Control of a crude oil preheat furnace

The temperature of a crude oil provided from a crude oil preheat furnace to a crude tower is substantially minimized by controlling such temperature so as to maintain a desired minimum overflash flow in the crude tower. Also, the temperature of each of the individual coils in the preheat furnace are balanced with respect to each other. Such minimization and balancing results in substantially optimal utilization of the fuel provided to the preheat furnace which results in considerable economic savings with respect to the operation of the preheat furnace.

This invention relates to control of a preheat furnace. In one aspect this 
invention relates to method and apparatus for substantially minimizing the 
temperature of the crude oil feed provided from a crude oil preheat 
furnace to a crude tower so as to substantially minimize the cost of 
operating the crude oil preheat furnace. In another aspect, this invention 
relates to method and apparatus for balancing the individual coil 
temperatures in the crude oil preheat furnace. In still another aspect 
this invention relates to method and apparatus for maintaining the flow 
rate of the crude oil feed through each of the individual coils in the 
crude oil preheat furnace substantially equal. 
Crude oil is typically preheated in a furnace prior to being provided to a 
crude tower or separation of the various components contained in the crude 
oil feed. This preheating step represents a substantial economic cost in 
the processing of a crude oil feed because substantial fuel is required to 
supply the heat required. It is thus desirable to operate the furnace as 
economically as possible so as to minimize the cost of processing the 
crude oil feed. 
It is thus an object of this invention to provide method and apparatus for 
controlling a crude oil preheat furnace so as to substantially minimize 
the cost of operating the crude oil furnace. 
In accordance with the present invention, method and apparatus is provided 
whereby the temperature of the crude oil provided from a furnace to a 
crude tower is substantially minimized by controlling such temperature so 
as to maintain a desired minimum overflash flow in the crude tower. Such 
minimization of the crude oil temperature results in considerable economic 
savings with respect to the operation of the preheat furnace. 
Also in accordance with the present invention, method and apparatus is 
provided whereby the temperature of each of the individual coils in the 
preheat furnace are balanced with respect to each other. Such balancing 
results in substantially optimal utilization of the fuel provided to the 
preheat furnace which also contributes to minimizing the cost of operating 
the preheat furnace. 
Other objects and advantages of the invention will be apparent from the 
foregoing brief description of the invention and the claims as well as the 
detailed description of the drawings which are briefly described as 
follows:

The invention is described in terms of a crude oil preheat furnace having 
four coils. However, the invention is applicable to furnaces having 
different numbers of coils. As an example, the overflash control aspect of 
the present invention would be applicable to a furnace having only one 
coil. 
A specific control system configuration is set forth in FIG. 1 for the sake 
of illustration. However, the invention extends to different types of 
control system configurations which accomplish the purpose of the 
invention. Lines designated as signal lines in the drawings are electrical 
or pneumatic in this preferred embodiment. Generally, the signals provided 
from any transducer are electrical in form. However, the signals provided 
from flow sensors will generally be pneumatic in form. Transducing of 
these signals is not illustrated for the sake of simplicity because it is 
well known in the art that if a flow is measured in pneumatic form it must 
be transduced to electrical form if it is to be transmitted in electrical 
form by a flow transducer. Also, transducing of the signals from analog 
form to digital form or from digital form to analog form is not 
illustrated because such transducing is also well known in the art. 
The invention is also applicable to mechanical, hydraulic or other signal 
means for transmitting information. In almost all control systems some 
combination of electrical, pneumatic, mechanical or hydraulic signals will 
be used. However, use of any other type of signal transmission, compatible 
with the process and equipment in use, is within the scope of the 
invention. 
A digital computer is used in the preferred embodiment of this invention to 
calculate the required control signals based on measured process 
parameters as well as set points supplied to the computer. Analog 
computers or other types of computing devices could also be used in the 
invention. The digital computer is preferably an OPTROL 7000 Process 
Computer System from Applied Automation, Inc., Bartlesville, Okla. 
Signal lines are also utilized to represent the results of calculations 
carried out in a digital computer and the term "signal" is utilized to 
refer to such results. Thus, the term signal is used not only to refer to 
electrical currents or pneumatic pressures but is also used to refer to 
binary representations of a calculated or measured value. 
The controllers shown may utilize the various modes of control such as 
proportional, proportional-integral, proportional-derivative, or 
proportional-integral-derivative. In this preferred embodiment, 
proportional-integral-derivative controllers are utilized but any 
controller capable of accepting two input signals and producing a scaled 
output signal, representative of a comparison of the two input signals, is 
within the scope of the invention. 
The scaling of an output signal by a controller is well known in control 
system art. Essentially, the output of a controller may be scaled to 
represent any desired factor or variable. An example of this is where a 
desired flow rate and an actual flow rate is compared by a controller. The 
output could be a signal representative of a desired change in the flow 
rate of some gas necessary to make the desired and actual flows equal. On 
the other hand, the same output signal could be scaled to represent a 
percentage or could be scaled to represent a temperature change required 
to make the desired and actual flows equal. If the controller output can 
range from 0 to 10 volts, which is typical, then the output signal could 
be scaled so that an output signal having a voltage level of 5.0 volts 
corresponds to 50 percent, some specified flow rate, or some specified 
temperature. 
The various transducing means used to measure parameters which characterize 
the process and the various signals generated thereby may take a variety 
of forms or formats. For example, the control elements of the system can 
be implemented using electrical analog, digital electronic, pneumatic, 
hydraulic, mechanical or other similar types of equipment or combinations 
of one or more such equipment types. While the presently preferred 
embodiment of the invention preferably utilizes a combination of pneumatic 
final control elements in conjunction with electrical analog signal 
handling and translation apparatus, the apparatus and method of the 
invention can be implemented using a variety of specific equipment 
available to and understood by those skilled in the process control art. 
Likewise, the format of the various signals can be modified substantially 
in order to accommodate signal format requirements of the particular 
installation, safety factors, the physical characteristics of the 
measuring or control instruments and other similar factors. For example, a 
raw flow measurement signal produced by a differential pressure orifice 
flow meter would ordinarily exhibit a generally proportional relationship 
to the square of the actual flow rate. Other measuring instruments might 
produce a signal which is proportional to the measured parameter, and 
still other transducing means may produce a signal which bears a more 
complicated, but known, relationship to the measured parameter. Regardless 
of the signal format or the exact relationship of the signal to the 
parameter which it represents, each signal representative of a measured 
process parameter or representative of a desired process value will bear a 
relationship to the measured parameter or desired value which permits 
designation of a specific measured or desired value by a specific signal 
value. A signal which is representative of a process measurement or 
desired process value is therefore one from which the information 
regarding the measured or desired value can be readily retrieved 
regardless of the exact mathematical relationship between the signal units 
and the measured or desired process units. 
Referring now to the drawings and in particular to FIG. 1, a crude oil feed 
is illustrated as flowing through conduit means 12. The crude oil feed 
flowing through conduit means 12 is divided into two parts which flow 
through conduit means 14 and 15 respectively. In like manner, the crude 
oil flowing through conduit means 15 is divided into two parts which flow 
through conduit means 16 and 17 respectively and the crude oil flowing 
through conduit means 14 is divided into two parts which flow through 
conduit means 18 and 19 respectively. The crude oil flowing through 
conduit means 18 and 19 is recombined at conduit means 21 after passing 
through the furnace 11. In like manner the crude oil flowing through 
conduit means 16 and 17 is recombined at conduit means 23 after passing 
through the furnace 11. The crude oil flowing through conduit means 21 and 
23 is combined in conduit means 24 and is provided through conduit means 
24 to the crude tower 26. 
Conduit means 16, 17, 18 and 19 are considered individual coils of the 
furnace 11. Fuel is supplied through conduit means 31 to a burner 32 which 
supplies heat to coils 18 and 19. In like manner, fuel is supplied through 
conduit means 41 to a burner 42 which supplies heat to coils 16 and 17. 
Signal 51, which is provided from the crude tower 26, is representative of 
the actual overflash flow rate in the crude tower 26. The overflash in the 
crude tower is the feed which is flashed above the feed tray where the 
feed enters the crude tower in excess of what is required to satisfy the 
flow rates required of the side draw streams from the crude tower (the 
side draw streams are not illustrated since the side draws play no part in 
the description of the present invention.). Any excess overflash must be 
compensated for by increasing the heat removal in the crude tower 26 and 
thus it is desirable to maintain the overflash flow at some low value such 
as about 3 to about 3.5% above the minimum level required to maintain the 
desired side draw flow rates. The overflash flow rate can be measured by 
employing a liquid trap tray immediately above the feed tray and measuring 
flow in external piping that carries liquid from the trip tray to the feed 
tray. 
The process measurements required for control of the temperature of the 
crude oil flowing through conduit means 21 and the temperature of the 
crude oil flowing through conduit means 18 and 19 as well as the flow rate 
of crude oil through conduit means 18 and 19 is identical to that required 
for the lower portion of the furnace 11. Thus, for the sake of convenience 
and clarity, only the process measurements required for the upper coils 18 
and 19 will be described hereinafter. However, it should be recognized 
that these same process measurements would be required for the lower coils 
16 and 17. 
Temperature transducer 53 in combination with a temperature sensing device 
such as a thermocouple, which is operably located in conduit means 21, 
provides an output signal 54 which is representative of the actual 
temperature of the crude oil flowing through conduit means 21. Signal 54 
is provided from the temperature transducer 53 as an input to computer 
100. 
In like manner, temperature transducers 56 and 57 in combination with 
temperature sensing devices such as thermocouples, which are operably 
located in conduit means 19 and 18 respectively, provide output signals 58 
and 59 respectively which are representative of the actual temperature of 
the crude oil flowing through conduit means 19 and 18 respectively. Signal 
58 is provided from temperature transducer 56 as an input to computer 100. 
In like manner, signal 59 is provided from temperature transducer 57 as an 
input to computer 100. 
Flow transducer 61 in combination with the flow sensor 62, which is 
operably located in conduit means 19, provides an output signal 63 which 
is representative of the actual flow rate of crude oil through conduit 
means 19. Signal 63 is provided from the flow transducer 61 as an input to 
computer 100. 
In like manner, flow transducer 65 in combination with the flow snesor 66, 
which is operably located in conduit means 18, provides an output signal 
68 which is representative of the actual flow rate of crude oil through 
conduit means 18. Signal 68 is provided from the flow transducer 65 as an 
input to computer 100. 
The temperature of the feed flowing through conduit means 24 is essentially 
maintained by manipulating the flow of fuel through conduit means 31 and 
41. The balancing of the temperature between the coils 18 and 19 is 
accomplished by manipulating the split of the crude oil flowing through 
conduit means 14. Also, to the extent possible, the flow rate of crude oil 
through coils 18 and 19 is maintained substantially equal by manipulating 
the split of the crude oil flowing through conduit means 14. This control 
is accomplished by utilizing three control signals provided from computer 
100 in response to the described process measurements. These control 
signals are briefly described hereinafter. 
Signal 71 is provided from computer 100 as the setpoint input to the flow 
controller 72. Signal 71 is representative of the flow rate of fuel 
through conduit means 31 required to maintain the actual temperature of 
the crude oil flowing through conduit means 21 substantially equal to a 
temperature which will result in a desired minimum overflash flow in the 
crude tower 26. In like manner, a similar control signal is provided as 
control signal 74 from computer 100 to flow controller 75. Manipulation of 
the temperature of the crude oil flowing through conduit means 21 and 23 
in this manner will result in a desired minimum crude oil temperature for 
the crude oil flowing through conduit means 24 which results in a desired 
minimum overflash flow rate. 
Flow transducer 77 in combination with the flow sensor 78, which is 
operably located in conduit means 31, provides an output signal 79 which 
is representative of the actual flow rate of fuel flowing through conduit 
means 31. Signal 79 is provided as the process variable input to the flow 
controller 72. 
In response to signals 71 and 79, the flow controller 72 provides an output 
signal 81 which is responsive to the difference between signals 71 and 79. 
Signal 81 is scaled so as to be representative of the position of the 
control valve 82, which is operably located in conduit means 31, required 
to maintain the actual flow rate of the fuel through conduit means 31 
substantially equal to a desired flow rate represented by signal 71. 
Signal 81 is provided from the flow controller 72 as a control signal for 
the control valve 82 and the control valve 82 is manipulated in response 
thereto. 
Flow transducer 84 in combination with the flow sensor 85, which is 
operably located in conduit means 41, provides an output signal 86 which 
is representative of the actual flow rate of fuel flowing through conduit 
means 41. Signal 86 is provided as the process variable input to the flow 
controller 75. 
In response to signals 74 and 86, the flow controller 75 provides an output 
signal 88 which is responsive to the difference between signal 74 and 86. 
Signal 88 is scaled so as to be representative of the position of the 
control valve 89, which is operably located in conduit means 41, required 
to maintain the actual flow rate of the fuel through conduit means 41 
substantially equal to a desired flow rate represented by signal 74. 
Signal 88 is provided from the flow controller 75 as a control signal for 
the control valve 89 and the control valve 89 is manipulated in response 
thereto. 
Control signal 91 is provided from computer 100 to control valve 92 which 
is operably located in conduit means 19. Signal 91 is scaled so as to be 
representative of the position of control valve 92 required to maintain 
the actual temperature of the crude oil flowing through conduit means 19 
substantially equal to the temperature of the crude oil flowing through 
conduit means 18. Also, if required, signal 91 may be biased so as to 
prevent the flow rate of the crude oil flowing through conduit means 19 
from deviating by some desired margin from the average of the crude oil 
flow rates through conduit means 16, 17, 18 and 19. Control valve 92 is 
manipulated in response to signal 91. 
In like manner, computer 100 provides control signal 94 to control valve 95 
which is operably located in conduit means 18. Signal 94 is similar to 
signal 91 and control valve 95 is manipulated in response to signal 94. 
Control signal 97 is provided from the computer to control valve 98 which 
is operably located in conduit means 17. In like manner, control valve 
signal 99 is provided from computer 100 to control valve 102 which is 
manipulated in response to control signal 99. As has been previously 
stated, the generation of signals 97 and 99 will not be described in 
detail because this generation is identical to the generation of signals 
91 and 94 except for their application to different coils. 
Referring now to FIG. 2, signal 51, which is representative of the actual 
overflash flow rate in the crude tower 26, is provided as the process 
variable input to the overflash controller 201. The overflash controller 
201 is also provided with a set point signal 202 which is representative 
of the desired overflash flow rate in the crude tower 26. 
In response to signals 51 and 202, the overflash controller 201 provides an 
output signal 204 which is responsive to the difference between signals 51 
and 202. Signal 204 is scaled so as to be representative of the 
temperature of the crude oil flowing through conduit means 24 required to 
maintain the actual overflash flow rate substantially equal to the desired 
overflash flow rate represented by signal 202. 
In some cases, signal 204 could be supplied directly to the temperature 
controller 207 as the set point input. An example of one case in which 
this could be easily done is the case where there is only one coil in the 
furnace. However, where there are multiple coils in a furnace such as 
illustrated in FIG. 1, application of signal 204 as the set point to each 
of the required temperature controllers could result in different exit 
temperatures for each coil because of temperature controller offset and 
different process dynamics within each coil. It is thus desirable to bias 
signal 204 in such circumstances as will be described hereinafter. 
Signal 54 which is representative of the actual temperature of the crude 
oil flowing through conduit means 21 is supplied as the process variable 
input to controller 211 and also to the temperature controller 207. 
Controller 211 is also supplied with a set point signal 212 which is 
representative of the average of the temperatures of the crude oil flowing 
through conduit means 21 and 23. While this signal is represented as a set 
point, it should be recognized that this signal would be supplied by 
process measurement. 
In response to signals 54 and 212, controller 211 provides an output signal 
214 which is responsive to the difference between signals 54 and 212. 
Signal 214 is a bias signal which compensates for factors such as 
temperature controller offset and different process dynamics e.g. delays 
in different coils. Signal 214 is provided from controller 211 to the dead 
time compensation block 216. 
Because each coil contains significant dead time on the order of about 2 to 
about 3 minutes, it is preferred to include dead time compensation for 
stability. The preferred dead time compensation is preferably a well-known 
Smith predictor. After passing through the dead time compensation block 
216, the bias signal represented by signal 214 is supplied as signal 217 
to the summing block 218. Signal 204 and 217 are summed in summing block 
218 to establish signal 219. Signal 219 is representative of the 
temperature of the crude oil flowing through conduit means 21 required to 
maintain a desired crude oil temperature for the crude oil flowing through 
conduit means 24 when the temperature control of the present invention is 
applied to all coils and also to maintain the temperature of the crude oil 
flowing through conduit means 21 substantially equal to the temperature of 
the crude oil flowing through conduit means 23. Signal 219 is provided 
from the summing block 218 as the set point input to the temperature 
controller 207. 
In response to signals 54 and 219, the temperature controller 207 provides 
an output signal 71 which is responsive to the difference between signals 
54 and 219. Signal 71 is representative of the flow rate of fuel through 
conduit means 31 required to maintain the actual temperature of the crude 
oil flowing through conduit means 21 substantially equal to the desired 
temperature represented by signal 219 as has been previously described. 
Signal 71 is provided as an output control signal from computer 100 and is 
utilized as previously described. 
Signal 58, which is representative of the actual temperature of the crude 
oil flowing through conduit means 19, is provided as a first input to the 
averaging block 231 and is also provided as the process variable input to 
the temperature controller 232. In like manner, signal 59 which is 
representative of the actual temperature of the crude oil flowing through 
conduit means 18, is provided as a second input to the averaging block 231 
and is provided as the process variable input to the temperature 
controller 234. 
The averaging block 231 provides an output signal to 235 which is 
representative of the average of the temperatures of the crude oil flowing 
through conduit means 18 and 19. Signal 235 is provided as the set point 
input to the temperature controller 232 and also as the set point input to 
the temperature controller 234. 
In response to signals 58 and 235, the temperature controller 232 provides 
a output signal 236 which is responsive to the difference between signals 
58 and 235. Signal 236 is scaled so as to be representative of the 
position of the control valve 92 required to maintain the actual 
temperature of the crude oil flowing through conduit means 19 
substantially equal to the average temperature represented by signal 235. 
It is noted that signal 236 could be provided directly to the contol valve 
92. However, it is desirable to also maintain the flow rate of crude oil 
through conduits 18 and 19 substantially equal to the average flow rate of 
crude oil through all coils of the furnace 11. This is accomplished by 
summing signal 236 with a bias signal produced as described hereinafter. 
Signal 63, which is representative of the actual flow rate of crude oil 
through conduit means 19, is provided as the process variable input to the 
flow controller 238. The flow controller 238 is also supplied with a set 
point signal 239 which is representative of the average flow rate through 
all of the coils of the furnace 11. Signal 239 is also supplied as a set 
point input to the flow controller 241 and would be a calculated average 
of measured variables since all of the flow rates through each of the 
coils in the furnace 11 would be available to computer 100 if the entire 
control system were illustrated. 
In response to signal 63 and 239, the flow controller 238 provides an 
output signal 243 which is responsive to the difference between signals 63 
and 239. Signal 243 is considered a bias signal. However, because it is 
not possible to balance both the flows and temperatures due to 
interactions, flow controller 238 preferably contains a dead band such as 
750 barrels per hour. Thus, signal 243 will have a magnitude of 0 unless 
the flow rate of crude oil through conduit means 19 varies from the 
average value by more than the dead band value. Only when the actual flow 
rate varies from the average flow rate by more than the dead band value 
will signal 243 have a magnitude. 
Signals 236 and 243 are summed in a summing block 245 to establish signal 
91 which has been previously described. Signal 91 is provided as a control 
output from computer 100 and is utilized as has been previously described. 
In response to signals 59 and 235, the temperature controller 234 provides 
an output signal 251 which is responsive to the difference between signals 
59 and 235. Signal 251 is scaled so as to be representative of the 
position of control valve 95 required to maintain the actual temperature 
of the crude oil flowing through conduit means 18 substantially equal to 
the average temperature represented by signal 235. As was the case with 
signal 236, signal 251 could be applied directly to the control valve 95 
but is preferably biased by summing signal 257, which is generated as 
previously described for signal 243, with signal 251 in the summing block 
258 to establish signal 94. The magnitude of signal 94 has been previously 
described. Signal 94 is provided as a control output from computer 100 and 
is utilized as previously described. 
In summary, control based on the overflash flow rate in the crude tower is 
utilized in accordance with the present invention to maintain a minimum 
temperature for the crude oil feed flowing through conduit means 24. At 
the same time, the temperature of the crude oil feed flowing through 
conduit means 21 and 23 is balanced and the temperature of the crude oil 
feed flowing through coils 16, 17, 18 and 19 is balanced. Also, to the 
extent possible, the flow rate of crude oil through coils 16, 17, 18 and 
19 is balanced. Control in this manner results in a substantially optimum 
operation of the furnace 11 both from a process standpoint and an economic 
standpoint. 
The invention has been described in terms of a preferred embodiment as 
illustrated in FIGS. 1 and 2. Control components which can be utilized in 
the practice of the invention as illustrated in FIG. 1 such as flow 
transducers 61, 65, 77 and 84; flow controllers 72 and 75; temperature 
transducers 53, 56 and 57 and control valves 82, 89, 92, 95, 98 and 102 
are each well-known, commercially available components such as are 
illustrated at length in Perry's Chemical Engineers Handbook, 4th Edition, 
Chapter 22, McGraw-Hill. 
While the invention has been described in terms of a preferred embodiment, 
reasonable variations and modifications are possible by those skilled in 
the art. Such variations and modifications are within the scope of the 
present invention, as claimed.