Special Report
LNG, NGL and Alternative Feedstocks M. BHARGAVA, C. NELSON, J. GENTRY and V. SIDDAMSHETTI, GTC Technology US LLC, Houston, Texas
Maximize LPG recovery from fuel gas using a dividing wall column Refiners are challenged to recover LPG from mixed fuel gas streams due to the difficulty of separating the lighter components from bulk gas. As a result, many valuable components are lost to a fuel stream or flare. To maintain profitability, it is essential to direct all of the crude oil components to the optimum disposition. This practice is becoming more significant due to increasing LPG demand in some countries and the supply of lighter crudes in countries such as the US. In a refinery, fuel gases are produced from various types of units, including fluid catalytic crackers, catalytic reformers, hydrotreaters, delayed cokers, and crude distillation units. 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. Some of these systems are licensed from technology suppliers, and others are available in the public domain. These conventional technologies have major challenges to maximize the recovery of LPG-range material beyond 95 wt%, while at the same time being highly energy efficient. To cover this engineering gap, a solution has been developed to maximize LPG recovery and reduce energy consumption. Indepth details of this process solution are provided, with a case study comparing an existing refinery’s LPG recovery scheme with the application of the new design to achieve better process performance and a higher return on investment. A key element of the technology is the use of a dividing wall column (DWC) to overcome the inherent inefficiency associated with the traditional methods of processing fuel gas for LPG recovery. TABLE 1 shows the basic process performance and a simple payback period for an investment using the DWC system. New technology solution. A recently developed process uses
the DWC technology to optimize the overall operation and enhance C3+ recovery. The DWC can separate a multi-component feed into three or more streams within a single column. The deethanizer and depropanizer columns in a traditional LPG recovery system are replaced with one column using a dividing wall to achieve higher C3+ recovery at lower operating temperatures and pressures. As a result, both capital investment and operating costs for grassroots and revamped applications are reduced.
is used here to maximize LPG recovery from refining fuel gas. A simplified flow diagram of the process is shown in FIG. 2. The diagram shows a single column with a dividing wall for deethanizer and depropanizer operation, in the place of two conventional columns. The vertical wall separates the top of the column into two sections, with one side used as an abTABLE 1. Economics of the DWC system Variables
New LPG recovery technology
Overall propane recovery, wt%
97+
LPG recovered, bpd
1,350
Number of columns
1
Material of construction
Carbon steel
Turboexpander and refrigeration system
Not required
Net benefit, $MM
10.1
Total installed cost, $MM
15
Simple payback, months
18 Fuel gas
Gas processing Gas H2 Light straightrun naphtha
Heavy naphtha
Atmospheric crude distillation column
Gas
Hydrotreater
Gas H2
Naphtha hydrotreater
Catalytic reformer
Kerosine
Diesel
H2
Isomerate
Reformate Gas H2 Hydrotreating unit
H2
Kerosine/ jet fuel
Diesel
Hydrotreating unit Gas H2
Gas
FCC feed treater
FCC unit
Desalted Vacuum distillation crude oil column Atmospheric residue
Butanes
C5/C6 isomerization
Gas H2
Gas
LPG
Sweetening unit
Gas H2
FCC naphtha
Hydrotreater
FCC gasoline
FCC fuel oil Gas Coker gas oil Delayed coker
Process description. The patented separation process with
the DWC concept through a non-cryogenic absorption system
FIG. 1. Typical fuel-gas-producing units in a refining complex. Hydrocarbon Processing | JANUARY 201539
LNG, NGL and Alternative Feedstocks sorption section and the other side used for fractionation. The process is designed to separate lighter C2– components (noncondensables), 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+ as desired for specific applications. The feed is supplied to the absorption section of the DWC, where non-condensables and water are concentrated in the overhead and passed through a partial condenser. Condensed vapors are collected in the overhead drum for separating out the sour water, and then circulated back to the column as reflux. Non-condensables from the overhead drum are removed as vapor product and routed to the refinery fuel gas header. The section above the feed location acts as an absorption section, where a separate heavy liquid stream is introduced to recover C3 and C4 components from the C1 and C2 components. The liquid, which serves as a solvent for minimizing C3 loss, can simply be the heavier components from the feedstream. In this case, the heavy liquid for absorption is a slip stream from the bottoms material of the DWC.
The other side at the top of the DWC 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, while the remaining liquid is withdrawn as LPG product. The overhead pressure of the column is 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 bypass pressure control loop. A single thermosiphon reboiler is provided at the bottom of the column to supply the duty required to distill C3 components. 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, while the remaining liquid is removed from the system. Application case study. The aforementioned process has
TABLE 2. Fuel gas composition of the feed Liquid
Vol%
Hydrogen
0.05
H2O
0.01
CO2
0.01
H2S
1.23
Methane
0.6
C2
5.12
C3
14.95
C4
28.19
C5
36.06
C6
+
13.81
Total
100
been applied to a real-world case. A new simulation model has been created to review the existing LPG recovery scheme, the process disadvantages, and the application of a DWC to enhance the overall process performance. After an in-depth study and detailed analysis of the simulation results, the key advantages of the advanced DWC process show great improvement in LPG recovery and a dramatic reduction in both capital and operating costs compared to the closest alternative technology. Project scope. The objective of the study was to maximize LPG recovery (> 96 wt%), reduce hydrogen sulfide (H2S) in the product (< 40 ppm) and minimize operational costs (no refrigeration) with a higher energy-efficiency solution from a mixture of fuel gas. The fuel gas to the unit comes from two sources and is mixed in a feed drum at an operating pressure of 160 psig, before being supplied to the LPG recovery scheme. The design basis for the maximum utilization of the existing process scheme includes:
Fuel gas C2-
Fuel gas
Water
Fuel gas
LPG Water
Fuel gas
Deethanizer
Feed
470 psig
Depropanizer
250 psig
C4 and heavies Heavies slip stream for absorption
FIG. 2. Simplified flow diagram for maximizing LPG recovery from fuel gas using a single column.
40JANUARY 2015 | HydrocarbonProcessing.com
C4 and heavies
FIG. 3. Simplified process diagram of the existing process scheme.
LPG
LNG, NGL and Alternative Feedstocks tion would come at the expense of a high utility requirement, leading to higher operating costs.
1. Feedrate at 10,000 bpd 2. Cooling water to be used for overhead condensation (no refrigeration) 3. Minimize operating pressure 4. Use existing columns for new design; if not feasible, design new columns 5. Minimize H2S in LPG product. The fuel gas feed composition is shown in TABLE 2. Existing process. A simplified flow diagram of the existing
process is shown in FIG. 3. The existing process uses two separate columns, at operating pressures of 250 psig and 470 psig, for separating C3– material and then recovering C2– fuel gas and LPG products. The overall process is able to recover only 55 wt% of the propane and leaves a higher content (180 ppm) of H2S in the LPG product. The primary reasons for the low recovery rate in the existing process are lower operating pressure and a partial condenser used in the first two columns, both of which contribute to propane loss in the overhead gas streams of both columns. A logical solution to counteract the problem and enhance the recovery rate is to increase the operating pressure and use refrigeration to condense the overhead gas. However, this solu-
Study for advanced solution. The existing process was 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 was carried out in four stages. Process Scheme 1. In the first-stage study (shown in FIG. 4), a new depropranizer and an existing deethanizer column were used at an increased operating pressure of 390 psig (up from 250 psig). The new depropranizer helped recover 92% of the propane, but the existing deethanizer column remained inefficient due to its lower column dimensions and the usage of cooling water for overhead gas condensing. Therefore, the overall C3 recovery achieved was only 76%, with 160 ppm of H2S in the LPG product. The total reboiler heat duty required for this case was 18.1 MMBtu/hr. Process Scheme 2. The second-stage study (shown in FIG. 5) further enhances the recovery by using two new columns for the deethanizer and depropranizer, at a reduced operating pressure of 250 psig. Also, an absorption operation is included at the top of the deethanizer column for minimizing propane loss. Fuel gas
Fuel gas Fuel gas
Water
LPG New deethanizer
Deethanizer To DIB column
Feed
250 psig New depropanizer
250 psig
470 psig
New depropanizer
390 psig
Feed
Water
Heavier hydrocarbons
C4 and heavies
LPG
FIG. 4. New depropanizer at higher pressure, plus existing deethanizer.
FIG. 5. New deethanizer based on absorption, plus new depropanizer.
TABLE 3. Economic advantages of the existing and modified process schemes
Variables
Existing scheme
Process Scheme 1
Process Scheme 2
Process Scheme 3
New depropanizer at higher pressure, plus existing deethanizer
New depropanizer based on reboiled absorption plus existing deethanizer
Enhanced LPG recovery technology
Overall propane recovery, wt%
55
76
97
97
Total duty requirement, MMBtu/hr
22
20
28
20
LPG product rate, bpd
883
1,267
1,445
1,445
LPG benefit/yr, $MM
Existing
7
10.2
10.2
Net benefit/yr, $MM
Existing
4.2
9.2
10.1
Total installed cost, $MM
Existing
17
23
15
Simple payback, months
Existing
48
30
18
42JANUARY 2015 | HydrocarbonProcessing.com
LNG, NGL and Alternative Feedstocks The absorption effect here is achieved with the introduction of a heavier stream consisting of C5 and C6 components at the column top to absorb C3+ material stripped along with C1 and C2 components. This modified process helps achieve higher C3 recovery of 96.9 wt% with just 40 ppm of H2S in the LPG product. However, this comes at the expense of a higher reboiler duty of 28 MMBtu/hr and the addition of two new columns. Part of the duty in the deethanizer is used to build a concentration peak of C3 component in the bottom area of the column, which is remixed with the heavier components. Process Scheme 3. In the final stage, to further reduce energy consumption and minimize capital costs, an advanced process solution using a single top DWC was designed. This DWC solution eliminates the need for a new depropranizer column. A single column is used for both the deethanizer and depropranizer operations. This single-column solution comes at a significantly lower reboiler duty of 20.3 MMBtu/hr, while at the same time maintaining a higher C3 recovery of 97 wt% and 40 ppm of H2S in the LPG product. FIG. 2 shows the simplified process scheme for the enhanced LPG recovery technology. The economic analyses of the various stages of this study in comparison with the existing scheme as the base case are shown in TABLE 3. The product specifications achieved with respect to the design target are shown in TABLE 4. The calculations for the total investment are based on the equipment cost estimated using 2013 US Gulf Coast prices, the equipment
TABLE 4. Target product specifications vs. design for LPG and fuel gas products Component Propane, lv% Ethane, lv% H2S, ppm Butanes and heavier, psig Vapor pressure at 100°F, lv%
Specification target
Achieved design
> 90
99
<1
<1
< 123
< 40
2.5
0.7
< 208
176
size based on the required equipment list, and the installation factors for each type of equipment. With a small capital investment of $15 MM and a simple payback period of 18 months, the DWC process solution is able to improve propane recovery from 55 wt% to 97 wt%, while also being highly energy efficient. This new process solution provides several advantages and benefits over conventional cryogenic or non-cryogenic LPG recovery processes: • Higher C3 recovery • Lower H2S in LPG product • Lower operating pressure • Lower-temperature heat duty requirement for reboiler • No external solvent requirement for absorption • No refrigeration or cryogenic conditions required for enhanced process performance • Applicable for grassroots and revamp projects.
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