Oct-2014

Integrated hydrogen management

Redesigning an existing hydrogen system leads to an integrated, reliable and flexible supply

SAÅ A POLOVINA, DANIJELA HARMINA and ANA GRANIC Å ARAC
INA Rijeka refinery

Article Summary

Hydrogen has become one of the most important refining energy media and its efficient use is of the highest priority. Therefore, refineries are forced to exploit their existing hydrogen sources to the maximum and continually increase their hydrogen sources.

This article describes the integration of two hydrogen systems at INA Rijeka refinery: one system linked to a catalytic reforming unit as a source of hydrogen and the other linked to a hydrogen generation unit based on steam reforming. The integration of these two hydrogen systems involves three different purities of hydrogen-rich gas. Hydrogen-rich gas produced by catalytic reforming is used in the refinery processes naphtha hydrotreating (NHT), isomerisation, kerosene hydrotreating (KHT1) and gasoil hydrotreating (GHT2). The hydrocracking unit’s requirement for make-up gas of high purity in large amounts relies on the hydrogen generation unit. The two sources of hydrogen and two systems of purification for hydrogen-rich gases give rise to three purity levels for hydrogen-rich gas:
•    Gas from the catalytic reforming unit of 73-75 vol% purity, depending on the catalyst cycle stage (start-of-run or end-of-run)
•    Gas after absorption with a hydrogen content of 83-85 vol% 
•    Gas after purification in the pressure swing absorption (PSA) unit, with a hydrogen content of 99.99 vol%.

The current situation
Make-up hydrogen for the hydro- cracking unit comes from the hydrogen generation unit which uses natural gas as feed. Make-up hydrogen for isomerisation, NHT, KHT1 and GHT2 comes from catalytic reforming.

In order to integrate these two hydrogen systems, and to achieve better utilisation of the produced hydrogen, the systems are connected via two pipelines with two manual valves. One pipeline supplies hydrogen produced in catalytic reforming (after obligatory purification in the PSA unit) as make-up for the hydrocracking plant; the other pipeline supplies hydrogen from the hydrogen generation unit as make-up for the isomerisation, NHT, KHT1 and GHT2 plants.

Shut-off for both pipelines relies on ordinary manual valves. Problems in the operation of one plant can lead to shutdowns of other plants connected to this integrated hydrogen system.

The hydrogen system in the catalytic reforming plant starts with the high pressure (HP) separator where hydrogen-rich gas is separated from unstabilised gasoline. The amount of hydrogen-rich gas produced depends on feed composition and process conditions in the reactor section. The purity of the produced gas depends on the pressure and temperature at the HP separator and on ambient conditions, especially temperature.

After physical separation, hydrogen-rich gas enters the suction vessel of the booster compressors where it achieves the required pressure and is distributed to the consumers.

A PSA unit is located between the HP separator and the booster compressors. This is used for purifying the reformer’s hydrogen-rich gas. Pressures at the HP separator, at the inlet of the PSA unit, and at the suction line of the booster compressors are regulated with the same pressure controller.

In addition to pressure control of these positions, the control valve has a safety role. In case of problems in the catalytic reforming process, fast depressurisation of the HP section in the refinery fuel gas system would be crucial.

This type of pressure control has significant disadvantages. Fine-level flow control of hydrogen-rich gas discharges to the fuel system cannot be implemented. At 1% valve opening, the flow rate reaches around 2000 Nm3/h. If the process is operated in a mode in which the regulator valve is opened or closed, large amounts of hydrogen are discharged to the refinery fuel system.

Booster compressors are the main part of the hydrogen system since they maintain the pressure required for normal operation of hydrogen consumers and high pressure absorption. Hydrogen at two different levels of purity is fed to the suction side of the compressors. If the PSA unit is operating, hydrogen purity is 99.99%; if not, hydrogen-rich gas is fed from the reforming HP separator.

After cooling and passing through the knockout drum, one part of the hydrogen-rich gas is distributed to the isomerisation and NHT plants and the rest goes through the absorption column. After purification, hydrogen-rich gas goes to the KHT and GHT units at a purity of 83-85%.

Make-up gas for the hydrocracking unit must be at 99.99% purity, which means that it can only come from the PSA unit. If the PSA unit of the reforming plant is not in service, excess hydrogen-rich gas cannot be sent to the hydrocracker due to insufficient purity. Excess produced gas is discharged to the fuel system.

If the PSA unit is operating and the produced hydrogen is at 99.99% purity, this is distributed through booster compressors to the consumers. Excess hydrogen from the PSA, together with hydrogen from the hydrogen generating unit, becomes make-up gas for the hydrocracker. The economic impact of this solution can be measured in the reduction of demand for expensive natural gas feedstock in the hydrogen generating plant.

Since two separate hydrogen systems are connected by a manual valve, this solution carries significant risks to the normal operation of both hydrogen systems. The risk is much higher for operation of the hydrocracker; in the event of a PSA shutdown, the connection between these hydrogen systems must be closed immediately, so the hydrocracker is without substantial amounts of make-up hydrogen. In order to compensate for the reduced volume of make-up gas, the hydrogen generating unit must significantly raise its capacity. Thus, sudden loss of the required amount of make-up hydrogen may lead to shutdown of the hydrocracker.

If operational problems in the hydrocracker lead to a shutdown of the PSA unit, problems in the old hydrogen system are less challenging. At the shutdown of the PSA unit a bypass is automatically opened to ensure the necessary quantity of gas for compressor operation. In this way, continuous supply of make-up hydrogen is ensured to all consumers connected to the pressure side of the compressor. By increasing the flow to the absorption column, a higher level of make-up hydrogen purity can be ensured. To ensure continuous hydrogen supply to all consumers, integration of two separated hydrogen systems is vital. The existing scheme of the hydrogen system in Rijeka refinery is shown in Figure 1.