US3022635A

US3022635A - Method and apparatus for controlling periodically reversed heat transfer devices

Info

Publication number: US3022635A
Application number: US767005A
Authority: US
Inventors: Duran L Hagier; Martin L Kasbohm
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1958-10-13
Filing date: 1958-10-13
Publication date: 1962-02-27
Anticipated expiration: 1979-02-27
Also published as: GB889498A; DE1877536U; FR1240182A

Description

Feb. 27, 1962 D. L. HAGLER ETAL 3,022,635

. METHOD AND APPARATUS FOR CONTROLLING PERIODICALLY REVERSED HEAT TRANSFER DEVICES Filed Oct. 13, 1958 4 Sheets-Sheet 1 ACCUMULATOR No.2

ACCUMULATOR No.1

ACCUMULATOR TEMPERATURE AT INTERMEDIATE POINT (IN C.)

. TIME W 1 PERlODi PERIODZ CYCLE I INVENTORS DURAN L. HAGLER MARTIN L. KASBOHM Feb. 27, 1962 o. L HAGLER ETAL 3,022,635

METHOD AND APPARATUS FOR CONTROLLING PERIODICALLY REVERSED HEAT TRANSFER DEVICES Filed Oct. 13, 1958 4 Sheets-Sheet 2 INVENTORS DURAN L. HAGLER MARTIN L. KASBOHM A T TORNE V Feb. 27, 1962 HAGLER TAL 3,022,635

METHOD AND APPARATUS FOR CONTROLLING PERIODICALLY REVERSED HEAT TRANSFER DEVICES Filed Oct. 15, 1958 4 Sheets-Sheet 3 6 99 295 A 300 i 39 z 3 w- 7 .6: CIRCUITS INVENTORS 4 Shets-Sheet 4 DURAN L. HAGLER MARTIN L. KASBQHM WAM z; /v r D. L HAGLER ETAL METHOD AND APPARATUS FOR CONTROLLING PERIODICALLY REVERSED HEAT TRANSFER DEVICES Feb. 27, 1962 Filed Oct. 13, 1958 United States Patent 3,022,635 METHOD AND APPARATUS FOR CONTRGLLING PERIODICALLY REVERSED HEAT TRANSFER DEVICES Duran L. Hagier, Kenmore, N.Y., and Martin L. Kasbohm, Indianapolis, Ind., assignors to Union Carbide Corporation, a corporation of New York Filed Oct. 13, 1958, Ser. No. 767,005 13 Claims. (Cl. 62--21) This invention relates to a method of and apparatus for controlling the operation of periodically reversed heat transfer devices employed for effecting heat exchange between gaseous fluids, particularly when the gaseous fluid to be cooled contains condensable material.

Cold accumulators or regenerators are often employed for cooling gaseous fluids for low temperature processes. Such devices are particularly useful when the gaseous fluid to be cooled contains condensable material which is deposited on the heat storage surfaces from the gaseous fiuid when it passes through one of a set of such accumulators. Simultaneously, a cold outflowing gas to be warmed is passed outwardly through another of the set of accumulators and effects evaporation of the condensable material that was deposited when that accumulator was used to cool inflowing gaseous fluid.

For example, such heat transfer or storage devices may be used in a process for the low-temperature separation of air to provideoxygen and nitrogen component gases. The latter cold gases discharged from the separation column may pass through cold accumulators to effect cooling of the compressed air. During the air cooling step, water vapor and low-boiling condensables such as carbon dioxide and hydrocarbons are frozen out of the air and deposited on the heat storage surfaces. The air may flow in through one accumulator while a purge stream, for example, the colder nitrogen, flows outwardly through another accumulator. The flows are periodically reversed so that the nitrogen flows out through an accumulator through which the air had flowed during a previous period of operation. Thus, the operation of cold accumulators proceeds in a cyclic manner and is usually accomplished by means of automatic valves which direct the inlet and purge gas streams alternately between the accumulators. A cycle is a complete program of valve operation and includes an inlet gas flow period and a purge gas flow period for each accumulator in the set;

The operation of cold accumulators or regenerators is described in more detail in US. Patent No. 1,970,299 of M. Fr'anld.

The successful operation of cold accumulators or regenerators over an extended period of time depends upon the system being "self-cleaning with respect to any condensable matter which is deposited in the heat storage mass. The measure of self-cleaning is the ability of a periodically reversed heat transfer system to purge itself of low-boiling condensables deposited within the system by the inlet gas to the extent that an excessive pressure drop does not build up over a desired operating period, eg, one year. The self-cleaning characteristics at any given level within the condensable matter deposition zone are dependent upon the relative condensable matter carrying capacities of the inlet feed gas and the product streams flowing past that level. The carrying capacity of the outgoing product stream must be at least equal to that of the inlet feed gas in order to prevent progressive accumulation of condensable matter deposits. The condensable matter carrying capacity of either stream is a function of its temperature, and the relative carrying capacity of the vtwo streams is therefore dependent on the temperature difference. In many applications, e.g. air

' inlet gas to purge gas flow in all passages.

3,022,635 Patented F eb. 27, 1962 separation processes, the specific heats of the inlet gas and purge gas vary to unequal degrees with changing temperature. The specific heat of the inlet gas may be approximately equal to that of the purge gas at the Warm end but considerably higher than the purge gas at the cold end. This causes excessive temperature differences at the cold end of the accumulator which'are extremely unfavorable for removal of deposits. In such cases, a self-cleaning condition may not be achieved by simply passing the same molar volumes of purge and inlet gas through the accumulators. In the prior art. this problem is alleviated, that is, the accumulators may be made selfcleaning, through the expedient of unbalancing the inlet and purge flow rates. At least two basic methods of flow unbalance are used: either by increasing the cold fluid flow relative to the warm gas flow in the colder section of the passages, or by reducing the quantity of incoming gas passed though the colder section. The effect of such flow unbalance is to reduce the average temperature differences in the deposition zone by altering the ratio of total outflow to total inflow and thereby create conditions favorable for self-cleaning in the reversing heat transfer system.

In addition to providing the requisite amount of flow unbalance or self-cleaning capacity for a reversing heat transfer system, it is also necessary to properly proportion the cleaning capacity, between the accumulators so that all accumulators in the system are maintained in a self-cleaning state.

In a cold accumulator, the temperaturet variations and the mass flow ratio are closely related. This fact may be utilized in proportioning the gas flow quantities among a set of accumulators so as to maintain the same ratio of When the flow ratios are properly equalized, it may be said that each accumulator is receiving its share of the cleaning capacity of the system, and similar temperature difference patterns are simultaneously established. Equalization control is therefore the maintenance of'similar temperature difference patterns throughout the lengths of two or more periodically reversed accumultors. If such stable temperature difference patterns are not maintained, one accumulator will be cleaned more thoroughly than necessary, and this will be done at the expense of another accumulator which willaccumulate excessive deposits of condensables. Furthermore, dissimilar temperature diiference patterns will allow the warm end temperature difference of one accumulator to increase, resulting in a greater loss of refrigeration in the product stream.

One currently used method of accumulator cycle control requires a multiple cam timer, usually with a separate cam provided for operating each switching valve. Signals are transmitted from the cams to valves either 'electricallyor pneumatically. The principal disadvantage of such timer controlled cycles is their lack of flexibility, which makes it difficult to perform graduated adjustments to suit variations of temperature and pressure in the inlet and purge gas streams. Such variations may affect the heat exchange efliciency and self-cleaning ability of a given accumulator, and flexibility is needed in the adjustment period to compensate for moderate changes.

One presently used system for equalization control requires two alternate sets of timing cams with the first set adjusted to favor warming one accumulator and the second adjusted to favor warming the other of the accumulator pair. Approximate equalization is maintained by manually switching the control from one set of cams to the other, thereby limiting the maximum deviation from perfectly equalized conditions. Accumulator temperatures or pressure drops are observed to indicate need for a control change.

It will be apparent that the degree of efliciency which cur.

can be obtained by manual equalization control with two sets of cams is dependent on the care and attention of the operator. Experience has shown that the average midpoint (lengthwise) .temperature of an uncontrolled cold accumulator pair may drift apart as much as 20 C. over the interval of a few reversing cycles. Frequent attendancesis required to obtain reasonably efficient operation, and in actual practice occasional periods of considerable deviation in average temperature dilference oc- When severe thermal upsets occur as might be caused by momentary malfunctioning of one of the switching valves, the double-cam system is further limited in the rate of correction which can be efiected by switching sets of cams. The corrective force available in a set of cams designed to accommodate moderate process variable deviation may be insufiicient to correct a severecondition rapidly enough to prevent permanent fouling by deposited condensables in one accumulator.

control variable.

In present practice with the double-cam system, severe upsets are corrected by stopping the timer motor momentarily in the proper reversing period to favor equalzing the temperature differences between the accumulators.

This is a manual treatment requiring promptness and skill onthe partof the operator.

In: another well-known system, equalization is maintained automaticallyby varying the proportion of warm inlet gas to each accumulator by means of throttling valves, using the cold end temperature difference as the Disadvantages of throttling type systems are their complexity and' expense, and, the probability of pressure surges on the system due to periodic restriction of the inlet gas flow.

A principal object of the present invention is therefore to provide an improved method of and apparatus for controlling the operation of periodically reversed accumulators employed for effecting heat exchange between gaseous fluids. More specifically, it is an object of the present invention to. provide amethod of and apparatus for automatic. cycle and equalization control is periodical- 1y reversed heat storage' devices such as cold accumulators, so that excessive accumulation of condensable mateiral is avcided therein. A further object is to provide an automaticrcycle and equalizationscontrol system in which the'temperature differences between pairs of periodically reversed heat'storage devices are cont-rolled within predetermined limits so as to minimize refrigeration losses in such devices. A still'further object is to provide an automatic cycle and equalization control systween temperature and cycle time of a pair of periodically reversed accumulators;

FIGURE 2 'is a diagrammatic view of an exemplary portion of the controlling apparatus employing an elecsystem for cooling a compressed air stream containing low-boiling condensable material such as carbondioxide by means of cold air components such as gaseous oxygen and nitrogen, it is to be understood that the invention is equally suitable for other gases containing low-boiling condensables, such as' crude hydrogen containing the lighter weight hydrocarbons.

The end regions of a cold accumulator, e.g. the coldest and warmest zones, are not idealregions for measuring temperature variations for control purposes. There are two main reasons for this situation: first, because the range of temperature variation in these regions is relatively small, necessitating sensitive measuring devices, and second, because the temperature changes in these regions are slow or delayed in--refiecting a deviation from equalized conditions. It has been found that in a periodically reversed accumulator in which low-boiling condensables are deposited from cooling inlet gas, the ideal point for sensing temperature for control purposes is a thermal level intermediate the warm end and the coldest zone of thepacked bed, and preferablyslightly warmer than the dew point of the low-boiling condensable material. apr essure of about '75 -p,s. i-.g. the optimum sensing level is about 1(}(-) C. If two or more cold accumulators are equalized, their intermediate levels will all be at approximately the same average temperature. If thermal deviation should occu'rphowever, it has been found that the immediate and most pronounced observable etfect is an increasing difference between the average temperatures at the aforementioned intermediate temperature level of a pair of accumulators. For example, in an air separation system. a 1 C. deviation in average ternperatures, may be observed. between the warm ends of a pair of accumulators while at an intermediate point a deviation of.25 C. may occur. Conversely, if the average temperature deviation between the intermediate levels of a pair is restricted to. a maximum of coldest zone of the accumulators, Control at this level also f acilitates more efficient heat transfer and improved self-cleaning as aresult of smaller deviations from perfectly equalized conditions. Anadvantage of controlling apparatus for controlling the operation of a pair of cold accumulators according to the invention, in which fluidfilled bellows comprise the process, variable comparison means; 7 FIGURE 3 is a fragmentary diagrammatic view of portion of the controlling apparatus employing an electrical bridge circuit as the process variable comparison means;

FIGURE 4 is a fragmentary diagrammatic view of a portion of the controlling apparatus employing a fluidfilled U-tube manometer as the process variable comparison means;

FIGURE 5'is a fragmentary diagrammatic view of electrically powered switches which may be used to actuate the gas flow reversing mechanism, and mechanically actuated holding means for maintaining the desired valve control circuit;

FIGURE 6 is a fragmentary diagrammatic view of a the temperatures at "a thermal level slightly above the dew point of the low-boiling condensables is that the temperature sensing means willnot be coated with such material, as would be the case vifthe temperature. were sensed within the dew point range of such condensables. If such coating occurs, the temperature measurement is aifected; by the coating and an erroneous condition not trulyrepresentative of the gas being processed is monitored. Thus, if suitable compensations to the temperature measurement are not made, a control system based on such variables may result in inefi'icient operation of the accumulators, Therefore, it is preferable to avoid the necessityof such compensations and their accompanying complexity, by sensing the operating variable at an intermediate level which is slightly warmer than the dew point of the condensable material.

According to the present invention, a method of cycling and thermally equalizinga plurality of periodi- For example, if inlet air isprovided at cally reversed accumulators or regenerators is employed for cooling an initially warm inlet gas containing lowboiling condensable material by a cold purge gas which is initially free of the condensable material. The condensable material is condensed from the initially warm inlet gas and is substanially completely evaporated into the initially cold purge gas, such evaporation being assisted by maintaining a relatively small difference between the average temperatures of the cooled inlet gas and the cold purge gas prior to warming. The periodic reversal is efiected in a succession of periods during one of which the initially warm inlet gas flows from the warm end to the coldest zone of a first accumulator while the initially cold purge gas flows from the coldest zone to the Warm end of a second accumulator. During a subsequent period of the cycle, the initially warm inlet gas flows from the warm end to the coldest zone of the second accumulator while the initially cold gas flows from the coldest zone to the warm end of the first accumulator. The temperature is continuously sensed at a thermal level in each accumulator intermediate the warm end and coldest zone. The sensed temperatures are continuously compared, and the warm and cold gas flows are switched, i.e. the period is advanced, when the temerature difference reaches a predetermined value. In the case of most gases processed by the method of the present invention, the inlet gas, e.g. compressed air, has a higher specific heat than the purge gas, e.g. low pressure air components. Also, the total mass of gas flowing from the coldest zone towards the warm end of the accumulators is preferably greater than the total mass of gas flowing in a direction from the warm end towards the coldest zone of such accumulators.

It can be seen from the foregoing discussion that in order to maintain an efiicient self-cleaning system, the temperature is sensed at an intermediate point between a warm end and the coldest zone of the accumulator where normal temperature changes are great and where deviations from normal temperatures are magnified, as contrasted with the warmest and coldest zones where temperatures tend to remain more stable regardless of the performance of the accumulators.

The temperatures may, for example, be sensed by fluid-filled bulbs placed in the accumulators at the desired intermediate level. The bulbs may communicate pneumatically with a fluid-filled U-tube manometer as the temperature comparison means, the latter having upper level electrical contacts in either arm of the U-tube as the means for actuating the gas fiow reversing mecha nism. Alternatively, a pair of fluid-filled bellows communicating with the bulbs may comprise the temperature comparison means, and a moving electric contact connected with and actuated by movement of the bellows pair may serve as the means for actuating the gas flow reversing mechanism.

Still another alternative sensing device is a temperature sensitive solid element having a suitable coefficient of thermal expansion. A dimensional change in the element caused by varying temperature may be employed to actuate an electric contact at the desired temperature level.

Thermocouples or variable resistance elements such as temperature sensitive wires may also berused to sense temperature in the accumulators and such elements could, for example, form part of an electrical bridge, i.e. the Wheatstone type, as the temperature comparison means. Electric contacts may be operated by the bridge and may in turn actuate the gas flow reversing mechanism.

The temperatures for two accumulators which are continuously monitored and compared will vary in a cyclic, convergent-divergent pattern. Similarly, the rather delicate switch mechanism actuated by the temperature sensing devices w ll oscillate in a cyclic manner. After closing of one of the switches initiates a cycle change, the temperature swing in the accumulator bed will cause the switch to open almost immediately, which would adverse- 1y affect the flow reversing mechanism and the cycle. To avoid this situation, the present invention contemplates holding means for maintaining a steady electrical signal to the flow reversing mechanism even though the switch which closed to initially establish the circuit has reopened after such establishment. The holding means maintaining the signal during one period of the cycle is released only when the predetermined temperature difference is again reached. When this condition occurs, the switch moves to a second position which actuates the gas flow reversing mechanism to initiate the next period of the cycle. The second electrical circuit established thereby is then maintained by the holding means, which may be either mechanically or electrically actuated and released.

Referring now to the drawings and particularly to FIG- URE l, the temperature of the fluids in a pair of accumulators at the previously described intermediate sensing level is plotted against the cycle time. Separate curves for each accumulator are obtained which criss-cross in an alternately convergent-divergent pattern. In each curve, the upwardly extending portions, denoting rising temperature, coincide with the inlet air flow period when the heat storage mass is being warmed. Conversely, the downwardly extending portions, denoting falling temperature, coincide with the purge gas fiow period when the heat storage mass is being cooled. When the temperature difference reaches a predetermined maximum value, e.g. 25 q C., the flow reversing mechanism is actuated to prevent further divergence of the temperatures. The temperatures now begin to swing in the opposite direction since the accumulator formerly being warmed is now being cooled and the accumulator formerly being cooled is now being warmed. As long as the respective temperatures of the inlet warm gas and the oppositely flowing cold purge gas are relatively steady, it is not necessary to relate the temperature difference measurements to an absolute reference temperature in order to avoid an overall temperature drift of the system. In this way the cycle time is set and the accumulators are forced to run in balance. It has been found experimentally that the control system of the present invention permits operation of cold accumulators with intermediate level temperature variations held to 2 C. or less. The control system of the present invention is in no way related to time, and thus eliminates the necessity of expensive and complicated timing mechanisms. However, as long as there are no upsets in the heat exchange system, the limiting temperature diiierences will be reached at reasonably regular intervals so that the flows will be reversed regularly, e.g. every 2 to 3 minutes. if thermal or flow upsets do occur, the relative lengths of the two periods of each cycle will vary as required to maintain equilibrium conditions.

FIGURE 2 illustrates the invention by an embodiment adapted to control the reversible operation of a pair of cold accumulators N-l and N-2 used to cool and clean air'by an outflowing cold product of air separation, such as the nitrogen product by reversing the flows between the accumulator packing beds.

Air under a relatively low pressure, for example, 75 p.s.i.g., is supplied through a conduit it to branches l1 and 12 which connect conduit 16 to the warm ends of the cold accumulators N-l and N2. The cooled air alternately leaves the cold ends of the accumulators through conduits l3 and 14 which join an air delivery conduit 15. The nitrogen product is supplied as a purge gas through conduit 16 connected to

branches

17 and 16 that conduct the nitrogen to the cold ends of the accumulators N-l and N-Z. The warmed nitrogen flows from the accumulators through branch conduits l9 and 2! that connect to a discharge conduit 21. The conduits associated with the warm ends of the accumulators are controlled by a gas flow reversing mechanism including reversing valves which preferably are stop valves that are motor operated, for example, by electrically responsive means. Thus, conduit 11 has therein an electrically actuated stop valve trical contacts 6t) and 61.

, converging.

reversing valve 23-while the branches? and 26 for nitrogen are controlled. by reversing

valves

24 and 25. The connections associated with the cold ,end of the accumulators are preferably supplied with automatically operated nonreturn valves or check valves indicated at 26, 27, 28, and 29 interposed in the

respective branches

13, 14, 17, and 18. The check valves 26 and '27'are arranged to permit outflow only of cold cleaned air from accumulator N-l or N-2 while the

check valves

28 and 29 are arranged to permit inflow only of cold nitrogen gas to the accumulators N1 or N2 as purge gas.

The temperature of the fluids passing through the intermediate zone of the cold accumulators N1 and N-2 is continuously sensed by fluid-filled

bulbs

39 and 31, respectively, embedded in the accumulator packing mass 32. The bulbs communicate with fluid-filled bellows 33 .manner as shown in FIGURE 1, and this results in cyclic 1 expansion and contraction of

bellows

33 and 34 of FIGURE 2; This in turn causes'a corresponding cyclic movement of switch 39 which alternately closes elec-

Contacts

60 and 61 energize and de-energize valve circuits A'and B respectively containing relay coils 44 and 45.

Coils

44 and 45 control the position of

relay contacts

46 and 47 such that when coil 4 is energized, contacts 46 are thereby closed and 47 are opened. When coil 45 is energized, contacts 46 V are opened and 47 are closed;

Relay contacts

46 and 47 in turn actuate automatic

flow reversing valves

22, 23, 24, and 25.

The position or contacts shown in FIGURE 2 corresponds to a time in the cycle when. accumulators N1 and N-Z are approaching their warm and cold limits, 7 respectively. Switch 39 is moving in a direction to close circuit through electrical conduit 49, contacts 47, branch line 51, valve 23, branch conduit 52, and conduit 53. It

1 also completes the circuit to valve 24 through

branch conduits

54 and 55. This opens, air inlet valve 23 on:

accumulator N2 and nitrogen outlet valve 24 on accumulator N-l. Simultaneously, the opening of relay contact 46 de-energizes the circuits to air inlet valve 22 on accumulator N-l and to nitrogen outlet valve 25 on accumulator N-2 through similar

electrical branch conduits

56, 57, 58, and 59.

7 The flow reversal described above corresponds to the end of period 1 of FIGURE 1. Immediately after the flow reversal, the temperatures stop diverging and begin 7 The convergence continues through the point of equality and until. the temperatures again diverge in the opposite direction.

perature difierence, pole 38 will close contacts 60 and thus complete valve circuit A containing coil 44 through

electrical conduits

41, 42, and 48. This will again reverse the position of

relay contacts

46, 47 which inturn will reverse the flows through accumulators N-l and N-2.

In FIGURE 3,'

therma1 resistance elementsv

130 and 131, eg calibrated Wires instead of gas-filled bulbs, are located in the accumulator beds to provide the temperature sensing means, and form part of a bridge circuit 132 which includes

resistances

133 and 134, as well as power source 135. When a limiting temperature difference is reached between the

thermal resistance elements

130 and 131, the current flow through galvanometer coil 180 will be sufiicient to deflect pole 138 of single pole-double throw switch 139 to contact either point A or B, thereby stablishing the corresponding valve control circuit and actuating the valve reversing mechanisms as previously described.

FIGURE 4 illustrates still another control apparatus embodiment of the present invention, in which

fluid conduits

235 and 236 are connected to fluid-filled bulbs in a pair of accumulators and communicate by means of

adapters

281 and 282, respectively, with

arms

283 and 284 of mercury-filled U-tube manometer; 285. Bypass connection 236 and valve 287 therein are provided 'as communicating means between

arms

283 and 284 to equalize the pressure in such arms before'the accumulator system is placed in operation. Valve 283 in base 289 of the U-tube 285 permits drainage of the mercury fluid therefrom, or adjustment of the level as desired. Electrodes 290 and 291'are provided in U-tube arms 283 and 284-, respectively, at equal heights above the base 289,

. justment in the mercury level.

At a point corresponding 7 to the end of period 2 in FIGURE 1, e.g. at 25 C. temand

electrical conduits

294 and 295 provide the necessary electrical connections between such electrodes 'an'd'suitable valve reversing meanssuch as valve circuits A and B, respectively, of FIGURE 2. Electrode 292 in the base of U-tube285. and electrical conduit 293 connected thereto provides the necessary contact to complete both valve circuits. 7 7

'The U-tube control system operates as follows: a temperature change in one of the gas-filled bulbs results in a change of pressure which produces a corresponding ad- As one accumulator warms and the other cools, the difference in mercury levels in the two manometer arms increases until contact is made with one of the

electrodes

290 or 291. Since mercury is an electrical conductor, the corresponding valve circuit isrestablished and the flow reversing mechanism actuated. It is important that the manometer be equalized bymeans of the bypass connection 286 prior to commencement of operation when the accumulators have equal intermediate temperatures. This assures that the temperature swings in all accumulators will be about the same and hence, the temperature dilference patterns will be similar. The total permissible temperature'swing of the accumulators may be adjusted by varying the quantity of mercury in the manometer. Reducing the quantity of mercury will require a greater temperature difference to cated that

contacts

46 and 47 of the valve reversing a relay are held inthe desired position throughout a given period in spite of the fact that switch 39 may reopen immediately due to the cyclic variation of the'temperatures. FIGURE 5 illustrates a toggle-type latch suitable as a mechanical'holding means for the valve reversing relay. The toggle device is illustrated as a spring 301 compressed between pinned anchor member 302 and projection303 on relay shaft 296. As shown in the drawing,

relay contact 297 is held forcibly closed without assist- 'ance from'solenoid coils 295, due to the horizontal component of the spring force being exerted towards the left on shaft 296 by spring 301. When coil 299 is energized, the shaft 296 moves to the right against the force of spring 301 so that contacts 297 open and contacts 360 close. Movement of shaft 296 moves projection 3.03 past dead center so that the force of spring 301 is now exerted toward the right and maintains pressure on contacts .300. Mechanical pressure may thus be maintained on

contacts

297 or 300 even though

coils

295 and 299 are de-energized.

FIGURE 6 illustrates a control system utilizing. an electrical bridge of the Wheatstone type as the temperature comparison means and electrically actuated means for holding the valve control circuits A and B. Temperatures are measured by

resistance elements

130a and 131a located physically at a suitable intermediate level in each of a pair of accumulators. When the limiting temperature difference between the accumulators is reached sufficient current flows through galvanometer coil 13011 to deflect pole 138a of single pole-double throw switch 13911 to contact point Bas illustrated. Relay coil 304 is energized and shaft 305 is pulled towards such coil, thereby releasing holding pole 306 from contacts 307, and closing control valve circuit pole 308 against contacts 309 to establish control valve circuit B and actuate the gas flow reversing mechanism. At the same time, holding pole 310 attached to shaft 311 drops by gravity against contacts 312 and an electrical holding circuit 313 for coil 304 is established thereby making valve control circuit B independent of contact B in switch 139a. Thus, when pole 138a leaves contact point B because of temperature swing, the circuit to coil 304 will not be broken. The latter will occur only when the temperature swing reaches the opposite extremity so that pole 138a reaches contact A of double throw switch 139a. At this point, relay coil 314 is energized and shaft 311 is drawn towards such coil thereby separating pole 310 from contacts 312. This breaks holding circuit 313, deenergizes relay coil 304 and opens valve control circuit B. With coil 304 de-energized, pole 306 drops against contacts 307 by gravity and simultaneously control valve circuit pole 315 drops against contacts 316 to establish valve control circuit A. Closing contacts 307 establishes electrical holding circuit 317 so that when pole 138a moves away from contact A of switch 139a, valve control circuit A is not opened.

FIGURE 7 illustrates one embodiment of the present invention which may be used to control the operation of periodically reversed cold accumulators of the double end type, as described in United States Patent No. 2,735,278 to P. K. Rice. That is, instead of introducing a warm inlet air stream at one end and progressively cooling-such air through the entire length of the accumulator bed for withdrawal at the opposite end as a cold gas, the warm inlet air is introduced at each end of the accumulator and withdrawn at a cold center zone. Simultaneously, the cold purge gas enters the cold center zone of the other accumulator and flows in either direction towards the warm ends for discharge therefrom. In the operation of such an accumulator, both ends must be cycled coincidently, but since their flow ratios are subject to variation as in separate accumulators, they may have different equalization requirrnents. This problem is solved by continuously sensing the temperature at the previously described intermediate level of each end of the double-end accumulator, comparing the two temperatures, and adjusting the inlet air flow between the two warm ends so that the sensed temperatures are equal at all times. The two ends of the double-end accumulator are then equalized.

Two double-end accumulators are controlled together in the previously described manner; that is, the intermediate level temperature in one double-end accumulator is compared with the intermediate level temperature of the other accumulator. When the temperature difference reaches a predetermined value, the inlet air and the cold purge gas flows are reversed. It is to be noted that in double end accumulator pairs there are four intermediate sensing levels whereas in the more conventional single end accumulator pairs there are only two intermediate sensing levels.

Referring now more specifically to FIGURE 7, a con- '10 trol system for the reversible operation of a pair of double-ended cold accumulators N-3 and N-4 is shown. Low pressure air is supplied from a central manifold (not shown) and passed through

inlet conduits

401a and 402a to the warm ends 403 and 404, respectively, of accumulator N-3. The inlet air flows toward the cold center zone 405 where it is discharged through conduit 406 as cold, clean gas and passed to a rectification column (not shown) for separation therein. Simultaneously, cold product gas, e.g. nitrogen, supplied through conduit 407 and control valve 408 therein, enters the cold center zone 409 of accumulator N-4 and flows toward the warm ends 419 and 411 for discharge through

conduits

412 and 413, respectively, as warm impurity laden purge gas. The temperautre in each end is continuously sensed at intermediate levels 414 and 415 of accumulator N3, and

intermediate levels

416 and 417 of accumulator N-4 by sensing means 418, 419, 420, and 421 for control of the double end accumulator pair. The sensing means may, for example, be electric resistance elements of the type described in conjunction with FIGURE 4. The temperatures sensed by

elements

418 and 419 are transmitted by

electrical conduits

422a and 423a, respectively, to flow controller 424a where the temperatures are compared. Signals are transmitted from flow controller 424a through

conduits

425a and 426a to throttling

valves

427a and 428a, respectively, to adjust the air inlet flows in conduits 4 01 and 402 so that the sensed temperatures are substantially identical for both sections of the accumulator.

Throttling valves may be either pneumatically or electrically operated; alternatively, flow controller 424a may provide a visual comparison of the sensed process variable and throttling valves 427 and 428 may be manually adjusted on the basis of this visual comparison. The two sections of accumulator N-4 are also equalized in the 'air inlet flow to the two sections of accumulator N-3 is adjusted. Alternatively, the temperatures could be sensed only during the purge stroke, and the nitrogen outlet flow adjusted by means of

valves

443 and 444. In the latter case,

electrical conduits

425a and 426a would run from flow controller 424a to

valves

443 and 444, respectively.

With the two sections of each accumulator equalized, the accumulator pair may be equalized and cycled in the previously described manner. The temperatures sensed by elements 419 and 421 in the accumulators N-3 and N4, respectively, are transmitted by electrical conduits 434 and 435 to swing controller 436 where the temperatures are compared, for example, in the manner described in conjunction with FIGURE 3. When the temperature ditterence reaches a. predetermined value, either valve circuit A or B is actuated and the flow reversing mechanism is placed in operation. For example, when air is flowing through accumulator N-3 and nitrogen through accumulator N4, the former will warm and the latter will cool until the limiting temperature difierence is reached, at which point signals will be transmitted through conduit 437 to close

air inlet valves

438 and 439 communicating with accumulator N-3, and

nitrogen outlet valves

440 and 441 communicating with accumulator N4. Simultaneously, signals are transmitted from swing controller 436 through conduit 442 to open nitrogen out- .let

valves

443 and 444 communicating with accumulator N3, and

air inlet valves

445 and 446 communicating with accumulator N-4. Thus, the inlet air is switched to accumulator N-4 and the cold nitrogen flow is switched to accumulator N-3, and the valve circuit is maintained by a holding mechanism which may be either mechanically or electrically operated as illustrated in FIGURES 5 and 6, respectively. When the temperatures have swung sutfi- V s ngers ciently for the opposite extremity to be reached, the valve positions are again reversed by signals transmitted from swing controller 436 through

conduits

437 and 442.

.tions of said first accumulator so that the temperatures Although preferred embodiments of the invention have t free of the condensable material, for'condensing such material from the inlet gas, and for substantially completely evaporating such material into the purge gas while maintaining a relatively small temperature difierence between the cooled inlet gas and the cold purge gas prior to warming, the reversal being effected by a succession of com- I coldest zone of a first accumulator while the initially cold purge, gas flows fromtthe coldest zone to a warm end of a second accumulator and during at least one subseperature within each of said accumulators at a thermal level intermediate a warm end and the coldest zone thereof, continuously comparing the sensed temperatures, and reversing'the warm inlet and cold purge gas flows between said accumulators each time in response to the difference between the temperatures reaching a predetermined value; and independently of a fixed time interval. V I

2. A method according to claim'l for cycling andthermally equalizing a plurality of periodically reversed accumulators, in which said cold purge gas has a lower specific heat than said warm inlet gas and in which the total mass of. gas flowing from the coldest zone towards the warmend of said accumulators is greater than the total'massof gas flowing in a direction from the warm end towards the coldest zone of said accumulators.

3..- A method according to claim 1 for cycling and thermally equalizing a plurality of periodically reversed accumulators in which said temperature is sensed at a thermal level slightly warmer than the dew point of said low-boiling condensable material.

4. A method of cycling and thermally equalizing periodically reversed accumulators employed for cooling an initially warm inlet gas containing low-boiling condensable material by a coldpurge gas which is initially free of the condensable materialand has a lower specific heat than the warm gas, for condensing such material from the inlet gas, and for substantially completely evaporating such 7 material into the purge gas while maintaining 'a relatively small tempeerature'difference between the cooled inlet gas and the cold purge gas prior to warming, the reversal being effected by a succession of complete reversal cycles including a period during which the initially warm inlet gas flows from each warm end to the cold center zone of a first accumulator while the initially cold purge gas flows from the cold'center zone to the warm ends of a second accumulator and during at least one subsequent period of the cycle the initially warm. inlet gas news from each warm end to the cold center zone of said second accumulator and the initially cold purge gas flows from the cold center zone to the warm ends of said first accumulator, which method comprises continuously sensing the temperatures at corresponding thermal levels-intermediate eachwarm end and said cold center zone of said first accumulator, continuously comparing the sensed temperatures of said first accumulator, adjusting-the'flows of at least one-of the gasespassing through the twosecsensed at said corresponding thermal levels therein are substantially identical, continuously sensing the temperatures at corresponding thermal levels intermediate each warm end and said cold center zone of said second accumulator, comparing the sensed temperatures of said second accumulator, adjusting the flows of at least one of the gases passing through the two sections of said second accumulator so that the temperatures sensed at said corresponding thermal levels therein are substantially identical, continuously comparing a temperature sensed at an intermediate thermal level of said first accumulator with a temperature sensed at a corresponding intermediate thermal level of said second accumulator, and reversing the inlet and purge gas flows between said first and second accumulators each time in response to the difierence between the last mentioned intermediate level temperatures reaching a predetermined value; and independently of a fixed time interval.

5. Apparatus for controlling the operation of periodically reversed accumulators that cool an initially warm inlet gas containinglow-boiling coudensable material by an initially cold purge gas that is initially free of such material and has a lower specific heat than the cold gas, which apparatus comprises, in combination with gas flow reversing mechanism for periodically interchanging the flows of the warm inlet gasand the cold purge gas through countercurrent flow accumulators; so that the condensables deposited in an accumulator from-the inlet gas during one flow period are almost completely evaporated and swept out of said accumulator by thetpurge gas during a subsequent flow of the cycle; means for contemperatures, and means foractuating, said gas flow reversing mechanism in response-to saidsignal and producting'a signal when thedifference between said temperatures reaches a predetermined value.

6. Apparatus according to claim 5' for controlling the operation of periodically reversed accumulators, in which fluid-filled bulbs comprise the means for continuously sensing the temperatures, a fluid-filled U-tube manometer communicating with such' bulbs comprises said' means for continuously comparing the temperatures, and upper levelelectrical contacts in either arms ofrthe U tube comprise said means for actuating the gas flow reversing mechanism.

7. Apparatus according to claim 5 for controlling the operation'of periodically reversed accumulators, in'which fluid-filled bulbs comprise the means for continuously sensing the temperatures, a pair of fluid-filled bellows communicating with the bulbs comprise said means for continuously comparingthe temperatures; and an electric contact communicating with and" actuated by movement' of the bellowspair comprises said means for actuating-the gas-flow reversing mechanism.

8. Apparatus according to'claim 5 for controlling the operation of periodically reversed accumulators, in which variable resistance elements comprise the means for continuously sensing the temperatures, a bridge circuit including said variable resistance elements comprises said means for continuously comparing the temperatures, and an'electric contact means operable by said bridge circuit comprises said means for actuating the gasflow reversing mechanism. a

9. Apparatus according to claim 5" for'controllingithe operation of periodically reversed accumulators, in which 10. Apparatus according to claim 9 for controlling the operation of periodically reversed accumulators, in which said holding means is mechanically actuated and released.

11. Apparatus according to claim 9 for controlling the operation of periodically reversed accumulators, in which an electric holding circuit comprises said holding means.

12. Apparatus for controlling the operation of periodically reversed accumulators that cool an initially warm inlet gas containing low-boiling condensable material by an initially cold purge gas that is initially free of such material, which apparatus comprises, in combination with gas flow reversing mechanism for alternately interchanging the flow of warm inlet gas from each warm end to the cold center zone of a first accumulator and the flow of cold purge gas from the cold center zone to each warm end of a second accumulator so that the condensables deposited from the inlet gas in an accumulator during one flow are almost evaporated and swept out of said accumulator by the urge gas during the following flow of the cycle, means for continuously sensing the temperatures at corresponding thermal levels intermediate each warm end and said cold center zone of said first accumulator, means for comparing the sensed temperatures of said first accumulator, means for adjusting the inlet gas flow to each Warm end of said first accumulator so that the temperatures sensed at corresponding thermal levels therein are substantially identical, means for continuously sensing the temperatures at corresponding thermal levels intermediate each warm end and said cold center zone of said second accumulator, means for comparing the sensed temperatures of said second accumulator, means for adjusting the inlet gas flow to each warm'end of said second accumulator so that the temperatures sensed at corresponding thermal levels therein are substantially identical, means for continuously comparing a temperature at an intermediate thermal level of said first accumulator with a temperature at a corresponding thermal level of said second accumulator, and meansv for actuating said gas flow reversing mechanism in response to said signal and producing a signal when the difference between the last mentioned intermediate level temperatures reaches a predetermined value.

13. Apparatus for controlling the operation of periodically reversed accumulators that cool an initially warm inlet gas containing low-boiling condensable material by an initially cold purge gas that is initially free of such 14 material, which apparatus comprises, in combination with gas flow reversing mechanism for alternately interchanging the flow of warm inlet gas from each warm end to the cold center zone of a first accumulator and the flow of cold purge gas from the cold center zone to each warm end of a second accumulator so that the condensables deposited from the inlet gas in an accumulator during one flow are substantially completely evaporated and swept out of said accumulator by the purge gas during the fol lowing flow of the cycle, means for continuously sensing the temperatures at corresponding thermal levels intermediate each warm end and said cold center zone of said first accumulator, means for comparing the sensing temperatures of said first accumulator, means for adjusting the purge gas flowing from each warm end of said first accumulator so that temperatures sensed at corresponding thermal levels therein are substantially identical, means for continuously sensing the temperatures at corresponding thermal levels intermediate each warm end and said cold center zone of said second accumulator, means for comparing the sensed temperatures of said second accumulator, means for adjusting the purge gas flow from each warm end of said second accumulator so that the temperatures sensed at corresponding thermal levels therein are substantially identical, means for continuously comparing a temperature at an intermediate thermal level of said first accumulator with a temperature at -a corresponding thermal level of said second accumulator, and means for actuating said gas flow reversing mechanism in response to said signal and producing a signal when the difierence between the last mentioned intermediate level temperatures reaches a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 1,055,699 Bibbins Mar. 11, 1913 2,058,491 Noble Oct. 27, 1936 2,407,036 Snavely Sept. 3, 1946 2,455,320 Stephens Nov. 30, 1948 2,730,875 Ranke Ian. 17, 1956 2,735,278 Rice Feb. 21, 1956 FOREIGN PATENTS 206,161 Australia Feb. 8, 1957 343,017 Great Britain Jan. 28, 1931 372,777 Great Britain May 11, 1932 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,022,635 February 27 1962 Duran L. Hagler et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 12, line 30, strike out "period"; column l3 line 20, for "urge" read purge column 14, line 13, for "sensing" read sensed Signed and sealed this 9th day of October 1962.

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patent: