Process Description
The Crude Oil Stabilizer unit removes hydrogen sulfide and light gases such as methane, ethane, and propane from raw crude oil received from a Gas Oil Separation Plant (GOSP) to produce a stabilized crude oil which is safe to store and transport at atmospheric pressure. These volatile compounds are present in the raw crude oil produced by a GOSP because the final stage of gas/oil separation in these plants occurs at pressures much higher than atmospheric pressure. As a result, a significant concentration of these volatile compounds are dissolved in the raw crude produced by the GOSP.
Simtronics’ Crude Oil Stabilizer simulator employs two methods of stabilization. Both methods involve treating the unstabilized crude oil in a distillation column.
Unstabilized crude oil is pumped from a GOSP to the Crude Oil Stabilizer unit by the Unstabilized Crude Pumps, P-101A/B. The pumps are needed to push the unstabilized crude from the GOSP through a long pipeline to the Crude Oil Stabilizer unit. The unstabilized crude oil enters Stabilizer Feed/Bottoms Exchanger, E-103, and is then fed to the top tray of Stabilizer Column, T-102.
Overview – Stabilization by Application of Heat
The first method of stabilization uses the application of heat in the Stabilizer Reboiler, E-105, at the base of the Stabilizer Column, T-102, to warm the crude oil up so that the vapor pressure of these volatile compounds is increased. This heating generates a vapor stream of the volatile compounds up the column. This vapor stream also aids in stripping out nearly all of the volatile compounds in the feed. The warm vapor from the top of the Stabilizer Column is then cooled in Light Ends Condenser, E-104, using air. Because the Stabilizer Column is not refluxed like conventional distillation columns, some of the moderately volatile compounds in the feed such as butane and pentane will also be stripped from the crude oil feed because of the higher temperatures established at the top of the Stabilizer Column with heat applied to the Reboiler. These heavier compounds and any water vaporized from the feed will condense in the Light Ends Condenser, E-104, which is an air cooler.
The outlet stream from E-104 is separated into its liquid and vapor phases in the Light Ends Separator, D-104. The vapor, referred to as sour gas because of its high concentration of hydrogen sulfide, is taken off to a treater at battery limits to remove the hydrogen sulfide before recovery as a fuel gas. The heavier condensed compounds are collected in D-104 and the condensed water is separated and collected in a boot. The light ends are pumped by Light Ends Pumps, P-140A/B, to a storage tank at battery limits for recovery as liquid fuels and/or chemical feedstocks. The water from the boot of D-104 is sent off to a water treating unit at battery limits for disposal/reuse.
All the crude oil from the bottom tray of T-102 is routed through E-105 by gravity where it is heated using hot oil from battery limits. The heated crude oil from E-105 enters the bottom of T-102. The vapor generated in E-105 separates from the crude oil and returns to the bottom tray of the Stabilizer Column. The heated, stabilized crude is collected in the base of T-102 and cooled in Stabilizer Feed/Bottoms Exchanger, E-103, which preheats the feed, thereby minimizing hot oil usage in E-105. The stabilized crude is cooled in Stabilized Crude Cooler, E-106, which is an air cooler. The cooled, stabilized crude oil is pumped to storage at battery limits by Stabilized Crude Pumps, P-151A/B.
Overview – Stabilization by Gas Stripping
Raw crude oil from a GOSP can also be stabilized by using an externally supplied stripping gas such as natural gas (mostly methane) instead of generating the stripping gas using the application of heat in the Stabilizer Reboiler, E-105. The stripping gas is introduced to a gas distributor located below the bottom tray of Stabilizer Column, T-102. In the stripping mode of operation, no heat is applied to the reboiler. This has several advantages over stabilization by application of heat:
- No hot oil system is required (lower site investment and operating costs)
- There is no significant loss of light ends from the crude oil (higher stabilized crude value)
- Easier to operate (Light Ends system, Reboiler and air coolers do not have to be operated)
The drawback to the gas stripping mode of operation is the need for a supply of clean stripping gas to be available. This gas can come from a natural gas pipeline or from recycling recovered gas after it has sweetened (i.e. the hydrogen sulfide has been removed). If the stripping gas comes from a pipeline, the sweetened gas can be returned to the pipeline.
Unstabilized Crude Oil Pumping & Preheating
Unstabilized crude oil from a Gas/Oil Separation Plant (GOSP) in the field is pumped through a long pipeline to the Stabilizer Feed/Bottoms Exchanger, E-103, by Unstabilized Crude Pumps, P-101A/B, which are electric motor-driven centrifugal pumps. The design flow of unstabilized crude oil is 20.0 MBPD. The unstabilized crude oil from the pipeline is normally at 60 DEG F and is preheated to 162 DEG F by E-103. The unstabilized crude oil is sent to the top tray of Stabilizer Column, T-102. The unstabilized crude oil normally has a hydrogen sulfide concentration of 500 weight PPM and its API gravity is normally 32.8.
Stabilized crude oil from the bottom of T-102 is cooled from 195 DEG F to 83 DEG F in E-103.
Crude Oil Stabilizer
Preheated unstabilized crude oil from Stabilizer Feed/Bottoms Exchanger, E-103, is sent to the top tray of Stabilizer Column, T-102, which consists of 20 distillation trays. The unstabilized crude oil is distributed evenly onto the top tray and contacts rising light gases and some heavier hydrocarbons such as butane, pentane and hexane which are vaporized in the Stabilizer Reboiler, E-105. The contacting of the unstabilized crude oil with these vapors causes the crude oil to heat up as it falls through the trays in T-102 which, in turn, causes some of these heavier compounds to condense before reaching the outlet at the top of T-102. The Stabilizer Column operates at low pressure (12.1 PSIG) which is low enough to allow release of the hydrogen sulfide and light gases from the crude oil at reasonable temperatures while also being high enough to provide for transport of sour gas to the downstream treater at battery limits.
The following is the normal temperature profile of the Stabilizer Column:
Tray no. 20 (top) 159 DEG F
Tray no. 13 162 DEG F
Tray no. 7 164 DEG F
Tray no. 1 (reboiler inlet) 173 DEG F
Bottom (reboiler outlet) 195 DEG F
The bottom temperature is the key variable to reducing the hydrogen sulfide concentration in the stabilized crude. The bottom temperature also affects how much water is driven from the crude oil, which increases its sales value. A maximum hydrogen sulfide concentration of 10 weight PPM is permitted in the stabilized crude. During normal operation the hydrogen sulfide concentration is 5.7 weight PPM in reboiler mode and 4.9 weight PPM in stripping mode.
Operation at these temperatures also produces a significant flow of light ends product overhead. Depending on the sales value of these light ends relative to their value if they remain in the stabilized crude oil, it may be desirable to increase or decrease E-105 outlet temperature while maintaining on-spec hydrogen sulfide concentration. The economics of making a temperature adjustment will also depend on the crude composition.
The E-105 uses hot oil from battery limits to heat the crude oil. The hot oil is supplied at 500 DEG F and 50 PSIG and returns to battery limits from E-105 at 214 DEG F. The flow of hot oil is normally 71 GPM.
The stripping gas flow rate is zero during normal operation in reboiler mode. Stripping gas is also used as a purging fluid at shutdown.
Stabilized Crude Pumps
Warm stabilized crude oil from the bottom of Stabilizer Column, T-102, is first cooled by preheating Stabilizer Column feed in Stabilizer Feed/Bottoms Exchanger, E-103, and is then pumped by Stabilized Crude Pumps, P-151A/B, which are motor-driven, centrifugal pumps. Stabilized crude from P-151A/B is further cooled in Stabilized Crude Cooler, E-106, which is an air cooler. From E-106 the stabilized crude oil is sent to storage at battery limits. The flow rate of stabilized crude oil to storage is 18.67 MBPD. The temperature of the stabilized crude from E-106 is normally 76 DEG F. The API density of the stabilized crude is normally 29.4 and the hydrogen sulfide content is 5.7 weight PPM.
Light Ends Recovery
Vapor from the top tray of the Stabilizer Column, T-102, is routed to the Light Ends Condenser, E-104, which is an air cooler that cools the stripped light gases and hydrogen sulfide and condenses heavier hydrocarbons such as butane and pentane (light ends) in the column vapor. The outlet temperature of E-104 is normally 75 DEG F.
The cooled light gases and condensed light ends and sour water (containing hydrogen sulfide) are separated by gravity in the Light Ends Separator, D-104. D-104 normally operates at 10.0 PSIG. Light ends liquid collected in D-104 is pumped to storage at battery limits by the Light Ends Pumps, P-140A/B, which are electric motor-driven centrifugal pumps. The normal production rate of light ends liquid is 0.63 MBPD.
The sour water produced from E-104 is collected in the boot of D-104 and sent to a storage tank outside battery limits for subsequent treating to remove dissolved hydrogen sulfide before disposal.
The vapor (sour gas) separated in D-104 is sent to a fuel gas treater outside battery limits for hydrogen sulfide removal prior to being used as a utility fuel gas. The flow of sour gas is normally 31.8 MSCF/H. The hydrogen sulfide concentration of sour gas is normally 3.809 VOL %.
Instrumentation
BasicBasic Controls: Unstabilized Crude Pumps
The unstabilized crude from the GOSP is pumped by P-101A/B whose motors are controlled by switches HS-101A and HS-101B, respectively. The unstabilized crude charge temperature from the pipeline is indicated on TI-101 and the feed flow rate is controlled by FIC-101 which adjusts control valve FV-101 on the discharge of Unstabilized Crude Pumps, P-101A/B. The hydrogen sulfide concentration of the unstabilized crude oil is indicated on AI-101. The API density of the unstabilized crude is indicated on AI-102.
The unstabilized crude oil outlet temperature from Stabilizer Feed/Bottoms Exchanger, E-103, is indicated on TI-130. The temperature of stabilized crude oil from Stabilizer Column, T-102, is indicated on TI-136 while the temperature of stabilized oil leaving E-103 is indicated on TI-137.
Hand controller HIC-130 adjusts the position of three-way valve HV-130. When HIC-130’s output is 0%, no feed flow is bypassed around E-130. When HIC-130’s output is 100%, all the feed flow is bypassed around E-130. Normally, the output of HIC-130 is 0%.
Basic Controls: Crude Oil Stabilizer
The temperature of the unstabilized crude oil feed to Stabilizer Column, T-102, is indicated on TI-130. The overhead pressure of T-102 is indicated on PI-131. TI-131 indicates the temperature of vapor leaving the top of T-102. TI-132 indicates the temperature on Tray #13, TI-133 indicates the temperature on Tray #7 and TI-134 indicates the temperature of crude oil leaving tray #1 of T-102.
TIC-135 controls the temperature of the crude oil leaving Stabilizer Reboiler, E-105, by adjusting the setpoint of the hot oil flow controller, FIC-160. FIC-160 is normally in cascade mode and adjusts the position of hot oil flow control valve FV-160. The temperature of hot oil supplying E-105 is indicated on TI-160 and its pressure is indicated on PI-160. The temperature of hot oil leaving E-105 is indicated on TI-161.
XA-160 indicates the status of the reboiler interlock I-160. When XA-160 is in the trip state it will generate an audible alarm and lock hot oil flow controller in manual mode with an output of 0% to prevent flow of hot oil through E-105. See the section on Interlocks for more details about I-160 operation.
The pressure drop across all the distillation trays of T-102 is indicated on PDI-135. Stripping gas flow is controlled by FIC-132 by adjusting the position of valve FV-132. Normally FIC-132 is not used in reboiler mode. The temperature of the stabilized oil leaving the bottom of T-102 and going to Stabilizer Feed/Bottoms Exchanger, E-103, is indicated on TI-136.
The level of stabilized crude oil in the bottom of T-102 is controlled by LIC-139 which adjusts the setpoint of stabilized crude oil flow controller, FIC-151.
Basic Controls: Stabilized Crude Pumps
The temperature of stabilized crude oil from Stabilizer Feed/Bottoms Exchanger, E-103, is indicated on TI-137. Stabilized crude oil is pumped to storage at battery limits by Stabilized Crude Pumps, P-151A/B, whose motors are controlled by switches HS-151A/HS-151B, respectively. The fan motor for Stabilized Crude Cooler, E-106, is controlled by switch HS-106. The opening of the air louvers on E-106 are adjusted by HIC-151. Adjusting HIC-151’s output will adjust the air flow through E-106. TI-138 indicates the temperature of the stabilized crude oil leaving E-106.
The flow of stabilized crude oil to storage is controlled by FIC-151 which adjusts the position of control valve FV-151. FIC-151 receives its setpoint from Stabilizer Column, T-102, bottoms level controller LIC-139 when in cascade mode. The hydrogen sulfide content of the stabilized crude oil is indicated on AI-151. The API density of the stabilized crude oil is indicated on AI-152.
Basic Controls: Light Ends Recovery
The temperature of the vapor leaving the top of Stabilizer Column, T-102, is indicated on TI-131 and the pressure at the top of T-102 is indicated on PI-131. An independent pressure instrument PAH-131 is used as an input to the Reboiler interlock I-160.
The fan motor for Light Ends Condenser, E-104, is controlled by switch HS-104. The opening of the air louvers on E-104 are adjusted by HIC-140. Adjusting HIC-140’s output will adjust the air flow through E-104. TI-140 indicates the temperature of the light ends/light gas mixture leaving E-104.
The pressure of Light Ends Separator, D-104, is controlled by PIC-140 which adjusts the position of control valve PV-140. The flow of sour gas through PV-140 is indicated on FI-140 and the hydrogen sulfide concentration of the sour gas is indicated on AI-140.
The level of light ends liquid in D-104 is controlled by LIC-140 which adjusts the setpoint of light ends flow controller FIC-141 which is normally in cascade mode. An independent level instrument LAH-141 is used as an input to the Reboiler interlock I-160.
The temperature of light ends liquid leaving D-104 is indicated on TI-141. The motors of Light Ends Pumps P-140A/B are controlled by switches HS-140A/HS-140B, respectively. The flow of light ends liquid to storage is controlled by FIC-141 which adjusts the position of control valve FV-141.
The level of the water in the boot of D-104 is controlled by LIC-141 which adjusts the position of control valve LV-141.