(100c) Development of an Advanced Control Philosophy for Dry Gas Combined Steam Reforming, to Operate the Plant Efficiently during Feed Composition Changes

Steam reforming of natural gas is one of the very important processes in the world now, as hydrogen, a key product of the reforming, continues to be in high demand for various petrochemical and chemical industries. As the gas reserves are depleting in the world, existing reforming plants are open to use natural gas from different sources. Different natural gas sources offer different compositions in the gas. This wide range of feed natural gas composition has significant effect on the performance of the steam reformers. In order to operate the reforming process at optimum conditions, precise control of reformer temperature, and steam intake are very crucial. Disturbances in the feed gas create instability in the reforming process. Furthermore, some steam reformers are operated with carbon dioxide added to the reformer feed, to have a lower hydrogen to carbon mole ratio, as required for downstream methanol or plasticizer alcohols processes. These dry gas combined steam reformers are operated at higher temperatures and lower steam to carbon ratios than that of pure steam reformers. Hence, frequent feed composition disturbances imply greater in-stability in the dry gas combined steam reformers in terms of reformer performance, reformer temperature, methane slip, fuel gas consumption and excess air in reformer box.

The existing control philosophy of the reformers cannot cope with the feed disturbances and can be effected by big swings in the reformer temperature, methane slip, fuel gas and steam & carbon dioxide flows in case of dry gas combined steam reformer. These disturbances may also have adverse effects on the plant efficiency, capacity, safety and reliability, and the plant integrity. In order to have a stable operation and cope with these disturbances, most of the existing plants are opting for advanced process control possibilities like Aspen DMC+ etc

In the paper, a new feed forward control strategy is described that can control the dry gas combined steam reforming process efficiently and effectively for a large composition variations. The basic methodology of the new control strategy is to control the steam reforming key parameters like reformer temperature, stream flow, process air, CO2 flow, and Fuel gas flow, based on the carbon count of the natural gas in a feed forward way. Additionally, a small correction to the individual manipulators is done in a feedback control philosophy. This feed-forward & feedback control philosophy enables the operating plant to react to the disturbances quickly and efficiently. Hence, the plant performance disorder is kept minimum and huge swings in the parameters are avoided during the feed disturbances. Also, with this new control strategy, the variations in the plant parameters are reduced and stable and steady production of the hydrogen can save the energy about 0.5 GJ/ton of H2 produced from the reforming process. More than the energy, the process operates more steadily and downstream processes that depend on the reforming process operate steadily as well. This developed strategy is tested and implemented and has improved the plant reliability and integrity.