MAXIMISING PYGAS

OPERATIONS PRODUCTIVITY

YUZOH SATOH, CLARIANT, JAPAN, STEFAN BREJC, AND PAIGE MARIE

MORSE, CLARIANT, GERMANY, AND IAN BUTTRIDGE AND JOSEPH GENTRY, GTC TECHNOLOGY, USA, OUTLINE THE

RECENT CATALYST AND EQUIPMENT DEVELOPMENTS CAPABLE

OF MAXIMISING PYGAS OPERATIONS PRODUCTIVITY.

Pyrolysis gasoline is a common byproduct in the production of ethylene, particularly when heavier feeds are used. This mixture, typically referred to as pygas, has many

reactive components and impurities that make it unsuitable for gasoline blending or other purposes without further treatment. After full hydro treatment and desulfurisation, the pygas can be upgraded into valuable petrochemical feedstocks such as benzene, toluene, and xylenes.

The raw pygas from the cracker has high potential for gum formation, in addition to existing gums, which creates a significant challenge in processing. The hydrotreatment process is highly exothermic so careful selection of equipment and catalyst can have a major impact on the reliability of operations. Recent commercial operations using OleMax® 600 catalyst from Clariant with a newly designed distributor, developed by GTC Technology, has demonstrated very stable operations for first stage diene hydrogenation with a much more uniform temperature profile and longer operating lifetime than other systems.

Pyrolysis gasoline contains hydrocarbons chains of C5 and higher and with varying levels of saturation. Processing proceeds in two stages: diolefin removal, followed by olefin and sulfur removal.

The first stage converts diolefins (or dienes) primarily, but also removes acetylenes and styrene by selective hydrogenation. Effective conversion in this stage is critical to downstream operations, enabling such processing benefits as:

n Improved oxygen stability and colour. n Reduced gum content. n Reduced fouling in the downstream hydro

desulfurisation unit.

OleMax 600 is a high performance palladium based catalyst for this first stage hydrogenation. It uses a spherical alumina carrier and exhibits very high activity and excellent physical durability. Clariant has a long history of expertise in selective hydrogenation technology, building from predecessor company Süd-Chemie.

OleMax 600 is particularly effective at reducing styrene levels and gum formation in first stage processing. The formation of insoluble gums is a particular concern, as it is the primary reason for catalyst deactivation. This catalyst can be used in single or double bed systems.

In the single bed system, typical operations combine heavy gasoline from the quench tower and light gasoline from the debutaniser bottoms. The feed is pumped to reactor pressure, mixed with recycle and then introduced to the reactor. Hydrogen is added to the reactor to maintain system

Reprinted from September 2013 HYDROCARBON ENGINEERING

pressure. Part of the reactor effluent is cooled and recycled to the feed, to limit the temperature rise and vaporisation across the catalyst bed.

The vapour fraction of the reactor effluent is cooled and condensed. The vent containing the excess hydrogen can be used as a hydrogen source for the second stage hydrotreater, or may be

directed to a Claus unit for sulfur recovery. The condensed vapour product and the balance of cooled liquid reactor effluent are fed to a depentaniser or directly to the second stage pygas hydrotreater unit. It is challenging to maintain styrene conversion above 90% per pass in the single bed system.

The two bed design is used for large scale reactors when higher diene and styrene conversion are required. Two configurations are common in the dual bed system. One process configuration recycles most of the reactor effluent back to the feed as a top dilution, which limits the temperature rise and the vaporisation across the catalyst bed.

The other configuration option uses an additional side quench that recycles product to the second bed also, as shown in Figure 1. As the top bed is more exposed to feed contaminants, it can be replaced more frequently than the bottom catalyst bed.

GTC Technology has invented a much improved distributor design that enables even and efficient mixing of the feed to the catalyst beds, and thus much more stable and efficient operations. The distributor is designed to distribute the liquid evenly across the tower cross section, while maximising the number of drip points. Figure 2 shows a simplified diagram of distributor operations, and illustrates the benefits of a drip point multiplying effect. It is important to note that physical constraints often do not allow a large amount of drip points or risers in conventional arrangement.

The GTC distributor combines unique drip point design and placement to achieve minimal cross sectional temperature variance, and greater operational stability.1 Operations with this distributor design have four to seven times higher effective drip point density, without excessively small holes, than typical designs and can achieve an effective drip point density of 200 - 300/m2.

Figure 1. Pyrolysis gasoline 1st stage (two bed system).

Figure 2. Increased drip points in the distributor provide improved liquid spreading and much better catalyst coverage.

Figure 3. Stable temperatures achieved across both reactor beds using only top recycle.

Figure 4. Top bed temperature profile for first few months of operation, with the GTC distributor and with alternate equipment. NOTE: Same temperature and height scales used for both charts.

Reprinted from September 2013HYDROCARBON ENGINEERING

GTC uses a modified Moore-Rikovena method to evaluate the geometry of each distributor. This method has been used in distillation columns to calculate the distribution quality (DQ) of any given design as a fractional percentage, with 100% reflecting the ideal design. The primary drip point will be completed using an equilateral triangular pitch. However some issues prevent a perfect flow pattern, as noted below:

n The reactor is round, thus at the periphery the pitch will be slightly distorted, and spacing to the reactor wall can cause dead areas.

n The distributor must be assembled in parts with a bolted removable construction.

n In some large reactors, the required support beams also distort the ideal pattern.

An optimised design will minimise the effect of these issues as much as possible, and the DQ calculation can be used to evaluate each design. An optimised layout enables distribution quality approaching 90%. The efficiency of operation is further enhanced by the use of a mixer between the reactor beds. This mixer collects and redistributes the liquid from the upper bed and mixes it with any quench feed. The liquid from the top reactor bed and the quench is brought into a central distribution point, mixed, and then sent to the distributor below. This process ensures that the lower bed achieves maximum performance, and also that any issues with the upper bed caused by catalyst deactivation or hot spots do not propagate down to the second bed. This is achieved with minimal reactor volume.

The OleMax 600 catalyst has excellent activity and stability and well balanced physical properties, which combine to increase cycle length without any process modifications. With the addition of the GTC distributor to the equipment, even longer cycle length is possible with further optimised temperature control.

The advantages of this catalyst system and distributor design are illustrated by results from commercial operations at an existing processing unit. The feedstock for this unit is raw pyrolysis gasoline and the unit was operated with a gradual inlet temperature increase to prevent leakage of outlet diene values and styrene levels. Diene values are maintained well below the target and styrene levels are less than 0.2 wt%.

The reactor demonstrated excellent temperature stability with minimal increase required after more than 200 days of service, as shown in Figure 3. The performance shows notable improvement over the previous catalyst and distributor specified by the original licensor of this unit, confirming superior catalyst stability and improved feed distribution.

This reactor stability is maintained throughout the run, as shown in Figures 4 and 5. The temperature profiles across the top and bottom beds, respectively, at one, two and three months of operation reflect only a gradual increase across the period. This contrasts with the previous system for this unit that exhibits a much less stable temperature profile.

One key performance advantage from the combined operation of OleMax 600 with the GT-599 distributor is the significant resistance to gum content in the feed. Based on current performance, the operating lifetime of the catalyst is approximately 25% longer when compared to previous operations without the GTC distributor. The expected performance improvement is illustrated in Figure 6, showing estimated cycle length improvement correlated with gum content in the feed.

Overall, the system utilising the new distributor from GTC with the OleMax 600 catalyst enables more stable operations with uniform temperature profile and longer operating lifetime. The value of this performance has been further verified with the recent selection of this system by two new customers.

References1. MOORE, F., and RUKOVENA, F., ‘Liquid and Gas Distribution in

Commercial Packed Towers’, 36th Canadian Chemical Engineering Conference, October 5 - 8, 1986 Paper 23b.

2. OleMax is a registered trademark of Clariant.

Figure 5. Bottom bed temperature profile for first few months of operation with the GTC distributor and with alternate equipment. NOTE: same temperature and height scales used for both charts.

Figure 6. Projected performance using GTC distributor with OleMax 600 catalyst.

Clariant international ltd

Rothausstrasse 614132 MuttenzSwitzerland

Catalysts

petrochemicals@clariant.com

www.Catalysts.Clariant.Com