Patent Application
20070267176
The invention will now be described in conjunction with the accompanying drawings in which:
In the drawings, like reference numerals indicate corresponding parts in the different figures.
This invention is directed to a method of mitigating fouling in heat exchangers, in general, and the devices for practicing the method. In a preferred use, the method and devices are applied to heat exchangers used in refining processes, such as in refineries or petrochemical processing plants. The invention is particularly suited for retrofitting existing plants so that the process may be used in existing heat exchangers, especially while the heat exchanger is on line and in use. Of course, it is possible to apply the invention to other processing facilities and heat exchangers, particularly those that are susceptible to fouling in a similar manner as experienced during refining processes and are inconvenient to take off line for repair and cleaning.
While this invention can be used in existing systems, it is also possible to initially manufacture a heat exchanger with the vibration inducing devices described herein and use the method in accordance with this invention in new installations.
Heat exchange with crude oil involves two important fouling mechanisms: chemical reaction and the deposition of insoluble materials. In both instances, the reduction of the viscous sublayer (or boundary layer) close to the wall can mitigate the fouling rate. This concept is applied in the process according to this invention.
In the case of chemical reaction, the high temperature at the surface of the heat transfer wall activates the molecules to form precursors for the fouling residue. If these precursors are not swept out of the relatively stagnant wall region, they will associate together and deposit on the wall. A reduction of the boundary layer will reduce the thickness of the stagnant region and hence reduce the amount of precursors available to form a fouling residue. So, one way to prevent adherence is to disrupt the film layer at the surface to reduce the exposure time at the high surface temperature. In accordance with this invention, the process includes vibrating the wall to cause a disruption in the film layer.
In the case of the deposition of insoluble materials, a reduction in the boundary layer will increase the shear near the wall. By this, a greater force is exerted on the insoluble particles near the wall to overcome the particles' attractive forces to the wall. In accordance with the invention, vibration of the wall in a direction perpendicular to the radius of the tube will produce shear waves that propagate from the wall into the fluid. This will reduce the probability of deposition and incorporation into the fouling residue.
Referring to the drawings,
It will be recognized by those of ordinary skill in the heat exchanger art that while a shell-tube exchanger is described herein as an exemplary embodiment, the invention can be applied to any heat exchanger surface in various types of known heat exchanger devices. Accordingly, the invention should not be limited to shell-type exchangers.
The controller 22 can be any known type of processor, including an electrical microprocessor, disposed at the location or remotely, to generate a signal to drive the dynamic actuator 20 with any necessary amplification. The controller 22 can include a signal generator, signal filters and amplifiers, and digital signal processing units.
The dynamic actuator 20 can take the form of any type of mechanical device that induces tube vibration while maintaining structural integrity of the heat exchanger 10. Any device capable of generating sufficient dynamic force at selected frequencies would be suitable. The dynamic actuator 20 can be single device, such as an impact hammer or electromagnetic shaker, or an array of devices, such as hammers, shakers or piezoelectric stacks. An array can be spatially distributed to generate the desired dynamic signal to achieve an optimal vibrational frequency.
Any suitable mounting device 21 can be used depending on the type of dynamic actuator 20. The mounting device 21 provides a mechanical link between the dynamic actuator 20 and the heat exchanger 10. It can be designed as a heat insulator to shield the dynamic actuator 20 from excessive heat. It could also be formed as a seismic mass. The mounting device 21 could also function as a mechanical amplifier for the dynamic actuator 20 if necessary.
The dynamic actuator 20 may be placed at various locations on or near the heat exchanger 10 as long as there is a mechanical link to the tubes 14. The flange 16 provides a direct mechanical link to the tubes 14. The rim of the flange 16 is a suitable location for connecting the dynamic actuator 20. Other support structures coupled to the flange 16 would also be mechanically linked to the tubes. For example, the header supporting the heat exchanger would also be a suitable location for the dynamic actuator 20. Vibrations can be transferred through various structures in the system so the actuator does not need to be directly connected to the flange 16.
As seen in
In the above applications in accordance with is invention, the actuation of a dynamic force creates tube wall vibration V and corresponding shear waves SW in the fluid adjacent the walls, as seen in
The inventors have determined through experimentation that mechanical vibration in accordance with this invention will considerably reduce the extent of fouling. With the proper vibration frequency, the thickness of the oscillating fluid can be made sufficiently small so that the fluid within the sub-laminar boundary layer, otherwise stagnant without shear waves, will be forced to move relative to the wall surface. The concept is shown in
An experiment was conducted using a commercially available unit used in the petroleum industry to measure fouling known as ALCOR Hot Liquid Process Simulator (HLPS) fouling test system. The test applied vibrational excitation to a heating rod with the driving force and frequency of the vibration shaker selected to excite the heating rod with sufficient relative motion between the fluid and vibrating surface while maintaining mechanical integrity and normal operation of the ALCOR unit. The applicable frequency ranged from a few Hz to 20,000 Hz, and the acceleration force at the driving point from a fraction of g to 20 g. Other values of driving force and frequency are also considered to be effective in minimizing fouling. The procedure of selecting optimal frequency is to identify a set of the natural frequencies and modes of the heating rod and to select a driving frequency that is close but not identical to one of the natural frequencies. Alternatively, a synthesized waveform can be generated such that multiples of vibration resonance of the heating rod could be excited.
The test feed was Arab Extra Light whole crude oil run through the ALCOR HLPS under once-through conditions at 3 ml/min under a nitrogen pressure using 370° C. (698° F.) surface temperature to induce fouling. The build up of foulant causes an insulating effect, much like in refinery heat transfer equipment. The insulating effect reduces the ability of the heated source to heat the fluid, and as a result the outlet liquid temperature decreases as more foulant is deposited. The reduction in outlet temperature is measured as Outlet Delta T. This is a standard that is measured over a 3 hour (180 minutes) period. The end fouling indicator is termed ALCOR Outlet Delta T180. The Delta T180 for Arab Extra Light has been typically between −57 and −63° C. in previous ALCOR tests without vibration.
Using the above vibration parameters, vibration was induced perpendicularly to the ALCOR heating rod. The final ALCOR Outlet Delta T180 for the Arab Extra Light whole crude oil was observed to be reduced to only 19° C., as shown in
Based on the vibration measurement and analysis of the tube bundles 12, the inventors determined that the tube-sheet flange 16 provides an effective mechanical link to the internal tubes 14 and can be used to exert mechanical excitation. Sufficient vibration energy can be transferred from the flange 16 to the tubes 14 at vibration modes. There are low and high frequency vibration modes of tubes. For low frequency modes (typically below 1000 Hz), axial excitation is more efficient at transmitting vibration energy, while at high frequency modes, transverse excitation is more efficient. The density of the vibration modes is higher at a high frequency range than at a low frequency range (typically below 1000 Hz), and vibration energy transfer efficiency is also higher in the high frequency range. Further, displacement of tube vibration is very small at high frequency (>1000 Hz) and insignificant for potential damage to the tubes.
Fouling mitigation by vibration is strongly dependent on wall shear stress parameters to quantitatively evaluate the effectiveness of different excitation methods. The wall shear stress of the tube due to wall vibration can be estimated by the following equation:
τw=CVw√(ρμω)
where C is a constant, ρ and μ are the fluid density and viscosity, Vw is the velocity amplitude of the wall vibration, and ω is the circular vibration frequency. Assuming a reference wall shear stress above which the fouling mitigation is significant, the ratio of the tube wall shear stress to the design target is expressed by the following equation:
τw/τref=(Vw/Vref)√(ω/ωref)
In accordance with the experiment described above, in one example a design target for wall shear stress was selected by using a calculated wall shear stress ratio of axial and transverse tube vibration by a 750N dynamic force applied axially (parallel to the tube axis) on the flange. The same amount of dynamic force was also applied transversely (perpendicular to the tube axis) on the flange. It was shown that in both cases tube vibration could be excited to a desirable degree for purposes of fouling mitigation at most vibration modes at which the wall shear stress ratio is >1.0. The displacement amplitude (in micrometers) of tube transverse vibration was generally much smaller at frequencies of above 100 Hz than the maximum allowable vibration displacement, which is typically around 0.025 inches or 600 microns for a design that avoids tube damage by vibration. For frequencies above 1000 Hz, the dynamic displacement of the tube is negligible in terms of potential vibration damage to the tube and supports.
It is advantageous to use high frequency vibration for fouling mitigation because (1) it creates a high wall shear stress level, (2) there is a high density of vibration modes for easy tuning of resonance conditions, (3) there is low displacement of tube vibration to maintain the structural integrity of the heat exchanger, and (4) there is a low offensive noise level.
Selection of the precise mounting location, direction, and number of the dynamic actuators 20 and control of the frequency of the amplitude of the actuator output is based on inducing enough tube vibration to cause sufficient shear motion of the fluid near the tube wall to reduce fouling, while keeping the displacement of the transverse tube vibration small to avoid potential tube damage. Obviously, the addition of a dynamic actuator 20 can be accomplished by coupling the system to an existing heat exchanger 10, and actuation and control of the dynamic actuator can be practiced while the exchanger is in place and on line. Since the tube-sheet flange is usually accessible, vibration actuators can be installed while the heat exchanger is in service. Fouling can be reduced without modifying the heat exchanger or changing the flow or thermal conditions of the bulk flow.
Various modifications can be made in the invention as described herein, and many different embodiments of the device and method can be made while remaining within the spirit and scope of the invention as defined in the claims without departing from such spirit and scope. It is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.
