Integration of a New Process Into an Existing Site: A Case Study in the Application of Pinch Technology

Abstract

New production processes often need to be located in existing sites. Existing sites usually have existing utility systems providing electric power and steam in a limited number of existing mains. A site will have an overall power demand, be nearly balanced, or have a power surplus. Steam consumption, steam generation, power consumption, and power generation of a new process need to be economically assessed against the existing situation on site. Site average and site marginal costs help in this assessment. Changes in overall site production pattern, throughput, energy tariffs, etc., complicate the issue. This paper describes a specific situation. A new Nitric Acid plant is to be located in an existing site. Nitric Acid processes are net energy exporters and contractors tend to offer three basic options: steam export, power export, or a mixed option. The paper discusses how integration of each option affects the balance of the overall site utility system. Boiler feedwater needs to be supplied to the Nitric Acid plant, condensate is returned, the site fuel consumption changes, etc. The true marginal operating cost savings (Nitric Acid processes are energy exporters) are established for each option. The paper then proceeds to consider alternatives. First, it considers whether improvements can be made in the Nitric Acid process and in the existing site utility system separately. Objective targets are set using Pinch Technology and energy cost benefits are improved by 60 percent. Second, the question is posed whether a more fundamental approach would be appropriate. Rather than accepting optimized but separate designs for the Nitric Acid process and the site, could we integrate the Nitric Acid process and the site by allowing individual heat sources and heat sinks in the process to interact with the site utility system individually? This may sound complex and impractical. However, the paper demonstrates how the problem so defined can be tackled using Pinch Technology and how an alternative Nitric Acid process design emerges that is optimized specifically for the site. The design is practical and the energy cost benefits are approximately doubled. The paper is written with two objectives in mind. First, it is hoped to demonstrate the benefits of “site-specific optimization.” Second, it is hoped to demonstrate the benefits of Pinch Technology. The problem is totally defined (all data are given) before a solution is attempted. Realistic and clear assumptions are made so that the overall objective is clearly defined in terms of cost. While such black-and-white conditions are not usually given in practice, the benefit is that the reader is free to attempt his or her own solution using either technique or method and compare costs. It is hoped that, in particular, this will stimulate the use of the technique of Exergy Analysis and lead to a constructive dialogue in the literature concerning the relative merits of the techniques of Exergy Analysis and Pinch Technology.