Oct-2014

Less is more: how a dividing wall column maximises LPG recovery

Maximising LPG recovery from fuel gas using a dividing 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.

Manish Bhargava, Cole Nelson, Joseph Gentry and Vamshi Siddamshetti
GTC Technology US, LLC

Article Summary

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 Figure 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 maximise the recovery of LPG range material beyond 95 wt%, while at the same time being highly energy efficient.

To cover this gap, a new solution has been developed to maximise 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 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.

GT-LPG MAX – a New Technology Solution
GT-LPG MAX is a new process developed by GTC Technology using DWC technology to optimise the overall operation and enhance C3+ recovery. DWC can separate a multi-component feed into three or more streams within a single column. The deethaniser and depropaniser 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.

Process Description
The patented separation process with the DWC concept through a non-cryogenic absorption system is utilised here for maximising LPG recovery from the fuel gas of refining units. A simplified flow diagram of the process is shown in Figure 2. The diagram shows a single column with a dividing wall for deethaniser and depropaniser 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.

The feed is supplied to the absorption section of the dividing wall column, where non-condensables (C2- material) and water are concentrated in the overhead and passed through a partial condenser. Condensed vapours 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 vapour 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 minimising 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 dividing wall column

The other side at the top of the dividing wall column is referred to as the fractionation section, which is concentrated with C3 components. The vapours 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 vapour bypass pressure control loop. A single thermosiphon reboiler is provided at the bottom of the column to supply the duty required to distil 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 Study
The 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 dividing 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 DWC process 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 maximise LPG recovery (>96 wt%), lower H2S in the product (<40 ppm), and minimise 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 utilisation of existing process scheme is summarised in Table 2. The fuel gas feed composition is shown in Table 3.

Current Process
A simplified process diagram of the existing process is shown in Figure 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.  

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 maximising the propane recovery at lower energy consumption is carried out in four stages as follows.

Process Scheme 1: New
Depropaniser at Higher Pressure plus Existing Deethaniser
In the first stage study, a new depropaniser and an existing deethaniser column were used at an increased operating pressure of 390 psig (up from 250 psig). The new depropaniser helped to recover 92% of the propane, but the existing deethaniser 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.