Apr-2006

Recovering pure aromatics

The recovery of pure aromatics feedstocks such as reformate or pyrolysis gasoline through solvent-based extractive distillation. The efficiency, purities and yields of a single-column extractive distillation configuration based on actual plant results are discussed

Thomas Diehl, Bärbel Kolbe and Helmut Gehrke
Uhde GmbH

Article Summary

The recovery of pure aromatics from refinery or petrochemical cuts (reformate or pyrolysis gasoline respectively) or coke oven light oil (COLO) is an established field of activity in the industry. Not only are high-purity aromatics produced for chemical processing, but the associated reduction of aromatics in gasoline is a highly welcome additional benefit in the face of ever more stringent environmental laws (for example, auto oil programme). The proprietary Uhde Morphylane extractive distillation process for the recovery of BTX aromatics achieves aromatic purities of 99.999 wt% and yields of more than 99.9 wt%.

Since 1968, more than 55 Morphylane plants with an overall production capacity in excess of 10 million mtpy of pure aromatics have been commissioned or are in the engineering and construction stage worldwide. Choosing the appropriate solvent has a profound effect on the economics of an extractive distillation process. Uhde’s proprietary solvent NFM (N-formylmorpholine) meets all the requirements for an ideal solvent for extractive distillation, such as:
— Optimum combination of selectivity and solvent efficiency
— High permanent thermal and chemical stability
— Low vapour pressure
— Low price
— No corrosive effect (carbon steel equipment)
— No toxicity
— Single-component solvent (no addition of water or chemical agents).

The molecular structure of Uhde’s proprietary solvent and some of its basic properties incorporated into the extractive distillation process are shown in Figure 1.

Classical two-column ED configuration
The “classical” Morphylane process consists of two columns: the extractive distillation (ED) column and the stripper column, as shown in Figure 1. The aromatics feed is added to the ED column between the stripping and rectifying sections, and the solvent NFM above the rectifying section. Solvent traces are separated from the non-aromatics in the solvent-recovery section. The non-aromatic components are drawn off at the top of the ED column.

The aromatics contained in the feed leave the ED column at the bottom together with the solvent. The aromatics are then stripped from the solvent in the stripper column. The condensed aromatics meet all industrially desired product specifications. The solvent is recycled to the upper part of the ED column from the bottom of the stripper column. The heat from the hot solvent is recovered almost completely by means of heat exchangers within the ED process.

Single-column ED technology
The ED process is already highly optimised and, due to its flexible design, the Morphylane process can be adapted easily to accommodate a wide range of feedstock and product qualities.1 More importantly, research and development work continues, with the aim of further reducing operating and investment costs. This has led to the development of a new, cost-effective thermally coupled column solution — the single-column ED technology.

The thermodynamic advantages provided by thermally coupled columns, and their resultant energy savings, have been known for decades and are well proven in theory.2-5 While conventional distillation sequences have been thoroughly investigated regarding the potential benefits of thermal coupling, Uhde’s development team was the first to look into the thermal coupling of ED columns.6,7 The principle of thermally coupled columns for extractive distillation involves replacing the classical two-column system (ED column and stripper) by one main column with a side rectifier, as described in Figure 2.

The upper part of the main column corresponds to the classical ED column, whereas the side rectifier, together with the bottom part of the main column, has the same functionality as the original stripper column. The extract is withdrawn from the vapour phase, where the concentration of high-boiling solvent is low and the aromatics are at their maximum concentration.

The next logical step is then to incorporate the side rectifier into the column shell of the main column, as shown in Figure 3, the result being the single-shell ED column. The thermodynamic behaviour of the configuration presented in Figure 3 is completely identical to the configuration in Figure 2.

The concentration profiles in Figure 4 clearly show the thermodynamic advantage of the new configuration, which results in smaller reboiler and condenser duties. Looking at the conventional configuration (Figure 4,  top), one can see that part of the aromatics enrichment already achieved is lost in the bottom part of the ED column. This can be avoided by using the new configuration (Figure 4, bottom). In addition to the energetic advantages, the new configuration reduces the number of main equipment items. The reboiler of the second column and the pump, which transfers the liquid from the bottom of the ED column to the stripper, is no longer needed.

Process design
A single-column ED configuration of the type previously discussed is shown in more detail in Figure 5. The complete column consists of five distillation sections:
— Solvent-recovery section (I) for removing solvent traces from non-aromatics
— Rectifying section (II)
— Stripping section (III)
— Side rectifier (IV) for removing solvent traces from aromatics
— Solvent stripping section (V).

The feed material is added above section III on the feed side. The non-aromatics are withdrawn from section I, which is the solvent-recovery section. When the aromatics have been stripped from the solvent in section V, the solvent is recycled to the column above section II. Intensive aromatics stripping is crucial for a high aromatics yield (ie, it drastically reduces the aromatics content in the solvent). The aromatics product is recovered at the top of section IV once the solvent traces have been removed in this section.