Oct-2013

Gasification for hydrogen supply

Integrating the gasification of liquid residues into a refinery balances the hydrogen demand of hydroprocessing units

MAX-MICHAEL WEISS, HELMUT HEURICH, DELPHINE ROMA and STEFAN WALTER
Air Liquide Global E&C Solutions/Lurgi GmbH

Article Summary

Hydroprocessing is used in refineries for producing transportation fuels such as gasoline and diesel from crude oil. These technologies ensure that the emissions from the fuels after they are burnt in a combustion engine are meeting today’s environmental requirements. The refining processes are consuming hydrogen. For instance, in a hydrotreater, impurities such as sulphur, nitrogen and metals are removed and replaced by hydrogen. In a hydrocracker complex, petroleum molecules are converted into smaller molecules, boiling in the diesel range, and saturated by adding hydrogen.

The required hydrogen can be produced inside the refinery using several technologies:
•    Steam reforming of natural gas, LPG naphtha, refinery off-gases (or a combination of those)
•    Gasification of liquid residues or pet coke
•    Recovery of hydrogen from naphtha reformer or hydroprocessing off-gases using membranes and/or pressure swing adsorption units (PSA).

Alternatively, a third-party industrial gas company can supply the required hydrogen over the fence.

This article describes Air Liquide Global E&C Solutions’ Lurgi Multi Purpose Gasification (MPG) technology as a reliable source for hydrogen supply to refineries. The North West Redwater (NWR) upgrader project in Alberta, Canada, is the latest example of integrating the gasification of liquid residues into a refinery for balancing the hydrogen demand of the hydroprocessing units in the complex.

The NWR complex processes bitumen, the heavy oil extracted from oil sands, from different sources and converts this extra-heavy oil, not into a synthetic crude but into high-value finished products such as ultra-low-sulphur diesel, which accounts for approximately 50% of the products and diluents recycled back to the bitumen producers. For the production of hydrogen, the project will utilise Lurgi MPG technology. The feedstock is unconverted oil from the hydrocracker. The gasification technology allows for the efficient elimination of upgrading by-products such as pet coke and/or heavy residues, solving any issues regarding marketing and logistics of these products for a “land-locked” refinery, as well as a significant reduction in natural gas and water consumption. The decision to use gasification as an integral part of the upgrading and refining processes enables the capture of a substantial portion of the produced CO2. This CO2 is of high purity and ideal for enhanced oil recovery (EOR).

North West Redwater upgrader project
A block flow diagram of the NWR complex is shown in Figure 1. The feedstock is a bitumen blend. In the atmospheric distillation process, most of the diluent is recovered. Together with the naphtha produced in the hydroprocessing units, it is recycled back to the bitumen producers. Vacuum residue is fed to the hydrocracker. The main product of the complex is ultra-low-sulphur diesel. The unconverted oil from the hydrocracker is used as feed for the MPG unit, which provides the hydrogen consumed in the hydrotreating and hydrocracking units. High-purity CO2 is recovered by a Lurgi Rectisol unit, and sold for EOR application and final storage.

Lurgi MPG technology
The special features of the MPG process are, inter alia, the very reliable burner and the quench (see Figure 2). The burner uses a special diffuser nozzle system to atomise the feedstock into fine droplets, which are required for good conversion. This differentiates it from other burner systems, which need a high-pressure differential for atomising into droplets. Due to the segmentation into several nozzles, the burner allows the simultaneous feed of two or more different feedstocks into the reactor via one burner. This is important for applications where the feeds are immiscible or, worse, tend to react with each other.

The second special feature is the quench technology. This allows for a feed of ash-containing feedstock. The ash, if present in the feedstock, melts in the special reactor, forming a slag, which flows along the refractory-lined wall. The hot gas and the slag, together with the high amount of soot, are shock-cooled by water injection in the quench pipe. The slag is vitrified to a non-leachable solid and is routed with the soot water to the solids separation system, designated as the metals ash recovery system (MARS) unit.

The MPG burner affords high feedstock flexibility. No limitations exist regarding the flash point of the feedstock. This is often an issue with highly viscous feedstock, which has to be heated to high temperatures before feeding it to the reactor, to obtain a low viscosity at the burner tip. If the feedstock has a low flash point, vaporisation of the low-boiling constituents of the feedstock starts in the burner nozzle, which disturbs the feedstock flow and results in damage to the burner tip. To avoid this, the MPG burner uses steam to atomise the oil into fine droplets in a proprietary diffuser nozzle system.

Thus, additional hydrocarbon vapours generated by heating the feedstock do not affect the feedstock flow at the burner tip. Due to the atomisation of the feedstock with steam, the burner can handle highly viscous feedstock and also particles in the low mm range. The MPG reactors afford good availability and reliability, thanks to a combination of long on-stream times and the option to restart the reactor without burner inspection. This increases the operational flexibility and reduces the downtime for maintenance work. The burner is equipped with a pressurised cooling water system, which gives an inherent safety to the burner operation. A surveillance system detects and records any deterioration of the front plate and the burner nozzles. This information is analysed and trips a safe shutdown of the reactor, if necessary. The burner creates a low pressure drop across the feed system, which allows for the use of inexpensive feedstock pumps. The burner is equipped with an integrated heat-up burner, so manipulation of burner system in the hot reactor is not necessary during startup.

An overview of MPG’s flexibility in operating modes is shown in Table 1.

Hydrogen production unit
Figure 3 shows a block flow diagram of the hydrogen unit. The main process steps are the conversion of feedstock in quench-type gasification reactors, ash and soot removal from the raw gas, the CO shift unit, gas cooling followed by acid gas removal and hydrogen purification. The hydrogen capacity of the unit is 3.2 MM mn3/d.