DME production and utilization data in Trinidad n Tobago
Description
The main objectives were to determine if a 15% allotment of M5 methanol plant’s annual production capacity would be sufficient to produce the total amount of DME required to achieve a 20 wt% blend of all major (household and transportation) fuels, namely LPG, diesel and gasoline in Trinidad annually, to determine an appropriate catalyst for DME production, to design and cost a DME reactor and to assess LPG-DME blends for use as a household fuel. Since there are no DME production plants currently in Trinidad, it was suggested that a DME unit be incorporated into the processing scheme of an existing methanol plant which would utilize a pure methanol feed to produce DME via methanol dehydration. Firstly, the total DME required for blending was calculated to be 149,600 MT/y using the annual consumption for each major fuel. Using 15% of M5’s annual production capacity, which corresponded to 300,000 MT/y of methanol, 194,045 MT/y of DME was able to be produced which met and surpassed the required 149,600 MT/y. A γ-Al2O3 catalyst which had a DME selectivity and methanol conversion of 99.97 and 90.07% respectively was utilized. Two catalysts were considered for use, namely HZSM-5 and γ-Al2O3, however γ-Al2O3 was ultimately chosen as it had less associated drawbacks, was more economically feasible and more optimized for the operating conditions of a gas phase reaction at 573K and 17bar. A multitubular fixed bed reactor which utilized boiling water cooling to maintain an isothermal operation was chosen as it was most suited to the process conditions, catalyst chosen and allowed for acute temperature control. The volume, length and diameter of the reactor were found to be 10.6m3, 7.32m and 1.916m respectively. The reactor contained 3254 tubes and required 6834 kg of catalyst. The reactor bare module, operating and catalyst costs were found to be USD $233,172, $33,772 and $106,615 respectively. The reactor duty was 11.168 GJ/h necessitating a cooling water flow rate of 7951.37 kg/h. The annual production cost of DME was estimated to be $58,213,500. A 20% DME-LPG blend was deemed most optimum for household fuel usage as it offers the most reduction in harmful CO and NOx emissions while sacrificing minimal calorific value and thermal efficiency. Additionally, a 20% blend is the maximum allowable concentration that does not require modifications to existing hardware or infrastructure thereby minimizing cost to the end user. One recommendation for future studies would be to utilize a pilot scale reactor to experimentally determine the rate constant as well as confirm the product composition flow rates since real world kinetics can differ from theoretical calculations. Different blend data are presented here.