Two bulk conveyors prove better than one
An energy assessment at Dow Chemical’s St. Charles Operations in Hahnville, La., has led to a 272,000-million-Btu/yr reduction in natural gas use, providing $1.9 million in annual savings; costs to improve operations were paid back in about six weeks. The U.S. Department of Energy (DOE) has named the petrochemical complex a 2007 Energy Champion Plant, an honor awarded only to the top energy savers in the country.
The 2,000-acre St. Charles facility, which has been in operation since 1966, produces more than 40 different products that go into a variety of consumer goods, and relies heavily on steam — for electricity generation and process duties. Dow has an active energy-management program which includes the corporate 2015 sustainability goal of reducing energy intensity by 25% from 2005 baseline, therefore we took advantage of a “Save Energy Now” assessment sponsored by DOE’s Industrial Technologies Program to gain insights on potential energy saving opportunities. A specialist qualified on DOE’s Steam System Assessment Tool (SSAT), Riyaz Papar of Hudson Technologies came to the site and worked with an empowered team comprised of plant operators and engineers. Once SSAT was installed on their computers, the team members modeled the facility and investigated a variety of what-if scenarios for energy savings.
This led to the identification of both near- and medium-term opportunities, based on payback periods. Implementing all of these could result in a more than $5 million annual savings.
With the help of SSAT, the team pinpointed four efforts to undertake first:
- Implementing a steam-trap repair project. An audit performed before the assessment had identified all failed steam traps. SSAT enabled us to quantify the value of implementing a repair program — annual savings exceeding 112,000 million Btu of natural gas and $880,000 in costs.
- Upgrading the steam-leak management program. Initial estimates from the assessment proved overly optimistic due to inaccurate measurements taken during data gathering. However, upon verifying the measurement system and performing quantitative analysis, the revised figures still pointed to savings greater than steam trap repair alone.
Improving insulation. Inspection revealed that several areas of the steam distribution network lacked sufficient insulation per Dow standards. Using 3EPlus, DOE’s insulation calculation program, the team estimated insulation losses to be about 1%, Reducing such losses to 0.1% promised savings in gas of more than 3,000 million Btu and $25,000 in costs.
Figure 1. Adding insulation, stopping leaks and repairing steam traps played key roles.
- Boosting condensate recovery. At the time of the assessment, Dow was recovering about half of the low pressure condensate. The SSAT indicated that a site-wide recovery rate of 75% was possible. This would cut gas use by nearly 88,000 million Btu and costs by almost $650,000.
The team also identified a number of other projects that offered somewhat longer payback but were definitely worth considering:
- Adding a blowdown heat-recovery exchanger. Blowdown was going to a flash tank to recover low pressure steam but lack of heat exchangers in the system meant that lots of thermal energy was still being lost. Putting in an exchanger upstream of the tank would enable capture of significant heat that could be used to preheat boiler makeup water. This would save about 31,000 million Btu of gas and $200,000.
- Preheating reactor feed with 75-psig steam. Replacing some of the 600-psig steam being used wouldn’t save natural gas but would allow more on-site electricity generation from the higher-pressure steam. This would reduce electricity purchases by almost 1,280 M Wh and nearly $80,000.
- Installing a back-pressure turbine drive. The site generates 600-psig steam but most applications only require 200-psig steam. Putting in a back-pressure turbine drive could generate electricity to power some critical equipment and save around 1,950 M Wh and more than $120,000.
The estimates the team developed using SSAT came very close to the benefits Dow actually achieved for the near-term projects. The steam-trap repair project provided annual energy savings of 109,000 million Btu and about $800,000 in costs. Efforts to combat leaks led to annual savings of 163,000 million Btu and more than $1.1 million. Implementation costs for both programs totaled about $225,000, so payback was achieved in little more than six weeks! Steam trap maintenance and leak management are now ongoing programs.
In the short-term, insulation inspection and repair is still being conducted with a minimum mandatory visual inspection of all plant distribution lines every three years. In addition, a condensate project has been planned and is part of the new capital spending plan for implementation in 2009.
Medium-term and additional long-term Dow driven energy efficiency and conservation projects have been captured in an opportunity tracking system and will be made part of the plant specific technology plan. These opportunities will be further developed and could be part of future capital spending plans.
Jason W. Gathright is senior improvement specialist, energy systems, and site energy intensity leader for St. Charles Operations, Hahnville, La., of Union Carbide Corp., a subsidiary of The Dow Chemical Co. E-mail him at JWGathright@dow.com.