The now closed Digital Equipment Corporation in Hudson, MA had several baths to develop trays of semiconductors following application of photoresists. They had a total of 12 baths, each with dimensions 0.51 m wide x 0.76 m deep x 0.76 tall. Each bath was filled with 240 kg of a solution containing 25% (mass) of tetramethyl ammonium hydroxide (TMAH) in water. You may assume that the baths have controls on them to maintain the liquid level and the TMAH concentration. The 12 baths are enclosed and ventilated with an air stream of 9.44 m/s at that provides a continuous air velocity immediately above the baths of 0.2 m/s. This exhaust is comingled with the total facility exhaust, which is 23.6 m/s. This combined exhaust exits the building through a 2.1 m diameter stack whose top is 8.5 m above ground level, resulting in a stack exit velocity of 6.81 m/s. The baths and all air flows and exhausts are at 25C. The process operates continuously. The configuration of the process is shown in Figure 1. 1. You would like to estimate the air emissions from these baths, including estimating what the absolute maximum value could be. a. If the exhaust from the bath enclosure were fully saturated in TMAH vapor, what would be the concentration of TMAH in the exhaust (in g TMAH/m?)? b. If the total exhaust (9.44 m/s) were at this concentration, what would be the emission rate (g TMAH/s)? C. The emission rate can be estimated assuming it is controlled by convection off a flat plate. Based on the velocity of the exhaust, the properties of the air, and the dimensions of the system show in Figure 1, what is the value of the Re and Sc numbers for the system? d. Is the system in the Turbulent or Laminar Regime? Which correlation for Sh is appropriate e. Using the appropriate correlation, what is the Sh and mass transfer coefficient for the system. f. Assuming that the TMAH concentration at the surface of the bath is at the vapor pressure and that the concentration of TMAH far above the bath is negligible, what is the emission rate based on this convection calculation. 8. How does this value compare with the value you calculated in (b)? Which do you think is more accurate and why? Some potentially useful properties of TMAH and the 25% solution include: - Vapor pressure of TMAH above the 25% solution at 25C = 0.023 bar Molar Mass of TMAH = 91.15 g/mol Density of 25% TMAH solution = 1014 kg/m3 Allowable Ambient Limit (AAL) of TMAH (short term) = 746 ug/m3 Density of air = 1.17 kg/m Viscosity of air = 1.85E-05 kg/m-s Diffusivity of TMAH in air = 2.6E-05 m/s Each bath: 0.51 m wide by 0.76 m deep. Total length of the 12 baths in the process: 6.12 m Total wetted area: 4.65 m Exhaust flow over the baths to the header duct on the right (0.2 m/s velocity, 9.44 m/s total flow). Figure 1: Process Configuration for HW4
