US3428527A

US3428527A - Method for the automatic control of the quality of the bottom and top product in a continuous distillation process

Info

Publication number: US3428527A
Application number: US510021A
Authority: US
Inventors: Johannes E Rijnsdorp; Johannes A Van Kampen; Hans Bollen
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1964-11-30
Filing date: 1965-11-26
Publication date: 1969-02-18
Anticipated expiration: 1986-02-18
Also published as: GB1119790A; NL6413882A; ES320123A1; DE1519707A1; BE673032A

Description

Feb. 18, 1969 J. E'. RIJNSDORP ET AL METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATION PROCESS Filed Nov. 26, 1965 CONTROLLER ACCUIULATQR JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN THEIR ATTORNEY Feb. 1 1969 J. E. RIJNSDORP E AL METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM 7 AND TO? PRODUCT IN A CONTINUOUS DISTILLATION PROCESS Filed NOV. 26, 1965 Sheet wmwODQwZSE. awk

En: 25 5: I! $3228 INVENTORS:

JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN BY: THEIR ATTORNEY Feb. 18, 1969 J. E. mmsoon ETAL 3,4 27

METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATION PROCESS INVENTORS:

JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN THEIR ATTORNE Feb. 18, 1969 J. E. RIJNSDORP ET AL 3,423,527 METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATION PROCESS Filed Nov. 26, 1965 Sheet WITT 57% M a .3 a

W e W L J Ml lllllll IIIAIILT 5:258 T; 5:15 i a 2L lNVENTORS:

JOHANNES E. RIJNSDORP THEIR ATTORNE;

Feb. 18, 1969 J. E. RIJNSDORP L METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATION PROCESS Filed Nov. 26, 1965 CONTROLLER 9 3 FJI.

Sheet 5 of '7 FIG.

CONTROLLER Q n CONTROLLER INVENTORS:

JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN B WW 7/ THElR ATTORNE Feb. 18, 1969 J. E. RIJNSDORP ETAL 3, 8,5

METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATION PROCESS FIG. 6

INVENTORS JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN THEIR ATTORNE Feb. 18, 1969 J. E. RIJNSDCRP ETAL 3,428,527 METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATION PROCESS Filed NOV. 26, 1965 Sheet m a m.

ECEEEE L 5252. Nm

lll rl l 5:228 I I l L 2 INVENTOR$= JOHANNES E, RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN THEIR ATTORNEY United States Patent 3,428,527 METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATION PROCESS Johannes E. Rijnsdorp, Johannes A. van Kampen, and Hans Bollen, Amsterdam, Netherlands, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Nov. 26, 1965, Ser. No. 510,021 Claims priority, application Netherlands, Nov. 30, 1964, 6413882 US. Cl. 203-1 5 Claims Int. Cl. B01d 3/42, 3/14 ABSTRACT OF THE DISCLOSURE Automatically controlling a continuous distillation process for the separation of feed stream into a top and a bottom product stream in a column using reflux and reevaporation. Upon a control action for keeping the quality of the bottom product constant the quality of the top product does not change. This is achieved by analyzing the quality of the bottom product and adjusting the feed flow rate to maintain the quality thereof and adjusting some of the remaining process variables to maintain the liquid levels in the accumulator and in the bottom of the column between desired limits and the ratio between the vapor flow and the liquid flow in the rectifying section of the column at a constant value.

The invention relates to a method and apparatus for automatically controlling a continuous distillation process for the separation of an intake mixture into a top and a bottom product stream in a column using reflux and reevaporation whereby upon a control action for keeping the quality of the bottom product constant, the quality of the top product does not change or changes only very slowly.

If often occurs in technical practice that both the top product and the bottom product withdrawn from a continuous distillation process have to meet specific requirements as regards quality. For example, it may be required that the top product contain no more than a specified small amount of impurity, while the bottom product should as far as possible be free from the main constituent of the top product. An example of such a situation in the oil industry is the removal of a valuable light product, such as isopentane, from a mixture of hydrocarbons where the purity of the isopentane must be at least 97% and where the bottom product may contain no more than 2% isopentane.

In such a distillation process, it may also be of importance to weight the quality of the bottom product highest, for instance, if that product has to be supplied against specification. Accordingly, continuous direct control of the quality of the bottom product must be provided to maintain the bottom product at its desired value. However, at the same time it is also desirable to permit the quality of the top product to vary as little as possible.

In order to reach this aim of maintaining the quality of the bottom product on specification while permitting the top product quality to vary very little, it is attempted in controlling processes of this type to prevent dynamic coupling between the stripping and the rectifying sections of the column. That is to say, when adjusting the process variables to a specific ratio between the vapor flow and the liquid flow in the stripping section of the column in order to maintain the quality of the bottom product, the ratio between the vapor flow and the liquid flow in the rectifying section of the column should be disturbed as ICC little as possible. In this way it is ensured that no or substantially no deviation occurs in the quality of the top product as a result of that control action in the stripping section of the column.

J. K. Kennedy in his thesis, The Control of Distillation Columns, Manchester, 1963, on pages 173 and 192, describes a control scheme by which it is ensured that to a certain extent decoupling of the stripping section from the rectifying section of the column occurs upon a control action for maintaining the quality of the bottom product. According to this scheme the degree of reevaporation is adjusted by a quality meter for the product on one of the trays in the column while the reflux ratio is kept at a desired value by means of a ratio controller. The intended decoupling is here obtained due to the fact that because of the ratio control at the top of the column the reflux flow follows any change in the vapor flow over the top of the column, and in such a way that the ratio between those flows remains constant. Accordingly, the quality of the top product also ordinarily remains substantially constant. However, if there are disturbances that require a permanent change of the process conditions, for example, a different composition of the feed, then the set value of the ratio controller has to be changed. Such a change can be carried out automatically, for example, with the aid of a signal derived from a meter or analyzer for measuring the quality of the top product.

With the mode of control described by Kennedy the feed stream can be adjusted to an independent value. It is not possible, however, with this control method to adjust the top product flow, the reflux flow, the bottom product flow or the heat flow to the reboiler to an independently controlled value.

The process according to the invention, which is likewise aimed at preventing dynamic coupling between the two column sections as far as possible, provides for the possibility of making a choice from the four last-mentioned flows for adjusting one of those flows at an independent value, as a result of which the variety of the applications is increased. In addition, compared with the mode of control described by Kennedy, an equivalent or improved decoupling between the column section is obtained, depending on the variant chosen.

Briefly, the characteristics of the process according to the invention are that:

(a) the quality of the bottom product is controlled by a controller which adjusts the feed stream flow, and

(b) the degree of reevaporation, the degree of preheating the feed, the top product flow, the bottom product flow, the degree of reflux and/or the degree of cooling in the condenser are adjusted in such a way that the liquid levels in the accumulator and in the bottom vary between desired limits and that the ratio between the vapor flow and the liquid flow in the rectifying section of the column is as far as possible maintained at a desired predetermined value.

If the quality of the bottom product does not meet the requirement or requirements made, then the feed stream is increased or decreased as the need may be until the ratio between the vapor flow and the liquid flow in the stripping section of the column that is necessary for the desired quality is reached. The correcting action therefore concerns the liquid flow in the stripping section of the column and since this flow has no influence on the ratio between the vapor flow and the liquid flow in the stripping section of the column that is necessary for the desired quality is reached. The correcting action therefore concerns the liquid flow in the stripping section of the column and since this flow has no influence on the ratio between the vapor flow and the liquid flow in the rectifying section of the column, it follows that the desired decoupling between the two column sections has been obtained.

According to the invention this mode of control is combined with control systems which in themselves are known, for those process conditions which must be controlled.

It is possible by an appropriate choice of those combinations to satisfy various wishes as regards process conditions. This flexibility within the range of the invention is likewise the result of the fact that the control action for maintaining the quality of the bottom product primarily concerns the liquid fiow in the stripping section of the column.

If the temperature of the feed stream when entering the column is higher than the boiling temperature on the feed tray, then the vapor flow will be larger in the rectifying section of the column than in the stripping section. If the temperature of the feed stream is lower, then that vapor flow will be smaller. In both cases the size of the vapor flow in the rectifying section of the column will change upon a change of the feed stream. Accordingly, as the reflux ratio is higher, the effect on the ratio between the liquid flow and the vapor flow in the rectifying section becomes smaller, that effect even becoming negligibly small. However, it is possible within the range of the invention totake account of this secondary effect in such a way that the occurring change of the size of the vapor flow in the rectifying section is rapidly compensated.

The invention will be more clearly understood from the following detailed description of the invention when taken in conjunction with the accompanying drawings where- FIGURE 1 is a diagrammatic representation of a control system according to the invention for a distillation apparatus wherein the quantity of heat supplied to the distillation column is maintained at an independently controlled value;

FIGURE 2 shows an alternate embodiment of the control scheme illustrated in FIGURE 1;

FIGURE 3 is a diagrammatic representation of a control system according to the invention for a distillation apparatus wherein the top product stream is maintained at an independently controlled value;

FIGURE 4 shows an alternate embodiment of the con trol scheme illustrated in FIGURE 3;

FIGURE 5 is a diagrammatic representation of a control system according to the invention for a distillation process wherein the reflux flow is maintained at an independently controlled value;

FIGURE 6 is a diagrammatic representation of a control system according to the invention for a distillation apparatus wherein the bottom product flow is maintained at an independently controlled value; and

FIGURE 7 shows a modification of the control scheme illustrated in FIGURE 3 which is of particular advantage when the feed is introduced to the column at a temperature below the boiling temperature.

The apparatus for carrying out the distillation process control scheme mentioned above is illustrated in FIGURE 1 wherein there is shown a distillation column 1 of any suitable design which contains vapor-liquid contacting elements, such as a vertical series of bubble or valve trays. The feed is continually introduced in either a liquid or vapor form into the column 1 to an intermediate stage or feed tray through a pipeline 2 containing a preheater 3 if such is required; the top product in the vapor phase is discharged through a pipeline 4; and the liquid bottom product is discharged through a pipeline 5.

Connected to the top product column discharge line 4 is a condenser 6 which condenses the vapor in the line 4; the condensate then flows into a top product accumulator 7. The condensate collected in the accumulator is discharged through a pipeline 8 and a portion of the condensate is returned to the top of the column 1 for refiux by a pipeline 9, while the remainder of the discharged condensate is passed through a discharge pipeline 10 as the liquid top product output stream from the distillation process.

Reevaporation for the column 1 is eifected by passing a stream of liquid from the bottom of the column via a pipeline 11 to a reboiler 12 which may, for example, be a heat exchanger, and returning the heated stream, either entirely or partially in the vapor phase to the column l via a pipeline 13. Heat is supplied to the reboiler 12 via a pipeline 14. The particular manner in which reevaporation is attained, however, forms per se no part of this invention since reevaporation may be accomplished in other ways, e.g., with a heating coil in the bottom of the column.

Turning now to the control of the distillation process shown in FIGURE 1, as indicated above, the control schemes according to the invention are for processes wherein the feed flow rate is adjusted to maintain quality of the bottom product at a desired value. Accordingly, the feed fiow line 2 is provided with a control valve 15, the position of which is controlled or adjusted by means of a controller 16. In order to determine the quality of the bottom product, quality measuring instrument or analyzer 17, e.g., a process chro-matograph, is provided which continuously samples the bottom product flowing in the flow line 5 and produces an output signal corresponding to the measured quality to the the normal input of the controller 16 which compares the measured value of the quality of the bottom product with a signal related to the desired value of the bottom product quality, i.e., the set value of the controller 16. The set value input signal of controller 16, and of all other controllers or adjustable control elements shown in the drawings which do not have set values which are adjusted by a process variable, is indicated by means of an arrow, as indicated by the reference numeral 18. Any differences resulting from the comparison by the controller 16 will cause the output signal from the controller 16 to adjust the valve 15 in a manner to decrease the difference and hence adjust the feed flow rate for the distillation process to the value which will produce a bottom product of the desired quality may be maintained.

The bottom liquid product stream flow rate is adjusted by means of a controllable valve 19' connected in the pipe line 5. The valve 19 is controlled by means of a controller 20 (which compares the measured value of the liquid level in the bottom of the column, as determined by the liquid level gauge 21, with a signal proportional to the desired level in the bottom of the column, i.e., the set value of the controller. The controller 20' attempts to keep the liquid level in the bottom of the column within desired limits and hence any differences resulting from the comparison will produce an output signal from the controller 20 which tends to adjust the valve 19 in such a way that the difference is decreased. Thus, if at a given moment the measured value of the liquid level is larger than the set or desired value, the signal supplied from the controller 20 tends to open the valve 19 wider.

As also indicated above, according to this embodiment of the invention, the heat flow to the column via the reboiler is maintained at an independently controlled value. The control of the heat flow to the reboiler 12 is provided by a controller 22 whose output is connected to a controllable valve 23 in the flow line 14. The normal input of controller 23 is connected to the output of a meter 24 which continuously measures the quantity of heat supplied to the column through the reboiler 12. The meter 24 performs this measurement, in a manner well known in the art, by combining the output signals from a fiowmeter 25 in the line 14 and a pair of

transducers

26 and 27 which measure the temperature of the heating medium before and after passing through the reboiler 12. It should be noted that if the heating medium passing through flow line 14 has a constant enthalpy, for example, oil of a constant temperature or steam of a constant temperature and pres sure, then only the signal from flowmeter 25 need be used to obtain a sufiiciently accurate indication of the heat flow to the reboiler 12. The controller 22 compares the output signal from. heat fiowmeter 24 with the desired value of the heat flow, i.e., the set value of controller 22, and adjusts valve 23 to maintain the heat flow at the desired value.

By means of the control actions just described, the quality of the bottom product is maintained at the desired value. v'However, to maintain the quality of the top product, the ratio between the liquid and vapor flows in the rectifying or upper section of the column 1 must be maintained at a constant value by controlling the ratio between the reflux and top product flows. According to this embodiment of the invention, the reflux flow is adjusted by means of a controller 30 and a controllable valve 31 connected in the reflux flow line 9. The controller 30 compares the output signal from a liquid level gauge 3-2, which measures the liquid level in the accumulator 7, with the desired value of the liquid level in the accumulator and adjusts the valve 31 to maintain the liquid level in the accumulator between desired limits. The top product flow is controlled by a ratio controller 33 whose output is connected to a control valve 34 in the top product flow line 10. The two inputs of the ratio controller are provided by a pair of

flowmeters

35 and 36 which measure the reflux flow and the top product flow respectively. The ratio controller 33 compares the two input signals thereto and adjusts the top product flow via control valve 34 to maintain a predetermined ratio between the reflux and top product flows. With this control scheme any variation in the vapor flow in the rectifying section will result in a change in the liquid level in the accumulator which, via the gauge 32, the controller 30 and the valve 31, causes a corresponding change in reflux flow and via ratio controller 33 and valve 34 a proportionate change in the top product flow to restore the desired vapor-to-liquid flow ratio in the rectifying section of the column and thereby maintain the quality of the top product.

With this control scheme it is not important at what temperature the feed enters the column. The additional effect, if any, of a change of the vapor flow in the rectifying section of the column upon adjustment of the feed stream caused by the temperature thereof differing from the boiling temperature on the feed tray, is immediately compensated by the ratio control at the top of the column.

It should be noted that, although not shown, the set value of controller 33 can be derived from a meter and a controller for the quality of the top product. Moreover, it is also possible according to this control mode for the outputs of level controller 30 and ratio controller 33 to be interchanged, i.e., the output of level controller 30 can be connected to valve 111 when the output of ratio controller 33 is connected to valve 9.

Referring now to FIGURE 2, there is shown an alternate embodiment or variant of the control scheme of FIGURE 1. In this and any following figures, components previously used are represented by like reference numerals. According to this variation, the output of flowmeter 35 for the reflux flow is connected to normal input of a controller 37, which has its output connected to the control valve 31, of a controller 37 while the output of flow meter 36 for the top product flow is connected to the normal input of a controller 38 which has its output connected to the control valve 34. The set value inputs of the controllers 37 and 38 are connected to respective outputs of a ratio adjusting device 39 whose input in turn is connected to the output of liquid level controller 30. The ratio adjusting device 33, which may, for example, be a pair of amplifiers having a common input terminal and their respective gains related by a desired predetermined ratio, produces a pair of output signals which are both derived from the input signal thereto and whose magnitudes are related by a predetermined ratio. With this arrangement, upon a change of the liquid level in the accumulator 7 both the reflux flow and the top product flow will change in such a way that the ratio between those flows remains constant. In the event that it is required to change the predetermined ratio, then a quality controller for the top product (not shown) can supply the signal by which the ratio between the output signals from ratio adjusting device 39 is adjusted.

It is sometimes also desirable to maintain the top product flow at an independent value. This type of control may be desirable, for example, if the top product flow constitutes the feed for a following process which requires a feed stream flow rate having a specified value. The process according to the invention can provide for this variant in the case where the feed stream for the distillation process is introduced into the column at a temperature that is equal or approximately equal to the boiling temperature, by carrying out the control of the process in such a way that the control of the quality of the bottom product by adjustment of the feed stream is com.- bined with:

(a) Maintenance of the top product flow at an independently controlled value;

(b) Adjustment of the reflux flow and of the bottom product flow in such a way that the liquid levels in the accumulator and in the bottom of the column vary between desired limits;

(c) Maintenance of the heat flow to the reboiler at a predetermined value.

An example of a control scheme according to the invention for carrying out this variant is represented in FIGURE 3. According to this embodiment of the invention, the top product flow is independently controlled by means of the fiowmeter 36, controller 38 and valve 34. The controller 38 compares the output signal from flowmeter 36 with the desired value of the top product flow, i.e., the set value of controller 38, and adjusts valve 34 to reduce any diflerence therebetween. The feed flow, the reflux flow, the liquid level in the bottom of the column and the heat flow to the reboiler are controlled in the same manner as in FIGURE 1. With this control scheme, since the feed stream enters the column at a temperature which is equal or approximately equal to the boiling temperature, the control action in the loop comprising analyzer 17, controller 16', valve 15, flow line 2, column 11 and flo'w line 5 in order to maintain the quality of the bottom product does not affect the ratio between the vapor flow and the liquid flow in the rectifying section of the column. Moreover, since the vapor flow in the column is kept at a specified value by meter 24, controller 22 and control valve 23, the ratio between the liquid and vapor flows in the rectifying section of the column remains constant as long as the level in the accumulator 7 does not change. Of course, any changes in the liquid level in accumulator 7 which cause the liquid level to vary out of the desired limits is compensated (for by adjusting the valve 31 to restore the desired liquid-vapor ratio in the rectifying section.

It should be noted that with the control scheme of FIGURE 3, slow or permanent changes necessary for maintenance of the desired quality of the top product can be effected by changing the set value of controller 22. These changes can be done manually or automatically by means of a controller for the quality of the top product. Additionally, if the required change is large, for example, when changing to another intake mixture, then it is likewise advantageous 'for the set value for the controller 30 for the liquid level in the accumulator 7 to be derived from a meter measuring the quality of the top product. A control scheme incorporating both of these modifications is illustrated in FIGURE 4 'wherein an analyzer 40 for measuring the quality of the top product is provided. The output signal from analyzer 40* is fed to the normal input of a quality controller 4 whose output signal is passed to the set value inputs of

controllers

22 and 30. Preferably, as illustrated in FIGURE 4, a multiplier 42 is also provided in order to properly adjust the magnitude of the signal derived from controller 41 and supplied to controller 30 according to a desired ratio. With the additional control provided, if the vapor flow increases due to valve 23 being farther opened, then the liquid level in the accumulator 7 will rise, resulting in valve 31 being farther opened, thereby preventing unnecessary fluctuations in the ratio between the vapor flow and the reflux flow. Obviously, if the vapor flow should decrease, the valves 23 and 31 will be similarly further closed.

The process according to the invention also provides for a variant by which the reflux flow can be adjusted at an independent value. This may be of importance in relation to column properties, for example, if the column is adjusted to give the maximum production. The characteristic of this process is that the control of the quality of the bottom product by adjusting the feed stream is combined with:

(a) Maintenance of the reflux flow at an independent value;

(b) Adjustment of the top product flow in such a way that the liquid level in the accumulator varies within desired limits;

(c) Adjustment of the bottom product flow in such a way that the liquid level in the bottom varies between desired limits; and

(d) Maintenance of the heat flow to the reboiler at a predetermined value.

An example of this control scheme is given in FIGURE 5. The only difference between the scheme of FIGURE 5 and that of FIGURE 3 is that the manner of controlling the valves 31 and 34 is interchanged. That is to say, the valve 34 is controlled by level controller 30 While the valve 31, and hence the reflux flow, is controlled to an independently derived value by means of flowmeter 35 and controller 37. It should be noted that with this embodiment, as with that of FIGURE 3, the feed stream should be introduced to the column at a temperature approximately equal to the boiling temperature.

The process according to the invention further provides for a variant by which the bottom product flow can be adjusted at an independent value. This control mode may be of importance, for example, if the bottom product stream constitutes the feed for another process, which requires a feed stream flow rate having a specified value. This mode of control according to the invention is achieved by combining the control of the quality of the bottom product through adjustment of the feed stream with:

(a) Maintenance of the bottom product flow at an independently controlled value;

(b) Adjustment of the top product flow in such a way that the liquid level in the accumulator is maintained with desired limits;

(c) Adjustment of the reflux flow in such a way that the ratio between the reflux flow and the top product How is maintained at a desired value;

(d) Adjustment of the heat flow to the reboiler by means of a signal derived from a gauge measuring the liquid level in the bottom of the column and to increase heat flow to the reboiler when the liquid level in the bottom of the column rises and decrease heat flow when the column bottom liquid level drops.

FIGURE 6 represents a scheme for this variant of a basic control scheme. According to this embodiment of the invention, the reflux flow, the top product flow and the feed flow are controlled in the same manner as described with respect to FIGURE 1. The bottom product flow in flow line 5, however, is controlled by means of a controller 43, whose normal input is connected to a flowmeter 44 in the flow line 5 and whose output is connected to the control valve 19, to an independently derived value, i.e., the set value of controller 43, with this type of control, if the liquid level in the bottom of the column changes due to a change in the liquid flow in the stripping section caused by adjustment of the valve to maintain the quality of the bottom product, it is not possible to correct the column bottom liquid level in the usual manner via bottom product flow line 18, since the bottim product flow is maintained at a constant value. Accordingly, control of the liquid level in the bottom of the column is provided by passing the output signal from level controller 20 to the set value of input of heat flow controller 22. Controller 22 adjusts the liquid level in the bottom of the column '1 to its desired value by increasing the heat flow to the reboiler upon an increase in the liquid level in the bottom of column 1 and vice versa. A change of the liquid level in the bottom of the column is naturally accompanied by a change of the vapor flow through the entire column. Any change in the vapor flow in the rectifying section of the column is, however, immediately compensated for by the ratio controller 33. As with the embodiment of FIGURE 1, with this variant of the invention the temperature at which the feed stream enters the column is of no consequence. The set value for controller 33 may, if desired, be derived from a meter measuring the quality of the top product (not shown).

Furthermore, for the processes whereby the top product flow or the reflux flow is maintained at an independent value and in relation to which embodiments are given (FIGURES 3-5) which are tied to the requirement that the feed stream is introduced into the column at a temperature equal or approximately equal to the boiling temperature, within the range of the invention embodiments can be presented which are suitable for feed streams that enter the column at a temperature below the boiling temperature. In these embodiments the heat flow to the feed preheater is then adjusted, by a signal derived from a flowmeter measuring the feed stream and from the difference in temperature between the feed stream leaving the feed preheater and the liquid on the feed tray, in such a way that the contribution from the feed stream to the vapor flow in the rectifying section of the column is kept at a desired value. An example of this modification of an embodiment of a control scheme according to the invention wherein the top product flow is adjusted at an independent value, i.e., the control scheme of FIGURE 3, is given in FIGURE 7.

According to this modification, a meter 5'0 is provided which measures the difference in temperature between the feed stream leaving the feed preheater 3 and the liquid on the feed tray in the column 1. The output signal from the meter 50 and from a flowmeter 51 for measuring the feed flow rate are fed to respective inputs of a multiplier 52 which produces an output signal proportional to the product of the two input signals and an adjustable constant. The output from multiplier 52 is fed to the normal input of a controller 53 whose output controls the valve 54 in the flow line 55 for the supply of heating medium to the feed preheater 3. The controller 53 varies the position of the valve 54 to maintain a constant output from multiplier 52 and thereby maintain the contribution from the feed stream to the vapor flow in the rectifying section of the column at a desired value. In the absence of this additional control, if the temperature of the feed stream is below the boiling temperature, then a portion of the vapor flow in the rectifying section of the column will be condsensed by the feed stream. Accordingly, any change in the feed stream resulting from a control action according to the invention will cause the vapor flow in the rectifying section to change as well. This change of the vapor flow in the column is prevented by increasing the heat flow to the preheater 3 to a suflicient extent. A similar effect can be obtained with a feed stream above the boiling temperature, if the heat supply to the preheater is replaced by a heat discharge medium.

It should be noted that the combination of the control of the heat flow to the feed preheater with the control according to the invention whereby the reflux flow is maintained at an independent value (see FIGURE 5) is not represented by a figure. However, it is understood that the control system formed by units 50-55 can equally well be applied to this embodiment of the invention.

Naturally, the two last-mentioned modes of control, whereby the heat flow to the feed preheater is adjusted, can also be used if the temperature of the feed stream is equal or approximately equal to the boiling temperature on the feed tray.

In the aforementioned examples the manner in which the flow of cooling medium through the condenser 6 is adjusted is not indicated. However, systems for controlling the degree of cooling by the condenser are old and well known in the art and take many forms depending on the desired mode of control. For example, known systems are available for adjusting the cooling medium flow to control the pressure in the column or to optimize the heat economy of the process. This also applies to the adjustment of the heat flow to the feed preheater in those cases where adjustment of that heat flow is not used. These additional control systems can be combined with the process according to the invention.

It is not necessary for the quality meters to be connected directly to the top product flow or the bottom product flow. Sometimes it may be desirable, for example, in view of the accuracy of the quality meter or analyzer, to connect this meter to the liquid on a tray in the column. The control systems mentioned hereinbefore may be pneumatic, hydraulic, electric, electronic or mechanical systems or may constitute a mixture of these types.

We claim as our invention: 1. In a process for the continuous distillation of an input feed stream into top and bottom products streams in a multi-stage distillation column in which overhead vapors are condensed in a top product accumulator and both reflux in the upper portion and re-evaporation in the lower portion of the column are applied, wherein the process variables include a feed stream flow rate, a degree of preheating of the feed stream, a reflux flow rate, a top product stream flow rate, a bottom product stream flow rate, a degree of cooling in the condenser, and a quantity of heat supplied to the reboiler for re-evaporation; a control method comprising:

continuously analyzing the quality of the bottom product to determine any deviation between the measured and desired values of quality of said bottom product;

adjusting the feed stream flow rate to maintain the quality of the bottom product at the desired value;

adjusting the reflux flow and the top product flow to maintain the liquid level in the top product accumulator within desired limits and the ratio between the top product flow and the reflux flow at a specified value;

adjusting the bottom product stream flow to maintain the liquid level in the bottom within specified limits; and

maintaining the quantity of heat to the reboiler at a constant value.

2. The method of claim 1 wherein the feed stream is introducted into the column at a temperature below the boiling temperature of said feed stream and including the steps of:

maintaining the reflux flow at a constant value;

adjusting the top product flow to maintain the liquid level in the accumulator between specified limits;

adjusting the bottom product flow to maintain the liquid level in the bottom of the column between specified limits;

maintaining the heat flow to the reboiler at a fixed value;

measuring the flow rate of the feed stream;

detecting the difference between the temperature of the feed stream leaving the feed preheater and the liquid on the column feed tray; and

utilizing the measured value of the feed flow and the value of said temperature diflerence to adjust the heat flow to the feed preheater to maintain the contribution from the feed stream to the vapor flow in the rectifying section of the column at a predetermined value.

3. The method of claim 2 wherein the top product flow is maintained at a constant value and the reflux flow is adjusted to maintain the liquid level in the accumulator between desired limits. 4. The method of claim 2 wherein the step of utilizing the value of the feed stream flow and the temperature difference to adjust the flow of heat to the feed preheater comprises:

multiplying a signal corresponding to the measured value of the feed flow by a signal corresponding to 5 the value of the said temperature difference and by a predetermined constant; passing the resulting product signal to the normal input of a controller; and adjusting the heat flow to the feed preheater by means of the output signal from said controller to maintain said product signal at a predetermined value.

5. The method of claim 3 wherein the step of utilizing the value of the feed stream flow and the temperature difference to adjust the flow of heat to the feed preheater comprises:

multiplying a signal corresponding to the measured value of the feed flow by a signal corresponding to the value of the said temperature difference and by a WILBUR L. BASCOMB, 111., Primary Examiner.

US. Cl. X.R.