A METHOD FOR MODELING AND MONITORING FOULING Field of Invention

The present invention relates to a method for modeling and monitoring the amount of fouling in a heat exchanger. Prior Art

Plants such as petroleum refineries make use of heat exchangers for providing heat transfer between the fluids used therein. Heat exchangers are typically structured to provide heat exchange between the fluids without involving any physical contact therebetween. According to the field of use and other requirements, heat exchangers with different structures are used. One of these is a "shell and tube type" heat exchanger comprising a plurality of tubes in a shell. In tubular heat exchangers, a fluid is passed through the tubes disposed in the shell and another fluid trough the shell, thereby providing heat exchange between these fluids.

Heat exchangers may foul up in time due to the fluids passing there through. Fouling, in turn, leads to a slowdown in the heat exchange between the fluids, namely it reduces the efficiency of the heat exchanger. For this reason, it is necessary to measure the amount of fouling (or the amount of heat exchange) in the heat exchanger in certain time intervals to keep the heat exchanger running at high efficiency. This measuring operation, however, cannot be conducted physically while the heat exchanger is operating. Therefore, various methods have been proposed and used for modeling the amount of heat exchange in heat exchangers according to the prior art. In these methods, data on the physical properties of the respective heat exchanger, data on the temperature and on the flow rate of the respective fluids, environmental factors, and many other parameters are used such that the amount of heat exchange is modeled using some complex formulas or equations.

In the patent document WO2008094757A1 according to the prior art is disclosed a method for measuring the amount of fouling in a heat exchanger. According to that method, the temperature of fluids which enter and leave the heat exchanger is measured at different times. Measured values are evaluated statistically (e.g. mean, standard deviation, etc.) and the amount of fouling in the heat exchanger is calculated accordingly. That method, however, is time taking and laborious since it requires many measurements for providing a correct fouling calculation.

Brief Description of Invention

A method is developed according to the present invention for modeling and monitoring the amount of fouling in a heat exchanger comprising at least one shell and at least one tube disposed in the shell to provide heat exchange between the fluids of different temperatures passed through said shell and tube, respectively. Said method comprises the steps of assigning an inlet and outlet temperature difference for each of the fluids passed through the tube and shell; calculating an iterative outlet temperature for each fluid using the assigned temperature differences and the measured inlet temperatures of the fluids; determining an expected outlet temperature for each fluid so as to equate it to the calculated iterative outlet temperature; calculating a mean temperature value for each fluid; calculating an iterative clean heat transfer coefficient for the heat exchanger using the physical properties under the calculated mean temperatures of the tube and shell; calculating a logarithmic mean temperature difference using the inlet temperatures and iterative outlet temperatures of the fluids; updating the iterative outlet temperatures for each fluid by solving some energy equations of the tube, the shell and of the heat exchanger using the calculated iterative clean heat transfer coefficient, the calculated logarithmic mean temperature difference, the physical parameters of the heat exchanger, the physical parameters of the tube and the shell, and the inlet temperatures of the fluids; comparing the updated iterative outlet temperatures to the expected outlet temperatures; returning to the step of determining an expected outlet temperature for each fluid if the difference between the updated iterative outlet temperature and the expected outlet temperature for at least one of the fluids is above a predefined value; calculating instantaneous values for the heat exchanger using the updated iterative outlet temperatures if the difference between the updated iterative outlet temperature and the expected outlet temperature for each fluid is below said predefined threshold value; and calculating the amount of fouling in the heat exchanger using the calculated instantaneous values and the calculated iterative clean heat transfer coefficient. By virtue of the method according to the present invention, the outlet temperatures of the fluids used in a heat exchanger are calculated iteratively. Thus, a fouling coefficient is calculated in a rapid and reliable manner for the heat exchanger using instantaneous outlet temperatures determined in the same manner. Additionally, a time-dependent fouling behavior can be monitored for the heat exchanger by repeating said method in certain time intervals and comparing the calculated fouling values of the heat exchanger.

Object of Invention The object of the present invention is to develop a method for modeling the amount of fouling in a heat exchanger.

Another object of the present invention is to develop a method, by which the amount of fouling in the heat exchanger is modeled in a simple manner.

A further object of the present invention is to develop a method for real-time monitoring the heat exchanger in terms of fouling.

Still a further object of the present invention is to develop a method for monitoring the time-dependent fouling behavior of the heat exchanger.

Description of Figures

Illustrative embodiments of a method for modeling and monitoring the amount of fouling in a heat exchanger according to the present invention are illustrated in the accompanying figures briefly described hereunder.

Figure 1 is a perspective section view of a heat exchanger to which the method according to the present invention is applied.

Figure 2 is a flowchart of the method according to the present invention.

The components in said figures are individually referenced as following:

Heat exchanger (E) Shell (S)

Tube (T)

Assigning a temperature difference (100)

Calculating an iterative outlet temperature (101 )

Determining an expected outlet temperature (102)

Finding mean fluid temperatures (103)

Calculating an iterative clean heat transfer coefficient (104)

Calculating a logarithmic mean temperature difference (105)

Updating the iterative outlet temperature (106)

Comparing the iterative temperature to the expected temperature (107)

Finding instantaneous values (108)

Calculating the amount of fouling in the heat exchanger (109)

Description of Invention

Plants such as petroleum refineries, where fluids at different temperatures are used, give place to heat exchangers for providing heat transfer between the fluids. Heat exchangers provide for the heat exchange between the fluids without involving any physical contact therebetween. However, heat exchangers can foul up in time due to the fluids and foreign matters possibly contained in the fluids. This fouling reduces the running efficiency of the heat exchanger. For this reason, a method is developed according to the present invention for modeling the amount of fouling in a heat exchanger.

The method developed according to the present invention is used for modeling the amount of fouling in a heat exchanger (E) (also known as shell and tube type heat exchanger in the art), as illustrated in Figure 1 , comprising a shell (S) having a hollow tubular structure and at least one tube (T) disposed in said shell (S) to provide heat exchange between the fluids at different temperatures passed through said shell (S) and tube (T), respectively. Said method, of which a flowchart is given in Figure 2, comprises the steps of assigning an inlet and outlet temperature difference for each of the fluids passed through the tube (T) and the shell (S) (100); calculating an iterative outlet temperature for each fluid using the assigned temperature differences and the measured inlet temperatures of the fluids (101 ); determining an expected outlet temperature for each fluid so as to equate it to the calculated iterative outlet temperature (102); calculating a mean temperature value for each fluid (103); calculating an iterative clean heat transfer coefficient for the heat exchanger (E) using the physical properties under the calculated mean temperatures of the tube (T) and the shell (S) (104); calculating a logarithmic mean temperature difference using the inlet temperatures and iterative outlet temperatures of the fluids (105); updating the iterative outlet temperatures for each fluid by solving some energy equations of the tube (T), the shell (S) and of the heat exchanger (E) using the calculated iterative clean heat transfer coefficient, the calculated logarithmic mean temperature difference, the physical parameters of the heat exchanger (E), the physical parameters of the tube (T) and the shell (S), and the inlet temperatures of the fluids (106); comparing the updated iterative outlet temperatures to the expected outlet temperatures (107); returning to the step of determining an expected outlet temperature for each fluid (102) if the difference between the updated iterative outlet temperature and the expected outlet temperature for at least one of the fluids is above a predefined value; calculating instantaneous values for the heat exchanger (E) using the updated iterative outlet temperatures if the difference between the updated iterative outlet temperature and the expected outlet temperature for each fluid is below said predefined threshold value (108); and calculating the amount of fouling in the heat exchanger (E) using the calculated instantaneous values and the calculated iterative clean heat transfer coefficient (109). Since the outlet temperatures of the tube (T) and the shell (S) are calculated by an iterative method according to the present invention, the amount of fouling of the heat exchanger (E) can be determined in a rapid and reliable manner.

In a representative embodiment of the method according to the present invention, the values assigned to the cold fluid and the hot fluid (which are cold or hot relative to each other) in the step of assigning a temperature difference (100) are represented by "deltl " and "delt2", respectively. Then, in the step of calculating the iterative outlet temperatures (101 ), these values are calculated using the following equations [1] for the hot fluid and cold fluid, respectively.

T _t _ _ol = T _t _ _{i +} deltl (1 )

T _s _ _ol = T _s _ _t - deltl Here, the term "T _t-0 i " is the iterative outlet temperature of the fluid passed through the tube side, "T _t -i" is the inlet temperature of the fluid passed through the tube side, "T _s-01 " is the iterative outlet temperature of the fluid passed through the shell side, and "T _s-i " is the inlet temperature of the fluid passed through the shell side. In the step of determining the expected outlet temperatures (102), these values of the fluids are equated to the calculated iterative outlet temperatures, respectively, as indicated in the following equations [2].

T t-oold = T t-ol (2)

-oold = T so

Here the term "T _t - ₀ oid" is the expected outlet temperature of the fluid passed through the tube side and "T _s - ₀ oid" is the expected outlet temperature of the fluid passed through the shell side. In the step of finding the mean fluid temperatures (103), these values are calculated by finding the average of the inlet temperatures and the iterative outlet temperatures, respectively, as indicated in the following equations [3].

Here the term "T _ave -t" is the average temperature of the fluid passed through the tube side and "T _aV e-s" is the average temperature of the fluid passed through the shell side. After the average temperature values are found, the iterative clean heat transfer coefficient of the heat exchanger (E) is calculated (104) in the following equation [4] by using the physical properties at the average temperature values of the tube (T) and the shell (S) at these temperatures (e.g. if hot fluid is passed through the shell (S), the physical properties of the shell (S) at the average temperature of the hot fluid).

Here the term "U _est " is the iterative clean heat transfer coefficient of the heat exchanger (E), "OD _t " is the outer diameter of the tube (T), "ID _t " is the inner diameter of the tube (T), "hi" is the heat transfer coefficient of the tube (T), "k _tUb e" is the thermal conductivity of the tube (T), and the term "h ₀ " is the heat transfer coefficient of the shell (S). The thermal conductivity (k _tUb e) of the tube (T) is a value which depends on the material from which the tube (T) is produced and can be found in the literature.

In calculating the heat transfer coefficient of the tube (T), the following equations ([5] and [6]) of Seider-Tate proposed in 1936 are used. These are as follows, for fluids with high viscosity (Re<2100):

and for the turbulent flow remaining above the transition region (Re> 10,000; 0.7<Pr<160 ve L/Di>10):

As for the calculation of the heat transfer coefficient of the shell (S), the following sequence is used.

j _H = 0.5 x l + ^- | x (0.08 x Re - ⁶⁸²¹ + 0.7 x Re ¹⁷⁷² ) (7)

ID

Here, the shell's (S) equivalent diameter and the mass velocity are used in calculating the Re number for the shell (S).

The clearance factor (C) and the equivalent diameter (D _e ) vary depending on the outer tube diameter (OD _t ) of the heat exchanger, the layout configuration of tubes, and the pitch size (P _T ).

Here, the Nu number and the heat transfer coefficient for the shell (S) are calculated using the following equations (9).

0.14

Nu _s = j _H x Pr,

μ (9)

h xD„

Nu„ x k.

D Then, the logarithmic mean temperature difference is calculated (105) using the inlet temperatures and the iterative outlet temperatures of the fluids as illustrated in the following equation [10].

Here "LMTD" is the logarithmic mean temperature difference. Then, the following energy equations [11] are formulated for the tube (T), the shell (S), and the heat exchanger (E).

Q = m _t xcp _t x(T _t _ _ol -T _t _ _i ) (11)

Q = m _s cp _s (T _s _ _ol -T _s _ _i )

Q = Fx U _EST xAx LMTD Here the term "Q" is the transferred heat , " m _t " is the mass flow rate of the fluid passed through the tube side, "cp _t " is the specific heat of the fluid passed through the tube side, "m _s " is the mass of the fluid passed through the shell side, "cp _s " is the specific heat of the fluid passed through the shell side, F is the correction factor in calculating the logarithmic temperature difference, and "A" is the surface area of the heat exchanger. As a result of interpreting any two of the energy equations above, the iterative outlet temperatures of the hot and cold fluids, respectively, can be updated, e.g. as indicated in the following equations [1 1 ] (106). m _t x cp _t x (T _t _ _ol - T _t _ _t ) = m _s x cp _s x (T _s _ _ol - 7 ) (1 1 )

m _t x cp _t x (T _t _ _ol - T _t _ _; ) = F x U _EST x A x LMTD

Then, the updated iterative outlet temperatures and the expected outlet temperatures calculated previously are compared as indicated below in [12] (107), and it is determined if the iterative outlet temperatures are calculated correctly with an acceptable error margin. abs{T _s _ _ol - T _s _ _oold ))TH _l (12)

abs{T,_ _ol - T _t _ _oold ))TH ₂ Here, the terms "ΤΉ-ι" and "TH ₂ " are predefined threshold values, respectively. If one of the statements above is fulfilled, it is returned to the step of determining the expected outlet temperature (102) based on that the calculated iterative outlet temperature is not calculated with an acceptable error margin. If neither of these statements is fulfilled, it is assumed that the calculated iterative outlet temperature is correct with an acceptable error margin, namely that the calculated iterative outlet temperature represents the instantaneous (operational) outlet temperatures of the fluids. Once the instantaneous outlet temperatures are determined, the clean heat transfer coefficient of the heat exchanger (E), the instantaneous logarithmic mean temperature difference and a fouling heat transfer coefficient are calculated using the following equations [13].

U _c = U _est (13)

Q = m _t x cp _t x (T _t _ _op - ) = U _d x A x LMTD,

Here the term "U _c " is the heat transfer coefficient of the heat exchanger (E) under clean conditions, "LMTD _op " is the instantaneous logarithmic mean temperature difference of the heat exchanger (E), "T _t-0 p" is the instantaneous outlet temperature of the fluid passed through the tube side, "T _s-op " is the instantaneous outlet temperature of the fluid passed through the shell side, " m _t " is the mass velocity of the fluid passed through the tube side, and "Ud" is the heat transfer coefficient of the heat exchanger (E) under fouling conditions. Finally, the amount of fouling of the heat exchanger (E) can be calculated instantaneously (109) using the following equation [14]. Here, the term "R _f " represents the fouling coefficient of the heat exchanger.

In a preferred embodiment according to the present invention, the fouling coefficient of the heat exchanger (E) is calculated in certain time intervals. As a result of comparing these calculated values, a time-dependent fouling behavior can be monitored for the heat exchanger (E).

According to another preferred embodiment of the present invention, in the step of assigning inlet and outlet temperate differences for the fluids (100), the temperature differences are assigned as a constant value (e.g. 10°C). When the constant value is selected as a reasonable value for the heat exchanger, the iteration number of the method can be kept below a certain number even if a fouling calculation is made for different fluids at different temperatures in the heat exchanger (E). Thus, the method can provide rapid and reliable results. According to an alternative embodiment of the present invention, the temperature differences in the step of assigning inlet and outlet temperature differences to the fluids (100) is made equal to a certain proportion (e.g. 25%) of the temperature difference between the hot fluid and the cold fluid. Thus, it becomes also possible to further reduce the iteration number of the method and the method can provide faster results.

In another preferred embodiment according to the present invention, said threshold values are determined in the form of constant values (e.g. 0,1 °C). Thus, the method can be operated very accurately.

By virtue of the method according to the present invention, the outlet temperatures of the fluids used in the heat exchanger (E) are calculated iteratively. Thus, the fouling coefficient is calculated in a rapid and reliable manner for the heat exchanger using instantaneous outlet temperatures obtained in the same manner. Additionally, the time- dependent fouling behavior can be monitored for the heat exchanger (E) by iterating said method in certain time intervals and comparing the calculated fouling values of the heat exchanger (E).