat carrier medium valve for adjustment of the mass flow rate {dot over (m)}(W); (VI) a withdrawal unit configured to remove a second portion B12 of the liquid bottom product B1 with a mass flow rate {dot over (m)}(B12), where the withdrawal unit has a bottoms withdrawal valve for adjusting the mass flow rate {dot over (m)}(B12); (VII) a top condenser configured to condense the evaporated top product A1 to obtain a liquefied stream A2; (VIII) a recycling and withdrawal unit configured to return a first portion of the liquefied stream A2 to the distillation column with a mass flow rate {dot over (m)}(A21) and for removing a second portion of the liquefied stream A2 from the distillation column with a mass flow rate {dot over (m)}(A22), where the recycling and withdrawal unit comprises a reflux valve for establishing a reflux ratio r={dot over (m)}(A21)/{dot over (m)}(A22); and (IX) a closed-loop control unit comprising a reflux controller, a bottoms level controller, a stripping section temperature controller, a bottom temperature limiting controller and a mass flow controller for the heat carrier medium W; wherein the method comprises: (i) defining, for the reflux ratio r, a target value r.sub.TARGET within a range from r1 to r2, wherein is defined and the reflux controller uses the defined value r.sub.TARGET and the value for the mass flow rate {dot over (m)}(S), taking account of the minimum permissible value r1 for r, to calculate the setting of the reflux valve; (ii) defining, for the bottoms level H(B1), a target value H(B1).sub.TARGET within the range from H(B1)1 to H(B1)2, wherein the bottoms level controller uses this target value H(B1).sub.TARGET, the current value for the bottoms level at a given time, H(B1).sub.CURR, and the value for the mass flow rate {dot over (m)}(S) to calculate the setting of the bottoms withdrawal valve; (iii) defining, for the stripping section temperature T(AT), a target value T(AT).sub.TARGET within the range from T(AT)1 to T(AT)2, wherein the stripping section temperature controller uses the target value T(AT).sub.TARGET, the current value for stripping section temperature at a given time, T(AT).sub.CURR, and the mass flow rate {dot over (m)}(S) to calculate the setting of the heat carrier medium valve and transmits the setting of the heat carrier medium valve thus calculated thereto by means of the mass flow controller; (iv) defining, for the bottom temperature T(B1), a target value T(B1).sub.TARGET within the range from T(B1)1 to T(B1)2, wherein the bottom temperature limiting controller is set up such that when the temperature goes below the temperature T(B1)1, the setting of the heat carrier medium valve, with disablement of the setting of the heat carrier medium valve according to (iii), is altered such that the mass flow rate {dot over (m)}(W) is increased, and the setting of the heat carrier medium valve according to (iii) is re-enabled as soon as the bottom temperature T(B1) is again within the range from T(B1)1 to T(B1)2.

2. The method as claimed in claim 1, in which the closed-loop control unit comprises a flow measurement device for the substance mixture S supplied to the distillation column, with which the mass flow rate {dot over (m)}(S) is ascertained in step (i).

3. The method as claimed in claim 1, in which the heat carrier medium comprises steam.

4. The method as claimed in claim 1, in which the substance mixture S comprises benzene as substance A and nitrobenzene as substance B.

5. The method as claimed in claim 4, in which: r1=55; r2=65, H(B1)1=0.18×H(130); H(B1)2=0.42×H(130), where H(130) denotes the height of the column bottom measured from the lower boundary of the distillation column to the lower end of the stripping section, T(AT)1=166° C.; T(AT)2=172° C. and T(B1)1=168° C.; T(B1)2=173° C.

Description

[0041] The appended drawings show:

[0042] FIG. 1 a schematic diagram of a distillation column (1000), the operation of which can be controlled by the process of the invention;

[0043] FIG. 2 the distillation column from FIG. 1, with inclusion of process control devices according to the invention;

[0044] FIG. 3 the benzene content in the bottom product from a distillation column for separation of benzene from nitrobenzene, and the mid-column temperature in the event of a change in the feed mass flow rate of crude nitrobenzene by 20% with employment of the closed-loop control system of the invention (example 2) and without (example 1) the closed-loop control system of the invention.

[0045] There firstly follows a brief summary of various possible embodiments:

[0046] In a first embodiment of the process of the invention, which can be combined with all other embodiments, the closed-loop control unit comprises a flow measurement device (460) for the substance mixture S supplied to the distillation column, with which the mass flow rate {dot over (m)}(S) is ascertained in step (i).

[0047] In a second embodiment of the process of the invention, which can be combined with all other embodiments, the heat carrier medium is steam.

[0048] In a third embodiment of the process of the invention, which can be combined with all other embodiments, the substance mixture S comprises benzene as substance A and nitrobenzene as substance B.

[0049] In a fourth embodiment of the process according to the invention, which is a particular configuration of the third embodiment: [0050] r1=55; r2=65, [0051] H(B1)1=0.18×H(130); H(B1)2=0.42×H(130), where H(130) denotes the height of the column bottom measured from the lower boundary of the distillation column to the lower end of the stripping section, [0052] T(AT)1=166° C.; T(AT)2=172° C. and [0053] T(B1)1=168° C.; T(B1)2=173° C.

[0054] The embodiments outlined briefly above and further possible configurations of the invention are elucidated in detail hereinafter. The embodiments may be combined with one another as desired, unless the opposite is apparent from the context.

[0055] The appended FIG. 1 shows an example of a distillation column (1000), the operation of which can advantageously be controlled by the process of the invention. The substance mixture S to be separated is fed in laterally between rectifying section (120) and stripping section (110). The distillation column may of course also have further auxiliary units and peripheral devices known to the person skilled in the art (for example liquid collector, liquid distributor, pumps and the like). The gaseous top product A1 is condensed in top condenser (300), with guiding of a portion of the condensate (A21) as reflux back into the distillation column and withdrawal of another portion (A22) from the distillation column as top product. The mass flow ratio of reflux to withdrawal, {dot over (m)}(A21)/{dot over (m)}(A22), is referred to as reflux ratio r. The setting of the reflux ratio is undertaken with a reflux valve (320). In a departure from FIG. 1, the top condenser may also be integrated into the column body (100).

[0056] The liquid bottom product B1 fills the column bottom up to height H(B1), indicated by the dashed line. The distillation column (1000) is heated by means of a circulation evaporator (200) in which a first portion of the column bottoms discharged (namely B11) is heated indirectly with a heat carrier medium (W) and returned to the column bottom (130). Suitable heat carrier media are steam, condensate, and further liquid and gaseous heat carriers. The mass flow rate of the heat carrier medium {dot over (m)}(W) is adjusted by means of the heat carrier medium valve (210). A second portion of the column bottoms (namely B12) is discharged as bottom product, with establishment of the mass flow rate of the proportion discharged via the bottom withdrawal valve (230).

[0057] FIG. 2 shows the process control devices to be used in the context of the present invention. To simplify the drawing, some reference numerals from FIG. 1 are omitted here. The substance mixture S to be separated is first detected by means of a mass flow measuring device (460). This mass flow is utilized in order to calculate the target value for the reflux controller (410) via a defined reflux ratio (in the range of r1 to r2). The target value for the reflux ratio r.sub.TARGET is fixed, especially in the case of a design of the distillation column such that a very good separation can be achieved with minimal energy input. In order to assure reliable operation of the distillation column, it should be ensured that a minimum reflux ({dot over (m)}(A21).sub.MIN) is always assured. This ensures that the distillation column never runs dry. The minimum reflux ({dot over (m)}(A21).sub.MIN) is defined by choice of suitable design parameters in the design of the distillation column.

[0058] The level in the bottom of the column is controlled via the level controller (420), likewise as a function of the feed stream; this is accomplished by disturbance variable feedforward of the feed stream S.

[0059] The temperature of the stripping section T(AT) that determines the quality of the bottom product is ensured by means of a temperature controller (430). This controller uses the defined target value and the feed stream S to calculate the target value for the mass flow controller (450) for the heat carrier medium. Since it can happen in the case of significant changes in the feed stream that the bottom temperature falls below the necessary minimum temperature as a result of too small an amount of the heat carrier medium, the necessary minimum temperature is ensured by means of closed-loop temperature control (440) in the bottom of the column. This temperature controller intervenes when the temperature goes below the minimum temperature, deactivates the closed-loop control according to (iii) and reactivates it as soon as the bottom temperature is within the range of T(B1)1 to T(B1)2 again.

[0060] The heat carrier medium W used is preferably steam. However, it is likewise possible to use other heat carrier media, for example heat carrier oils.

[0061] The process of the invention is suitable for the operation of those distillation columns in which a low boiler A has to be separated from a higher-boiling product B.

[0062] An example is the separation of benzene and nitrobenzene, which is required in the context of a process for preparing nitrobenzene by nitration of benzene with nitric acid in the presence of sulfuric acid. Benzene here, especially in the adiabatically operated nitration processes that are customary nowadays, is typically used in stoichiometric excess over nitric acid, such that it has to be removed in the course of workup of the nitration product. The workup of the nitration product is typically accomplished by separating the reaction mixture present after nitration (nitration product), comprising nitrobenzene, unconverted benzene and sulfuric acid, into an aqueous sulfuric acid phase and an organic nitrobenzene phase in a first step, followed by a second step of a single-stage or multistage wash of the nitrobenzene phase, followed by a third step in which the excess benzene is separated from the washed nitrobenzene phase.

[0063] The excess benzene is typically recycled into the nitration, which results in accumulation of impurities present in the benzene originally used that pass through the nitration process essentially unchanged (such as aliphatic organic compounds in particular). Since these impurities are lower-boiling than nitrobenzene, they are distilled off together with the benzene. Excessive accumulation of these impurities is prevented by the discharge of purge streams and/or by breakdown of the impurities after a while.

[0064] This distillation problem for provision of purified nitrobenzene is advantageously implementable by the process of the invention. For this specific separation problem, preference is given to target values within the following ranges: [0065] Reflux ratio: r1 to r2 corresponds to 55 to 65; [0066] Bottoms level: H(B1)1 to H(B1)2 corresponds to 18% to 42% of the column bottoms level H(130) measured from the lower boundary of the distillation column to the lower end of the stripping section—see also FIGS. 1 and 2; [0067] Stripping section temperature: T(AT)1 to T(AT)2 corresponds to 166° C. to 172° C.; [0068] Bottom temperature: T(B1)1 to T(B1)2 corresponds to 168° C. to 173° C.

EXAMPLES

[0069] The results of the examples that follow were achieved with a distillation column for separation of excess benzene from nitrobenzene. The basic construction of the distillation column corresponded to FIG. 1. [0070] The feed stream S consisted of crude nitrobenzene that had merely been separated from the nitrating acid and washed. [0071] A stream of benzene and other lower-boiling compounds than nitrobenzene (especially aliphatic organic compounds) was removed overhead as stream A1. [0072] A stream of nitrobenzene largely freed of low boilers was obtained in the bottom as stream B1.

[0073] In both examples, the feed mass flow rate S was increased by 20% during the continuous operation of the distillation column.

Example 1 (Comparative)

[0074] The feed rate of the heat carrier W (steam) was controlled as a function of the mid-column temperature (tray 20). When this temperature dropped, the feed rate of steam was increased. FIG. 3 shows, on the ordinate axis, the feed stream S (thick dashed line, in kg/h), the mid-column temperature T (at tray 20, thick solid line, in ° C.) and the concentration of benzene (the benzene content c(Bz) expressed as a proportion by mass) in the column bottom (thin dotted line, in kg benzene/kg column bottoms) as a function of the time reported on the abscissa axis t (in h).

Example 2 (Inventive)

[0075] The feed rate of the heat carrier W (steam) was controlled in accordance with the invention (cf. also FIG. 2). FIG. 3 shows, in the same way as for example 1, the feed stream S (thick dashed line), the mid-column temperature T (thin solid line) and the concentration of benzene in the column bottom c(Bz) (thick dotted line). It can be seen that, by comparison with example 1, both the mid-column temperature and the benzene concentration in the bottoms are subject to distinctly smaller fluctuations.