Multi-substrate Use for Ethanol Production

The Indian Alcohol Industry is mostly dependent on sugar factories for molasses as raw material. This is a major issue relating to the production of alcohol. Till recent past there has been no thrust on the use of non-molasses substrates. However, with the present alcohol scenario in India, the need to use other substrates has become necessary due to increased demand and anticipated shortage of molasses as raw material.

In addition to the synthetic ethanol production from ethylene, there are three groups of raw material:

• Beet, sugar cane, sweet sorghum and fruits
• Starchy materials such as corn, milo, wheat, rice, potatoes, cassava, sweet potatoes, etc., and
• Cellulose materials like wood, used paper, crop residues, etc. Ethanol production from cellulosic material is not yet commercialized.

An Alternate Route
In view of above, the use of the equivalent amount of cane juice (especially secondary juice) directly for the fermentation and production of ethanol seems to be the most economical and value-added concept for sugar factories. The secondary juice has less sugar and carries impurities. This requires a substantial amount of steam for evaporation and induces more load on the sugar refining operation. By diverting this secondary juice of equivalent amount of surplus sugar production to produce alcohol will help the sugar industry in:

• reducing the load on evaporation of this diluted juice, thus reducing steam consumption for the sugar factory
• reducing the load on the refining operation
• producing best quality sugar for export as well
• utilizing secondary juice for the production of alcohol
• value addition in terms of additional revenue generation by selling alcohol

Cane Juice to Ethanol
The process of converting cane juice to alcohol is by the single continuous fermentation process developed by Alfa Laval. This process offers benefits such as a very high yield of alcohol per MT of cane crushed and a very low quantity of effluent generated from the plant.

Process Highlights

• High efficiency of fermentation to the tune of 90%
• Savings in sugar due to yeast recycling
• Reduction in water consumption by stillage recycling
• Massive reduction in the spent wash generation to the tune of 3 liters per liter of ethanol produced, without using any additional energy
• Distillation efficiency of 98.5 %
• Ultimately higher yields/recoveries to the tune of 8–10 liters/ton of cane crushed

Unit Operations
The unit operations involved in processing cane juice to produce alcohol are given alongside (see diagram).

Evaporation: Cane is crushed in the milling section and the juice obtained has about 14-16% w/w sugar. This juice is concentrated to about 35-40% w/w solids in the tubular evaporators. The concentrated juice is fed to the fermenter.

Fermentation: The fermentation of juice is carried out in the single fermenter at a temperature between 32 and 34°C, the optimum range for efficient fermentation. The fermenter temperature is maintained at the desired value by an external plate heat exchanger through which cooling water circulates. The yeast for the fermentation is initially (during the start-up of the plant) developed in the propagation vessel. Then a viable cell population is maintained by yeast recycling and continuous aeration of the fermenter.

A high osmotic pressure and the concentration of alcohol in the fermenter together keep off infection and minimize sugar losses. Stillage recycling also reduces the quantity of effluent spent wash and reduces the process water requirement of the plant.

Yeast Recycling: The process adopts recirculation of grown, active yeast. Sugar that would otherwise have been consumed in yeast growth, is made available for ethanol production ensuring high process efficiency. A yeast separator specially designed by Alfa Laval for this purpose is employed.

Distillation: The distillation technique employed for the recovery of alcohol from fermented juice is a fully integrated different pressure system with the lowest steam consumption and a high quality of alcohol.

Fermented wash with approx. 8% v/v ethanol from the wash holding tank is fed to the top of the degasifying column after preheating and spent stillage cooler. Overhead vapor of approx. 40% w/w from the degasifying column is then fed to the bottom of the heads column. Impure spirit with approx. 95% v/v is removed from the vent condenser of the heads column. Heads column bottoms are fed to the alcohol column for recovery of alcohol.

The wash column is heated through the forced circulation reboiler with the condensing vapors from the rectifying column, which is operated under pressure. Analyzer column vapors are condensed and fed to an extractive distillation column. This column is operated with high dilution to enable the removal of fusel oils. Vapors are condensed and sent to the recovery column for alcohol recovery.

The alcohol water stream from the bottom of the extractive distillation column is fed to the rectifying column. The rectifier vapors are used to heat the analyzer by using the pressure cascading technique. Ethanol (concentration of 96% v/v) is removed from the top three plates and fed to the demethylizer column for separating methanol. The product ENA is removed from the bottom of the methanol column and cooled in the product cooler. Anhydrous alcohol is produced from 96% RS by molecular sieve technology. Anhydrous ethanol with purity above 99.8–99.9 is produced in the system, using vapor phase adsorption with pressure swing for regeneration. This is the most economical technology for producing anhydrous alcohol.

Pioneer Project
Alfa Laval has entered into its first venture in India to produce ethanol from cane juice for M/s Shetimal Sahakari Prakriya Ltd, in Kolhapur, Maharashtra with a capacity 30,000 liters per day of rectified spirit/extra neutral alcohol/ anhydrous alcohol. This is expected to be on stream by November 2002.

Courtesy: Mr Satish Tandon, Managing Director
Alfa Laval (India) Ltd, Dapodi, Pune-411012 (India)
Ph: +91 (20) 7107100/4116346; Fax: 020-714 9114
Mr Tandon joined Vulcan Laval (as Alfa Laval India was then known) in 1967 after obtaining a degree in Chemical Engineering from IIT, Delhi, and rose up to the position of Managing Director in 1998. Alfa Laval AB, is a Swedish multinational engineering company with a global turnover of approx. USD 1.8 Billion USD with manufacturing facilities in Sweden, USA, China, Japan, Italy, Spain, Denmark, Poland and India.