US2588303A

US2588303A - Fractionator reflux control method

Info

Publication number: US2588303A
Application number: US762846A
Authority: US
Inventors: Stanley Clyde Page
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1947-07-23
Filing date: 1947-07-23
Publication date: 1952-03-04
Anticipated expiration: 1969-03-04

Description

Fracflona for Feed Fracrionafor' C. P. STAN LEY FRACTIONATOR REFLUX CONTROL METHOD Filed July 23, 1947 Reflux Cond Accumularar'.

: Liquid Lev.

Pump, Canl'raI/ar. fa 1 l- ,3 25 Mia/d Produci- 76 Tamp. Cont ME Q Turbine.

20 I! I F/OwConh. v i [8 2/ I r' Q 50 t]: 29 Reflux Cons. I I z 1 z'z L-lq.Lev- Con h, l

Aczumulafw q 2 69 l" INVENTOR. 8 69612 PL Gian/e5.

Gas Producr- Ari-" 3.

Patented Mar. 4, 1 952 FRACTIONATOR REFLUX CONTROL MET Clyde Page Stanley, Phillips, Tex.r assignor to Phillips Petroleum Company, a corporation of Delaware Application July 23, 1947, Serial No. 762,846"

2 Claims,

This invention is concerned with improvements in fractionating column control systems and methods of operating them.

This invention as disclosed herein is applicable to at least two types of fractionating systems characterized in the one case as a system for ob-' taining total condensation of overhead vapors and in the other case as obtaining partial condensation of the overhead vapors. In the former case the overhead product is a liquid and in the latter, in accordance with the operation of. the system herein disclosed, the overhead product is a vapor.

The broad object of this invention is to provide fractionating systems wherein the reflux temperature is controlled by regulating the temper-- ature of the cooling fluid of the reflux condensers as distinguished from the more common method of securing such control by regulating the quantity of cooling fluid supplied to the reflux condensers.

More specifically, it is an object of this invention to operate a fractionating system by regulating the speed of the fan on a mechanical draft cooling tower to which the cooling liquid is supplied at a constant rate to the reflux condensers.

In one form of the apparatus herein disclosed it is an object of this invention to control the speed of such fan in accordance with variations in the temperature of the reflux liquid.

In the other form of the apparatus herein disclosed it is an object of this invention to efiect the control of such fan in accordance with variations in the liquid level of the reflux liquid accumulator.

As will appear later, in the former system the overhead from the fractionating column is completely condensed while in the latter it is only partially condensed.

Other objects and all of the advantages of the several forms of the invention herein disclosed will be apparent from the following description in detail of the embodiments thereof illustrated. in the attached drawings.

This invention resides substantially in the combination, construction, arrangement, relative lolation of parts, steps and series of steps, all as hereinafter clearly set forth.

In the accompanying drawings:

Figure 1 is a diagrammatic and schematic illustration of the form of the system in which fan control is effected by variations in the reflux accumulator temperature; and

Figure 2 is a diagrammatic and schematic i1 lustration of the form of .the system wherein fan control is effected by variations in the liquid level in the reflux accumulator.

Before attempting to emphasize th advantages of this invention a detailed disclosure of the several embodiments illustrated in the drawings is in order. In the system of Figure 1. the fractionating column is generally indicated by reference numeral I. This column may be of conventional design and is preferably provided with the usual bubbletrays in number depending upon the complexity of the material to be fractionated and other conditions well understood by those skilled in the art which must be met in fractionati-ng a particular feed. The liquid to be fractionated is supplied to the column through the feed line 26.. As is of course .well understood, the fractionating column. is heated inany suitable manner to generate the required temperatures therein. The overhead vapors are withdrawn from the column through the line 2 and pass thereby into the condensing coils '1 of the cooling tower by means of the branch line 6'. Branch line- B is provided with a suitable motor actuated valve 8 such as the fluid pressure operated diaphragm valve diagrammatically illustrated. The valve 8' is controlled by means of a pressure controller 9 which is actuated by and in accordance with the pressure conditions in line 2' and therefore for all. intents and purposes the pressure in the column i. The controller 9' as will be well understood is set to maintain the proper pressure condition. in the column l for the particular mixture being fractionated. The condensate from the coils I is delivered by the line H) to the accumulator tank 3. Line 2 is connected to the top of this accumulator and has therein a valve 4 similar to the valve 8. Valve 4 is controlled by means of the pressure controller 5 similar to the controller 9 which in this case is actuated by and in accordance with pressure conditions in the accumulator 3. Atthis pointitmay be noted, even. though it may not be necessary, that valves suchas the

valves

4, 8, I8, 20 and 23 are well known articles of commerce as are the pressure controllers 5 and 9. It will also be understood that other types oi motor actuated valves canbe used as, for example, magnet valves and electric motor actuated valves, the pressure controllers being of course adapted as would be well understood in the art to control these types of electric valves.

The reflux condenser coils 1 may, as shown, form part of an atmospheric mechanical draft cooling tower employing water as the cooling liquid. Such tower has been diagrammatically illustrated as including. a housing II on which is mounted, in. communication therewith, a fan housing l2 in which is mounted a fan 13, for cooling the water sprayed over the coils l, and for discharging it at the upper end of the housing [2. As illustrated, this fan is driven from a steam turbine l4 through suitable shafting and gears as diagrammatically illustrated. The housing H is provided with a water feed line 21 and sprays for spraying water over the 00115 1 countercurrent to the induced air draft caused by the fan 13;

A steam line 15 supplies operating steam to the turbine [4 the control of which is effected by means of the diaphragm valve l8. The operation of this valve in turn is controlled by means of a temperature controller ll on which the temperature responsive member i6 is mounted so as to be immersed in the accumulator reflux in the accumulator 3 and thus to be subject to the temperature thereof. Controllers of this type are also well known in the art and in the case of a pressure fluid operated valve will be of a well known type for controlling a supply of air to and exhaust from the diaphragm motor. If the valve I8 is of the electric type the controller I! will be of suitable well known type to contro1 its operation.

A line 19 is connected from the bottom of the accumulator 3 to the top of the fractionating column I to return the reflux liquid thereto. Movement of the reflux liquid through this line is effected by means of a suitable pump 25. As is well known the rate of flow of reflux to the tower must be controlled. For this purpose there is included in the line l9 the diaphragm valve 20 and a rate of flow controller 2| for maintaining the rate of flow of reflux liquid through the line at a predetermined value. Rate of flow controllers of this type are also Well known and several forms suitable for the purpose are commercially available. Line [9 has a branch 22 also connected to the outlet of the pump 25 through which a portion of the reflux liquid or overhead fraction is withdrawn from the system for further use. The line 22 includes the diaphragm valve 23 which in this case is controlled by a liquid level controller 24 so that all reflux liquid in excess of a predetermined quantity in the accumulator 3 and the amount returned from the line I9 will automatically be discharged through the branch 22.

In the operation of this system the feed mixture is supplied to the column I at a constant rate through the line 26. The pressure controller 9 is set to maintain a desired pressure condition in the column by controlling the rate at which the overhead vapors are withdrawn through the line 2 and branch 6. These vapors are condensed in the condenser coils 'l which are cooled by means of a cooling liquid such as water supplied at a constant rate from the sprays on line 21. The overhead condensate is delivered from the coils I through the line in liquid form to the accumulator 3. The pump 25 is operated at a constant speed and the condensate is withdrawn therefrom and discharged into line I9. A portion thereof is returned by that line to the column I. The desired portion is predetermined by the setting of the rate of flow controller 2|. If liquid is accumulating in the tank 3 so that its level rises above the level predetermined by the adjustment and setting of the liquid level controller 24 valve 23 will be proportionally opened to discharge the excess through the line 22 still in the form of a liquid. If pump 25 is withdrawing the liquid faster than it is accumulating in tank 3 valve 23 will be closed by the liquid level controller 24. Valve 4 is operated in accordance with pressure changes in the tank 3 through the operation of the pressure controller} so that vapors from line 2 can be bled into the tank 3 to equalize the pressure therein. In other words, the valve 4 and its controller are provided to maintain a desired vapor pressure in the tank 3 over the accumulated liquid therein.

As the temperature of the condensate in the accumulator 3 varies from a desired predetermined value the temperature controller II will operate the valve [8 to vary the supply of steam to the turbine l4 and thus vary the speed of the fan [3. Changes in speed of fan 13 will vary the amount of air moving countercurrent to the constant quantity of cooling water being supplied so as to vary the temperature of the cooling water in the proper direction to bring the temperature of the condensate back to the predetermined value. Operating conditions are set so that the cooling water being supplied through the line 2'! will be in sufllcient quantity to completely flood the condenser coils 1. Any variations in operating conditions which would require a change in the quantity of cooling water supplied to the coil l are compensated for by changing the speed of the fan [3 and therefore the quantity of the air moving countercurrent to the cooling water. Thus, the temperature of the cooling water is varied to return operating conditions to those desired. An important advantage of this system is that there results a minimum wear on and power consumption by the cooling tower fan. Complete flooding of the condenser coils has the advantage of retarding scale formation which frequently results when.

the quantity of cooling water is varied when conditions require less than enough cooling water to flood the coils. Localized evaporation of the cooling fluid is prevented and thus scale deposition on the coil surface which would otherwise result is prevented. Additionally, the best principles of heat transmission indicate that it is desirable to maintain a relatively high velocity of cooling liquid with relatively low temperature mean difference for best efiioiency. This is accomplished by this system.

Similar parts in the system of Figure 2 have been given the same reference numerals. In this system the overhead line 2 is connected to the condenser coils 1 as before but has no branch line. The discharge from the condenser coil 1 is delivered by the line H) to the accumulator line 3 as before. The condensate is delivered in proportioned amount back to the column I by means of pump 25 and is under the control of valve 2|] and rate of flow controller 2| as before. The overhead product withdrawn from the system in this case is in vapor form and is discharged from the accumulator 3 by means of the line 28. The rate of overhead vapor withdrawal is controlled by means of a diaphragm valve 29 in the line 28 and the pressure controller 30 which is actuated by and inaccordance with pressure changes in the tank 3. Thus, the pressure in the tank is maintained constant and the overhead vapors are Withdrawn through the line 28 at a rate to maintain this condition. The tank 3 is provided with a liquid level controller 21 which in turn actuates diaphragm valve l8 to vary the rate of steam supply through line IE to the turbine 14.

In the operation of this system as before the quantity of cooling water supplied through the line 2! is constant. Variations in operating conditions which require changes in the rate of condensation in the coil 1 show up as variations in the liquid level in the tank 3. These variations in liquid level through the agency of the liquid level controller 2! actuate the valve I8 to vary the speed of the turbine and hence of the fan l3. Fan speed changes vary the amount of air mov-,

ing countercurrent to the cooling water and change its temperature in the proper direction to vary the rate of condensation so that a predetermined level of liquid is maintained in tank 3.

A differentiating characteristic of this system is that only a part of the overhead vapors are condensed and the overhead product is removed from the system through the line 28 in the form of a vapor. To state it another way, only sufficient of the overhead vapors are condensed to supply the quantity of reflux liquid which it is desired to return to the column. By doing this, the condensed liquid will contain the greatest portion possible of the heaviest components, thus making a more effective reflux for the column. This results in a liquid in the top of the column which is further from the critical conditions than would be the case if a greater portion of the overhead vapors were condensed with only a part returned as reflux. Thus, the liquid being returned to the column is of the highest surface tension of any reflux obtained in this quantity. Therefore, the column has more capacity for handling reflux and a better separation will be obtained.

An additional advantage occurs in the subsequent processing of the fractionator overhead product if that product is uniform in quantity and quality. Uniformity of quantity and quality will be more nearly approached if a steady stream of vapor is withdrawn from the reflux accumulator instead of alternate slugs of vapor and liquid as would be the case if a portion of the overhead product were taken off as a liquid. This is particularly true if the vapors are handled through a recompressor as of course no liquid can be tolerated in a vapor stream to a compressor.

In this system there still remains the advantage previously mentioned that the operating conditions can be set so that the condensing coils 1 are always flooded, thereby preventing scale formation. Likewise the most efficient heat transfer conditions are maintained as previously explained.

It will be noted that in operating both systems the quantity of cooling water remains constant and the quantity of countercurrent air is varied. This method of operating is believed to be novel in this art.

A specific example of operation of the system r Feed Gallons Kettle Overhead Per Day Product Product The column was operated at 3'75 pounds gage and the system was-set so that the volume of reflux was 550,000 gallons per day. The temperature of the kettle was 290 F. and the temperature of the column at the top 130 F. The

reflux temperature was 90 F. Thus, the controller 9 of the system of Figure 2 was set to maintain the above pressure in the column with a kettle temperature of 290 F. The pressure controller was set to maintain a pressure in the accumulator 3 corresponding to a reflux temperature of 90 F. Of course, the liquid level controller 21 was adjusted to meet these conditions. With the system thus set up and with a constant flow of cooling water through the line 21 the speed of fan l3 was varied in accordance with variations of the liquid level in the tank 3 to maintain the desired'reflux temperature of 90 F.

From the above description it will be apparent to those skilled in the art that the novel apparatus and methods herein disclosed. are capable of some variation without departing from the novel subject matter herein disclosed. I do not therefore desire to be strictly limited by the disclosure but only as required by the appended claims.

What is claimed is:

1. Amethod of fractionating a liquid mixture which comprises, continuously passing a liquid mixture to a fractional distillation zone at a constant rate, removing a stream of vapors from the top of said distillation zone, cooling said vapor stream in indirect heat exchange with a stream of water supplied at a constant rate, and a stream of air supplied at a variable rate, and controlling said cooling by varying the rate of air supplied and condensing a portion of said vapor stream to a liquid, passing the resulting vapor liquid mixture to an accumulation zone, the cooling of said liquid being controlled responsive to the level of liquid in said accumulation zone, and passing a stream of said condensed liquid at a constant rate from said accumulation zone to said distillation zone as a reflux liquid and as the sole liquid stream removed from said accumulation zone, controlling said cooling of said vapor stream responsive to the liquid level in said accumulation zone so as to maintain said liquid at a substantially constant level, and withdrawing from said accumulation zone a vapor stream as a product of the process.

2. A method of fractionating a liquid mixture which comprises continuously passing the liquid.

mixture to a fractional distillation zone at a constant rate, removing a stream of vapors from the top of said distillation zone, cooling said vapor stream in indirect heat exchange with a stream of water supplied at a constant rate, and a stream of air supplied at a variable rate, and controlling said cooling by varying the rate of air supplied, and condensing a portion of said vapor stream to a liquid, passing the resulting vapor liquid mixture to an accumulation zone, the cooling of said liquid being controlled responsive to the temperature of the liquid in said accumulation zone and passing a stream of said condensed liquid at a constant rate from said accumulation zone to said distillation zone as a reflux liquid, and controlling the cooling of said vapor stream responsive to the temperature of liquid in said accumulation zone while maintaining said liquid at a substantially constant level therein and withdrawing from said accumulation zone the product of the process.

CLYDE PAGE STANLEY.

(References on following page) EEEEEENcEs CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS 5 Name Date Leslie Oct. 8, 1929 Fisher Mar. 14, 1939 Bichowsky May 7, 1940 10 Number Name Date Wetter Feb. 11, 1941 Mollenberg Mar. 4, 1941 Dube et a1. 'Apr. 15, 1941 Carney Mar. 24, 1942 Dewey et a1 Apr. 20, 1943 Houghland et a1. Aug. 29, 1944 Legatski Feb. 6, 1945 Williams July 13, 1948

Claims

1. A METHOPD OF FRACTIONATING A LIQUID MIXTURE WHICH COMPRISES, CONTINUOUSLY PASSING A LIQUID MIXTURE TO A FRACTIONAL DISTILLATION ZONE AT A CONSTANT RATE, REMOVING A STREAM OF VAPORS FROM THE TOP OF SAID DISTILLATION ZONE, COOLING SAID VAPOR STREAM IN INDIRECT HEAT EXCHANGE WITH A STREAM OF WATER SUPPLIED AT A CONSTANT RATE, AND A STREAM OF AIR SUPPLIED AT A VARIABLE RATE, AND CONTROLLING SAID COOLING BY VARYING THE RATE OF AIR SUPPLIED AND CONDENSING A PORTION OF SAID VAPOR STREAM TO A LIQUID, PASSING THE RESULTING VAPOR LIQUID MIXTURE TO AN ACCUMULATION ZONE, THE COOLING OF SAID LIQUID BEING CONTROLLED RESPONSIVE TO THE LEVEL OF LIQUID IN SAID ACCUMULATION ZONE, AND PASSING A STEAM OF SAID CON-