Analysis of Alternative Processes for Anhydrous Ethanol Production, an Economic and Environmental Approach

KAHWAJI JANHO, MICHEL; GATICA, JORGE; HERNÁNDEZ, MARÍA ROSA; MELE, FERNANDO DANIEL; COLOMBO, MAURICIO A.

Pittsburgh

Conferencia; AIChE Annual Meeting 2012; 2012

AIChE

The production of sugar is one of the oldest agro-industrial activities. It includes the production of sugar cane, sugar and dairy products: paper, alcohol, energy, etc. In addition to the production of sugar for domestic and international markets, one of the activities that is growing in importance is the production of alcohol from molasses. Energetic integration has also resulted in bringing power generation from bagasse as a secondary activity in modern sugar-cane processing industries.A new chapter for bio-energy products associated with the manufacture of sugar has gained relevance as blending gasoline with ethanol is becoming part of energetic policies in most industrialized countries. Indeed, energy products from renewable sources are receiving increasing attention, and this has generated a renewed stimulus to examine alternative processes for bio-ethanol production. It has become common for sugar mills to have complementary distilleries with production capacities ranging from 50 to 400 liters per day of non-anhydrous alcohol.At sugar plants, steam is generated in boilers using as fuel bagasse-cellulosic fiber waste obtained crushing the sugar cane- and it is used to give useful heat and shaft work to the plant, but it can also be used to generate electricity with export opportunities to the electrical network. The great number of process alternatives outlines a serious decision making problem in order to take advantage of the resources.At steady state conditions the amount of bagasse obtained is more than enough to supply the demands of useful heat and energy. Thus these industries can advantage of the emergence of a new potential market, as they have an increased potential for generating a surplus of electricity for sale to the grid.In this paper, a classic bio-ethanol distillery is adopted as case-study and simulated using AspenPlus® and UniSim ® commercial simulators. Alternative process flow sheets of varying complexity are analyzed aiming to reduce power consumption and effluent generation.The process simulation allows a systematic analysis that leads to a good combination of all the values of process variables. In particular, since they demand significant energy influx, an important subsystem for analyzing is the separation of alcohol-water mixtures, There are also different technologies for this dehydration. In this paper we consider azeotropic distillation and hybrid schemes with membrane separation. The development of a simulation module for calculating the membranes performance,typically not included in commercial process simulators,The consumed electric energy and steam depends on the adopted scheme and they have, depending to their source or generation policy, an associated cost and environmental impact. Among the energy systems that deserve special attention, cogeneration in the sugar industry must be pointed out. This part of the process makes efficient use of a common fuel for the generation of heat and power. As a result, the environmental impact and costs associated with the use of electricity and useful heat change appreciably, such as anhydrous alcohol production schemes can be achieved with a lesser environmental at the expense of decreasing the economic benefit which would result from selling electricity to the grid.Cogeneration is not new in the sugar-cane processing industry. Indeed, typically co-generation refers to the generation of heat as the main objective and the generation of electrical and mechanical power as secondary objectives. To date, however, no versatile tool that would allow analyzing economical feasible alternatives of anhydrous alcohol production, bearing in mind environmental issues, is available.In addition, the explicit consideration of environmental issues offers an alternative route to explore trade-offs. Life Cycle Analysis (LCA) is used in this work. Unlike most environmental approaches to design that focus on reducing the generation of residuals, but without taking into account the impact associated to related processes, LCA allows considering environmental impact as an integral part of the problem. The software used to evaluate the environmental impact is SimaPro®.Alternative schemes that meet the same specifications of purity and recovery in effluent streams, are examined in this paper. Of particular relevance is the assessment of costs and environmental impact of these processes. A baseline case is defined in our case-study, and structural and parametric changes are made. Different scenarios with respect to economic and environmental issues taking into account the specifications of the final product. Near-optimal scenarios are formulated by defining an operating window for some of environmentally friendly technologies available. The impact of different operating policies for the production of anhydrous alcohol on the environment is examined in detail.The membrane area, the number of plates in the distillation units, the heat exchange area, and the pressure manipulators are considered as structural variables. Among the most relevant operational variables considered one can mention reflux ratio, pressure ratio, as well as compositions and flow rates of streams.