Anton Babik
Chemko s.p.
Strazske, Slovakia
Josef Kurt
Uhde GmbH
Dortmund, Germany
Chemko s.p. has implemented Uhde GmbH's new combined autothermal reforming (CAR) process into an existing hydrogen plant at its refinery in Strazske, Slovakia.
The new technology uses a combination of steam reforming and partial oxidation processes to produce synthesis gas or hydrogen for use in refinery or petrochemical processes.
CAR PROCESS
Steam reforming technology has been used to generate synthesis gas for many years. Although this technology has continually improved in recent years, no revolutionary changes have taken place.
The CAR process, however, represents an innovation in this sector. The process combines the two known reformer principles:
- Steam reforming (convective type)
- Partial oxidation.
The two processes are integrated into a new concept that utilizes a single pressure vessel.
The 19-tube CAR reactor has been in operation at Chemko's Strazske refinery since 1991.
Fig. 1 shows the basic flow scheme of the process. A mixture of steam and part of the desulfurized primary feed (natural gas) is reformed first in the primary reforming section by catalytic reaction. This endothermic reaction takes place in the externally heated reformer tubes.
The mixture of hydrogen, carbon monoxide, carbon dioxide, residual methane, and steam discharged from the primary reforming section is subjected to further conversion in the partial oxidation zone by adding an oxidant and the remaining portion of the gas (secondary feed).
The adiabatic temperature in the partial oxidation zone is about 1,300 C. This temperature and, consequently, the residual methane content in the product gas, are a function of the quantity of oxidant added and thus can be controlled.
The sensible heat of the product gas is used for indirect heating of the primary reforming section of the CAR reactor. The product gas composition is determined by the composition of the feed gas, the overall steam-to-carbon ratio, which is related to the carbon content in the hydrocarbons, and the process pressure.
Firing of fuel to provide the absorption duty, which is customary in conventional reformers, is not required in this process. Consequently, there are no flue gas emissions in the gas generation unit.
PROCESS DEVELOPMENT
Uhde began developing the combined autothermal reformer in 198283. One of the crucial points during the development was the design of the partial oxidation chamber.
Optimum mixing of all streams involved in the secondary reaction (in the partial oxidation zone) is necessary for successful process design. Uhde developed models to describe the hydrodynamics and the mixing of streams in the CAR reactor.
The models were verified using the cold-flow model unit.
The next step was the construction and operation of a single-tube CAR pilot unit at UK-Wesseling, Germany. This pilot unit in 1987-88 provided important information for the design of a multitube reactor.
All major steps were thoroughly tested, including:
- Commissioning of the unit, equipped with the newly developed, water-cooled start-up burner
- Reduction of catalyst at low pressure
- Partial load operation
- Hot restart
- Unit shut-down.
The water-cooled nozzles, which had been developed recently, are used to add oxidant (pure oxygen) and secondary feed to the partial oxidation chamber. The nozzles operated safely and reliably. The design efficiency of the heat exchanger was verified.1
Further tests were conducted using the cold-flow model, the aim of which was optimizing the partial oxidation chamber and the arrangement and dimensions of the nozzles.
All the information gained from the operation of the pilot unit, and from the cold-flow model tests, provided the basis for the design criteria and the scale-up philosophy. A computer program was developed to serve as an engineering tool.
REACTOR
The 19-tube CAR demonstration reactor has been operated commercially by Chemko s.p. for more than 2 years. The reactor is a combination of a convective reformer and a partial oxidation unit, integrated in one vessel (Fig 2).
The convective reformer section consists of:
- 19 reformer tubes filled with catalyst
- 19 enveloping tubes
- 1 sandwich tubesheet
- 2 single tubesheets.
The partial oxidation section comprises:
- The partial oxidation chamber
- 6 water-cooled oxygen nozzles
- 3 water-cooled secondary feed nozzles
- 19 reformer-tube discharger nozzles.
The pressure vessel of the CAR reactor is lined on the inside with ceramic insulating material and enveloped by a water jacket. The water-cooled start-up burner is installed in the lower part of the reactor.
CONVECTIVE PERFORMER
The mixture of natural gas and steam is distributed via the sandwich type tubesheets to the 19 reformer tubes rifled with catalyst. The reformer tubes are heated from the outside by the reformed gas leaving the partial oxidation chamber.
The upper portion of each reformer tube is enclosed in an enveloping tube to improve heat transfer. The reformed gas discharged through the annular gap above the sandwich tubesheet is collected in a chamber before leaving the CAR reactor.
The enveloping tubes are positioned by the sandwich tubesheet and the two single tubesheets, and are kept a certain distance from each other. The reformer tubes are positioned by spacers installed in the enveloping tubes.
The increased outer diameter of the reformer tubes at their hot ends, and the honeycomb-type bricklining in this area (leaving a very small clearance) assume the function of a further tubesheet.
This setup ensures that horizontal movement of the freely suspended reformer tube ends is eliminated, to a great extent, without impeding any longitudinal expansion of the reformer tubes.
The tube bundle is supported by a sandwich tubesheet placed on brackets.
Fig. 3 shows the installation of a complete tube bundle which has been assembled in the workshop in horizontal position. This method of assembly can also be used for larger units.
PARTIAL OXIDATION
Oxygen and secondary feed are admitted to the partial oxidation chamber by nozzles installed at a defined angle, thus causing a vortex flow in this chamber.
The reformed gas leaving the reformer-tube outlet nozzles is drawn into the partial oxidation zone by the lower pressure (caused by the vortex) in the fluidized zone.
The reformed gas reacts with the oxygen and the secondary feed. Following the reaction, the reformed gas flows along the wall into the tube bundle (Fig, 4).
The design of the partial oxidation chamber, the dimensioning of all nozzles, and the arrangement of the nozzles was performed with the aid of the computer-design program.
A water-cooled burner is used to heat the reactor during start-up. This burner remains in the reactor while in operation. The heating process is controlled by a flame monitor installed in the burner.
DEMONSTRATION UNIT
In the summer of 1989, Chemko and Uhde signed a contract for the design, supply, installation, and operation of a CAR demonstration unit. The unit was to replace one of the two partial oxidation units in operation at the refinery.
The location was particularly suitable because all specification-grade feedstocks and utilities - such as desulfurized hot natural gas, pressurized oxygen, and steam - were available. This reduced the cost of peripheral installations.
Trial operation began after only 15 months. Following successful performance test runs, the CAR unit was fully integrated into the hydrogen plant in mid-1991 and has been supplying an the hydrogen required for the cyclohexanone plant since then. 2
The performance test run was successfully completed in August 1991 (Table 1). the integration of the CAR unit into the existing hydrogen plant is illustrated in Fig. 5.
It took no more than 14-15 hr after commissioning the start-up burner to reach full hydrogen production.
The replacement of the partial oxidation unit by the CAR unit has reduced consumption of oxygen by about 35% and natural gas by about 15%.
So far, the demonstration reactor has accumulated an on-stream time of more than 17,000 hr. The operation of the unit has, however, been interrupted numerous times without any prewarning. These interruptions were caused by failure of the facilities outside the battery limits of CAR.
CAR 19 was restarted repeatedly and brought back to full production without any problems. This proves the reliability of CAR as well as its effectiveness at preventing the equipment and catalyst from being damaged.
The CAR reactor can be restarted after an interruption of up to 8 hr without requiring the start-up burner to be operated (hot restart). About 3-4 hr are required until hydrogen is again supplied to the cyclohexanone plant after a hot restart of the CAR reactor following an interruption.
Furthermore, the CAR unit is characterized by a high flexibility and can be operated at loads between 30 and 100% without any problems.
REFERENCES
- Marsch, H.D., and Thiagarajan, N., "CAR: A new reformer technology," Ammonia Plant Safety, Vol. 29, 1989.
- Marsch, H.D., and Thiagarajan, N., "CAR - Demonstration unit on stream," Ammonia Plant Safety, Vol. 33, 1993.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
