Boosting energy efficiency is important to help manage and reduce energy use, and cut maintenance costs. As one of the five most-energy-intensive industrial sectors, the chemical industry has much to gain by improving energy efficiency. No other sector produces such a wide spectrum of end products with sensitive operating costs. Because it's not possible to cover the many types of chemical plants in one column, here we'll focus on examples of energy efficiency improvements found in petrochemical facilities.
Process heating, distillation, evaporation, absorption and cooling are typical processing operations in the petrochemical industry. Energy efficiency improvements in this sector begin with the following state-of-the-art applications: good housekeeping, process management, optimized steam network, process integration, heat cascading, mechanical vapor recompression, heat exchangers, adjustable speed drives, high-temperature heat recovery, and low-temperature heat recovery. Other, more advanced applications include selective steam cracking membranes, catalyst upgrades, new reactors, and better water treatment techniques such as reverse osmosis and demineralization.
In a petrochemical complex near the Louisiana-Texas border, we identified a flash-steam recovery opportunity. In one building, condensate from the process reactors and other heat exchangers was routed to the common condensate receiver in that plant. Flash steam was separated and vented at this receiver to eliminate bi-phase flow. Condensate was pumped to the utility department. Because venting from this receiver was significantly high, condensate sources were further investigated and we discovered one of the reactor exchangers was heated with 350-psig steam.
The condensate from this reactor and its associated supply line steam traps were collected separately but routed to this same condensate receiver and were causing excessive flash-steam venting. We recommended installing a suitably sized high-pressure flash tank with a level control valve upstream to the existing condensate receiver to save heat and condensate. The flash steam from the new tank was routed to the steam supply line to the feed preheating exchanger that was supplied continuously with low pressure steam.
Though the existing condensate return system matches standard design practices (such as collecting different pressure level condensate in segregated lines), and the receiver for flash separation and pumping was good, adding a high-pressure flash tank enhanced design practices further. Process engineers can always look for opportunities to improve their standard design practices by considering energy efficiency.
In this same plant, the condensate recovery was significantly lower than its potential. This is a common problem observed in most processing plants. Condensate drains were identified in all five buildings and the two tank farms. In the process plants, simple piping additions, and in the tank farms, additional condensate receivers and pumps, helped improve the condensate return by an additional 120 gpm and raised the overall condensate recovery at the plant by 9%.
The percent increase of condensate recovery could have been higher if the plant's payback criteria was little more than the cut of level. Process engineers should always be aware of their plant's condensate return potential and its actual return level. Once aware of the potential, they should investigate why condensate isn't returned and find economic ways to boost the return levels.
In one of the large reactors at this plant, the process charge was heated in a tank and then transferred to the reactor for further heating to the required temperature to initiate a reaction. Because the reaction was exothermic, the reactor charge was cooled to maintain the reaction rate. Hence, cooling water from the tower circulated to the reactor for the next 2½ hours to maintain the reactor temperature. Because more heat is rejected from the process to the cooling tower in each batch, and the batch process had a dedicated charge preheating vessel, potential existed for possible heat integration between batches. As a result, we recommended modifying the cooling circuit's piping connections to enable preheating the process charge in the charge tank during the exothermic reaction cooling phase. The piping modifications also enabled retaining the cooling water exchanger in case of excessive heat removal from the reaction, so that the reaction process is not disturbed.
VEN V. VENKATESAN is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@putman.net