GTC TECHNOLOGY · column. The deethanizer and depropanizer columns in a traditional LPG recovery...

GT-UWCSM

How a Uniting Wall Column Maximizes LPG Recovery

Engineered to Innovate®

GTC TECHNOLOGY

WHITE PAPER


GT-UWCSM

Joseph C. Gentry PE and Roomi Kalita

GTC Technology US, LLC Please send inquiries to [email protected] or visit GTC’s web site at www.gtctech.com.

Maximizing LPG Recovery from Fuel Gas Using a Uniting Wall Column

Refiners have a challenge 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 is becoming more significant due to increasing LPG demand in some countries and the supply of lighter crudes, as for example in the U.S.

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 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.

Fig. 1: Typical Fuel Gas Producing Units in a Refining Complex

Fuel GasLique�ed Petroleum Gas

Gas H2

Gas

Gas

AtmosphericResidue

DesaltedCrude Oil

Coker Gas Oil

FCC Fuel Oil

FCC Gasoline

FCC Naphtha

DieselDiesel

Kerosine

HeavyNaphtha

Light Straight-Run Naphtha

AtmosphericCrude

DistillationColumn

Kerosene/Jet Fuel

Butanes

Isomerate

Reformate

Gas H2Gas H2

Gas H2 Gas H2

Gas H2

NaphthaHydrotreater

Catalytic Reformer

C5/C6IsomerizationHydrotreater

Merox TreatersGas Processing

FCC Unit Hydrotreater

DelayedCoker

VacuumDistillation

Column

Gas H2

HydrotreatingUnit

Gas H2

HydrotreatingUnit

FCC Feed Treater


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To cover this gap, a new solution has been developed to maximize LPG recovery and lower energy consumption. This document provides in-depth details of this novel process solution, with a case study comparing an existing refinery’s LPG recovery scheme with the application of the new design to achieve a better process performance and higher ROI. A key element of the technology is the use of a uniting wall column (UWC) 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 UWC system.

GT-UWC – a New Technology Solution: GT-UWC is a new process developed by GTC Technology using the uniting wall column technology to optimize the overall operation and enhance C3+ recovery. The UWC 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 uniting 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.

Process Description: The patented separation process with the UWC concept through a non-cryogenic absorption system is utilized here for maximizing LPG recovery from the fuel gas of refining units. A simplified flow diagram of the process is shown in Fig. 2. The diagram shows a single column with a 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 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.

Variables Units GT-UWCSM

Overall Propane Recovery wt % 97+

LPG Recovered BPD 1350

Number of Columns / Material of Construction – 1 / Carbon Steal

Turbo-Expander & Refrigeration System – Not Required

Net Benefit $MM 10.1

Total Installed Cost $MM 15

Simple Pay Back Months 18

Table 1: Economics of UWC System


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The feed is supplied to the absorption section of the uniting wall column, where non-condensables (C2- material) 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, C2 components. The liquid, which serves as a solvent for minimizing C3 loss, can simply be the heavier components (C5+ or heavier) from the feed stream. In this case, this heavy liquid for absorption is a slip stream from the bottoms material of the uniting wall column.

Heavies Slip Stream for Absorption

LPG Product

C4 and Heavies

Fuel Gas

Fuel Gas (C2-)

Water

Fig. 2: Simplified Flow Diagram for Maximizing LPG Recovery from Fuel Gas Using Single Column


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The other side at the top of the uniting 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, 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; 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 can be further processed into butanes and C5+ heavies.

Application Case StudyThe aforementioned process has been applied to a real-world case: a new simulation model has been created to review the existing LPG recovery scheme, the previous process’s disadvantages, and the application of a uniting wall column to enhance the overall process performance. After an in-depth study and detailed analysis of the simulation results, the key advantages of the advanced UWC pro-cess show great improvement in LPG recovery and a dramatic reduction in both capital and operating costs compared to the closest alternative.

Project Scope: The objective of the study was to maximize LPG recovery (>96 wt%), lower 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 maximum utilization of existing process scheme is summarized in Table 2. The fuel gas feed composition is shown in Table 3.

Design Basis

1. Feed rate at 10,000 BPD

2. Cooling water to be used for overhead condensation (no refrigeration)

3. Minimize operating pressure

4. Utilize existing columns for new design; if not feasible, design new columns

5. Minimize H2S in LPG product

Table 2: Design Basis


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Current Process: A simplified process diagram of the existing process is shown in Fig. 3. As seen, the existing process uses two separate columns at 250 psig and 470 psig operating pressures for separating C3- material and then recovering C2- fuel gas and LPG products. This overall process is able to recover only 55 wt% of the propane and leaves a higher content of H2S (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. A logical solution to counteract the problem and enhance the recovery is to increase the operating pressure and use refrigeration to condense the overhead gas. However, this would be at the expense of a high utility requirement, leading directly to higher operating costs.

Feed Composition

Flow rate [bbl/day] 10,000

Liquid Volume %

Hydrogen 0.05

H2O 0.01

CO2 0.01

H2S 1.23

Methane 0.60

C2 5.12

C3 14.95

C4 28.19

C5 36.06

C6+ 13.81

TOTAL 100

Table 3: Fuel Gas Composition of the Feed to UWC


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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 is carried out in four stages as follows.

Process Scheme 1: New Depropanizer at Higher Pressure plus Existing Deethanizer In the first stage study, a new depropanizer and an existing deethanizer column were used at an increased operating pressure of 390 psig (up from 250 psig). The new depropanizer helped to recover 92% of the propane, but the existing deethanizer column remained inefficient due to its lower column dimensions and usage of cooling water for overhead gas condensing. Thus, the overall C3 recovery achieved is only 76% with 160 ppm of H2S in the LPG product. The total reboiler heat duty required for this case was 18.1 MM Btu/hr.

Fig. 3: Simplified Process Diagram of Existing Process Scheme

Feed

C4 & Heavies LPG

Fuel Gas

Water

Fuel Gas

250 psig 470 psig

Dep

ropa

nize

r

Dee

than

izer


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Process Scheme 2: New Depropanizer Based on Reboiled Absorption plus Existing Deethanizer The second stage study further enhances the recovery by using two new columns for the deethanizer and depropanizer columns 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. 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 96.9 wt% with just 40 ppm H2S in the LPG product. But this comes at the expense of higher reboiler duty of 28 MM Btu/hr and the addition of two separate 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 re-mixed with the heavier components.

Fig. 4: New Depropanizer at Higher Pressure plus Existing Deethanizer

Feed

To DIB Column LPG

Fuel Gas

Fuel Gas

390 psig 470 psig

New

Dep

ropa

nize

r

Dee

than

izer


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Process Scheme 3: GT-UWC In the final stage, to further reduce the energy consumption and minimize the capital costs, an advanced process solution utilizing a single top uniting wall column was designed. This UWC solution eliminates the need for a new depropanizer column. A single column is utilized for both deethanizer and depropanizer operations. This single column solution comes at a remarkably lower reboiler duty of 20.3 MM Btu/hr, while at the same time maintaining a higher C3 recovery of 97 wt% and 40 ppm of H2S in the LPG product. Figure 2 displays the simplified scheme of GT-UWC.

Fig. 5: New Deethanizer Based on Absorption Plus New Depropanizer

Feed

HeavierHydrocarbon

C4 & Heavies

Fuel Gas

Water LPG250 psig 250 psig

New

Dee

than

izer

New

Dep

ropa

nize

r


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Process Scheme 1 Process Scheme 2 Process Scheme 3

Variables UnitsExisting Scheme

New Depropanizer at Higher Pressure plus Existing Deethanizer

New Depropanizer Based on Reboiled

Absorption plus Existing Deethanizer

GT-UWCSM

Overall Propane Recovery

Wt % 55 76 97 97

Total Duty Requirement

MM Btu/hr

22 20 28 20

LPG Product Rate BPD 883 1267 1445 1445

LPG Benefit/Year $MM Existing 7 10.2 10.2

Net Benefit/Year $MM Existing 4.2 9.2 10.1

Total Installed Cost

$MM Existing 17 23 15

Simple Pay Back Months Existing 48 30 18

Table 4: Economic Advantages of the Existing and the Modified Process Schemes

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; 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 cost estimated using 2017 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 18 months, the UWC process solution is able to improve propane recovery dramatically from 55 wt% to 97 wt%, while also being highly energy efficient.


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This new process solution provides several advantages and benefits over conventional cryogenic or non-cryogenic LPG recovery processes, including:

• 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

Table 5: Target Product Specifications vs. Design for the LPG and Fuel Gas Products

Component Units Specification Target Achieved Design

Propane lv% >90 99

Ethane lv% <1 <1

H2S ppm <123 <40

Butanes & Heavier psig 2.5 0.7

Vapor Pressure @100F lv% <208 176

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