Method And System For Detecting Coking Growth And Maldistribution In Refinery Equipment (Patent Application ExxonMobil Research and Engineering)

CATEGORY: COKERS & COKING 
Method And System For Detecting Coking Growth And Maldistribution In Refinery Equipment (Patent Application ExxonMobil Research and Engineering)
United States Patent Application 20150268078
September 24, 2015
Assignee: ExxonMobil Research and Engineering
Abstract
Systems and methods for detecting coking in a wash bed of a vacuum pipe still with a sensing cable including an optical fiber sensor array aligned with a heating element disposed in the vessel. An optical signal interrogator is configured to measure a first temperature profile at a plurality of sensor locations to determine a flow distribution. An excitation source is configured to propagate at least one heat pulse through the heating element and the optical signal interrogator is configured to measure a second temperature profile corresponding to the heat pulse at the sensor locations. A control unit is configured to detect coking by determining one or more properties of the media exposed to the sensing cable at each of the plurality of sensor locations based on the second temperature profile corresponding thereto.
Description
FIELD
[0002] The presently disclosed subject matter relates to methods and systems for detecting coking and flow maldistribution in a wash bed of a vacuum pipe still distillation tower. More particularly, the presently disclosed subject matter relates to detecting coking and flow maldistribution in a wash bed of a vacuum pipe still distillation tower using a sensing cable including an optical fiber sensor array aligned with a heating element.
BACKGROUND
[0003] Components of certain equipment, such as that used in the petroleum and petrochemical industry, which includes the exploration, production, refining, manufacture, supply, transport, formulation or blending of petroleum, petrochemicals, or the direct compounds thereof, are often monitored to maintain reliable operation. However, such components can involve harsh conditions, such as high temperature, high pressure, and/or a corrosive environment, making it difficult or costly to obtain reliable measurements.
[0004] Detection of coking formation in a wash bed of a vacuum pipe still (VPS) distillation tower can allow operators to alter operating parameters to increase utilization of the bed and thus enhance operations. For example, detecting coking formation at an early stage and knowing its location within the wash bed in the VPS distillation tower can allow for mitigation strategies such as increasing the flow rate of wash oil to remove the coking.
[0005] Conventional techniques for detection of coking/fouling, and/or corresponding maldistribution resulting from such coking in such equipment as catalytic hydroprocessing reactors, can include monitoring temperature distribution to identify hotspots and infer flow distribution. Such techniques often rely on multiple thermocouples to monitor temperature distribution, e.g., inside fixed bed catalytic hydroprocessing reactors. However, the number of thermocouples used for hot-spot detection within a VPS wash bed or reactor catalyst bed can be limited by the space inside the bed and the cost of installation and maintenance. Thus, it can be difficult to provide adequate coverage inside the fixed bed space for sufficient hot spot detection. Likewise, flow conditions inferred from the limited point temperature measurements provided by thermocouples, constrained by the physical size of the thermocouples as well as the cost of installation and maintenance, can be inaccurate.
[0006] Other techniques to detect coking, and/or corresponding flow maldistribution, can include monitoring the delta pressure between the top and bottom of the wash bed. However, this technique is not without disadvantages, such as for vacuum tower wash beds, where the pressure drop is typically only on the order of a few mmHg in these wash beds when coking occurs. Thus, pressure measurement can be a highly unreliable indicator of coking. Similarly, temperature differentials between bulk temperatures have also been used to detect coking. However, this technique involves a gross measurement and thus not necessarily accurate.
[0007] Accordingly, there is a continued need for improved techniques for detecting coking/fouling growth and maldistribution in components of refinery equipment such as a wash bed of a VPS distillation tower.
SUMMARY
[0008] The purpose and advantages of the disclosed subject matter will be set forth in and apparent from the description that follows, as well as will be learned by practice of the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and attained by the methods and systems particularly pointed out in the written description and claims hereof, as well as from the appended drawings. To achieve these and other advantages and in accordance with the purpose of the disclosed subject matter, as embodied and broadly described, the disclosed subject matter includes systems and methods for detecting coking in a wash bed of a vacuum pipe still. Although the disclosures herein may be described in relation to the use in a wash bed of a vacuum pipe still, such technology as described herein will generally also be applicable in a similar manner to the installation and use in detecting localized coking, as well as flow maldistributions, in a catalytic hydroprocessing reactor catalyst bed.
[0009] In accordance with one aspect of the disclosed subject matter, a method for detecting coking in a wash bed of a vacuum pipe still includes providing within a vacuum pipe still a sensing cable including an optical fiber sensor array aligned with a heating element and measuring a first temperature profile of the sensing cable at a plurality of sensor locations. The method includes determining a flow distribution of fluids within the vacuum pipe still by identifying a first set of sensor locations of the sensing cable exposed to vapor and a second set of sensor locations of the sensing cable exposed to liquid based on the first temperature profile. The includes propagating at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and media, including the fluids, exposed to the sensing cable. The method includes measuring, over time, at least a second temperature profile of the sensing cable corresponding to the heat pulse at each of the plurality of sensor locations of the optical fiber sensor array. The method includes detecting coking by determining one or more properties of the media exposed to the sensing cable at each of the plurality of sensor locations based on the second temperature profile corresponding thereto.
[0010] In certain embodiments, measuring the first temperature can further include propagating a heat pulse through the heating element along at least a portion of the sensing cable and, for each sensor location, measuring at least a heating temperature measurement during propagation of the heat pulse over the sensor location, a peak temperature measurement, and a cooling temperature measurement after propagation of the heat pulse over the sensor. Determining the flow distribution of the fluids exposed to the sensing can include calculating a difference in the heating temperature measurement, the peak temperature measurement, the cooling temperature measurement, or combination thereof, between sensor locations, wherein the difference indicates a change in fluid characteristic proximal at least one of the plurality of sensor locations if the difference exceeds a predetermined threshold.
[0011] As embodied herein, measuring the second temperature profile corresponding to the heat pulse at each of the plurality of sensor locations can include, for each sensor location, measuring a plurality of temperatures over a period of time upon arrival the heat pulse at the sensor location. Detecting coking can include, for each temperature profile, performing a regression of the plurality of temperatures over a logarithm of corresponding measurement times for a predetermined time window in the period of time to generate a slope and an intercept of the regression, wherein the slope and the intercept indicate a coking deposit proximal the sensor location. Additionally or alternatively, detecting coking can include, for each temperature profile, generating a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time, applying a transform to the time derivative to generate a complex spectrum, and determining an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum indicate a coking deposit proximal the sensor location. Detecting coking can further include generating a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency, and determining an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum indicate a coking deposit proximal the sensor location.
[0012] In certain embodiments, determining the flow distribution of the fluids can further include detecting a misdistribution condition in the wash bed by monitoring the second temperature profile corresponding to each of the plurality of sensor locations, and comparing the monitored temperature profiles to predetermined temperature profiles corresponding to a desired operation condition. Alternatively, determining the flow of the fluids within the vacuum pipe still can further include detecting a misdistribution condition in the wash bed by monitoring the second temperature profile corresponding to each of the plurality of sensor locations and at least a third temperature profile corresponding to each of the plurality of sensor locations, and comparing the second and third temperature profiles to detect a change in operation condition. The sensing cable can disposed in a grid configuration within the wash bed, and the method can include generating a multi-dimensional flow distribution based on the first temperature profile corresponding to each sensor location.
[0013] In accordance with another aspect of the disclosed subject matter, a system for detecting coking in a wash bed of a vacuum pipe still includes a sensing cable including an optical fiber sensor array aligned with a heating element disposed in the wash bed, the optical fiber sensor array having a plurality of sensor locations. The system includes an optical signal interrogator coupled with the optical fiber sensor array and adapted to receive a signal from each of the plurality of sensor locations and configured to measure a first temperature profile of the sensing cable at the plurality of sensor locations. The system includes a control unit, coupled with the heating element and the optical signal interrogator, configured to determine a flow distribution of fluids within the vacuum pipe still by identifying a first set of sensor locations of the sensing cable exposed to vapor and a second set of sensor locations of the sensing cable exposed to liquid based on the first temperature profile. The system includes an excitation source coupled with the heating element configured to propagate at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and media, including the fluids, exposed to the sensing cable. The optical signal interrogator is configured to measure, over time, a second temperature profile of the sensing cable corresponding to the heat pulse at each of the plurality of sensor locations on the optical fiber sensor array. The control unit is configured to detect coking by determining one or more properties of the media exposed to the sensing cable at each of the plurality of sensor locations based on the second temperature profile corresponding thereto.
[0014] As embodied herein, the optical signal interrogator can be configured, for each of the plurality of sensor locations, to measure a plurality of temperatures over a period of time upon arrival of the heat pulse at the sensor location. The control unit can be configured, for each temperature profile, to perform a regression of the plurality of temperatures over a logarithm of corresponding measurement times for a predetermined time window in the period of time to generate a slope and an intercept of the regression, wherein the slope and the intercept indicate a coking deposit proximal the sensor location. Additionally or alternatively, the control unit is configured, for each temperature profile, to generate a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time, apply a transform to the time derivative to generate a complex spectrum, and determine an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum indicate a coking deposit proximal the sensor location. The control unit can further be configured to generate a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency, and determine an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum indicate a coking deposit proximal the sensor location.
[0015] In certain embodiments, the control unit can further be configured to detect a misdistribution condition in the wash bed by monitoring the second temperature profile corresponding to each of the plurality of sensor locations, and compare the monitored temperature profiles to predetermined temperature profiles corresponding to a desired operation condition. Alternatively, the control unit can be further configured to detect a misdistribution condition in the wash bed by monitoring the second temperature profile corresponding to each of the plurality of sensor locations and at least a third temperature profile corresponding to each of the plurality of sensor locations, and comparing the second and third temperature profiles to detect a change in operation condition. The sensing cable can be disposed in a grid configuration within the wash bed, and the control unit can be further configured to generate a multi-dimensional flow distribution based on the first temperature profile corresponding to each sensor location.
[0016] It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the disclosed subject matter claimed. Also, as noted, although, for simplicity purposes, the disclosures herein may be described in relation to the use in a wash bed of a vacuum pipe still, such technology as described herein will generally also be applicable in a similar manner to the installation and use in detecting localized coking, as well as flow maldistributions, in a catalytic hydroprocessing reactor catalyst bed.
[0017] The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the disclosed subject matter. Together with the description, the drawings serve to explain the principles of the disclosed subject matter.
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