GTC Technology White Paper GT-LPG Max SM Maximizing LPG Recovery from Fuel Using a Dividing Wall Column Engineered to Innovate
GT-LPG Max SM Maximizing LPG Recovery from Fuel Using a Dividing Wall Column - Single Column (low capital cost) and Higher Energy Efficiency Solution (low operating cost) Over the last several years, refiners have faced increasing challenges maximizing the recovery of high valued products from process off-gases that are either used as fuel gas or flared. These off-gases used as fuel gases hold a high composition of valuable C3, C4 components. With refiners facing increased pressure to remain profitable, there has been a high focus on maximizing recovery of LPG range material from the fuel gases. In a refinery, fuel gases are produced from various units such as fluid catalytic crackers, catalytic reformers, hydrotreaters, coker units, crude units, etc. A typical configuration of fuel gas producing units in a refining complex is shown in Fig. 1. There are many processes available for LPG recovery (either through cryogenic or absorption systems) of which some are licensed and others are available in the public domain. However with all these conventional technologies available, there are major challenges in maximizing the recovery (over 95wt %) of an LPG range material while at the same time being highly energy efficient. To cover this engineering gap, a new solution has been developed that maximizes LPG recovery and lowers energy consumption. Following are in-depth details of this process with a case study highlighting the analysis of an existing refinery s LPG recovery scheme and the application of this advanced solution for best process performance and higher ROI. These results help in understanding the best way to optimize operations and maximize LPG recovery from fuel gases. A key element in this study is dividing wall column (DWC) selection to overcome some of the challenges commonly associated with processing fuel gas and recovering higher LPG product economically way using traditional methods. Table 1 shows basic process advantages and a simple payback period for implementing the DWC system. 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 2
Processing Fuel Merox Treaters Butanes Liquefied Petroleum Light Straight-Run Naphtha Kerosine C 5 /C 6 Isomerization Isomerate Atmospheric Crude Distillation Column Heavy Naphtha Kerosine Diesel Naphtha Hydrotreater Hydrotreating Unit Catalytic Reformer Diesel Oil Reformate Hydrotreating Unit Kerosene/ Jet Fuel Crude Oil Desalter Atmospheric Residue Atmospheric Oil Vacuum Distillation Column FCC Feed Treater Delayed Coker FCC Unit FCC Naphtha FCC Fuel Oil Coker Oil Hydrotreater FCC oline Hydrocracking Unit Hydrocracked oline Asphalt Oxidation Asphalt Fig. 1: Typical Fuel Producing Units in a Refining Complex 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 3
GT-LPG Max a New Technology Solution: GT-LPG Max is a new process developed by GTC Technology using the DWC application to optimize the overall operation and enhance C3 recovery. It offers a top dividing wall column for separating a multi-component feed into three or more streams within a single column, thus eliminating the need for a second column. The process lowers both capital investment and operating costs for grassroots and revamped applications. The Deethanizer and Depropanizer columns are replaced with one single column and a single reboiler that uses a dividing wall to achieve higher C3+ recovery at lower operating temperatures and pressures. Variables Units GT-LPG Max Overall Propane Recovery wt % 97+ LPG Recovered BPD 1350 Number of Columns / Material of Construction Turbo-Expander & Refrigeration System 1 / CS Not Required Net Benefit $M 9 Total Installed Cost $M 15 Simple Pay Back Months 20 Table 1: Economics of DWC System Process Description: The patented separation process with the diving wall column concept through a non-cryogenic absorption system is utilized here for maximizing LPG recovery from fuel gas streams of refining units. A simplified process flow diagram for recovering higher propane material from fuel gas is shown in Fig. 2. The process diagram shows a combined single column with a dividing wall for Deethanizer and Depropanizer operation in place of two conventional columns. The dividing wall column uses a vertical wall to divide the top of the column into two sections, where one side is used as an absorption section while the other side is used for fractionation. The process is designed to separate lighter C2- components (non-condensables), intermediate C3 boiling range components and heavier C4+ material in a single distillation column. The butane plus material can be further fractionated to produce butanes and C5 plus as desired for specific applications. The hydrocarbon stream from the feed drum is pumped to a feed-effluent heat exchanger and feed preheater as required, to pre-heat it to bubble point at the feed tray conditions. The preheater will use either MP steam or another existing hot process stream as a heating medium. The preheated feed is supplied to the absorption section of the diving wall column where non-condensables (C2- material) are concentrated in the overhead and passed through a partial vent condenser. Condensed vapors are 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 4
collected in the overhead drum for separating sour water (present in feed) and then circulated back to the column as reflux. Non-condensables from the overhead drum are separated as overhead product and routed as fuel gas to the refinery fuel gas header. The section above the feed point location acts as an absorption section, where a separate heavy liquid stream is introduced to recover C3, C4 components stripped along with C1, C2 components. This liquid, which serves as a solvent for minimizing C3 loss, can simply be the heavier components (C4+ or C5+) from the feed stream. In this case, this heavy liquid for absorption is supplied from the following two sources. Overhead condensed material at the absorption side of the dividing wall Bottoms material from the dividing wall column from which a slip stream is pumped back to the top section for absorption while the remaining liquid is introduced to the debutanizer column for separating butanes from C5 plus heavies The other top side of the dividing wall column is referred to as the fractionation section, which is concentrated with C3 components. The vapors from the overhead of the fractionation side are condensed in a water cooled condenser and collected in an overhead receiver. A portion of this liquid is circulated back to the column as reflux and the remaining liquid is withdrawn as LPG product. The liquid level in the receiver is controlled by cascading with the LPG product flow sent to the BLs. Overhead pressure of the column is set at 250 psig and controlled by a pressure control loop installed on the line to the fuel gas header at the absorption side, while the pressure in the overhead receiver on the fractionation side is controlled by a hot vapor by-pass pressure control loop. A thermosiphon reboiler is provided at the bottom of this column to supply enough of the duty required to distill C3 components at the bottom using either MP steam or existing hot process stream as a heating medium. The heat input to the reboiler is regulated by controlling the steam flow cascaded to the column bottom tray temperature controller. A slip stream from the bottom product is pumped to the top of the adsorption section as a solvent or absorbing medium for minimizing C3 loss while the remaining liquid is cooled down in an air/water cooled exchanger and is supplied as a feed stream to the De-Butanizer column for separating butanes from C5 plus heavies. The column bottoms liquid level is controlled by a level control loop that is cascaded with the net bottoms product rate. For feed streams enriched with a higher C1 composition in the fuel gas, an absorber before the dividing wall column will be used to separate C1 components first and then the separated C2+ material from the absorber bottoms is introduced as feed stream to the dividing wall column for recovering higher LPG range material. 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 5
Fuel (C2-) LPG Product Fuel Heavies Slip Stream for Absorption C4 and Heavies Fig. 2: Simplified Flow Diagram for Maximizing LPG Recovery from Fuel Using Single Column with GT-LPG Max Application Case Study The aforementioned process has been applied to a real-world case: a new simulating model has been created to review in detail the existing LPG recovery scheme, process disadvantages and application of a dividing wall column to enhance the overall process performance. After in-depth study and detailed analysis of the simulation results, the key advantages of the advanced DWC process has shown great improvement for LPG recovery and a dramatic reduction in both capital and operating costs. Project Scope: The objective of the study is to maximize LPG recovery (>96wt%), lower S in the product (<10 ppm) and minimize operational costs (no refrigeration) with a higher energy efficiency solution from a mixture of two fuel gas streams. The fuel gas to the unit comes from two sources and 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 6
is mixed in a feed drum at an operating pressure of 160 psig before being supplied to the LPG recovery scheme. The fuel gas feed composition is shown in Table 3 and the design basis for maximum utilization of existing process scheme is summarized in Table 2. Design Basis 1. Feed rate at 9057 BPD 2. Cooling water to be used for overhead condensation (no refrigeration) 3. Column pressure not to exceed 390 psig Table 2: Design Basis Feed Composition Flow rate [bbl/day] 9057 Liquid Hydrogen 0.08 Methane 0.69 Ethane 5.3 S (Note 1) 0.31 Propane 15.14 Propene 0.1 i-butane 8.66 i-butene 0.19 n-butane 19.15 tr2-butene 0.29 22-Mpropane 0.01 cis2-butene 0.1 i-pentane 16.73 2M-1-butene 0.04 1-Butene 0.1 Feed Composition Flow rate [bbl/day] 9057 Liquid n-pentane 18.17 33M-1-butene 0.02 2M-2-butene 0.09 Cyclopentane 1.07 22-Mbutane 0.45 23-Mbutane 1.17 2-Mpentane 4.16 3-Mpentane 2.28 n-hexane 4.18 Mcyclopentan 0.46 Benzene 1.04 O 0.01 CO2 0.01 Ethylene 0.02 TOTAL 100 Table 3: Fuel Composition of the Feed to DWC 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 7
Current Process: A simplified process diagram of the existing process is shown in Fig. 3. As seen, the existing process uses three separate columns at 250 psig and 470 psig operating pressures for separating C3- material and then recovering C2- fuel gas and LPG products. This whole process is able to recover only 55wt% of propane and also contributes to higher S composition of 180 ppm in the LPG product. The primary reasons for such low recovery in the existing process is due to lower operating pressure and a partial condenser used in the first two columns which contribute to propane loss in the overhead gas streams of both columns. An immediate solution to counteract the problem and enhance the recovery is to increase the operating pressure and use refrigeration for total condensing of the overhead gas. However this would be at the expense of a high utility requirement which leads directly to higher operating costs. Fuel Fuel 250 psig 250 psig 470 psig Depropanizer Stripper Depropanizer Deethanizer Feed C 4 + LPG Fig. 3: Simplified Process diagram of existing Process Scheme 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 8
Study for Advanced Solution: The existing process is evaluated in detail to determine the root cause of the propane loss. Then an in-depth study for maximizing the propane recovery at lower energy consumption is carried out in four stages as explained below. A combined simplified process diagram depicting all four new process schemes is shown in Fig. 4. 1. In the first stage study, a new Depropanizer and an existing Deethanizer column is used at an increased operating pressure of 390 psig (up from 250 psig). Though the use of the new Depropanizer helped to recover propane of 92wt% in the first pass, the inefficiency of the existing Deethanizer column was due to its lower column dimensions and usage of cooling water for overhead gas condensing. Thus, the overall C3 recovery achieved is only 76.3wt% with 156 ppm of S in the LPG product. Total reboiler heat duty required for this case is 18.1 MM Btu/hr. 2. The second stage study further enhances the recovery by using two new columns for Deethanizer and Depropanizer columns at a reduced operating pressure of 250 psig. Also an absorption operation is included at the top of the Deethaninzer column for minimizing propane loss. The absorption effect here is achieved through the introduction of a heavier stream consisting of C5, C6 components at the column top to absorb C3+ material stripped along with C1, C2 components. This modified process helps to achieve higher C3 recovery of 96.9wt% with just 7 ppm S in the LPG product. But this comes at an expense of higher reboiler duty of 28 MM Btu/hr and the addition of two separate new columns. 3. To eliminate/minimize the thermodynamic inefficiency that exists in the 2nd stage process with two new columns, a modified process flow is studied in the third stage. In this scheme, a sidedraw from the Depropanizer column consisting of at least 50% propane vapors is withdrawn and introduced as a separate stream at the bottom of the Deethanizer column. This new stream provides enough of the heat duty required to boil off all the C2- components at the Deethanizer bottom thereby potentially eliminating the need of boiling the complete C3+ range material in a conventional column bottom using a reboiler. With this modified scheme, the required total heat duty is largely reduced while at the same time being able to maintain higher propane recovery similar to the second stage. 4. In the final stage, to further reduce the energy consumption and minimize the capital costs, an advanced process solution utilizing a single top dividing wall column is been designed. This DWC solution eliminates the need for a new Depropanizer column and a single column is utilized for both Deethanizer and Depropanizer operations. This single column solution comes at a remarkable lower reboiler duty of 20.3 MM Btu/hr while at the same time maintaining a higher C3 recovery of 97wt% and 7 ppm of S in the LPG product. The economic analysis of the various stages of this study in comparison with the existing scheme as the base case is shown in Table 4 and the product specification achieved with respect to the design target is shown in Table 5. The calculations for the total investment are based on the equipment 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 9
cost estimated using 2013 USGC prices, equipment size based on the required equipment list and installation factors for each type of equipment. Overall with a small capital investment of $15M and a simple payback period of 20 months, the DWC process solution is able to improve propane recovery dramatically from 55wt% to 97wt% while also being highly energy efficient. Fuel Heavier Hydrocarbon 250 psig 250 psig LPG Feed New Depropanizer Deethanizer C4 & Heavies Fuel Fuel 250 psig Fuel 250 psig Feed New Depropanizer 390 psig Deethanizer 470 psig Feed S/Deethanizer Stripper Depropanizer LPG To DIB Column LPG Fuel 250 psig LPG Feed C4 & Heavies Fig. 4: Simplified Process Schemes of All Four Stages of the Study 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 10
Variables Units Existing Scheme (Base Case) New Column + Existing Column Two New Columns Advanced DWC Solution Overall Propane Recovery Wt % 55 76 97 97 Total Duty Requirement MM Btu/ hr 22 18.1 28 20 LPG Product Rate BPD 745 1026 1350 1350 LPG Benefit/Year $M Existing 7.5 10 10 Net Benefit/Year $M Existing 6.6 8.7 9.0 Total Installed Cost SM Existing 17 30 15 Simple Pay Back Months Existing 31 42 20 Table 4: Economic Advantages of the Existing and the Modified Process Schemes Component Units Specification Target Achieved Design LPG Product Propane lv% >95 99.1 Ethane lv% <1 <1 S ppm <50 <10 Vapor Pressure @105F psig <219.5 176 Fuel Propane lv% minimum <3.0 Table 5: Target Product Specifications vs. Design for the LPG and Fuel Products Thus this new process solution provides several advantages and benefits over conventional cryogenic or non-cryogenic LPG recovery processes. Benefits include: Higher C3 recovery Lower S in LPG product Lower operating temperature and pressure No external solvent requirement for absorption No refrigeration or cryogenic conditions required for enhanced process performance 1001 South Dairy Ashford Suite 500 Houston, TX 77077, USA page 11