Apr-2011

A promoter for selective H2S removal: part II

Refinery trials of an MDEA promoter demonstrate low H2S lean loadings and the option for enhanced process stability during high-sulphur operation

Gerald Vorberg, Ralf Notz and Torsten Katz BASF SE
Wieland Wache and Claus Schunk Bayernoil Raffineriegesellschaft

Article Summary

The Bayernoil Refinery located in Vohburg, Bavaria, Germany, and the Gas Treatment Process Technology team of BASF SE, Ludwigshafen, jointly conducted a test with a new promoter system in order to improve the performance and energy consumption of the refinery’s amine system. This article provides an overview of the refinery’s setup and a summary of the test and the results. A more detailed description of the promoter’s characteristics appears in part I of this article (see PTQ Gas 2011).

Bayernoil is among the leading manufacturers of mineral oil products in Germany and Europe, with a high flexibility in processing operations and its range of products. Nearly two-thirds of the mineral oil products consumed in Bavaria come from Bayernoil’s process units in Neustadt and Vohburg. The two locations are connected by 11 pipelines and work as one establishment. The majority of the refinery’s supplies arrive via the transalpine pipeline (TAL), which starts at Trieste, Italy, and is routed via Ingolstadt to Karlsruhe for a total run of 759 km. The pipeline delivers crude supplies from Africa, Russia, Venezuela, Saudi Arabia, Norway and other countries to Bavaria. In the Bayernoil operation plants, the staff process crude oil into products such as mogas, diesel, jet fuel, LPG, heating fuels and bitumen for commercial and end users.

The plants perform a combination of distillation, conversion and upgrading steps. Figure 1 shows a simplified flow chart of the main processes and products.

Refinery amine systems
Amine systems in refineries may consist of multiple absorber systems connected to one or two regenerators. Complexity can increase because, in some cases, two or three amine systems are interconnected, on either the lean or the rich side, or even both sides. Additionally, liquid treatment of C3/C4 fractions to obtain LPG and further value products such as ethyl-tert-butyl-ether (ETBE, gasoline additive) has become a substantial part of most refineries’ acid gas removal units  (AGRU).

Those AGRUs have been expanded over the course of more than 40 years by adding more absorbers for dedicated hydrotreating units. Subsequent debottlenecking measures were partly based on solvent swaps (for instance, from DEA to MDEA) and continuous revamps in downstream Claus units. In today’s world- scale refineries, interconnected AGRUs with more than 15 absorbers are common.  

As a further challenge, refineries may not run for long with the same crude and change the output of dedicated value streams on a weekly or even daily basis.  System optimisation is not easy and rigorous process simulation of an amine system is difficult and sometimes impossible.

Bayernoil’s amine system
Bayernoil’s refinery in Vohburg is a so-called bitumenic refinery, able to process huge amounts of crude fractions with high boiling points in a vacuum distillation unit (see Table 1).

The amine system at Vohburg, called BTV (BeTriebsteil Vohburg, Engl. Plant Site Vohburg), comprises four absorbers in three operational parts (see Figure 2). For upgrading purposes, the solvent was changed from generic DEA to various MDEA-based solvents in 1995. Besides that, several revamps of absorber/desorber internals were carried out.

The amine system has a hold-up of approximately 40 tonnes (88 000 lb) and a maximum reboiler steam feed rate of 5.5 t/h (12 100 lb/h). The stripped acid gas is further processed in two Claus units (25–50 t/day, 55 000–110 000 lb/h) with a recovery rate of >97%. Table 2 gives an overview of individual absorber/regeneration conditions.

Challenge and intention
Processing bitumenic crudes with high amounts of various sulphur species has a range of specific impacts on individual units, in particular those where the treated gas/liquid specification is crucial; for instance, unit 14 with FCC tail gas absorber E1410 and LPG liquid treater E1411.

During the processing of sour, bitumenic crudes, FCC operation becomes more severe; the FCC reactor temperature and charges of offgas containing sour gas increase. In a similar way, LPG treatment is affected by higher sulphur concentrations, which requires more efficient amine treatment and proper amine/hydrocarbon-phase separation. In the worst case, sulphur and sulphur-containing solvent can carry over to further downstream units such as caustic and Merox treatment, ending up in the debutaniser and the C3 value chain.

Consequently, the processing of sour crudes with high sulphur charges (high-sulphur operation) needs to be controlled and responded to by an increase in the reboiler steam feed rate. In this context, the H2S amine lean loading is one of the guiding values.

Bayernoil and BASF jointly decided to conduct a test run with a new promoter formulation added to the generic MDEA solvent using dosing equipment. The promoter formulation itself was provided as an MDEA-diluted premix. The concentration was adjusted and controlled during the test period. This promoter system is non-volatile and is stable at elevated process temperatures. Thus, promoter losses were only expected by removal through the filter system or losses of the amine itself.

Based on pilot tests, a drop in H2S lean loading was expected, particularly during high-sulphur operation, as were energy savings or potential capacity increases. As it turned out, a piping connection upstream of the LPG treater E1411 (on the pressure side of the LPG pump) was found to be the only feasible dosing point. Prior to the test run, two operational optimisation phases were conducted to exclude operational phenomena or other effects that could not be attributed to the test phase.

Apart from the performance of the entire amine system, the review and test programme also focused on FCC tail gas absorber E1410 and LPG treater E1411 due to their higher sensitivity to changing levels of sulphur charge.