Process Description
Simtronics’ Advanced Distillation simulator program represents a typical industrial distillation plant consisting of a distillation column, feed storage tank, two product storage tanks, an off-spec tank and a flare system. The feed to the unit is a raw light gasoline feed produced from primary fractionation of natural gas liquids (NGL). In the petroleum industry, the distillation column is commonly called a debutanizer since it removes butane and lighter compounds from the feed to produce a stabilized (less volatile) liquid product.
The feed mainly contains a range of alkane hydrocarbons: propane through hexane. The purpose of the unit is to separate the lighter components (principally butane and lighter components) from the heavier components (pentane and heavier components). All products are removed as liquids and are routed to product storage tanks. Off-Spec product can be transferred to the Off-Spec Storage Tank. A flare system is included in the model and is present in the event that a small amount of light gas is produced. The flare system is intended to burn any light gas produced during operations.
A full range of operations can be learned and practiced on the Distillation Column with Tank Farm and Flare System simulator. These include normal, startup, shutdown, and emergency shutdown procedures.
The rest of this manual describes the Advanced Distillation simulation and provides detailed operating procedures and exercises for training.
Hydrocarbon feed enters the process and is stored in a spherical Feed Tank, V-101.
The feed is pumped to Distillation Column, T-401. To effect separation in the Distillation Column, heat is provided to the process by:
- A Feed Preheater, E-400, in which heat is recovered from the distillation column bottoms by the distillation column feed.
- A Reboiler, E-402, via hot oil.
Distillation Column overhead vapor is a reasonably pure butane stream containing a small amount of propane. The overhead vapor is condensed in Condenser, E-401, by cooling water before separation in the Reflux Drum, D-401. The Reflux Drum pressure is controlled by the Condenser and its bypass control valve. Uncondensed vapor in the Reflux Drum, if any, is sent to the flare system. Condensate from the Reflux Drum is pumped by Reflux Pumps, P-401A/B, with a portion of the liquid returned as reflux to the Distillation Column. The balance of the condensate is pumped to the Light Product Tank, V-901.
The Light Product Tank provides short term product storage prior to being pumped by the Light Product Pump, P-901, for delivery.
The Distillation Column operates at a pressure sufficiently high to transfer its bottoms liquid to atmospheric product storage. As indicated earlier, heat is recovered by heating feed with distillation column bottoms in E-400. If necessary, Distillation Column bottoms is further cooled by cooling water in the Bottoms Cooler, E-403 before entering the Heavy Product Tank, V-902.
The Heavy Product Tank provides short term product storage prior to being pumped by the Heavy Product Pump, P-902, for delivery.
Alternatively, any off-spec product can be transferred to the Off-Spec Tank, V-903, for reprocessing by sending material back to the Feed Tank, V-101, or to another processing unit.
The Flare System collects pressure safety valve (PSV) releases from all four tanks and from vent and PSV release from the Distillation Column in the Flare Header. The releases, typically vapor, are sent to the Liquids Knock Out Drum, D-951, to separate any liquid releases that may occur in case a vessel is overfilled. Any liquid collected in the drum is pumped by Knock Out Liquid Pumps, P-951A/B, to liquids disposal at battery limits. Remaining vapor from D-951 is sent to the Seal Drum, D-952, where the vapor is contacted with utility water to create a seal that prevents air from back-flowing into V-951 and the flare header. Water is collected on the spillover side of the Seal Drum and pumped to water disposal by Seal Water Pumps, P-952A/B. Vapor from the Seal Drum is combined with fuel gas in the Flare Stack, FS-952, where it is burned before being released to the atmosphere.
Feed Tank
The Feed Tank, V-101, receives raw feed from Unit A at battery limits under flow control from the upstream unit. The Feed Tank is designed to combine fresh feed and rerun feed from the Tank Farm. Fresh Feed normally enters the Feed Tank via XV-101A. Fresh feed may also bypass the Feed Tank and pass on to the Distillation Column (T-401) through XV-101B. Normally, no rerun from Off-Spec Tank, V-903, is processed.
Under startup, shutdown and upset conditions the unit may produce off-spec product. If desired, and as noted above, the off-spec product from the Tank Farm can be rerun by combining with fresh feed in the Feed Tank.
The pressure of V-101 floats depending on the vapor pressure of the tank contents. In case of overpressure due to high vapor pressure or if the tank is overfilled, PSV-101 will release gas and/or liquid to the Flare Header.
Feed System
Liquid from the Feed Tank is transferred to the Distillation Column via the Distillation Feed Pumps, P-400 A/B. The feed pumps are electric motor driven pumps and each has the same capacity. Normally, only one pump is in service.
Feed which bypasses the Feed Tank via XV-101B is combined with the pump discharge before entering the Feed Preheater, E-400. The Feed Preheater recovers heat from the Distillation Column bottoms (light gasoline) which facilitates component separation downstream in the Distillation Column and reduces the heat input required from hot oil.
The direct feed line is used in case the Feed Pumps, P-400A/B, are out of service or in circumstances where the feed rate to the Distillation Column needs to be less than the flow rate from Unit A at battery limits. In this latter circumstance, the Feed Pumps should normally be stopped.
Distillation Column
The Distillation Column, T-401 consists of 36 distillation trays with the feed introduced on tray 24 (bottom-most tray is tray 1). The feed liquid is distilled into a butane product produced in the overhead section of the Distillation Column and a liquid product (principally pentane) from the bottom of the column.
Additional heat is supplied to the Distillation Column by the Reboiler, E-402. The reboiler uses hot oil to provide supplemental heat for component separation.
The Distillation Column operates at a pressure sufficiently high so that pumping the column bottoms product to storage is not required. The heavy liquid bottoms product is normally used to preheat to the Distillation Column. Residual heat is removed by cooling water in the Bottoms Cooler, E-403. The cooled heavy product is normally routed to Heavy Product Tank V-902. Alternatively, it can be routed to the Off-Spec Storage Tank, V-903, or directly to the Feed Tank, V-101, for reprocessing.
Top vapor from T-401 is sent to Condenser E-401. The Distillation Column is protected from overpressure by PSV-401 which routes any release to the Flare Header. The set pressure of PSV-401 is 185 PSIG.
Overhead System
Typically, all the vapor from the top of the Distillation Column T-401 is condensed in Condenser, E-401. The condenser uses cooling water from battery limits to cool and condense the overhead vapor down to 91 DEG F.
The condensed liquid from E-401 enters the bottom of the Reflux Drum D-401. The elevation of E-401 relative to D-401 is such that the condensate it produces will form a liquid seal between the shell-side inlet of E-401 and the vapor space of D-401. The liquid level within E-401 will change until the condensing rate matches the overhead vapor flow rate from T-401. Increasing the pressure in D-401 will back up liquid in E-401 which, in turn, will cause a lower condensing rate in E-401. Because of the lower condensing rate, the pressure builds in T-401 until it can push the liquid level in E-401 lower to increase the condensing rate until it balances with the overhead vapor rate from T-401. Therefore, changing the pressure of D-401 will change the pressure of T-401. In order to increase the pressure of D-401, a hot vapor bypass line is provided around E-401 to admit hot vapor from the overhead of T-401 directly to the vapor space of D-401. To decrease the pressure of D-401, some of the vapor from D-401 may be vented to flare. A split-range control loop is used to automatically control the hot vapor bypass and flare control valves. In case the pressure of D-401 cannot be controlled, a second automatic pressure vent to flare is provided. The Reflux Drum is protected from overpressure by PSV-402 which routes any release to the Flare Header. The set pressure of PSV-402 is 185 PSIG.
The liquid collected in D-401 is product butane and is pumped by Reflux Pumps P-401A/B under level/flow control to the Light Product Storage Tank, V-901. Before entering V-901, that stream is cooled by cooling water in E-404, the Overhead Cooler. A separate portion of the reflux is under flow control with the liquid returning to the Distillation Column. The reflux pumps are electric motor driven pumps and are of the same capacity. Normally, only one pump is in service.
Product and Off-Spec Tanks
There are three spherical storage tanks which make up the product portion of the Tank Farm (the other is the Feed Tank V-101).
The Light Product Tank, V-901, is supplied by the Reflux Pumps, P-401A/B, via the Overhead Cooler, E-404. The Light Product Tank contains liquid with a high butane content. Liquid is removed from the Light Product Tank by the Light Transfer Pump, P-901, under level control to Unit C. The pressure of V-901 floats depending on the vapor pressure of the tank contents. In case of overpressure due to high vapor pressure or if the tank is overfilled, PSV-901 will release gas and/or liquid to the Flare Header.
The Heavy Product Tank, V-902, is supplied by the Distillation Column, T-401, bottoms via the Bottoms Cooler, E-403. The Heavy Product Tank contains liquid rich in pentane with some hexane and a small amount of butane. Liquid is removed from the Heavy Product Tank by the Heavy Transfer Pump, P-902, under level control to Unit E. The pressure of V-902 floats depending on the vapor pressure of the tank contents. In case of overpressure due to high vapor pressure, or if the tank is overfilled, PSV-902 will release gas and/or liquid to the Flare Header.
The Off-Spec Tank, V-903, can accept feed from the Reflux Drum, D-401, or the Distillation Column, T-401, bottoms. The Off-Spec Tank will contain off-spec liquid. This liquid is removed from the Off-Spec Tank by the Off-Spec Transfer Pump, P-903, under level control. The liquid is transferred to Unit D or returned for rerun to the Feed Tank, V-101. The pressure of V-903 floats depending on the vapor pressure of the tank contents. In case of overpressure due to high vapor pressure or if the tank is overfilled, PSV-903 will release gas and/or liquid to the Flare Header.
Flare System
Any releases to the Flare Header are routed to the Liquids Knock Out Drum, D-951. This includes:
- Flare system purge nitrogen
- Releases from the Distillation Column, T-401, pressure relief valve, PSV-401
- Releases from the Reflux Drum, D-401, pressure relief valve, PSV-402
- D-401 vapor via
- PV-412B, the split range pressure release control valve
- PV-413, the high-pressure control valve
- Releases from the Feed Tank, V-101, pressure relief valve, PSV-101
- Releases from the Light Product Tank, V-901, pressure relief valve, PSV-901
- Releases from the Heavy Product Tank, V-902, pressure relief valve, PSV-902
- Releases from the Off-Spec Tank, V-903, pressure relief valve, PSV-903
Normally, there is a nominal flow of nitrogen added at the end of the Flare Header to sweep any leaks from the PSVs or small vent flows from Reflux Drum, D-401, to D-951. This also inhibits the infiltration of air from the Flare Stack, FS-952, into the header.
The combined feeds from the Flare Header enter the Liquids Knock Out Drum, D-951, where liquid, if any, will accumulate at the bottom of the drum. This liquid will be transferred from the Liquids Knock Out Drum by the Knock Out Liquids.
Pumps, P-951A/B, under level control. The Knock Out Liquids Pumps are electric motor driven pumps and are of the same capacity. Normally, no pump is in service. An interlock system will automatically start P-951A if the level of D-951 exceeds 30% and will automatically start P-951B if the level exceeds 60%.
Vapor from the Liquids Knock Out Drum, D-951, feeds the left side of the Seal Drum, D-952 which is normally completely full of water. Under no-release conditions, the nitrogen sweep gas to the Flare Header will bubble through the water in the left side of the Seal Drum and then flow to the Flare Stack, FS-952. The water seal prevents infiltration of air back into D-951 and the Flare Header under no-release conditions.
The Seal Drum is partitioned with a vertical baffle separating two sides. Utility water under flow control feeds the left side of the drum. Once the left side is water full, water will spill over the baffle to the right side of the Seal Drum. A small opening is cut in the base of the baffle to allow drainage of water from the seal-side at shutdown. Water collected in the right side of the Seal Drum will be pumped to disposal via Seal Water Pumps, P-952A/B, under level control. The Seal Water Pumps are electric motor driven pumps and are of the same capacity. Normally, only one pump is in service.
The vapor from D-951 will combine with draft air entering at openings in the base of the Flare Stack. The draft air provides oxygen which will burn fuel gas fed to a burner ring within the stack. The Flare Stack contains a Pilot Flame Generator (X-955) which provides a steady ignition flame at the burner ring. A small gas flow keeps the Flare Stack warm so as to have a sufficient flow of draft air in case of a release of hydrocarbons into the Flare Header. During a release to the Flare Header, the temperature of the Flare Stack will increase which will, in turn, provide more draft air.
Instrumentation
Feed Tank Controls and Instruments
Total fresh feed to the process unit is indicated on FI-100. The feed rate is normally controlled in Unit A at battery limits when feed is routed to Feed Tank, V-101. Switch XV-101A is used to open and close the feed to tank isolation valve XV-101A. Switch XV-101B is used to open and close the feed tank bypass valve XV-101B. Normally XV-101A is open and XV-101B is closed.
The liquid level in spherical Feed Tank, V-101, is displayed on level controller LIC-101 which is cascaded to the downstream feed flow controller FIC-400 for the Distillation Column, T-401. The pressure in V-101 is indicated on PI-101.
Feed System Controls and Instruments
Pumps P-400A/B transfer feed to the Distillation Column, T-401. Switches HS-400A and HS-400B are used to change the states of the motors for Distillation Feed Pumps P-401A and P-401B, respectively.
The temperature and pressure of the combined feed (from Feed Tank V-401 and its bypass) are indicated on TI-400 and PI-400, respectively. The feed analyzer provides four composition measurements:
- AI-400A – Mole % Propane
- AI-400B – Mole % Butane
- AI-400C – Mole % Pentane
- AI-400D – Mole % Hexane
As indicated earlier, the total feed to the Distillation Column is controlled by FIC-400 (receives the output signal from LIC-101) and adjusts the valve opening of FV-400. FIC-400 is normally in cascade mode. In the case where the Feed Tank is bypassed and Unit A directly feeds into the discharge of P-400A/B, FIC-400 should be placed into either automatic or manual mode. Also, pumps P-400A/B should be switched off in this mode of operation unless they are needed to empty the Feed Tank. When FIC-400 is in automatic mode, the feed to the Distillation Column will be controlled by FIC-400. The setpoint of FIC-400 should be at least a bit less than the demand flow of the flow controller in Unit A (normally 1950 BPD210 M3/D); otherwise the controllers will fight each other. In case FIC-400 is in manual mode, the flow controller in Unit A will try to regulate the feed flow. In this case, make sure that the feed control valve FV-400 is fully open to allow Unit A’s controller to properly regulate the flow. Distillation Column feed temperature (tube side pass) is indicated on TI-402. The temperature at the outlet of the shell side pass (Distillation Column Bottoms) can be observed on TI-415.
Distillation Column Controls and Instruments
As indicated earlier, the feed temperature to the Distillation Column is displayed on TI-402.
The pressure at the top of Distillation Column T-401 is indicated on PI-403. A second independent pressure indicator, PAH-403, is used as a trip sensor for the Distillation Column Reboiler Interlock I-401. The temperature of vapor leaving the top of T-401 is indicated on TI-403.
TI-404, TI-405, TI-406 and TI-407 indicate the temperatures of trays 15, 13, 8 and 6 of T-401, respectively (recall that the bottom of the column is tray 1 while the top of the column is tray 17). TIC-408 controls the temperature of tray 3 by adjustment of the setpoint of hot oil flow controller FIC-004. FIC-004 will adjust the valve opening of FV-004. The temperature of tray 1 liquid to the Distillation Column Reboiler is indicated on TI-409. The temperature of the NGL leaving the Reboiler, E-402, is indicated on TI-410. The temperature of T-401 bottoms is indicated on TI-411.
For the Reboiler, E-402, the hot oil supply pressure is indicated on PI-001. The temperature of the hot oil supply is indicated on TI-001. The flow of hot oil to E-402 is controlled by FIC-004 which adjusts the position of FV-004. The temperature of hot oil leaving E-402 is indicated on TI-004. If the Distillation Column Reboiler Interlock I-401 is active, FIC-004 will be locked in manual with an output of 0% to prevent flow of hot oil to E-402.
The flow of Distillation Column bottoms is managed by HIC-415 which controls a 3-way valve. At an output of 100%, all of the bottoms flow will be directed to E-400, the Distillation Column Preheater. When the output of HIC-415 is 0%, all of the bottoms flow will bypass E-400 entirely and feed directly into Bottoms Cooler, E-403.
The tube side inlet temperature for E-403 is indicated on TI-416. After transferring heat to cooling water in E-403, the tube side outlet temperature from E-403 is displayed on TI-417. The cooling water supply temperature is indicated on TI-021 and the cooling water outlet temperature from E-403 is indicated on TI-025. The cooling water flow rate can be adjusted with hand controller HIC-025 which adjusts the opening of control valve HV-025.
The pressure drop across the top section of T-401 is indicated on PDI-401. The pressure drop across the lower section of T-401 is indicated on PDI-402.
The level of liquid in the bottom of T-401 is controlled by LIC-411 which adjusts the setpoint of bottoms flow controller FIC-411. FIC-411 controls the bottoms flow from T-401 to Bottoms Storage by adjustment of the position of FV-411. The butane content of the bottoms from T-401 is indicated on AI-411.
Overhead Controls and Instruments
The process stream outlet temperature from Distillation Column Condenser E-401 is indicated on TI-412. The cooling water flow to E-401 is controlled by HIC-024 with the cooling water outlet temperature displayed on TI-024.
The pressure of Reflux Drum, D-401, is controlled by PIC-412 which is a split range controller. PIC-412 controls both PV-412A (0-50%) and PV-412B (50-100%). Only one of these two valves will be open at any point in time. When the Reflux Drum pressure is low, PV-412A is open (PV-412B is closed). Conversely, when the Reflux Drum pressure is high, PV-412B is open (PV-412A is closed). The graph below shows the relationship between PIC-412 and the positions of the valves it controls.
The flow of gas through PV-412B to flare is indicated on FI-412. A second independent pressure controller PIC-413 adjusts the position of PV-413. PIC-413 should have a higher pressure setpoint than PIC-412 and will only open PV-413 if PIC-412 is unable to control the pressure of D-401.
The level of liquid in Reflux Drum D-401 is controlled by LIC-412 which adjusts the setpoint of overhead product flow controller FIC-414. An independent level is indicated on LAH-412 which is used for the Reboiler Interlock I-401.
The temperature of liquid leaving D-401 is indicated on TI-413. Switches HS-401A and HS-401B are used to change the states of the motors for Reflux Pumps P-401A and P-401B, respectively. The flow of reflux to T-401 is controlled by FIC-413 which adjusts the position of FV-413. The flow of light product to storage is controlled by FIC-414 which adjusts the position of FV-414. The concentration of pentane and higher components in the light product indicated on AI-414.
HIC-026 adjusts the position of Overhead Cooler, E-404 cooling water supply valve HV-026. The temperature of light product leaving E-404 is indicated on TI-418. The temperature of the cooling water return is indicated on TI-026.
Product & Off-Spec Tanks Controls and Instruments
Light Product from Reflux Drum, D-401 bottoms can be forwarded to the:
- Light Product Tank, V-901, by opening isolation valve XV-901, or
- Off-Spec Tank, V-903, by opening isolation valve XV-903A.
Heavy Product from Distillation Column, T-401 bottoms can be forwarded to the:
- Heavy Product Tank, V-902, by opening isolation valve XV-902, or
- Off-Spec Tank, V-903, by opening isolation valve XV-903B, or
- Feed Tank, V-101, by opening isolation valve XV-101C.
Product is transferred from the Light Product Tank, V-901, by level controller LIC-901 which adjusts valve LV-901. Switch HS-901 is used to change the state of the motor for the Light Transfer Pump P-901. Product flow is displayed on FI-901 and is transferred to Unit C. The pressure in V-901 is indicated on PI-911.
Product is transferred from the Heavy Product Tank, V-902 by level controller LIC-902 which adjusts valve LV-902. Switch HS-902 is used to change the state of the motor for the Heavy Transfer Pump P-902. Product flow is displayed on FI-902 and is transferred to Unit E. The pressure in V-902 is indicated on PI-912.
Off-Spec material is transferred from the Off-Spec Tank, V-903, by level controller LIC-903. Switch HS-903 is used to change the state of the motor for the Off-Spec Transfer Pump P-903. Liquid flow is displayed on FI-903. Off-Spec material can be forwarded to:
- Feed Tank, V-101, by opening isolation valve XV-101D for reprocessing, or
- Unit D at battery limits by opening isolation valve XV-903C.
The pressure in V-903 is indicated on PI-913.
Flare System Controls and Instruments
HIC-960 adjusts the position of valve HV-960 which will admit a small nitrogen purge stream to the Flare Header. The nitrogen flow is indicated on FI-960.
The pressure of the Liquids Knock Out Drum, D-951 is shown on PI-951. The liquid level in D-951 is controlled by LIC-951, which adjusts valve LV-951. The flow of liquid from the bottom of D-951 is measured by FI-951. Switches HS-951A and HS-951B are used to change the state of the motor for the Knock Out Drum Liquids Pumps, P-951A and P-951B, respectively. An independent level for D-951 is indicated on LAH-951 which is used for the KO Drum Interlock I-951.
Overhead vapor flow from the Liquids Knock Out Drum to the Seal Drum, D-952 is indicated on FI-953. The liquid level on the left side of the Seal Drum is indicated on LI-953. Utility water supply is controlled by FIC-961 which manipulates valve FV-961.
The right side of the seal drum is liquid level controlled by LIC-952 which adjusts LV-952. The flow of seal water from the Seal Drum, D-952, is measured by FI-952. Switches HS-952A and HS-952B are used to change the state of the motor for the Seal Water Pumps, P-952A and P-952B, respectively.
The flare system has a pilot generator, X-955, which is turned on and off with switch HS-955. The flow of pilot fuel gas to X-955 is shown on FI-955. XI-955 is a measure of pilot flame intensity. Flare gas is supplemented with a small stream of fuel gas which is under flow control by FIC-954 which adjusts the valve opening of FV-954. XI-956 provides an indication of flame intensity in the Flare Stack. TI-955A and TI-955B are indicators for temperatures in the Flare Stack, FS-952.