SPM-2920 Renewable Diesel Hydrotreater

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Process Description

Renewable Diesel Hydrotreater (RDHT)
Simtronics’ Renewable Diesel Hydrotreater (RDHT) simulator enables detailed training of the operating principles of RDHT units becoming commonly found in the petroleum refining industry. RDHT units convert renewable (agriculturally sourced) oil feedstocks into a high quality, very low-sulfur diesel fuel product referred to as renewable diesel oil (RDO). These oil feedstocks typically are byproducts of the food processing industry and, in some cases, virgin vegetable oils. These oils are pre-processed before use in a RDHT to remove solids such as fats and food particles which would be harmful to the equipment of the RDHT.

The oil feedstocks also have a significant oxygen content which results in a very high consumption of hydrogen in the RDHT Reactor compared to traditional refinery hydroprocessing processes (e.g., hydrodesulfurization). The RDHT reactions also give off much more heat than is typical of traditional hydroprocessing processes. As a result, the RDHT process employs a high rate of recycled RDO product to absorb the reaction heat in the RDHT reactor to avoid excessively high temperature rises.

The simulated RDHT process employs a four-bed catalytic reactor to convert the feed oil in the presence of hydrogen at moderately high temperatures and at higher pressures compared to traditional refinery hydrotreating processes. Higher pressures result in higher hydrogen partial pressures in the reactor which promote the conversion of the feed oil into a diesel oil. These reactions also produce carbon dioxide, carbon monoxide and a significant amount of water due to the high oxygen content of the feed oil. 100% of the feed oil is converted to diesel oil in the RDHT Reactor.

The reactor effluent is cooled by preheating recycled RDO from the downstream and using an air cooler. The condensed unstabilized RDO is collected and sent to the to a distillation tower (Stripper) to remove absorbed light gases from reactor section. The stabilized RDO product is cooled and sent to storage.

A full range of operations can be learned and practiced on the RDHT simulator. These include normal, startup, shutdown, and emergency shutdown procedures.

The rest of this manual describes the simulated RDHT process and provides detailed operating procedures and exercises which can be used for learning RDHT operations.

Purpose
The purpose of the Renewable Diesel Hydrotreater (RDHT) is to convert the vegetable-based oil feed to a renewable diesel oil (RDO) product.

Importance
Many hydrodesulfurization (RDHT) units in refineries are being converted to process renewable feedstocks such as vegetable-sourced oils from food processing industries. These RDHT units are reducing the demand for crude oil-based diesel oil for motor fuel. The RDHT units are more complicated in their design due to the recycling of RDO back to the RDHT reactor in order to keep the temperature rise across the reactor from getting too high from the higher heat released in RDHT reactions.

Major Equipment
The RDHT process consists of two main sections: the Reactor Section and the Stripper Section.

The Reactor Section consists of:

  • Feed Surge Drum, D-101
  • Fresh Feed Pumps, P-101A/B
  • Recycle Pumps. P-171A/B
  • Hydrogen Compressor, K-101
  • Spillback Cooler, E-180
  • Recycle Liquid/Reactor Effluent Heat Exchangers, E-101A/B/C
  • Recycle Heater, F-101
  • Renewable Diesel Hydrotreating Reactor, R-101
  • Reactor Effluent Air Cooler, E-102
  • Reactor Effluent Separator, D-102
  • Recycle Compressor Knockout (KO) Drum, D-103
  • Recycle Compressor, K-102

The Stripper Section consists of:

  • Stripper Feed/Bottoms Heat Exchanger, E-103
  • Stripper, T-102
  • Overhead Condenser, E-104
  • Stripper Reflux Drum, D-104
  • Stripper Reflux Pumps, P-102A/B
  • Stripper Reboiler Pumps, P-105A/B
  • Stripper Reboiler, F-102
  • Stripper Bottoms Pumps, P-103A/B
  • Product Cooler, E-106

Feed System
Treated vegetable oil is received from battery limit and fed to the Feed Surge Drum, D-101 (omitted on Schematic 2) and is pumped from D-101 by Fresh Feed Pumps P-101A/B directly to the Renewable Diesel Hydrotreating Reactor R-101. The design feed flow is 16.0 MBPD of vegetable oil. The distribution of feed to the first three beds of R-101 is roughly equal.

Recycle RDO System
Recycled renewable diesel oil (RDO) is taken from the base of the Stripper, T-102, after being cooled in Stripper Feed/Bottoms Exchanger, E-103, and pumped back to the reactor section by Recycle Pumps, P-171A/B. The design recycle RDO flow is 24.0 MBPD. This gives a theoretical fresh feed volume concentration of 40% at design, or a recycle to feed volume ratio of 1.5. The recycled RDO is split and sent to Recycle Liquid/Reactor Effluent Heat Exchanger No. 1, E-101A and Recycle Liquid/Reactor Effluent Heat Exchanger No. 2, E-101B to recover heat from the outlet stream of R-101. 62.5% of the recycle RDO flows to E-101A and the balance flows to E-101B.

Hydrogen System/Recycle Liquid Preheating
Fresh makeup hydrogen from Hydrogen Compressor, K-101, provides 2.019 MMSCF/H of hydrogen to the RDHT reactor loop. The design concentration of feed hydrogen is 99.6 volume %. About one-sixth (16.5%) of the fresh hydrogen is sent directly to E-101A and combines with recycle RDO upstream of the heat exchanger. The balance of the makeup hydrogen is sent to the recycle gas distribution header where it mixes with hydrogen-rich recycle gas from the Recycle Compressor, K-102. Most of the combined hydrogen and recycle gas is fed to E-101B and combines with recycle RDO upstream of E-101B. The preheated recycle RDO and recycle gas from E-101B are further heated in Recycle Liquid/Reactor Effluent Heat Exchanger No. 3, E-101C by the hot effluent from R-101.

The Hydrogen Compressor is a reciprocating type and includes a spillback line and spillback cooler.

Recycle Liquid Heater
Recycle RDO and fresh hydrogen from E-101A are then heated in Recycle Heater, F-101, which is a gas-fired process heater with two separate tube passes. F-101 normally runs at minimal fuel firing because of the high amount of heat recovered from the RDHT Reactor outlet in preheat exchangers E-101A, B and C. The outlet RDO/fresh hydrogen stream from F-101 is then combined with the outlet recycle RDO/recycle gas from E-101C and the portion of vegetable oil feed sent to Bed No. 1 of R-101. This combined stream normally operates at 738 PSIG and 535 DEG F and flows into the top of R-101.

R-101 RDHT Reactor

Hydrodeoxygenation Reaction
R-101 is a fixed-bed reactor employing several beds of catalyst designed for hydrodeoxygenation of vegetable oils. Vegetable oils mainly consist of triglycerides which have a chemical structure essentially consisting of three linear chains of fatty acid each held together at one of their ends by oxygen atoms connected to a short chain of three carbons as illustrated in the diagram below.

The hydrodeoxygenation reaction requires high hydrogen partial pressure and moderately high temperature to break the oxygen-carbon bonds of the triglyceride molecule. The hydrogen combines with the oxygen atoms in the triglyceride molecule, forming water (H2O) which results in the separation of the fatty acid chains and the triple carbon chain into individual compounds. The disassociated fatty acid chains are further saturated with hydrogen to form alkanes (linearly chained hydrocarbons) that are similar to the components of diesel oil derived from the distillation of crude oil. The triple hydrocarbon chain is saturated with hydrogen to form propane.

In addition to the hydrodeoxygenation reaction, there are some undesirable side reactions that occur to a lesser extent. These result in the production of carbon monoxide and carbon dioxide as byproducts. Also, some cracking of the carbon chains occurs, resulting in a some smaller-chained hydrocarbons like those hydrocarbons found in conventional gasoline.

Heat of Reaction Considerations
The hydrodeoxygenation reaction releases large amounts of heat, owing to the high number of oxygen bonds that are broken and owing to the fairly large amount of hydrogen saturation of the disassociated compounds. In conventional hydrotreating of crude oil-based hydrocarbons, the heat of reaction and the amount of reaction occurring is much lower so that reactor temperatures are controlled by injecting cold quench recycle gas into the reactor between beds.

This type of temperature control is not economically feasible in Renewable Diesel Hydrotreaters because the flow rates of quench gas would be extraordinarily high, requiring many recycle compressors to satisfy the quench gas demand. This would also incur high recycle gas compression costs. Therefore, RDO product is recycled to the RDHT reactor to absorb this large amount heat so as to avoid an excessively high temperature rise across the reactor. Quench gas lines to inject recycle gas to between the four beds are included in the RDHT design for fine temperature control of a reactor bed, if needed. This especially useful for adjusting the temperature of the last catalyst bed.

The first three catalyst beds receive roughly equal flows of fresh feed. Each of these beds contains hydrodeoxygenation catalyst and each bed effectively converts 100% of the fresh feed routed to it. This approach distributes the hydrodeoxygenation reaction through the three beds while permitting some flexibility in adjusting the split to each bed in case of changes in the catalyst activity (reaction rate) of the individual catalyst beds.

The normal outlet temperature of the RDHT Reactor, R-101, is 662.6 DEG F making the apparent temperature rise across the reactor to be 127.6 DEG F.

Isomerization Reaction
The fourth bed of R-101 contains a catalyst formulated to convert the linear alkanes produced by the hydrodeoxygenation reaction of the top three beds into branched alkanes. This is done to improve the cold flow properties of the RDO product. Linear alkanes will solidify (i.e., create wax) at a higher temperature than branched alkanes. This makes the cloud point property (i.e., the temperature at which wax is produced) of the produced RDO too high for commercial distribution as a motor fuel. The branching reaction is called isomerization. Isomers are structurally different forms of compounds having the same chemical formula (e.g., normal octane and isooctane). They mostly differ in their physical properties (e.g., boiling point, freezing point). The isomerization reaction has no effect on gas composition and has a small heat of reaction, especially relative to the heat of reaction of the hydrodeoxygenation reaction.

Catalyst Sulfiding
The catalysts in the RDHT Reactor need to be sulfided to maximize their catalytic activity. It is assumed that a suitable sulfiding agent such as dimethlydisulfide (DMDS) has been injected into the feed at an appropriate concentration to keep the catalysts in the sulfided state. DMDS breaks down in the reactor to form hydrogen sulfide (H2S) which then keeps the catalyst sulfided. Therefore, H2S will be present in the gas produced from the reactor.

Reactor Effluent Cooling

Reactor Effluent Heat Recovery
The hot effluent from the bottom of R-101 at 662 DEG F is sent to the Recycle Liquid/Reactor Effluent Heat Exchanger No. 3, E-101C, to preheat the portion of the recycled HDO routed through E-101B and E-101C. The hot side outlet stream from E-101C is routed to E-101A to preheat recycled HDO destined for F-101. Finally, the hot side outlet stream from E-101A is routed to E-101B. From E-101B, the reactor effluent stream at 417 DEG F is sent to Reactor Effluent Air Cooler, E-102. The cooling of the reactor effluent in E-101C/B/A results in partial condensation of the produced RDO from R-101.

Reactor Effluent Cooling
The Reactor Effluent Air Cooler, E-102, cools and condenses the remainder of the RDO and water in the reactor effluent leaving E-101B. The normal outlet temperature of E-102 is 92 DEG F. Most of the unreacted hydrogen along with CO, CO2 and methane remains in the vapor phase at the outlet of E-102 and is separated from the condensed RDO in High Pressure Separator, D-102. Water (deaerated condensate) is injected into the feed to E-102 to remove any water-soluble compounds produced in the RDHT Reactor such as ammonia, hydrogen chloride and organic salts. The normal water injection rate is 125 GPM. This water and water produced in the reactor is separated out in the boot section of D-102 and is sent to a sour water treating unit. The flow of water from D-102 to battery limits is 176 GPM.

High Pressure Separator
The High-Pressure Separator, D-102, separates vapor from the liquid hydrocarbon and water phases in the outlet stream from E-102. D-102 also separates the water and hydrocarbon liquid phases. The water settles into the boot of D-102 while the hydrocarbon phase consisting of product RDO and dissolved gases is collected in the horizontal section. D-102 normally operates at 685 PSIG. The separated gas from D-102 is rich in hydrogen and sent to the Recycle Compressor Knockout (K.O.) Drum, D-103. A portion of the gas is taken off to a hydrogen recovery unit at battery limits. This serves as the purge stream for the reactor loop. Purging keeps non-condensable gases such as methane, CO, CO2 and hydrogen sulfide from building to excessive concentrations in the recycle gas.

Water from the boot of D-102 is taken off to a sour water treating unit at battery limits as described in the previous section.

The RDO containing dissolved light gases (unstabilized RDO) is taken off to the Stripper Feed/Bottoms Heat Exchanger, E-103. Note that the flow rate of unstabilized RDO is much higher that the net product flow rate of RDO from the unit because of the high recycle flow rate of product RDO from the Stripper back to the reactor.

Recycle Compressor
The Recycle Compressor is a reciprocating type and is used to recirculate the hydrogen-rich gas from the High-Pressure Separator, D-102, back to the RDHT Reactor, R-101. By recycling this gas, a high hydrogen concentration is maintained throughout the RDHT Reactor catalyst beds even though the hydrodeoxygenation reaction is consuming much of the makeup hydrogen brought in from the Hydrogen Compressor. The normal flow of recycle gas is 3.426 MMSCF/H.

Fresh hydrogen from the Hydrogen Compressor, K-101, is mixed with the recycle gas from the Recycle Compressor prior to distribution of the mixed gas. The normal fresh hydrogen flow rate is 1.686 MMSCF/H. Most of this mixed gas flows to Recycle Liquid/Reactor Effluent Heat Exchanger No. 2 E-101B. Depending on conditions, a portion of the recycle gas will be used for fine temperature control of the last three beds of the RDHT Reactor (these lines are not shown on Schematic 2).

The Recycle Compressor is also used during startup to help heat up the RDHT Reactor R-101.

Stripper Section
Condensed RDO collected in the High-Pressure Separator, D-102, contains a fairly significant fraction of dissolved hydrogen, hydrogen sulfide and other light gases owing to the elevated pressure of the Reactor Section. As such, the RDO cannot be sent directly to storage. To remove these light gases (i.e., to stabilize the RDO product) the liquid is sent to the Stripper Column, T-102, where it undergoes distillation to separate out the lighter components as a product overhead gas. A large percentage (60%) of the stabilized RDO product from the bottom of the Stripper is recirculated to the reactor section and the balance is sent to storage.

Stripper Column Feed
The cool, unstabilized RDO from D-102 is heated to 426 DEG F in Stripper Feed/Bottoms Heat Exchanger E-103 using hot stabilized RDO product from the bottom of the Stripper Column, T-102. The flow rate of treated naphtha distillate from D-102 is 41.4 MBPD.

Heated, unstabilized RDO from E-103 enters Stripper Column, T-102, onto tray 15. T-102 operates at 35 PSIG which is much lower than the pressure in D-102. Because of the lower pressure and high temperature, the distillate flashes into a vapor fraction and a liquid fraction as it enters T-102. The vapor fraction mainly contains hydrogen, hydrogen sulfide and any other light gases that were dissolved in the RDO leaving D-102. The vapor fraction flows up through the top section of T-102 where any recoverable lighter distillate will be washed down the Stripper Column by reflux. The liquid fraction of the feed entering T-102 will fall through the bottom section of T-102 where vapor generated by Stripper Reboiler, F-102, will strip any unflushed hydrogen and other light gases.

Stripper Column Bottoms
RDO leaving the bottom-most tray of T-102 is routed into the bottom of the Stripper Column. RDO is circulated to the Stripper Reboiler F-102 by Stripper Reboiler Pumps, P-105A/B. Steam is injected into the RDO flowing to F-102 (not shown on schematic 2) to improve heat transfer in F-102 and to provide stripping steam for T-102. Hot unvaporized RDO from F-102 falls back into the bottom of T-102 while the vapor from F-102 flows up into the bottom-most tray of T-102. F-102 is a fired heater that uses fuel gas.

The hot, stabilized RDO product from the bottom of T-102 is pumped out by Stripper Bottoms Pumps P-103A/B and Recycle Pumps P-171A/B. The hot RDO product at 502 DEG F is cooled in Stripper Feed/Bottoms Heat Exchanger E-103 before reaching the pumps to ensure there will not be any problems with excessively hot liquid flashing at the pumps’ suction. The treated product leaves E-103 at 181 DEG F and is pumped by Stripper Bottoms Pumps P-103A/B through Product Cooler E-106 and sent to storage. E-106 uses cooling water as its cooling fluid. The normal flow rate of RDO to storage is 15.9 MBPD.

The normal flow rate of RDO recycle pumped from E-103 to the reactor loop is 24.0 MBPD.

Stripper Column Overhead
Vapor leaves the top of T-102 at 200 DEG F and flows through Overhead Condenser E-104 which is cooled using cooling water. The process stream leaves E-104 at 93 DEG F. Most of the light gases stripped from the feed remain as vapor leaving E-104 and are separated from the liquid in Stripper Reflux Drum D-104. A small portion of the gases are still dissolved in the overhead liquid, but after they are refluxed to T-102 they will recycle back up the top of T-102 and through E-104 until they are completely removed in D-104.

Stripper Reflux Drum D-104 separates the vapor, hydrocarbon liquid and liquid water phases leaving E-102. The vapor is taken off as sour gas (i.e., it contains hydrogen sulfide) to the fuel gas treater for removal of H2S prior to use as a fuel gas. The design rate of sour gas is 97.0 MSCFH. D-104 normally operates at 30 PSIG.

The hydrocarbon liquid collected in D-104, which mainly consists of propane through light gasoline components, is refluxed back to T-102 under level control. Stripper Reflux Pumps P-102A/B are used to pump the reflux back up to the top of T-102. Normally, all of the liquid is refluxed to T-102. The normal flow rate of reflux is 1.56 MBPD. If desired, a portion of the reflux can be taken off to storage. This permits extraction of lighter liquid components out of the unstabilized RDO product entering T-102.
The water collected in the boot of D-104 is taken off to the sour water treating unit

Instrumentation

Basic Controls: Feed System 1/2

D-101
PI-100 indicates the supply pressure of fresh feed at battery limits. FI-100 indicates the flow rate of fresh feed through LV-101.

LIC-101 controls the level of feed distillate in the Feed Surge Drum D-101 by adjusting the opening of the feed supply valve LV-101.

PIC-101 controls the pressure of the nitrogen blanket of D-101 by adjusting the nitrogen supply valve PV-101A or the vent to flare valve PV-101B via a split-range control. When the output of PIC-101 is 50% both valves are closed. When the output moves to 100%, PV-101A fully opens and PV-101B stays closed. When the output moves to 0%, PV-101A stays closed and PV-101B fully opens.

P-101A/B
HS-101A/B are switches that control the motors of Fresh Feed Pumps, P-101A/B, respectively. If a motor trips, the respective switch will move to the STOP state.

FIC-101 controls the flow rate of feed through P-101A/B by adjusting the position of control valve FV-101. This is a minimum flow controller and FV-101 is normally closed. The setpoint of FIC-101 is set to 8.0 MBPD to avoid dead-heading the pumps during low flow operation.

TI-101 indicates the temperature of the feed drawn from D-101. PI-102 indicates the discharge pressure on the common line of P-101A/B.

Basic Controls: Feed System 2/2

FIC-102 controls the total flow rate of fresh feed to RDHT Reactor R-101. The output of FIC-102 is then multiplied by the outputs from three ratio setpoint stations, RSP-103, RSP-104 and RSP-105 to compute setpoints in % of range for the three individual reactor fresh feed flow controllers, FIC-103, FIC-104 and FIC-105. The operator needs to ensure the outputs of RSP-103, RSP-104 and RSP-105 sum up to 100% in normal operation where all three feed flow rates will be moved proportionally to each other in response to the action of flow controller FIC-102.

The design total flow rate of fresh feed to R-101 is 16.0 MBPD. RSP-103 and RSP-104 are normally set to 33% while RSP-105 is set to 34%.

Each of the three fresh feed flow controllers FIC-103, FIC-104 and FIC-105 adjust the position of their respective control valves, FV-103, FV-104 and FV-105. Normally all three controllers are in cascade mode.

FFI-102 indicates the computed volume percentage of fresh feed to the reactor. This is based on the following formula:

FFI-102.PV = FIC-102.PV / (FIC-102.PV + FIC-171.PV) x 100%

XV-102 is a switch that is used to manually open and close fresh feed isolation valve XV-102. XV-102 will be locked in the CLSD position in the event of a high temperature in the reactor as indicated on I-115. I-115 will also lock all four feed controllers into manual mode with an output of 0% upon activation of I-115.

Basic Controls: Recycle Pumps
HS-171A/B are switches that control the motors of Recycle Pumps, P-171A/B, respectively. If a motor trips, the respective switch will move to the STOP state.

PI-171 indicates the discharge pressure on the common line of P-171A/B. TI-171 indicates the temperature of the recycle RDO from P-171A/B.

FIC-171 controls the total flow rate of recycle RDO to RDHT Reactor R-101 via all four feed lines. The output of FIC-171 adjusts the position of control valve FV-171. The flow rate through FV-171 is indicated on FI-173.

FIC-172 controls the flow rate of recycle RDO to E-101A. The output of FIC-172 adjusts the position of control valve FV-172. The PV of FIC-172 is used by F-101 interlock I-110 as a low-flow sensor. FX-172 indicates the computed % of trip flow for FIC-172’s PV.

FIC-174 controls the flow rate of recycle RDO directly to Bed No. 2 of R-101. The output of FIC-174 adjusts the position of control valve FV-174. Normally FIC-174 is in manual mode with an output of 0%. This line is used to adjust the temperature profile of the reactor beds or to quickly quench Bed No. 2 in case of a problem.

FIC-175 controls the flow rate of recycle RDO directly to Bed No. 3 of R-101. The output of FIC-175 adjusts the position of control valve FV-175. Normally FIC-175 is in manual mode with an output of 0%. This line is used to adjust the temperature profile of the reactor beds or to quickly quench Bed No. 3 in case of a problem.

Basic Controls: Hydrogen Compressor

K-101
HS-102 is a switch that controls the motor of Hydrogen Compressor K-101. If the motor trips, HS-102 will move to the STOP state.

HIC-102 adjusts the flow of K-101 using a suction valve unloading system that permits continuous capacity control of the reciprocating compressor. This is accomplished by momentarily holding open the suction (inlet) valves of the compressor (there are separate valves to each side of the double-acting piston). The system allows the operator to adjust the time the valves are momentarily held open. When HIC-102 output is 0% the suction valves are continuously held open, and no gas is compressed as the suction gas is allowed to move freely in and out of the cylinder of the compressor. When HIC-102 output is 100% the suction valves are never held open by the system and full flow of the compressor is achieved. The system is calibrated to make compressor flow respond nearly linearly with respect to the output of HIC-102.

PI-104 indicates the pressure of the supply hydrogen. TI-104 indicates the temperature of the supply hydrogen. AI-104 indicates the hydrogen concentration of the supply hydrogen in volume %.

PI-105 indicates the compressor discharge pressure. TI-105 indicates the compressor discharge temperature. TAH-105 indicates the same and is an independent sensor for the protective system of the compressor (see Interlocks).

XA-102B indicates the status of the mechanical monitoring system of the compressor and is tied into the protective system of the compressor (see Interlocks).

XA-102 indicates the status of the interlock I-102 (see Interlocks). XA-102 can also be used to manually trip the Hydrogen Compressor by changing the switch’s position from OK to TRIP.

HIC-106 adjusts the opening of the hydrogen supply valve HV-106 to the recycle gas distribution header. HV-106 should normally be 100% open. FI-106 indicates the flow rate of fresh hydrogen to the recycle gas distribution header.

FIC-107 controls the flow rate of fresh hydrogen to E-101A. The output of FIC-107 adjusts the position of control valve FV-107.

Compressor spillback control valve PV-120 is controlled by PIC-120 on the High-Pressure Separator, D-102. This is used to adjust the flow rate of fresh hydrogen to the recycle gas distribution header through control valve HV-106. Opening PV-120 will recycle (spill back) gas from the discharge of K-101 back to its suction, thereby decreasing the discharge pressure and, in turn, decreasing the flow rate through HV-106. Hydrogen is normally used/lost in the reactor section as follows:

    • Hydrogen consumption in the RDHT Reactor, R-101
    • Hydrogen in the hydrogen bleed flow, FIC-121, from the High-Pressure Separator, D-102
    • Hydrogen dissolved in the unstabilized RDO from the High-Pressure Separator, D-102 (released in Stripper T-102)

These uses/losses of hydrogen from the reactor loop determine the makeup hydrogen flow requirement through HV-106 and FV-107. Because the flow rate through FV-107 is controlled by FIC-107, the flow rate of hydrogen through HV-106 is effectively controlled by PIC-120.

E-180
HIC-180 adjusts the position of cooling water control valve HV-180 on Spillback Cooler, E-180. TI-180 indicates the outlet temperature of hydrogen spillback from E-180.

Basic Controls: Recycle Heating

E-101A
HIC-108 adjusts the opening of the recycle gas supply valve HV-108 to the inlet of E-101A. HV-108 should normally be fully closed and is mainly used at startup to circulate recycle gas through E-101A and then on to the Recycle Heater, F-101. FI-108 indicates the flow rate of recycle gas through HV-108. The PV of FI-108 is used by F-101 interlock I-110 as a low-flow sensor. FX-108 indicates the computed % of trip flow for FI-108’s PV. For more information about the interlock.

HIC-104 is used to adjust the position of nitrogen supply valve HV-104. This is normally used during pre-startup preparation and shutdown to sweep hydrogen and oxygen out of the Reactor Section. The pressure of the nitrogen supply is 100 PSIG.
TI-106 indicates the temperature of the combined recycle RDO/hydrogen mixture to the cold side of E-101A. PI-106 indicates the inlet pressure of the cold side of E-101A.

E-101B
HIC-109 adjusts the opening of the recycle gas supply valve HV-109 to the inlet of E-101B. HV-109 should normally be fully open. FI-109 indicates the flow rate of recycle gas through HV-109.

PI-172 indicates the inlet pressure of the cold side of E-101B. TI-172 indicates the temperature of the combined recycle RDO/recycle gas mixture to the cold side of E-101B.

TI-173 indicates the outlet temperature from the cold side of E-101B. TI-176 indicates the temperature of the reactor effluent stream from E-101A to the hot side of E-101B. TI-177 indicates the temperature of the reactor effluent stream from E-101B to E-102.

E-101C
TI-174 indicates the outlet temperature from the cold side of E-101C. TI-175 indicates the temperature of the reactor effluent stream from E-101C to E-101A.

Basic Controls: Recycle Heater, F-101

HIC-112A adjusts the valve position of HV-112A on the “A” tube pass through F-101. HV-112A is outfitted with a mechanical minimum stop set at 30% to avoid accidental no-flow through the tube pass either by operator misoperation or valve failure.

HIC-112B adjusts the valve position of HV-112B on the “B” tube pass through F-101. HV-112B is also outfitted with a mechanical minimum stop set at 30%.

TI-110A indicates the outlet temperature of the “A” tube pass of F-101.

TI-110B indicates the outlet temperature of the “B” tube pass of F-101.

TIC-110 controls the combined outlet temperature of F-101 by adjusting the setpoint of the fuel gas flow controller FIC-110. TIC-110 normally operates in cascade mode and receives its setpoint from TIC-111 on the inlet line to RDHT Reactor, R-101. When TIC-110 is not in cascade mode and TIC-111 is in manual mode, TIC-110 will initialize the output of TIC-111 to maintain bumpless control when TIC-111 is placed into cascade mode. The setpoint of TIC-110 tracks its PV when in manual mode. The protective system for F-101 will lock TIC-110 in manual mode if the trip is active.

FIC-110 controls the fuel gas flow to F-101 via FV-110. It is normally in cascade mode and receives its setpoint from TIC-110. When FIC-110 is not in cascade mode and TIC-110 is in manual mode, FIC-110 will initialize the output of TIC-110 to maintain bumpless control when FIC-110 is placed into cascade mode. When in manual mode, the setpoint of FIC-110 will track its PV. The protective system for F-101 will lock FIC-110 in manual mode with an output of 0% if the trip is active.

TIC-111 controls the combined feed and recycle stream to the top of RDHT Reactor, R-101, by adjusting the setpoint of TIC-110. TIC-111 is normally in automatic mode. When TIC-111 is in manual mode, its setpoint will track its PV.

TAH-110 is an independent sensor of the combined outlet temperature of F-101 for the protective system of the Reactor Feed Heater F-101.

PI-110 indicates the pressure of the feed to RDHT Reactor, R-101. PAH-110 indicates the same and is an independent sensor for the protective system of the Recycle Heater F-101.

FAL-F101 indicates the computed flow for the protective system of the Recycle Heater, F-101. It is expressed as a % of trip setpoint and accounts for operation in gas-only, liquid-only and mixed feed modes. FAL-F101 indicates the maximum value of FX-172 and FX-108.

XA-110 indicates the status of Recycle Heater interlock I-110. XA-110 may also be used to trip the Recycle Heater manually by changing the switch from OK to TRIP.

Basic Controls: Reactor Control
Bed No. 1
TI-112 indicates the average mid-point temperature of Bed No. 1 of R-101. It is the average of the readings of TI-112 A/B/C (shown on Schematic 10). TI-113 indicates the temperature of the effluent from Bed No. 1.

Bed No. 2
TIC-114 indicates and controls the average mid-point temperature of Bed No. 2 of R-101. It is the average of the readings of TI-114 A/B/C (shown on Schematic 10). TIC-114 adjusts the setpoint of recycle gas flow controller FIC-113, if desired. The output of FIC-113 adjusts the position of control valve FV-113. Normally FIC-113 is in automatic mode and TIC-114 is in manual mode. A small flow rate of recycle gas keeps the line downstream of FV-113 purged so it does not become plugged with fresh feed being directly sent to Bed No. 2. TI-115 indicates the temperature of the effluent from Bed No. 2.

Bed No. 3
TIC-116 indicates and controls the average mid-point temperature of Bed No. 3 of R-101. It is the average of the readings of TI-116 A/B/C (shown on Schematic 10). TIC-116 can be used to adjust the setpoint of recycle gas flow controller FIC-114 in certain situations, if desired. The output of FIC-114 adjusts the position of control valve FV-114. Normally FIC-114 is in automatic mode and TIC-116 is in manual mode. A small flow rate of recycle gas keeps the line downstream of FV-114 purged so it does not become plugged with fresh feed being directly sent to Bed No. 3.

TI-117 indicates the temperature of the effluent from Bed No. 3.

Bed No. 4
TIC-118 indicates and controls the average mid-point temperature of Bed No. 4 of R-101. It is the average of the readings of TI-118A/B/C (shown on Schematic 10). TIC-118 is normally used to keep the temperature of Bed No. 4 from getting too high. This helps promote the isomerization reactions that take place in Bed No. 4. TIC-118’s output adjusts the setpoint of FIC-115. FIC-115 adjusts the position of control valve FV-115. Normally FIC-115 is in cascade mode and TIC-118 is in automatic mode with a setpoint of 700 DEG F. In normal operation, FV-115 is fully closed because the PV of TIC-118 is well below its setpoint.

Reactor Outlet
TI-119 indicates the temperature of the effluent from R-101. PI-119 indicates the outlet pressure of R-101.

Basic Controls: Reactor Monitoring

Bed No. 1
TI-112A/B/C indicate the temperatures of Bed No. 1 of R-101. They indicate at the same bed depth but vary radially by 120 degrees. TAH-112A/B/C indicate the same and are independent sensors for the protective system of the RDHT Reactor, R-101.
PDI-111 indicates the pressure drop across bed No. 1 of R-101.

Bed No. 2
TI-114A/B/C indicate the temperatures of Bed No. 2 of R-101. They indicate at the same bed depth but vary radially by 120 degrees. TAH-114A/B/C indicate the same and are independent sensors for the protective system of the RDHT Reactor, R-101.

PDI-112 indicates the pressure drop across Bed No. 2 of R-101.

Bed No. 3
TI-116A/B/C indicate the temperatures of Bed No. 3 of R-101. They indicate at the same bed depth but vary radially by 120 degrees. TAH-116A/B/C indicate the same and are independent sensors for the protective system of the RDHT Reactor, R-101.

PDI-113 indicates the pressure drop across Bed No. 3 of R-101.

Bed No. 4
TI-118A/B/C indicate the temperatures of Bed No. 4 of R-101. They indicate at the same bed depth but vary radially by 120 degrees. TAH-118A/B/C indicate the same and are independent sensors for the protective system of the RDHT Reactor, R-101.

PDI-114 indicates the pressure drop across Bed No. 4 of R-101.

Reactor Outlet
TAH-119 is an independent sensor from TI-119 and is used the protective system of the RDHT Reactor, R-101.

Reactor High Temperature Interlock
XA-115 indicates the status of Reactor High Temperature Interlock I-115. XA-115 may also be used to trip the fresh feed out manually by changing the switch from OK to TRIP. This will also trip the Recycle Heater, F-101.

Basic Controls: Cooling and Separation

E-102
FIC-111 controls the flow deaerated condensate injected into the reactor effluent just upstream of Reactor Effluent Cooler E-102.

HS-110A/B are switches that control the motors of the fans of E-102. If a motor trips, the respective switch will move to the STOP state.

TIC-120 controls the outlet temperature from E-102 by adjusting the louvers at the top of the cells of E-102. The louvers affect the air flow through each cell. Normally both fans are in service.

D-102
LIC-120 controls the level of unstabilized RDO in High Pressure Separator, D-102, by adjusting the setpoint of the unstabilized RDO flow controller FIC-120. The setpoint of LIC-120 tracks its PV when in manual mode.

FIC-120 controls the flow of unstabilized RDO to the Stripper Feed/Bottoms Heat Exchanger, E-103, by adjusting the position of control valve FV-120. FIC-120 is normally in cascade mode and receives its setpoint from LIC-120. When FIC-120 and LIC-120 are in manual mode, FIC-120 will initialize the output of LIC-120 to maintain bumpless control when placed into cascade mode. When in manual mode, the setpoint of FIC-120 will track its PV.

TI-121 indicates the temperature of the unstabilized RDO leaving D-102, upstream of FV-120.

LIC-119 controls the level of the water boot of D-102 by adjusting the position of control valve LV-119.

PIC-120 controls the pressure of the vapor space of D-102 by adjusting the spillback control valve PV-120 of Hydrogen Compressor, K-101 (see Basic Controls: Hydrogen Compressor above for details).

FIC-121 controls the flow rate of purge or hydrogen bleed from the reactor loop. FIC-121 adjusts the position of control valve FV-121.

HIC-120 adjusts the position of vent control valve HV-120. This valve is used to manually depressure the Reactor Section in case of an emergency. It is also used to vent the Reactor Section as needed during shutdown. Note that HIC-120 should be used with extreme care because it can easily upset the reactor operating conditions.

Basic Controls: Recycle Compressor

D-103
TI-122 indicates the temperature of recycle gas from D-102 to D-103.

LI-122 indicates the level of any liquid that has accumulated in Recycle Compressor Knockout Drum D-103. LAH-122 indicates the same and is an independent sensor for the protective system of the Recycle Compressor K-102.

HIC-122 adjusts the position of D-103 drain valve HV-122. HIC-122 is used to manually drain D-103. Normally HV-122 is closed.

PIC-122 controls the pressure of the gas leaving D-103 by adjusting the opening of the purge gas valve PV-122. PIC-122 is designed to open only if the suction pressure of the Recycle Compressor gets abnormally high. It has a setpoint of 800 PSIG.

FI-122 indicates the gas flow rate through PV-122 to the flare system.
AI-122 indicates the concentration of hydrogen sulfide in the gas leaving D-103 in units of volume ppm.

AI-123 indicates the concentration of hydrogen in the gas leaving D-103 in units of volume %.

K-102
HS-123 is a switch that controls the motor of Recycle Compressor K-102. If the motor trips, HS-123 will move to the STOP state.

HIC-123 adjusts the flow of K-102 using the same type of suction valve control system as described for the Hydrogen Compressor K-101. FI-123 indicates the flow rate of recycle gas through K-102.

PI-123 indicates the compressor discharge pressure. TI-123 indicates the compressor discharge temperature. TAH-123 indicates the same and is an independent sensor for the protective system of the compressor.

XA-123B indicates the status of the mechanical monitoring system of the compressor and is tied into the protective system of the compressor.

XA-123 indicates the status of the interlock I-123. XA-123 can also be used to manually trip the Recycle Compressor by changing the switch’s position from OK to TRIP.

Basic Controls: Stripper Bottom

E-103
HIC-130 controls the position of the three-way valve at the cold inlet of Stripper Feed/Bottoms Heat Exchanger E-103. Normally, the output of HIC-130 is set at 0% to line all the flow through E-103. Setting the output of HIC-130 to 100% will completely bypass flow around E-103.

T-102 Bottom
TI-130 indicates the temperature of the feed to Stripper Column T-102.

FIC-130 controls the flow of startup diesel oil T-102 by adjusting the position of control valve FV-130. FIC-130 is used to inventory T-102 at startup.

HIC-132 adjusts the position of nitrogen control valve HV-132. This is used at shutdown to purge T-102 with nitrogen.

PDI-135 indicates the pressure drop across the trays of the bottom (stripping) section of T-102.

TI-134 indicates the temperature of Tray 15.

TI-135 indicates the temperature of Tray 10.

TI-136 is indicates the temperature of Tray 4.

TI-139 indicates the temperature of the stabilized RDO from T-102 to E-103.

LIC-139 controls the level of the bottom of T-102 by adjusting the setpoint of the product flow controller FIC-151. When LIC-139 and FIC-151 are in manual mode, FIC-151 will initialize the output of LIC-139 to maintain bumpless control when placed into cascade mode. The setpoint of LIC-139 tracks its PV when in manual mode.

P-103A/B
HS-151A/B switches control the motors of Stripper Bottoms Pumps P-103A/B, respectively. If a motor trips, the respective switch will move to the STOP state.

E-106
HIC-151 controls the flow of cooling water to Product Cooler E-106 by adjusting the position of HV-151.

TI-151 indicates the temperature of the product leaving E-105 for storage.

AI-151 indicates the concentration of fresh feed in the RDO product in weight ppm. This normally reads 0. A non-zero value indicates the conversion of fresh feed in the RDHT Reactor is not 100%.

Basic Controls: Stripper Reboiler

P-105A/B
HS-161A/B are switches that control the motors of Stripper Reboiler Pumps, P-105A/B, respectively. If a motor trips, the respective switch will move to the STOP state.

FAL-161 is an independent flow instrument used for the protective interlock I-160 of the Stripper Reboiler, F-101.

FIC-161 controls the flow of RDO through P-105A/B by adjusting the position of control valve FV-161.

F-102
PI-160 indicates the pressure of the steam supply for Stripper Reboiler, F-102.

TI-160 indicates the temperature of the steam supply for F-102.

FIC-163 controls the flow rate of steam to F-102 by adjusting the position of control valve FV-163.

TI-161 indicates the inlet temperature of the steam/RDO mixture to F-102.
TIC-162 controls the outlet temperature of F-102 by adjusting the setpoint of the fuel gas flow controller FIC-160. The setpoint of TIC-162 tracks its PV when in manual mode. The protective system for F-102 will lock TIC-162 in manual mode if the trip is active.

FIC-160 controls the fuel gas flow to F-102 via FV-160. It is normally in cascade mode and receives its setpoint from TIC-162. When FIC-160 and TIC-162 are in manual mode, FIC-160 will initialize the output of TIC-162 to maintain bumpless control when placed into cascade mode. When in manual mode, the setpoint of FIC-160 will track its PV. The protective system for F-102 will lock FIC-160 in manual mode with an output of 0% if the trip is active.

TAH-162 is an independent temperature instrument used for the protective interlock I-160 of the Stripper Reboiler, F-101.

PI-162 indicates the outlet pressure of F-102.

XA-160 indicates the status of the interlock I-160 (see Interlocks). XA-160 can also be used to manually trip the Stripper Reboiler by changing the switch’s position from OK to TRIP.

Basic Controls: Stripper Top

T-102 Top
TI-131 indicates the temperature of the vapor leaving the top of Stripper Column T-102.

PI-131 indicates the pressure at the top outlet of T-102. PAH-131 indicates the same and is an independent sensor for the protective system of the Stripper Reboiler.

TI-132 indicates the temperature of Tray 22.

TI-133 indicates the temperature of Tray 18.

FIC-131 controls the flow of reflux to T-102 via FV-131. It is normally in cascade mode and receives its setpoint from LIC-140A. When FIC-131 and LIC-140A are in manual mode, FIC-131 will initialize the output of LIC-140A to maintain bumpless control when placed into cascade mode. When in manual mode, the setpoint of FIC-131 will track its PV. FIC-131 should normally have a flow rate equal to about 10% of the steady-state bottoms product flow rate FIC-151 to ensure good stabilization of the product.

The reflux rate is dependent on how stripping is being done in T-102. Stripping depends on how much fuel gas is being used in Stripper Reboiler F-102 and on how much steam is injected into the Reboiler. If there is not enough reflux, the setpoint of Reboiler outlet temperature controller TIC-162 should be gradually increased until the desired reflux flow is attained. If this is ineffective, gradually increase the steam flow using FIC-163. This adjustment may also require a corresponding adjustment of the cooling water flow to Overhead Condenser E-104 using HIC-140.

E-104
HIC-140 controls the flow of cooling water to Overhead Condenser E-104 by adjusting the position of HV-140. The flow of cooling water will affect the condensing rate in E-104.

TI-140 indicates the outlet temperature from E-104.

D-104
PIC-140 controls the pressure of Stripper Reflux Drum D-104 by adjusting the opening of PV-140 on the line from D-104 to the fuel gas treater.

FI-140 indicates the flow of gas from D-104 to the fuel gas treater through PV-140.

LIC-140A controls the hydrocarbon level of D-104 by adjusting the setpoint of reflux flow controller FIC-131 as described above. The setpoint of LIC-140A tracks its PV when in MANUAL mode.

LAH-140 indicates the top liquid level of D-104 and is an independent sensor for the protective system of the Stripper Column.

FIC-141 controls the flow of light top product to storage via FV-141. It is normally in cascade mode with 0% output and receives its setpoint from LIC-140B. When FIC-141 and LIC-140B are in manual mode, FIC-141 will initialize the output of LIC-140B to maintain bumpless control when placed into cascade mode.

LIC-140B shares the same PV with LIC-140A and, therefore, also controls the hydrocarbon level of D-104 by adjusting the setpoint of top light product flow controller FIC-141. LIC-140B is normally in automatic mode with a setpoint of 90% while LIC-140A has a setpoint of 50%. This way, LIC-140B sends light product to storage only when the Reflux Drum level gets very high. This will help avoid a trip of the Reboiler on high level in the Reflux Drum.

LIC-141 controls the level of the water boot of D-104 by adjusting the position of LV-141.

P-102A/B
HS-140A/B are switches that control the motors of Stripper Reflux Pumps P-102A/B, respectively. If a motor trips, the respective switch will move to the STOP state.

Interlocks Introduction

There are five protective interlock systems in the Renewable Diesel Hydrotreating Unit as follows:

  • I-102 – Hydrogen Compressor, K-101, Protective System
  • I-110 – Recycle Heater, F-101, Protective System
  • I-115 – RDHT Reactor, R-101, Protective System
  • I-123 – Recycle Compressor, K-102, Protective System
  • I-160 – Stripper Column T-102, Reflux Drum, D-104 and Reboiler, F-102, Protective System
  • Interlocks: Hydrogen Compressor Trip

I-102 – Hydrogen Compressor, K-101, Protective System
This interlock protects the Hydrogen Compressor against operation under conditions which will damage it. The inputs to the interlock are:

  • TAH-105 – K-101 discharge temperature (trips at 280 DEG F)
  • XA-102B – K-101 trouble (trips on TRIP state)
  • The status of the interlock is indicated on switch XA-102. XA-102 not only indicates the status of the interlock but is also used to reset the interlock if it is in the TRIP state and all trip inputs are clear. Also, manually switching XA-102 to the TRIP state will activate the interlock.

When activated, interlock I-102 will:

  • stop the motor for K-101 and lock it out from restarting until XA-102 is reset to the OK position
  • generate an audible alarm that the interlock has activated

Interlocks: Reactor Feed Heater Trip

I-110 – Reactor Feed Heater, F-101, Protective System

This interlock protects the Reactor Feed Heater against operation under conditions that will cause damage to the equipment. The inputs to the interlock are:

  • FAL-F101 – Computed feed trip flow (trips at 100% of computed flow)
  • TAH-110 – F-101 outlet temperature (trips at 850 DEG F)
  • PAH-110 – F-101 outlet pressure (trips at 1,000 PSIG)
  • I-115 – RDHT Reactor high temperature interlock

FAL-F101 is a computed % of trip flow that accounts for F-101 operation on all-liquid or all-gas. FAL-F101 indicates the larger of:

  • FX-108 = the flow rate of recycle gas to E-101A (FI-108) expressed as a % of the gas flow trip setting (setpoint = 1.0 MMSCF/H)
  • FX-172 = the flow rate of recycle RDO liquid to E-101A (FIC-172) expressed as a percent of the feed flow trip setting (setpoint = 5.0 MBPD)

The status of the interlock is indicated on switch XA-110. XA-110 not only indicates the status of the interlock but is also used to reset the interlock if it is in the TRIP state and all trip inputs are clear. Also, manually switching XA-110 to the TRIP state will activate the interlock.

When activated, interlock I-110 will:

  • lock F-101 fuel gas flow controller FIC-110 in manual mode with an output of 0%
  • lock F-101 outlet temperature controller TIC-110 in manual mode
  • lock R-101 inlet temperature controller TIC-111 in manual mode
  • generate an audible alarm that the interlock has activated
  • Interlocks: RDHT Reactor Trip

I-115 – RDHT Reactor, R-101, Protective System
This interlock protects the RDHT Reactor, R-101, against operation under conditions that will cause damage to the equipment. The inputs to the interlock are:

  • TAH-112A/B/C – R-101 bed no. 1 temperatures (trips at 850 DEG F)
  • TAH-114A/B/C – R-101 bed no. 2 temperatures (trips at 850 DEG F)
  • TAH-116A/B/C – R-101 bed no. 3 temperatures (trips at 850 DEG F)
  • TAH-118A/B/C – R-101 bed no. 4 temperatures (trips at 850 DEG F)
  • TAH-119 – R-101 outlet temperature (trips at 850 DEG F)

The status of the interlock is indicated on switch XA-115. XA-115 not only indicates the status of the interlock but is also used to reset the interlock if it is in the TRIP state and all trip inputs are clear. Also, manually switching XA-115 to the TRIP state will activate the interlock.

When activated, interlock I-115 will:

  • activate F-101 interlock I-110
  • lock fresh feed valve XV-102 in the CLSD position
  • lock FIC-102 in manual mode with an output of 0%
  • lock FIC-103 in manual mode with an output of 0%
  • lock FIC-104 in manual mode with an output of 0%
  • lock FIC-105 in manual mode with an output of 0%
  • generate an audible alarm that the interlock has activated

Interlocks: Recycle Compressor Trip

I-123 – Recycle Compressor, K-102, Protective System
This interlock protects the Recycle Compressor against operation under conditions which will damage it. The inputs to the interlock are:

  • TAH-123 – K-102 discharge temperature (trips at 280 DEG F)
  • XA-123B – K-102 trouble (trips on TRIP state)
  • LAH-122 – D-103 high liquid level (trips at 80%)

Interlock status is indicated on switch XA-123. XA-123 is also used to reset the interlock if it is in the TRIP state and all trip inputs are clear. Manually switching XA-123 to the TRIP state activates the interlock. When activated, interlock I-123 will:

  • stop the motor for K-102 and lock it out from restarting until XA-123 has been reset to the OK position
  • generate an audible alarm that the interlock has activated

Interlocks: Stripper Reboiler Trip

I-160 – Stripper Column, T-102, Reflux Drum, D-104, and Reboiler, F-102, Protective System
This interlock protects the Stripper Column, Reflux Drum and Reboiler against operation under conditions which may potentially cause a large release to flare (via pressure safety valve opening) and protects the Reflux Drum from causing a potentially unsafe condition in the downstream fuel gas unit due to carryover of liquid in the sour gas to the treater and protects the Reboiler from high temperature damage. The inputs to the interlock are:

  • PAH-131 – T-102 top pressure (trips at 180 PSIG)
  • LAH-140 – D-104 high liquid level (trips at 90%)
  • FAL-161 – Reboiler RDO flow (trips at 10 MBPD after a 10-second delay)
  • TAH-162 – Reboiler outlet high temperature (trips at 750 DEG F)

Interlock status is indicated on switch XA-160. XA-160 is also used to reset the interlock if it is in the TRIP state and all trip inputs are clear. Also, manually switching XA-160 to the TRIP state will activate the interlock. When activated, interlock I-160 will:

  • lock Stripper Reboiler fuel gas flow controller FIC-160 in manual mode with an output of 0%
  • lock Stripper Reboiler outlet temperature controller TIC-162 in MANUAL mode
  • generate an audible alarm that the interlock has activated