Apr-2007

Meeting changes in plant production targets

Overview of a syngas plant automatic load controller shows how reliability is maintained. The functionality built into the plant automation concept allows 
the plant to remain in operation, even in the event of a unit trip

Dieter Krenz
Linde AG

Article Summary

Medium- to large-scale chemical/petrochemical plants will probably always need operators, but plant automation can help with routine work such as plant load changes and make for a smooth and reliable plant operation. This article gives an overview of the principal functionality of a plant automation concept. Celanese’s synthesis gas plant in Oberhausen, Germany, designed, erected and commissioned by Linde LE, is used as an example.

Synthesis gas — raw material for the chemical industries
Synthesis gas is a mixture of carbon monoxide (CO) and hydrogen (H2). 
It is used as a raw material for 
numerous inorganic and organic substances such as ammonia, methanol and OXO alcohols. Besides steam reforming, partial oxidation (POX) is used for the production of synthesis gas.

A POX system reacts hydrocarbon feed with oxygen at a high temperature to produce a mixture of H2 and CO. Since the high temperature takes the place of a catalyst, POX is not limited to the lighter, clean feedstocks required for steam reforming. In general, H2 processing in a POX system depends on how much of the gas is to be recovered as H2, and how much is to be used as fuel. Where H2 production is a small part of the total gas stream, a membrane unit can be used to withdraw a H2-rich stream. This is then purified in a pressure swing adsorption (PSA) unit. Where maximum H2 is required, the entire gas stream may be shifted to convert CO to H2, and a PSA unit is used on the total stream.
POX feedstocks are hydrocarbons (eg, natural gas). Celanese’s synthesis gas plant is based on a POX process and equipped with Linde’s proprietary Automatic Load Control (ALC) plant automation system.

Processing natural gas to OXO or synthesis gas
Natural gas with a high content of methane is compressed, desulphurised, heated in a fired heater and mixed with a pure oxygen stream. The mixture of natural gas and oxygen converts in a POX reactor to synthesis gas. Some soot is also produced during startup, which is removed in a soot wash unit. Energy is recovered for the production of high-pressure steam. The synthesis gas from the POX of natural gas shows a H2/CO ratio of approximately 1.8.

In order to obtain the required product rates of OXO gas as well as pure H2, the following gas separation and purification steps have been selected during process design (Figure 1): CO shift; CO2 wash; membrane; and PSA.

Part of the synthesis gas is routed to the shift reactor, where most of the CO is converted with steam to H2 and CO2. The other part of the synthesis gas is cooled and mixed with the shifted gas. In a wash unit, the CO2 is removed to a very low level.

The CO2-free synthesis gas is routed to a membrane unit, where part of the H2 contained in the synthesis gas permeates through the selective membrane. The driving force for this separation is the pressure difference between the synthesis gas and the H2-rich permeate stream.

The non-permeate stream still contains some H2, but is rich in CO compared to the membrane feed. Most of the CH4 in the synthesis gas is also contained in the non-permeate stream. The H2-rich permeate stream contains some CO and CH4, which are removed in the downstream PSA unit. Besides a pure H2 product stream at high pressure, a low-pressure purge gas is obtained from the PSA unit.

The purge gas stream, which contains H2, CO and some CH4, is compressed and mixed with the CO-rich non-permeate stream, resulting in an OXO gas mixture showing the required H2/CO ratio of approximately 1.0.

The separation of the synthesis gas into permeate and non-permeate streams via the membrane is controlled via a pressure valve on the permeate side. The pressure can be adjusted in the whole operating range in order to maintain the desired H2/CO ratio in the OXO gas.

Production has high demands
Changing load is a frequent scenario in the Oberhausen synthesis gas plant. The plant needs an automation system that adjusts all necessary controller setpoints in order to follow the demands of the OXO gas and H2 product consumers in a smooth, reliable and fast manner. The ALC is already successfully working in several synthesis gas plants. In the Oberhausen plant, the basic features have been used to match the special process design.

Only product target values remain as operator inputs
At the plant, the operator enters the target values for the products. ALC calculates the basic operating point of the plant and carries out the transition of all necessary operating variables so that the desired target values are reached (Figure 2). The basic operating point is defined by the process variables, the load to the POX reactors, the split of flow through the CO shift reactor and cooling section, and the split of flow through the membrane and PSA.

All other relevant controller setpoints are derived from the actual process parameter during a load change. The control system distributes the total load to two POX reactors, or it uses only the one reactor that is operated in cascade.

Feed forward signals are used in the process units’ fired heater, POX reactors, soot wash, CO2 wash and membrane. Based on all independent plant variables, the intelligent feed forward calculates the dependent controller settings. The dynamical treatment of setpoints is different for a load increase and a load decrease. For example, the MDEA cycle flow rate is increased before the feed flow rate to the CO2 wash unit raises. When the plant load is decreased, the feed flow to the CO2 wash unit is first decreased and later the MDEA cycle flow rate. All feed forward basic data are stored in polygons, which represent the functional behaviour. For the compensation of those disturbances not measured, the basic data are manipulated by controllers (Figure 3).