METHOD AND APPARATUS FOR QUENCHING THE COKE DRUM VAPOUR LINE IN A COKER

Abstract: A method and apparatus for quenching the coke drum vapour line from a coke drum to the main fractionator in a coker unit whereby the volume of quench liquid prevents the drum vapour line from plugging with carbon-based deposits. A differential pressure control technique is utilized to quench the drum vapours being delivered to the fractionator. Vapour line quench control by differential pressure prevents over-quenching of the vapour line during a coke drum switch, unit start-up, or slowdown as well as under-quenching during drum warm-ups.

Description: METHOD AND APPARATUS FOR QUENCHING THE COKE DRUM VAPOUR LINE IN A COKER The invention relates to coker units and their operation, particularly in the quenching of the vapour line running from coke drums to a fractionator in a coker unit. Flow rate in a coke drum vapour line is influenced by several factors including quench injection rate, quench oil properties, coke drum temperature, vapour rate and pressure drop from the coke drums to the fractionator. In prior systems, the actual rate of liquid flowing out of the vapour line into the coker main fractionator varies during the coking cycle. Prior systems result in either of two undesirable conditions: (1) overquench, which reduces yields and possibly reduces unit feed rates, or (2) underquench, which leaves a vapour line without any liquid to flush the line out into the main fractionator and which will eventually shut down the coker as the vapour line cokes. Once the line cokes to the point of causing enough pressure drop from the coke drums to the main fractionator such that all the liquid evaporates, only a short time remains until the coker must be shut down - a very expensive event. In the prior systems, the quench cannot generally be adjusted to target its contribution to the recycle ratio. One prior method, the delta temperature control technique, could possibly target a contribution of the recycle ratio; however, the downstream temperature indicator (TI) must be located in the common part of the vapour line near the fractionator in order for this to work correctly. The problem with putting a TI in this location is that, in all likelihood, it will foul and become inaccurate. As described in the present disclosure, a TI located at the coke drum vapour line outlet into the fractionator is not accessible during operation but is easily cleaned while decoking a drum. Prior quench techniques do not consider pressure differential between the coke drum and the fractionator. The invention is a method and apparatus for quenching the coke drum vapour line which runs from the coke drum to the main fractionator in a coker unit. The unique part of this improved quench system is that it uses both pressure differential and unit feed rates to control quench rates for a given quench oil and unit feed quality. If the composition of the coker feed or the quench oil changes significantly, a new set of quench curves may be generated to ensure proper quenching of the coke drum vapour line. The purpose of quench is to prevent the drum vapour line from plugging with carbon- based deposits. Plugging of the vapour line causes a restriction in coker unit feed rates and ultimately leads to severely limiting coker feed rates until the plug is removed. In order to remove the vapour line plug, shut down of the unit is required which results in lost coker capacity, due to the gradual slowdown and subsequent shutdown of the coker unit, and in significant economic loss. A differential pressure control technique is utilized to quench the drum vapours going to the fractionator as opposed to a temperature, delta temperature, uninsulated line or fixed flow rate control technique as used in prior systems. Vapour line quench control by differential pressure prevents over-quenching of the vapour line during a coke drum switch, unit startup, or slowdown as well as preventing under-quenching during drum warm-ups. It improves the fractionator recovery time after a drum switch and the overall liquid product yield during the drum cycle which can be reduced by over-quenching. It also prevents the vapour line from drying out at anytime, an under-quenched condition, as long as the quench oil quality and conditions do not vary significantly. To overcome the above problems, a new delayed coker unit and a new process have been developed based on pressure differential and unit feed. Thus, the present invention relates to a delayed coker design as described in claim 1 and a new process as described in claim 3. FIGURE 1 is a schematic drawing of a coker unit which incorporates the instant invention. FIGURE 2 is a graph showing quench flow vs. pressure differential for the minimum and maximum feed rates for a typical coker unit and coker feed quality. The root cause of a coker vapour line plug is drying out of the vapour line. In particular, during coke drum warm-up, the vapour line may dry out due to the increased pressure drop from the coke drum to the fractionator if there is no increase in quench rate to prevent drying. This added pressure drop can cause all of the liquid to flash off inside the vapour line which leaves a layer of carbon residue with entrained coke fines. To reduce the risk of plugging the vapour line, the quench technique disclosed herein adjusts quench rates based upon pressure drop and unit feed rate. This delta pressure quench control technique greatly reduces the potential of the vapour line drying out and maintains a constant flow of liquid flowing out the end of the vapour line into the fractionator. It will generally increase yields vis-a-vis the prior art delta temperature quench control (if the vapour line temperature indicator (TI) is not located near the fractionator) , or the constant vapour temperature quench flow technique, at a much reduced risk of plugging the vapour line. These latter two prior art techniques rely on over-quenching for most of the drum cycle in order to prevent drying of the vapour line during drum warm-up. Or, if the TI is placed in an inaccessible portion of the vapour line, the TI can foul with coke and produce unreliable data, resulting in under-quenching. If the delta temperature quench control technique is to be reliable, accurate vapour line temperatures near the coker main fractionator are necessary; however, temperature indication in this portion of the vapour line is inherently unreliable since it is in this common portion of the vapour line where the vapour line will likely foul, producing unreliable temperature data. The fixed-quench rate vapour temperature control may result in under-quenching and a dry vapour line whenever a drum switch occurs, and this can lead to the formation of a plugged vapour line. The present invention overcomes three limitations of the quenched vapour temperature control technique used in prior systems: (1) the possibility of drying out the coke drum vapour line; (2) the inferior reliability of temperature indication in a coking environment to control the quench rate, and (3) the essential over-quenching necessary during most of the drum cycle if adequate quench is to be supplied during drum warm-up, when the pressure drop is usually at its highest. Also, the accuracy of the drum pressure indicator is easily verified during every drum cycle because the inactive drum is opened to the atmosphere, therefore the pressure indicator will read zero psig if working properly. However, the temperature transducer can certainly foul with coke, such that its accuracy is not easily verified between drum cycles, due to the metal not having time to cool to ambient verifiable conditions between cycles. Or if the TI is located in the common portion of the vapour line, one will not know if the TI is fouled, thus producing unreliable data to control quench rates. In the following discussion, two coke drums are illustrated and described. It will be appreciated that a coker unit may comprise more than two coke drums. Referring now to Figure 1, a typical coker unit comprises two coke drums 10 and 20, two coker furnaces 30 and 40, a main fractionator 50, a light gasoil stripper 60, a heavy gasoil stripper 70 and possibly a rectified absorber 80, all of which are known to those skilled in the art. In the instant invention, a computer controller 90 is additionally required to receive input data from the coke drums 10, 20, the fractionator 50 and the input feed rate indicator 100 and to generate control signals for controlling quench flow rate as will be subsequently described. Each of the coke drums 10, 20 contain pressure transducers 11, 21, respectively, which monitor the pressure inside the respective drums at all times and relay such data to the controller 90. It will be appreciated that, at any given time, one of the coke drums will be "active" (on-line) and the other will be off-line undergoing decoking and cleaning in preparation for the next cycle, as is well known to those skilled in the art. Likewise, the main fractionator 50 also includes a pressure transducer 51 for constantly monitoring the pressure therein and relaying such data to controller 90. In operation, a cold feed heavy oil such as 6-Oil at about 82 °C (180 °F) is fed through flow meter 102 and line 104 to fractionator 50, via line 104a to grid tray/spray unit 59 or via line 104b to the bottom of the fractionator 50. Concurrently, a hot feed, such as hot pitch at about 260 °C (500 °F) is fed through flow meter 103 and line 105 into the bottom of fractionator 50. Flow meter signals from flow meters 102, 103 are relayed through data lines 106, 107 respectively to the unit feed flow indicator 100. The resulting flow signal is relayed over data line 101 to the controller 90. The hot fractionator bottom stream is fed through line 54 to furnaces 30, 40, after injecting velocity steam at 33, 43, respectively, where it is circulated through tubes 31, 41, respectively, and heated up to about 488 °C (910 °F) . The bottoms must be severely thermally cracked, otherwise it will not coke, and will, instead, form tar. The hot fractionator bottoms exit the furnace tubes 31, 41 at 32, 42, respectively, at about 488 °C (910 °F) and are directed to the active coke drum, either 10 or 20. In the usual manner, the active coke drum 10 or 20 catches and retains carbon matter while hydrocarbons evaporate. It will be appreciated that this described apparatus is called a "delayed coker" since it requires a combination of residence time and temperature to form coke in the coke drums 10, 20. Pressure transducers 11 and 21 relay data over lines 11a and 21a respectively to the controller 90. Vapour from the active coke drum 10 or 20 is passed through one of the valves 18, 28 to the overhead coke drum vapour line 29. A quench liquid is also injected into vapour line 29 through inputs 12 or 13, flow meter 14 and valve 17 to form a mixture of quench oil and vapour in vapour line 29. Quench liquid 12 may be slop oil while quench liquid 13 may be a coker gasoil. Quench liquid flow rate through vapour line 29 is set by the quench flow indicator controller 15 which regulates valve 17 in response to a signal received from the controller 90 over control line 91 as will be subsequently explained. The quench oil/vapour mixture in vapour line 29 is injected at the bottom of fractionator 50 at 29a, where, in prior systems, a thermocouple may have been placed to detect and relay temperature data and to possibly be used for controlling the flow rate. As has been explained, this temperature tended to be unreliable since the thermocouple became coated with coke and became inaccurate. Main fractionator 50 includes a heavy gasoil pump-around exchanger 53 for cooling vapours and removing heat from the system. A circulation reflux unit also includes a pump-around exchanger 52 for cooling vapours and removing heat from the system further up the column 50. Exchanger 52 receives hot circulating reflux oil through line 52b and sends cooled circulating reflux oil back to fractionator 50 through line 52a. Exchanger 53 receives hot unstripped heavy gasoil through line 53b, and part of the hot heavy gasoil can possibly go back to the spray 59 through line 53c to prevent entrained coke fines from escaping into the overhead vapours. Cooled heavy gasoil from exchanger 53 is sent back to the fractionator 50 via line 53a where it is flowed onto tray 53d as part of the pumparound heat removal system. Heavy gasoil stripper 70 receives unstripped heavy gasoil from the fractionator 50 through line 74 and steam is injected through line 72 to form stripped heavy gasoil which is withdrawn by line 71. Steam and stripped-out heavy gasoil is recirculated to the fractionator 50 via line 73 where it flows onto tray 53d. Line 53c is an alternate source of liquid for spray 59 which, if used, reroutes the cold feed flowing in line 104 to the bottom of the fractionator 50 via line 104b along with the hot pitch through line 105. Spray unit/contacting trays 59 prevent entrained coke fines from escaping into the overhead vapours. Light gasoil stripper 60 may be used for receiving light unstripped gasoil through line 64 and steam through line 62. Light stripped gasoil is produced and is withdrawn through line 61 while the remaining vapours are sent back to the fractionator 50 through line 63. The overhead vapours in fractionator 50 are passed on to the overhead condenser 54 which removes heat from the overhead vapours. The condensed liquid passes to an accumulator 55 and wet gas compressor 56 compresses the wet gasses, such as methane, ethane, propane, and butane. The output of wet gas compressor 56 is transported through line 57 to the rectified absorber (RA) 80 where fuel gas is withdrawn at 82 and coker naphtha at 84, the latter being sent to a hydrotreating unit. The absorber 80 receives a lean oil input 83 which assists in the separation of ethane from propane. Line 81 contains the overhead liquid hydrocarbons that have been condensed in the overhead condenser 54. These liquids are either sent back to the main fractionator 50 as reflux or to the RA 80. Pressure transducer 51 continuously transmits the pressure inside fractionator 50 to the controller 90 over line 51a. As noted, the controller 90 receives continuous pressure signals from pressure transducers 11, 21 in coke drums 10, 20, respectively, and from pressure transducer 51 in fractionator 50, even from the off-line drum being decoked. The controller 90 also receives an input feed rate signal 101 (in barrels per day) from unit feed flow indicator 100. Controller 90 senses which of the drums 10, 20 is active (on-line), since the pressure in the off-line drum is lower than the pressure in the on-line drum. It then calculates the difference in pressure (DP) between the active drum (10 or 20) and the fractionator 50 pressure transmitted by pressure transducer 51. This DP is used by the controller 90, along with the feed flow rate 101, to calculate the quench flow rate which is required to be injected at 12, 13 in order to maintain a selected fresh feed liquid flow percentage of, say 5 vol%, in vapour line 29 at point 29a where the vapourline 29 intersects the main fractionator 50. This is a very important area of the vapour line to understand. If one does not understand what influences the amount of liquid in the vapour line at this point, one could potentially (1) overquench, i.e., too much liquid, which reduces liquid yields and increases coker unit recycle to the main fractionator bottoms and potentially could reduce coker unit throughput or (2) underquench, i.e., too little liquid, resulting in a dry, non-irrigated, vapour line which will foul with coke and eventually shut down the coker unit. Either one of these conditions is undesirable. A signal is sent over line 91 to the quench flow indicator controller 15 and valve 17 is automatically adjusted to maintain such selected flow rate. Quench rates needed to maintain a wetted line at various vapour line pressure differentials, and unit feed rates required to ensure a constant liquid rate flowing out of the vapour line 29 into the coker main fractionator 50 were calculated. A PRO/II general purpose process and optimization software by Simulation Sciences, Inc. was used to generate the data (PRO/II is a trademark) . This data is presented in Tables 1 and 2 below. Tables 1 and 2 were obtained via computer simulation of the coke drum vapour line thermodynamics. Based upon the measured coker feed product yields and quench liquid properties, a simulation was run to determine the quench rate needed to produce a constant percentage of unit recycle from liquid flowing out of the coke drum vapour line into the bottom of the main fractionator. The vapour line pressure drop was varied to determine the quench rate needed to maintain constant liquid flow into the main fractionator, while at premeasured product yields and quench oil properties. From Tables 1 and 2, the curves shown in Figure 2 were produced. Differential pressure drop (psi; 1 psi = 0.0689 bar) from the active coke drum to the main fractionator is used as the X axis and quench rate (bpd) as the Y axis. Once the curves are prepared for a particular coker, (for a given set of unit yields and quench oil properties) such information is used to control quench flows via computer control thereafter. TABLE 1 TABLE 2 Referring now to Figure 2, Tables 1 and 2 have been displayed in graph form for the maximum (28.5 MBPD) and minimum (14.5 MBPD) feed rates for a typical coker unit.

Claims: C L A I M S 1. A delayed coker comprising: an active coke drum having a pressure transducer for measuring the pressure within said drum, said coke drum being adapted to receive hot fractionator bottoms from a fractionator, to capture the carbon from said bottoms and to pass vapours from said bottoms to a vapour line; means for injecting a quench liquid into said vapour line; a fractionator, adapted to receive said vapours from said vapour line, to receive a hydrocarbon feed material thereinto and having means for measuring the pressure therein; a controller for receiving pressure signals from said coke drum and said fractionator and for calculating the pressure differential therebetween; means for generating a signal representing the feed rate supplied to said fractionator and supplying said signal to said controller; and means within said controller for evaluating said pressure differential and said feed flow input rate data and generating, in response thereto, a signal for controlling a selected amount of quench liquid to be injected into said vapour line. 2. The apparatus of claim 1 further including at least one additional coke drum in parallel with said active coke drum. 3. In a delayed coker unit having a coke drum and a fractionator connected by a vapour line, a method for measuring and controlling the amount of flow of quench liquid injected into said vapour line, comprising the steps of: measuring the pressure within said coke drum; measuring the pressure within said fractionator; measuring the total flow rate of a liquid feed supplied to said fractionator; supplying, to a controller, said measured pressures and said measured total flow rate of feed liquid being supplied to said fractionator; using coke drum vapour line thermodynamics to evaluate the relationship between said pressure differential and said feed flow input rate data; determining, from said relationship, the amount of quench liquid which must be supplied to said vapour line in order to maintain a desired flow rate of liquid through said vapour line and into said fractionator; generating, in response to said relationship, a signal for controlling a selected amount of quench liquid which must be injected into said vapour line in order to result in the desired flow rate of liquid through said vapour line and into said fractionator; and controlling the flow rate of quench liquid injected in said vapour line by supplying said generated signal to a supply valve for opening and closing said valve in response to said generated signal.

Inventors: HAIK STEPHEN MICHEL, HAIK, STEPHEN, MICHEL
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 2001
Main classification: C10B55
IPC codes: C10B41, C10B55, C10B57, C10G9