ゼオライト膜を用いたバイオエタノールの濃縮脱水プロセスの経済性評価 Energy and Cost Evaluation for the Concentration and Dehydration Process of Bioethanol by Zeolite Membranes

バイオエタノールの脱水にはPSA法が用いられ省エネルギー化が図られているが，PSA法はプロセス上の制約があるためこれ以上の低コスト化が難しい．一方，ゼオライト素材の膜分離法は一層の省エネルギー化の可能性があり，弱点であった耐水性・耐酸性素材の開発も進められている．本研究では，バイオエタノールの蒸留・膜脱水プロセスについてシミュレーション計算を行い，膜分離法が経済的に優位となる膜特性及び濃縮プロセスについて検討した．<br>透過速度の向上は，真空ポンプに負荷をかけない低エネルギーモード運転を可能にするため消費エネルギーの低減に有効であるが，蒸留塔の消費エネルギーが大きいためその低減比率は微小である．一方，透過速度の向上は膜モジュール本数を削減できるため設備コストの低減が可能になる．分離係数の向上は消費エネルギー，コストにほとんど影響しないが，分離係数の低下によるエタノールの損失を考慮すると10,000程度以上は必要と思われる．蒸留の負荷を下げるため蒸留塔出口濃度を下げると，消費エネルギーを大きく低減できコストも下がる．さらに，蒸留法とPSA法を<i>Q</i>=36 kg·m⁻²·h⁻¹, α=10,000程度の膜で置き換えればおよそ6 JPY·kg-EtOH⁻¹のコスト削減が可能となる．また，含水エタノール製造においても同様におよそ6 JPY·kg-EtOH⁻¹のコスト削減が可能となる．これらの検討結果から，膜分離法をPSA法のみでなく蒸留法に置き換えることにより経済性に優れたプロセスを構築できることがわかった．

The pressure swing adsorption method (PSA) is one of the energy-saving technologies and adopted for dehydration of bioethanol, however further progress in cost-saving is unlikely because the PSA method has some restrictions in its process. On the other hand, the zeolite membranes have the potentiality to improve energy consumption and the acid-resistant zeolite has been developing to overcome the weak property of materials.<br>In the present study, the required membrane performances and process applications were studied through the process simulations for the distillation and dehydration of bioethanol to achieve the economical advantages over the conventional processes.<br>Increase of the water flux is effective in reducing energy because it enables to operate in the low-energy mode without a vacuum pump. However, the reduction ratio is relatively a little because the distillation energy is majority in total energy. The equipment cost can be reduced due to the decrease of membrane modules. Fluctuations of separation factor have little effect on the energy consumption and cost, but the separation factor of about 10,000 is thought to be necessary for controlling ethanol leakage through the membrane. Reducing the ethanol concentration at the outlet of the distillation tower in order to lower the load of distillation can decrease the energy consumption significantly and also decrease the cost. Additionally, the cost reduction of 6 JPY kg-EtOH⁻¹ can be achieved if the membrane which has flux of 36 kg·m⁻²·h⁻¹ and separation factor of 10,000 is applied for the concentrating process substituted for distillation. The comparable cost reduction of 6 JPY kg-EtOH⁻¹ is realized for the hydrous ethanol production in the similar process of replacing distillation to membrane separation. These calculated cost reductions are enough for stimulating the progress of membrane separation for bioethanol enrichment.