Apr-2008

Front End CDHydro - raw gas hydrogenation achieved

Front End CDHydro applies catalytic distillation hydrogenation to the ethylene flowsheet to successfully accomplish raw, cracked gas hydrogenation.

Stephen J Stanley, Robert Gartside and Jose deBarros, Lummus Technology, a CB&I Company

Article Summary

The Front End CDHydro system features a depentaniser tower with a hydrogenation catalyst zone in the rectifying zone of the tower to convert the C2 to C5 acetylenes and dienes to olefins. By combining the numerous fixed-bed reactors into a single integrated step, investment costs are lower and plant reliability is improved. In addition, 30–35% of the pyrolysis hydrogen is removed by chemical reaction rather than by cryogenic separation. This reduces compressor power requirements by 10%. Furthermore, by using the butenes produced by the Front End CDHydro system as feed to a metathesis unit, the process chemistry downstream of the cracking heaters is further changed by producing up to one-third of the propylene by energy-neutral metathesis rather than by energy-intensive thermal cracking.

The Front End CDHydro system has been in semi-commercial operation at the Sinopec-Tianjin Ethylene Plant since 2005. The results obtained from this unit show high selectivity to olefins, continuous on-specification conversion and extended catalyst life.

The production of ethylene and propylene by steam cracking is a fairly mature technology. While improvements in the configuration of the pyrolysis module and the product recovery sections continue to increase the efficiency of the process, the process chemistry and fundamental flow sheet configuration has remained relatively unchanged. With rising crude oil prices and global warming concerns, olefins producers face a number of challenges. One is to reduce greenhouse gas emissions by reducing the fuel fired in the thermal cracking of feedstocks and by lowering the energy consumption of the product recovery section. Another challenge is to lower the significant level of investment associated with new steam cracking facilities. A third challenge is to improve the rate of return on these investments by enhancing the product slate produced from thermal cracking, thereby improving the operating margins.

To help meet these challenges, Lummus Technology began a research and development programme that focused on changing the process chemistry downstream of the pyrolysis module, and on a fundamental reconfiguration of the steam cracker flowsheet. This programme has led to many improvements that represent the first fundamental changes in the steam cracker flowsheet in over 25 years. These improvements include: the Front End CDHydro system, which is the subject of this paper; tertiary refrigeration, which supplies all refrigeration from 40°C to -140°C in a single refrigeration system; and olefins conversion technology. The latter utilises the butenes that are produced in the Front End CDHydro system as feed to produce propylene in an energy-neutral fashion using isomerisation and metathesis chemistry.

Catalytic distillation hydrogenation
In the conventional steamcracker flowsheet, the selective hydrogenation of C2 and heavier acetylenes and dienes is accomplished in a series of fixed-bed reactor systems. As shown in Figure 1, the CDHydro process combines the hydrogenation step with the distillation step, thereby eliminating separate fixed-bed reactor systems. The hydrogenation catalyst is placed in the rectifying zone of a distillation tower. The CDHydro process offers a number of advantages when compared to conventional fixed-bed reactor systems. Dimers and oligomers are fractionated from the catalyst zone, which prevents catalyst fouling and the loss of catalyst activity. Spare reactor beds can therefore be eliminated. Separate green-oil-removal facilities are not required. Higher selectivities are achieved since the catalyst-fouling rate is minimised. Since the reaction and distillation steps are combined, the equipment associated with the fixed-bed reactor system, including the reactors, feed heaters and effluent coolers, are eliminated. Investment is therefore lowered and operation efficiency is improved.

When applied to the ethylene plant flowsheet, the CDHydro process permits the hydrogenation of the C2 through C5 acetylenes and dienes in a single reaction zone. Fractionation of the dimers and oligomers from the catalyst bed prevents the rapid fouling that occurs in fixed-bed, “raw gas” reactors. The ability to combine the hydrogenation of a wide range of acetylenes and diolefins further reduces investment. Since feedstock and by-product requirements differ from plant to plant, there are two CDHydro process applications that have been developed for the ethylene flowsheet.

The CDHydro process can be applied for liquid feed and ethane, propane or butane feed plants where butadiene is recovered. In this application, the CDHydro process is used to selectively hydrogenate the MAPD in the depropaniser and to selectively hydrogenate the C4 acetylenes in the debutaniser. The selective hydrogenation of the C4 acetylenes eliminates the C4 acetylene removal steps in the butadiene extraction unit. (It can also be applied to expand existing butadiene facilities.) The selective hydrogenation of the MAPD in the depropaniser eliminates the separate fixed-bed hydrogenation system. For a 1000 kta capacity cracker, the investment cost savings for the ethylene plant and butadiene extraction unit is about $5 million.

For liquid feed plants, and gas feed plants where no more than 40% of the feedstock is ethane, where the butadiene is hydrogenated, and where the C4 olefins are recovered as feed for a metathesis unit or are recycle cracked to extinction, the CDHydro process can perhaps have its most significant impact on investment, piece count reduction and energy consumption. This is the Front End CDHydro configuration shown in Figure 2.

The cracked gas from the third stage of the charge gas compressor, after caustic wash and drying, is sent to a depentaniser tower. The depentaniser tower has a CDHydro bed in the rectification zone where C2 acetylene, MAPD, butadiene, C4 acetylene, and C5 acetylenes and dienes are selectively hydrogenated. This combined hydrogenation step is possible with the CDHydro process because the oligomers and dimers that are formed are fractionated from the catalyst zone and exit with the C6 and heavier material in the tower bottoms. About 30–35% of the pyrolysis hydrogen is removed by chemical reaction rather than by cryogenic separation. The depentaniser overhead flows to a single fixed-bed acetylene hydrogenation reactor to guard against acetylene breakthrough and ensure that the tight acetylene specification required in the ethylene product is achieved. Due to the much lower hydrogen content of the cracked gas, the reactor effluent can then flow either to the next stages of compression or directly to a lower pressure chilling train.

The CDHydro process effectively accomplishes raw gas hydrogenation of the acetylenes and dienes without the severe catalyst fouling and loss of selectivity to olefins that has caused earlier attempts at raw cracked gas hydrogenation to fail. With the Front End CDHydro process, the selectivity to hydrogenate the acetylenes and dienes is significantly improved over conventional hydrogenation. It achieves 80% conversion of acetylene to ethylene and 95% conversion of the methyl acetylene and propadiene to propylene.