Patent Application
20070267175
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 device for mitigating fouling in heat exchangers, in general. In a preferred use, the device is applied to heat exchangers used in refining processes, such as in refineries or petrochemical processing plants. Such processing generally involves whole crude oils, blends and fractions, which will be referred to collectively herein merely as crude oils for purposes of simplicity. 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 device described herein 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 sub-layer (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.
A controller 22 is preferably in communication with the dynamic actuator device 10 to control the forces applied to the heat exchanger. A sensor 24 coupled to the heat exchanger can be provided in communication with the controller 22 to provide feedback for measuring vibration and providing data to the controller 22 to adjust the frequency and amplitude output of the dynamic actuator device 10 to achieve shear waves in the fluid adjacent the tubes to mitigate fouling while minimizing any negative effect of the applied force on the structure integrity.
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 device 10 with any necessary amplification. The controller 22 can include a signal generator, signal filters and amplifiers, and digital signal processing units.
The dynamic actuator device 10 is designed to induce tube vibration while maintaining structural integrity of the heat exchanger. If desired, an array of dynamic actuators 10 can be spatially distributed to generate the desired dynamic signal to achieve an optimal vibrational frequency.
The base 26 also includes an impact surface 36 that is disposed adjacent to the impactor 30 and is made of any hard material, for example a steel block. The impact surface 36 can be a portion of the base 26 and integral with the support 28, it can be connected to the support 28, or it can be proximate to the base 26. It is important that the impact surface 36 be connected to structure that can directly transfer vibrations to the heat exchanger structure. To effectively transfer vibrations it is preferred that the structure is fixed in place. It is also possible to use an existing surface on the heat exchanger that can transfer vibrations to the tubes.
An actuator 38 is supported by the base 26 or can be disposed proximate to the base 26 adjacent to the impactor 30 so as to cause the impactor 30 to move with respect to the impact surface 36. The actuator 38 can be any mechanism that causes the impactor to move, especially to cause the ball 32 to move toward and away from the impact surface 36. In a preferred embodiment, the actuator 38 is an electromagnet that is driven by a controller 22, for example a controller with a pulse generator.
Preferably, the components of the dynamic actuator device 10 are formed as a unit, with the impactor 30, impact surface 36 and actuator 38 supported together to allow easy installation and efficient retrofit to an existing heat exchanger. By this, the device 10 can be simply attached to the desired system, such as a shell-tube heat exchanger, to impart vibrational energy to the system.
In operation, the actuator 38 retains the impactor 30 in a first position spaced from the impact surface 36, as seen in
In the preferred embodiment, the electromagnet 38 is charged and attracts the steel ball 32, as seen in
Of course, any device capable of creating vibrational energy may be used. For example, instead of a ball, the impactor could be formed as a hammer. The rod could be replaced with another type of movable support, such as a lever, swing arm, plunger or rotating support. It is also possible to actuate movement of the impactor by other means than an electromagnet, such as a small motor. A suitable motor can be electrically or pneumatically driven and can use a gear system and/or cam arrangement to cause movement that creates vibrational energy.
The pulse from the impactor 30 induces a longitudinal mode of vibration in the system when the dynamic actuator device 10 is mounted with the base 26 axially oriented with respect to the heat exchanger as shown by the mounting arrangement on flange 16 in
The controller 22 will preferably be connected to the sensor 24 to monitor the induced vibrations and control the frequency of the impacts and resultant vibrations to optimize shear waves adjacent to the heat exchange surfaces, in this case the tubes 14, while maintaining structural integrity of the system, as explained below.
The dynamic actuator device 10 may be placed at various locations on or near the heat exchanger 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 device 10. 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 device 10. 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 explained above and seen schematically in
In the above applications in accordance with this 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 inventive concept will considerably reduce the extent of fouling. With proper vibration frequencies, 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
Selection of the precise frequency will of course be dependent on the design of the heat exchanger and type of dynamic actuator employed. However, selection will be based on determining an optimum frequency that imparts enough energy to prevent buildup on the tube wall while avoiding damage to the heat exchanger parts. Ideally, the driving frequency will be different from the natural frequency of the heat exchanger part as matching the driving frequency to the resident mode of the device can create damage to the heat exchanger parts. An acceptable range of driving frequency would be about 200 Hz to about 5,000 Hz, more preferably about 500 Hz to 1,000 Hz, while avoiding the resonance frequency of the heat exchange structure.
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 10 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 device 10 can be accomplished by coupling the system to an existing heat exchanger, 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.
