Pyrolysis Value Upgrade with GT-Styrene®

noemail@gtchouston.com — Sun, 02 Mar 2008 18:00:00 GMT

When new petrochemical capacity for ethylene or derivatives is considered, most investors pay relatively little attention to the optimum scheme for processing the pyrolysis gasoline (pygas) byproduct. Pygas operators will usually recover the benzene and possibly the toluene from the C₆ + stream but will rarely extract other components. Recent advances in separations technology make it possible to recover both high-purity styrene and isomer-quality mixed xylenes from the pygas at a significant upgrade over their alternate disposition into motor fuel.

The typical view is that this area of the plant is a required treatment center so that the bulk of the material can be blended into motor gasoline. Rather than merely coping with the requirements, it is possible to design the pygas processing area to generate substantial additional income from the petrochemicals contained in this stream.

Optimization Scope for Pygas Treatment Schemes

Inherent inefficiencies and missed opportunities for production economics upgrades become obvious in an evaluation of conventional processing scheme.

The conventional pyrolysis gasoline processing plants will have the following unit operations:

Hytrotreating for removal of the olefin and di-olefins to reduce the tendency towards gum formation and to remove sulfur components
Deheptanizing to isolate a benzene/toluene concentrate stream
Extraction and purification of the benzene and toluene from the non-aromatic components

Pygas Upgrade Possibilities

The pyrolysis gasoline by-product from naphtha steam cracking plants contains a wealth of petrochemical components that could be valuable if they were recovered in purified form. Unfortunately, many of these species have boiling points close to other components, or they form azeotropes that make it impossible to use traditional means of separation. Here, extractive distillation (ED) is a viable alternative to product recovery rather than simply directing these products to fuel streams. ED is commonly used to purify butadiene and isoprene from the lighter fraction of the cracked stock. It can also be used to effectively separate styrene from the crude C₈ fraction. Styrene is present in sufficient quantity to be economically recoverable.

The pygas hydrotreating units are often plagued by catalyst fouling and consume a large amount of hydrogen. Styrene in particular is a gum-forming precursor in gasoline and must be saturated before sending the stream to the motor fuel pool. There are several advantages to recovering the styrene component before the hydrotreatment. First, the styrene component is upgraded from fuel value to petrochemical monomer value, which averages over US $300 per ton. Also, the hydrotreatment costs and operational difficulties are greatly reduced. Finally, the value of the remaining C₈ aromatics is enhanced because less of the unwanted ethylbenzene from styrene saturation is produced along with the xylenes. These items are discussed in the following sections.

Figure 2 shows the basic configuration of the pygas system that includes styrene recovery using GT-Styrene® process technology.

Heartcut fractionation first separates the C₈ fraction containing styrene from the lights and the heavies. This concentrate stream is sent to the extractive distillation unit. Xylenes and close- boiling non-aromatics go overhead in the extractive distillation column (EDC) while styrene and solvent remain in the bottoms.

Figure 3 shows conventional pygas processing with GT-Styrene® process technology integrated.

Figure 4 shows the operations within the GT-Styrene® process scheme. This includes a mild hydrogenation reactor to convert the phenylacetylene into additional styrene. Further styrene purification, as required, takes place in the finishing section of the process. The lean solvent is routed back to the EDC. Styrene monomer of 99.9 wt% purity can be achieved with this process. Styrene recovery from pygas cannot be made by conventional means. The close boiling point of styrene to orthoxylene and other components preclude the use of classical distillation as a separation method. However, a solvent-based extractive distillation system can extract and purify styrene from the heartcut of the pygas. The table below shows the change in relative volatility that is made to various pygas components by using a selective solvent, Techtiv-200, in an extractive distillation operation. This allows the styrene to be separated and purified to the petrochemical product specifications.

Conventional Styrene Production Scheme from Petrochemical Feedstock

Some facilities use an unorthodox approach to make styrene from pygas components. Since the C₈ fraction of hydrotreated pygas contains a disproportionately-high content of ethylbenzene, this is sometimes routed to a hydrodealkylation (HDA) unit. The HDA unit converts the ethylbenzene and xylenes into benzene plus fuel gas. Benzene is then reacted with ethylene to produce the ethylbenzene again. The ethylbenzene is dehydrogenated to produce styrene, which is then purified from the dehydrogenation reactor effluent. This configuration (Figure 5), is not unusual in the industry, and may even be complicated by having to transport the streams between processing sites, incurring additional costs and logistical problems. Situations such as these can certainly be optimized with addition of facilities to recover styrene.

Economic Debits in Pygas Processing

Pyrolysis gasoline processing units experience a common set of maladies that place economic debits on the process:

Fouling of the selective hydrogenation catalyst
H 2 consumption
Conversion of the styrene to ethylbenzene
Inferior gasoline quality

The underlying reason for these problems is the reactive, gum-forming components, which must be removed from the pygas stream. Styrene contributes more to this phenomenon than any other component in the pygas. The easiest way to control the situation has historically been to hydrotreat the entire cut, which converts the styrene into ethylbenzene and the non-aromatic unsaturates into the corresponding olefins and paraffins. Although this method can achieve the objective of reducing the gum-forming constituents, it does so at a substantial cost.

The first cost is the operating expense to hydrotreat the entire stream. Costs include the utilities, hydrogen feedstock, depletion of the catalyst, and in some situations, a reduction in the cracker operating rates in order to regenerate the catalyst. A second area of cost is the penalty to the xylenes quality caused by conversion of the styrene into ethylbenzene. This creates a disproportionately-high ethylbenzene concentration in the C₈ aromatics fraction. Consequently, hydrotreated pygas xylenes are rarely used for paraxylene production, and the pygas aromatics extraction units only recover the benzene and toluene. In addition to the cost penalties in traditional pygas processing schemes, there is a tremendous missed opportunity to recover and upgrade the styrene to petrochemical value.

Fundamentals of Styrene Recovery Technology

Styrene recovery technology must deal with a wide variety of components in the pygas. Some of the notable ones are shown in Figure 6.

GT-Styrene® process technology can cleanly separate and purify styrene from these and other components in the pygas by a combination of conventional distillation, extractive distillation, selective hydrogenation, and finishing treatment.

Impact of GT-Styrene® on Pygas Xylenes Quality

Table 2 shows a comparison of the C₈ aromatic isomer distribution for three cases. In each case, there is approximately a 1:2:1 ratio of the paraxylene, metaxylene, and orthoxylene, which is related to the thermodynamic equilibrium among these isomers in the thermal cracking or reforming process. However, the ethylbenzene content is quite different. Reformer xylenes usually have less than 1 part EB to 4 parts xylene isomers while hydrotreated pygas xylenes will have approximately an equal percentage of EB. The high-ethylbenzene content in the pygas material diminishes the stream value as xylene isomer unit feedstock. The ethylbenzene displaces capacity in the paraxylene production units and adds to the recycle in the xylene isomerization loop. The PX content produced as a by-product from GT-Styrene® unit will be almost the same as that produced from the catalytic reformer.

Note, however, the third column. This represents the quality of xylenes derived from pygas when the styrene has been removed using GT-Styrene® process technology. Since there is no styrene being converted into ethylbenzene, the isomer distribution is more like that of a catalytic reforming unit. These xylenes are suitable for xylene isomer production and would thus command the higher market value for this type of material.

Optimal Operation of Pygas Treatment

The recovery of styrene provides economic and operational benefits that make the pygas section of an ethylene plant more profitable.

The main benefits to styrene recovery using GT-Styrene® are:

1. The styrene component is upgraded from fuel value to petrochemical value

2. The xylenes can be upgraded from motor fuel value to xylene isomer unit feedstock value.

3. Overall hydrogen consumption is reduced.

4. Catalyst fouling and operating costs in the selective hydrotreater unit are reduced.

5. The pygas hydrotreating area can be debottlenecked, to gain capacity for ethylene expansion.

The economics of styrene extraction from pyrolysis gasoline depend to a great extent on the quantity of styrene available in this stream. Units that produce at least 10,000 -15,000 tons/year of contained-styrene in the pygas would be candidates for application of the technology. This includes world scale crackers producing a minimum of 500,000 tpa ethylene from liquids feedstock. Table 3 shows the economics for styrene recovery based on 25,000 tons/year styrene capacity.

Other criteria that could enhance the value of a project for styrene recovery include:

1. Proximity to existing styrene producing unit or styrene consumer

2. Integration with xylenes processor

3. Relative capacity and constraints in the hydrogenation unit

4. Configuration of prefractionation and hydrogenation unit scheme; existence of idle fractionation equipment

5. Possibility of consolidating styrene fractions from nearby producers

Conclusion
GT-Styrene® process technology makes it possible for pyrolysis gasoline producers to generate income from petrochemicals, which would otherwise be downgraded into motor fuel. The system is primarily applicable to ethylene plants cracking liquids feedstock and can provide a favorable return on investment for most world scale units. The styrene produced by this means will always have the lowest feedstock cost and will therefore have a cost advantage over traditional units feeding benzene plus ethylene. Although the styrene extraction process provides a very good upgrade on pygas components, the feedstock availability will limit the styrene production volumes to a small, but profitable, fraction of the global capacity.

2-Mar-08 1:00 PM

GTC Technology, Inc

Pyrolysis Value Upgrade with GT-Styrene®