(674b) Heat and Power Optimization in Ammonia Plant

Retrofit study of process industries, for improving energy efficiency, requires methods and tools to understand interactions between the core process and utility system. In this paper, Pinch and Exergy Analyses, as a well-established conceptual method, has been applied to an existing ammonia plant, in order to reduce energy consumption of the process and consequently improve its efficiency.

Ammonia is one of the largest bulky chemicals in the world, and it is essential to meet the food production capacity. However, due to having high energy intensity (requiring 35-50 GJ energy to produce 1 ton of ammonia, depending on the raw material and the process) different measures should be taken to reduce the level of energy consumption.

As a common practice, one can divide ammonia plant into two parts, the hot-end and the cold-end. The hot-end consists of all plant sections that involve in preparing the gases needed for the ammonia reaction such as: Desulphurization, Primary Reforming, Secondary Reforming, Shift Conversion, Co2 Removal and Methanation. The cold-end section is mainly the Ammonia Synthesis and Refrigeration Cycles.

In this paper, hot section of an existing ammonia plant has been studied in details, using Pinch Analysis, and the cold section has been rechecked by the application of Combined Pinch & Exergy Analysis.

In hot section, two different options are investigated that both lead to a threshold condition and achieve maximum energy saving. The first one 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 cold section, retrofit study is dominated by reducing the amount of shaft work or power consumption in the refrigeration system. Exergy consumption of ammonia plant depends strongly on the design of Ammonia Synthesis Loop. Due to the thermodynamically limited degree of conversion of hydrogen-nitrogen mixture to ammonia, industrial practice for ammonia synthesis is to recycle significant quantity of reactants back to the reactor after the removal of ammonia at low temperature. This is called "Ammonia Synthesis Loop". 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 optimizing the refrigeration levels, reasonable saving (15%) in power consumption was observed without a need for new investment.