HP Steam Research Articles

Retrofit of ammonia plant for improving energy efficiency

The aim of this work is to perform a retrofit study of an ammonia plant, in purpose of improving energy efficiency. As a common practice, one can divide an ammonia plant into two parts: the hot-end and the cold-end. In the hot section, two different options are investigated that both lead to a threshold condition and achieve maximum energy saving. The first option covers only process-to-process energy integration, while the second option considers some modification in the convection section of the primary reformer through a new arrangement of the heating coils. Thus, a considerable reduction in cooling water, HP steam and fuel gas consumption is achieved. In the cold section, retrofit study is dominated by reducing the amount of shaft work or power consumption in the refrigeration system. Application of the Combined Pinch & Exergy Analysis revealed that part of the shaft work, which was originally being used, was inefficient and could have been avoided in a well-integrated design. Therefore, by proposing optimum refrigeration levels, reasonable saving (15%) in power consumption was observed without the need for new investment.

Spray enhanced demisting (SED)

A new innovative and efficient system has been studied, realized and tested by SWS's engineers for the production of top quality distillate TDS<0.1 ppm by evaporation of seawater in both MSF and MED desalination plants. This system is based on the combined effect of scrubbing the vapor outflowing from the demisters (with a spray of accelerated droplets of distillate) and of the mechanical coalescence in a further layer of demisters. The key factors of the main scrubbing process are the highest rate of hitting droplets velocity, the dimensions of the droplets and the number of droplets to be large enough to balance the statistically low impact efficiency. The arrangement of the system is very simple and has been protected by a patent since June 2001, registered by SWS. The benefits provided by this innovation are quite remarkable bothfor the dramatical reduction of the investment cost and for the high reliability of the plants whenever the fresh water product is requested for industrial services, such as make up to HP steam generators, and process water for the chemical industry being the suitable quality ensured without any further treatment of the distillate.

Exergy analysis proves viability of process modifications

Stork Comprimo has developed a commercially available exergy analysis program. With this program several exergy analyses were carried out. Most recently an exergy analysis of a reaction and distillation section within a refinery has been performed. In the reaction section endothermic reactions take place after which the product stream is cooled in a heat exchanger network whereafter the reaction products are separated in a distillation section. From the exergy analysis it can easily be notified that the main part of the losses occurs in the furnaces and distillation columns. A closer look at one of the distillation columns with the largest exergy losses shows that the main part of the losses occurs in the reboiler heated with a furnace. To reduce the exergy losses in this distillation column, Stork Comprimo proposed several process modifications. These modifications are: (1) decreasing operating pressure (⇒lower operating temperature), (2) HP steam reboiling instead of a furnace, (3) splitting feed streams, and (4) recompressing overhead. With these process modifications total exergy losses may be reduced by 70% that directly results in a primary fuel reduction of almost 40% for this column! Splitting of the feed streams has been implemented now and results in an energy saving of 10% and a more stable operation of the column.