Method for operating a distillation column

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 rTARGET within a range from r1 to r2, wherein is defined and the reflux controller uses the defined value rTARGET 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)TARGET within the range from H(B1)1 to H(B1)2, wherein the bottoms level controller uses this target value H(B1)TARGET, the current value for the bottoms level at a given time, H(B1)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)TARGET within the range from T(AT)1 to T(AT)2, wherein the stripping section temperature controller uses the target value T(AT)TARGET, the current value for stripping section temperature at a given time, T(AT)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)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.