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Jul-2018

Dividing wall columns for gas plants

Uniting two unit offerings into one column shell can lead refiners to improve the profitability of their gas plants.

MANISH BHARGAVA, ROOMI KALITA and DAVID KOCKLER
GTC Technology
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Article Summary
Dividing wall column (DWC) technology provides refiners with a unique way of improving the efficiency and profitability of traditional refining techniques. This technology, which has been in use since the 1940s, can be modified for various applications ranging from naphtha/reformate splitters to gas plants, and can even be used to transform established technologies like isomerisation and naphtha hydrotreating.

In this article, we discuss the distinct advantages that DWCs can offer to improve and modify traditional gas plant process schemes, with emphasis on top DWCs. DWCs are often characterised by the presence of a vertical wall in a regular distillation column. When the distinctive wall is present at the top of the column, the column can be referred to as a top DWC.

Top DWCs offer a host of advantages, especially in gas plants which are generally operated at high pressures. Unlike a conventional distillation column, the top of these columns is segregated into two separation zones within the same shell with no intermixing between the two zones. As a result, they can be operated in separate modes and basically behave as two independent columns. Absorption and distillation can be carried out in a single column. This principle forms the basis of uniting two unit operations into a single column shell known as the GTC Uniting Wall Column (GT-UWC).

The concept of top DWCs can also be implemented in a reformer unit gas plant and an FCC unit gas plant, wherein two or more columns operate in a sequence and under similar operating pressures and temperatures. The DWC will potentially combine the operation of two or more columns, thereby minimising both capital and energy consumption of the whole sequence.

Uniting wall columns in LPG recovery from off-gas
Liquefied petroleum gas (LPG) recovery units are an integral part of any refinery. In general, in an LPG unit two or more columns remove the C1-C2 components along with other non-condensable gases, while concentrating the C3-C4  components to generate LPG. Figure 1 shows a standard design consisting of a deethaniser column followed by a depropaniser column. The deethaniser column operates at a high pressure, wherein the C1-C2 components are removed as the top product. The C3 rich (or C3-C4) stream is removed in the next column. The recovery of LPG in this design is generally poor due to the high operating pressures and propane losses in the off-gas. In some processes, these losses are mitigated by using refrigeration in the overhead system. This increases the capital cost of the process.

These problems can be rectified by using a top DWC. Doing so allows the top section of the column to be operated under two different unit operations: absorption and distillation. The section of the column where the feed enters operates under reboiler absorption. The other section of the column separates a light liquid product and a heavy bottom product using regular distillation. This arrangement offers the following advantages.

First, use of absorption allows the column to be operated at a lower pressure. The absorbing solvent captures the valuable C3-C4 components in the off-gas and moves them towards the bottom of the column. These components are then concentrated on the other side of the DWC using distillation. For feeds which contain a good amount of C5 and heavier components, a portion of the heavy bottom product can be used as the absorption medium (see Figure 2a). Using the bottoms is highly beneficial when the feed contains at least 10 wt% of C5s. Table 1 shows a sample GT-LPG Max feed.1 If the C5 concentration in the feed is low, the bottoms solvent can be supplemented by an additional heavy oil solvent from nearby 
processes.       
 
A water-cooled partial condensation is used in the off-gas side of the column. This is highly economical compared to using refrigeration to mitigate C3-C4 losses.

Additionally, this DWC design sees a reduction in energy consumption by about 10-30% compared to traditional columns with better product recoveries (see Table 2).

Uniting wall columns in reformer unit gas plants
A typical reformer unit gas plant usually has a depentaniser column, followed by a debutaniser and a deethaniser. In Figure 3, the depentaniser column removes the C5- cut from the reformer bottoms as the top product. C6 and heavier cuts are removed as the bottom product. In the next column, the C5s are concentrated in the bottom of the column. The C4 and lighter cut is then processed in a deethaniser column, which removes an LPG cut at the bottom.

The three columns in this sequence operate at high pressures. Additionally, the debutaniser and deethaniser columns see partial overhead condensation. Due to these, a major portion of the LPG components (C3-C4) end up in the off-gas. These components can be recovered by use of a DWC configuration.

As Figure 4 shows, the operation of the deethaniser can be squeezed into the bigger depentaniser column by means of a dividing wall. The column has two distinct zones within: a pre-fractionation zone and a main fractionation zone. The pre-fractionation zone sees a combination of absorption and distillation. The lights are separated on this side as the off-gas, with the absorption medium reducing the liquid losses. On the main fractionation side, the C5 and lighter components are removed as the other top product. Similar to a GT-UWC design, a portion of the heavy 
bottoms (mainly C6-C7) is used as an absorption medium. The GT-UWC depentaniser column is operated at a lower pressure compared to the conventional column.

Since the majority of the lighter components is removed in the first column, the subsequent debutaniser column operates with a total overhead condenser. This column removes the LPG cut and the C4 cut. Table 3 compares the performance of the two configurations. For the same heating duties, the DWC configuration generates better product recovery while minimising the amount of equipment needed. The DWC configuration has a lower total installed cost over the conventional configuration.

Uniting wall columns in FCC unit gas plants
Figure 5 shows a traditional FCC unit gas plant. The process scheme primarily consists of absorber/stripper columns, where the gases and the liquid in the feed are separated. Unstabilised naphtha (supplemented with a portion of stabilised naphtha bottoms) prevents C3-C4 losses from the absorber column. The remaining C3-C4 is stripped using lean oil in the sponge absorber column. Sour fuel gas and rich oil are removed at the top and bottom respectively. The liquid product from the stripper column is processed in a debutaniser column. Sour LPG is extracted as the top product, while stabilised naphtha is recovered at the bottom.

The conventional design consists of three stages for the separation: the absorbing and stripping section followed by distillation in the main debutaniser column. Using GT-UWC technology, the three stages can be integrated into a single column (see Figure 6).
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