DETERMINATION OF TEMPERATURE SET
POINTS FOR CRUDE OIL
3.1 Introduction
Study of Temperature set point in Chemical industries happen to be an important feature. Here we give an illustration how fuzzy control method is adopted for finding precise temperature set point to distil different crude in an oil refinery. Oil that comes from the ground is called the “crude oil”. By cooking, the crude is converted to useful oil. Here the temperature set point plays a vital role at the time of cooking the crude oil. Since the quality and quantity of the crude is dependent on the
temperature set point, the crude oil refinery has different temperature, set points to distil different crudes. Here, this chapter tries to determine a precise temperature set point for the crude oil refinery to maximise the distillation of the crude and the quantity of the crude for long hours. This chapter six sections.
This study is significant because most of the crude oil
refineries have common type of operating systems. The analysis of this study is focussed on Kalundborg Refinery [Ebbesen (1992)]. Here we approach the problem of finding the precise 47
temperature set point for different crudes using fuzzy control theory. The data is taken from Kalundborg Refinery [Ebbesen (1992)].
In 1995, Friedman developed a Mass and Enthalpy balance
method and used it to improve the quality of crudes. In 1992, Ebbesen studied about the crude operating in Kalundborg
Refinery. He made some derivations from the theory of
Friedman. Finally he gave a range of temperature set points for the distillation of different crudes. However at the end of his study he made it clear that in the case of kerosene, 90% stayed within 10C of its set point of temperature, in the case of naphtha 95% distillation stayed within 10C of its set point of
temperature. After two hours, the quality during crude switches was different indicating a lower quality.
Here we establish the result using the data taken from
Ebbesen(1992). This gives temperature set point for the
distillation of kerosens, naphtha and gasoil. The range of temperature set points and the various percentage of distillation are converted into the fuzzy control theory. Here membership grade is assigned to each temperature set point and percentage of distillation and fuzzy control rule is used to each temperature set point and percentage of distillation. Finally center max-min rule is used, to find the precise temperature set point for kerosene, naphtha and gasoil.
3.2 Description of Crude Oil Refineries
Using the data available from Ebbesen(1992) of the Kalundborg oil refinery, we analyse the data via fuzzy rules and membership grades of fuzzy control theory method and find the precise temperature set points for different crudes to maximize the quality and distillation of crude for long hours. Crude oil refinery selects temperature set points randomly from the range of temperature set points for the distillation of the crude; as a result, the quality and quantity of the processed crude are maintained only for very few hours. Thus, to be more precise the aim to find the precise temperature set points for kerosene, 48
naphtha and gasoil using the data of Ebbesen (1992) from the Kalundborg oil refinery.
Kalundborg oil refinery operates with different crudes on a regular basis. Here this crude oil refinery distils kerosene, naphtha and gasoil. A schematic diagram of various streams is shown in the following figure [Ebbessen(1992)].
NAPHTHA
OVERHEAD
Q(TPA)
Q(REFLUX)
Q(MPA)
STEAM
KEROSENE
Q(EFA)
STEAM
LIGHT GASOIL
STEAM
HEAVY GASOIL
OVERFLASH
FIGURE 3.2.1: CRUDE REFINERY
where TPA – denotes the top pump-around, MPA – denotes mid pump-around and the BPA-bottom pump-around respectively.
Q-denotes the heat removed. These are mainly used for
controlling the temperature.
The random choice of temperature set points for different crudes with distillation taken from Ebbesen(1992) are described for kerosene, naphtha and gasoil.
49
Range of temperature set points for kerosene with the percentage of distillation
The crude oil refinery gives 2270 C-temperature set point for distillation of kerosene. The crude oil refinery selects randomly this 2270 C temperature set point from the given range of set points {2200 C, 2210 C, 2220 C, 2230 C, 2240 C, 2250 C, 2260 C, 2270 C, 2280 C, 2290 C, 2300 C}. This temperature set point 2270
C gives 90% distillation and it says within 10 C of its set point of temperature. The temperature graph is given below.
Graph 3.2.1: Graph depicting the 90% distillation of Kerosene 230
229
228
T
S
227
P
226
225
224
223 0
60
120
180
X axis– time in minute and Y axis set point of temperature for Kerosene. Set point was 227oC
Legend
TSP: Temperature set points
From this Ebbesen (1992) concludes that for the set point 2270
C the distillation of kerosene was 90%.
Range of temperature set point of naphtha with percentage of distillation
The crude oil refinery gives 1600 C-temperature set point for the distillation of naphtha. The crude oil refinery selects randomly 50
this 160o C temperature set point from the given range of set points {155o C, 156o C, 157o C, 158o C, 160o C, 161o C, 162o C, 163o C, 164o C, 165o C}. This temperature set point 160o C gives 95% distillation and it stays within 1o C of its set point of temperature.
The temperature graph is given below.
Graph 3.2.2: Graph depicting the 95% distillation of Naphtha 165
164
163
162
T
S
161
P
160
159
158
157 0
60
120
180
X axis– time in minute and Y axis set point of temperature for naphtha. Set point was 160oC
From this Ebbesen(1992) concludes that for the set point 1600 C
the distillation of naphtha was 95%.
Range of temperature set point of gasoil with the percentage of distillation
The temperature set point -4.50 C gives 95% distillation and it stays within 10 C of its set point temperature.
The temperature graph is given below.
51
Graph 3.2.3: Graph depicting the 95% distillation of gasoil
-2.00
-2.50
-3.00
T
S -3.50
P
-4.00
-4.50
-5.00
0
60
120
180
X axis– time in minute and Y axis set point of temperature for naphtha. Set point was –4.50oC
From this Ebbesen(1992) concludes that for the set point -4.50 C
the distillation of gasoil was 95%.
3.3 Determination of Temperature Set-Point of Kerosene Resulting in Better Distillation Using Fuzzy Control Theory The given possible ranges of temperature set points are {2200 C, 2210 C, 2220 C, 2230 C, 2240 C, 2250 C, 2260 C, 2270 C, 2280 C, 2290 C, 2300 C} and possible percentages of distillation are (88%, 89%, 90%, 91%, 92%} in case of kerosene as observed by Ebbesen(1992). As fuzzy control theory is the tool adaptable only when the past performance data is available, now this chapter considers the given possible range of temperature set points and distillation as the inputs of fuzzy control theory. To identify the precise temperature set points from the possible range of temperature set points, this chapter assigns membership grades to each input of fuzzy control theory. Here, the fuzzy control theory is used to find a precise temperature set point for kerosene.
52
In the procedure developed here membership grade is from the interval [0, 1] to the input of each temperature set point and each percentage of distillation. After membership grades are assigned to each input of temperature set points, the following graph results representing the membership grades of
temperatures set point.
223oC
225oC
227oC 229oC
231oC
M
S
G
0 T1 T2
T3
T4
T5 T6
T7
T8
T9 T10
temperature
FIGURE 3.3.1: Membership function of temperature set point After membership grades are assigned to each input of
distillation, the following graph results representing the membership grades of distillation.
88% 89%
90%
91%
92%
M
S
G
0
D1 D2
D3
D4
D5
D6
D7
D8
D9 D10 distillation
Legend
MSG: Membership Grade
T: Temperature
D: Distillation
FIGURE 3.3.2: Membership function of percentage of distillation The membership grade varies from 0 to 1.
For getting precise temperature set point for kerosene the throttle variables(The grade of membership) are qualified into five subsets. Here fuzzy rules are used to find the possible 53
percentage of distillation for each temperature set point and the Center Max-Min rule is used to find a throttle membership grade for the existing fuzzy rules. To get the grade of
membership to each existing fuzzy rule throttle variables are qualified into five subsets as follows:
N3 N2 Z P2 P3
M
S
G
-1 0.5 0
0.5
1
N3: Very Big Negative
N2: Big Negative
Z : Normal
P2: Big Positive
P3: very Big Positive
FIGURE 3.3.3: Throttle values
If T is 223oC and D is
87% then throttle is P3
Temperature
223oC
set points
225oC
227oC
If T is 223oC and D is
229oC
88% then throttle is P2
231oC
If T is 223oC and D is
88%
89% then throttle is Z
89%
Distillation
90%
91%
If T is 223oC and D is
92%
90% then throttle is N2
T: Temperature set point D: Percentage of Distillation
FIGURE 3.3.4:Fuzzy rules for the temperature set point 223oC
54
The following are the fuzzy rules :
Rule -1 : If T is 2230 C temperature set point AND D is 87%
THEN throttle is P3.
Rule -2 : If T is 2230 C temperature set point AND D is 88%
THEN throttle is P2.
Rule -3 : If T is 2230 C temperature set point AND D is 89%
THEN throttle is Z.
Rule -4 : If T is 2230 C temperature set point AND D is 90%
THEN throttle is N2.
We conclude the throttle value to the temperature set point for 2230 C, by the above stated rules, only rule -2, and rule-3 are applicable that is distillation is 88% and 89% respectively.
Rule-2
The throttle value to the temperature set point 2230 C for 88%
distillation is calculated using figures 4.2 and 4.3.
Throttle = (0.41+0.38)/2 = 0.395
The graphical representation of the membership grade of the temperature set point 2230 C for 88 percentage of distillation is as follows.
Graph 3.3.1: Graphical representation of Rule 2
1
0.41
M
S
G 0
Temperature
0.395
1
M
0.3
0
Throttle
S
G
0
Distillation
55
Graph 3.3.2: The two outputs are then defuzzified
by center max-min rule
1
M
S
G
0
Legend
MSG: Membership grade
■ ■ : 0.41 Membership grade
• • : 0.38 Membership grade
Rule -3
The throttle value to the temperature set point 2230 C for 89%
distillation from figures 4.2 and 4.3 is as follows
Throttle=(0.47+.45)/2=0.46
The graphical representation of the membership grade of the temperature set point 2230 C for 88% percentage of distillation is as follows:
Graph 3.3.3: Graphical representation of Rule 3
1
0.47
M
S
G 0
Temperature
0.46
1
M
0.4
0
Throttle
S
G
0
Distillation
56
Graph 3.3.4: The two outputs are then defuzzified
by center max-min rule
1
M
S
G
0
Legend
MSG: Membership grade
■ ■ : 0.47 Membership grade
• • : 0.45 Membership grade
Here, the Center Max-Min rule is used to find a precise
temperature set point.
Using Center Max-Min rule to find precise temperature set point for kerosene
Throttle(grade of membership) = m(P3) × Location(P2) + m(Z)
+ Location(N2) = 0.427. Graphs for the other rules have not been given explicitly but after calculations, the values are given as the same procedure is adopted.
If T is 225oC and D is
Temperature
223oC
88% then throttle is P3
set points
225oC
227oC
229oC
If T is 225oC and D is
231oC
89% then throttle is P2
If T is 225oC and D is
88%
90% then throttle is Z
89%
Distillation
90%
If T is 225oC and D is
91%
91% then throttle is N2
92%
T: Temperature D: Distillation
FIGURE 3.3.5:Fuzzy rules for the temperature set point 225oC
57
The fuzzy rule for distillation of kerosene:
Rule -1 : If T is 2250C temperature set point AND D is 88%
THEN throttle is P3.
Rule -2 : If T is 2250C temperature set point AND D is 89%
THEN throttle is P2.
Rule - 3 : If T is 2250C temperature set point AND D is 90%
THEN throttle is Z.
Rule - 4 : If T is 2250C temperature set point AND D is 91%
THEN throttle is N2.
Rule -2
The throttle value to the temperature set point 2250C for 89%
distillation is calculated as follows.
Throttle=
(.47+.45)/2=0.46,
Rule -3
The throttle value to the temperature set point 2250C for 90%
distillation is calculated as follows.
Throttle=
(.51+.23)/2=0.37,
Using Center Max-Min rule to find precise temperature set point for kerosene
Throttle(grade of membership) = m(P3) × Location(P2) +
m(Z) + Location(N2)
= 0.415,
If T is 227oC and D is
Temperature
223oC
89% then throttle is P3
set points
225oC
227oC
229oC
If T is 227oC and D is
231oC
90% then throttle is P2
If T is 227oC and D is
88%
91% then throttle is Z
89%
Distillation
90%
If T is 227oC and D is
91%
92% then throttle is N2
92%
FIGURE 3.3.6:Fuzzy rules for the temperature set point 227oC
58
The fuzzy rule for distillation of kerosene:
Rule -1 : If T is 2270C temperature set point AND D is 89%
THEN throttle is P3.
Rule -2 : If T is 2270C temperature set point AND D is 90%
THEN throttle is P2.
Rule - 3 : If T is 2270C temperature set point AND D is 91%
THEN throttle is Z.
Rule - 4 : If T is 2270C temperature set point AND D is 92%
THEN throttle is N2.
Here we calculate the throttle value to the temperature set point for 2270 C, by the above stated rules, only rule -2, and rule-3 are applicable that is distillation is 90% and 91%
respectively.
Rule-2
The throttle value to the temperature set point 2270 C for 90%
distillation is calculated as follows:
Throttle
=
(0.51+0.23)/2
= 0.37,
Rule-3
The throttle value to the temperature set point 2270 C for 91%
distillation is calculated as follows.
Throttle
=
(0.17+0.59)/2
= 0.38,
Using Center Max-Min rule to find precise temperature set point for kerosene
Throttle(grade of membership) = m(P3) x Location(P2) +
m(Z) + Location(N2)
=
0.375,
59
If T is 229oC and D is
Temperature
223oC
90% then throttle is P3
set points
225oC
227oC
229oC
If T is 229oC and D is
231oC
91% then throttle is P2
If T is 229oC and D is
88%
92% then throttle is Z
89%
Distillation
90%
If T is 229oC and D is
91%
93% then throttle is N2
92%
FIGURE 3.3.7:Fuzzy rules for the temperature set point 229oC
Here we obtain the throttle value to the temperature set point for 2290 C, by the above stated rules, only rule -2, and rule-3 are applicable that is distillation is 91% and 92%
respectively.
Rule-2
The throttle value to the temperature set point 2290 C for 91%
distillation is calculated as follows.
Throttle = (0. 17 + 0.59)/2 = 0.38,
Rule-3
The throttle value to the temperature set point 2290 C for 92%
distillation is calculated as follows.
Throttle = (0.4 + 0.4)/2 = 0.4,
Using Center Max-Min rule to find precise temperature set point for kerosene
Throttle(grade of membership) = m(P3) × Location(P2) +
m(Z)
+
Location(N2)
=
0.420,
We have taken the range of temperature set points for
distillation of kerosene from the crude oil refinery [Ebbesen 60
(1992)] to find a precise temperature set point. This data is analysed with rules of fuzzy control theory. The fuzzy rules expressed in terms of degree of membership grade to each temperature set point. Finally the ultimate membership grade was obtained using Center Max-Min rule for the distillation of kerosene.
It has been observed that the highest membership grade
using Center Max-Min rule was given to the temperature set point 2230C.
3.4 Determination of Temperature Set Point of Naphtha Resulting in Better Distillation using Fuzzy Control Theory The given possible ranges of temperature set points are {1550C, 1560C, 1570C, 1590C, 1600C, 1610C, 1620C, 1630C, 1640C,
1650C} and possible distillation are {93%, 94%, 95%, 96%, 97%} in the case of naphtha as observed by Ebbesen(1992).
Using this data as inputs of fuzzy control theory, we identify the precise temperature set points from possible range of
temperature set points. Now membership grade is assigned to the input of each temperature set point and each percentage of distillation. The following graph is represents the membership grades of temperature set point.
156oC
158oC
160oC 162oC
164oC
M
S
G
0 T1 T2
T3
T4
T5 T6
T7
T8
T9 T10
temperature
FIGURE 3.4.1: Membership grade of temperature set points After assigning membership grades in the interval [0,1] to each input of the percentage of distillation the following graph is 61
obtained representing the membership grades of percentage of distillation.
93% 94%
95%
96%
97%
M
S
G
0
D1 D2
D3
D4
D5
D6
D7
D8
D9 D10 distillation
Legend
MSG: Membership Grade
T: Temperature set point
D: Distillation
FIGURE 3.4.2: Membership function of percentage of distillation The membership grade varies from 0 to 1.
For getting precise temperature set point for naphtha the throttle variables(the grade of membership) quantified into five subsets. Here fuzzy rules are used to find the possible
percentage of distillation for each temperature set point and the Center Max-Min rule is used to find a throttle membership grade for the existing fuzzy rules. To get the grade of
membership to each existing fuzzy rule, throttle variables are quantified into five subsets as follows:
N3 N2 Z P2 P3
M
S
G
-1 0.5 0
0.5
1
N3: Very Big Negat