The present invention relates generally to a method of recovering hydrochloric acid from a product comprising a mixture of sugars and concentrated hydrochloric acid, and more particularly, to a method of recovering hydrochloric acid from the hydrolyzate product obtained from the acid hydrolysis of biomass. 
As a result of the continual depletion of nonrenewable energy sources and the rapid escalation of energy prices, various energy conservation measures have been encouraged and alternative energy sources have been proposed and studied.  Among the proposed alternative energy sources are those such as geothermal energy and solar energy, however, current research has focused more and more on the use of biomass as a viable energy source. 
Biomass is composed of three major materials: cellulose, hemi-cellulose and lignin in ratios of roughly 4:3:3.  This composition enables biomass to be a versatile alternative energy source since cellulose is a polymer of hexose rings and hemi-cellulose is a polymer composed of hexose and pentose rings.  These polymer chains may be broken and converted to sugars and other chemicals through various chemical, microbial or fermentation processes. 
The principal sources of biomass are agricultural crops, agricultural residues, forest products, municipal waste, raw sewage and manures from confined livestock operations.  Biomass is particularly attractive as an alternative energy source since it is available in large quantities and is renewable.  It can also be converted to a variety of chemicals and its conversion need not create air pollution problems.  Instead, such conversion can assist in alleviating municipal waste problems. 
To optimize the conversion of biomass to energy producing compounds and other chemicals, it is necessary to fractionate the crude biomass to the sugar monomers, glucose and xylose.  The most common method used in accomplishing this conversion is acid hydrolysis.  In general, acid hydrolysis of biomass requires either high temperatures and dilute acid or high acid concentrations and low temperatures to obtain acceptably high sugar yields.  The high temperature/dilute acid process has the advantage of not requiring acid recovery, because of the relatively inexpensive dilute acid being utilized.  However, such process has the disadvantage of suffering low sugar yields caused by the degradation of sugars at the high temperatures needed. 
The high acid concentration/low temperature process has produced reasonably acceptable sugar yields, but economic success of the process requires acid recovery.  Accordingly, it is recognized that for the conversion of biomass to sugars and other usable chemicals through the use of concentrated acids to be economically feasible, the process must include an efficient acid recovery procedure.  Typically, dilute acid processes involve acid concentrations of 5% or less, while high acid concentration processes involve acid concentrations of 10% or greater. 
The traditional approach used in recovering hydrochloric acid is through distillation or evaporation.  However, high temperatures associated with distillation cause sugar degradation, thereby lowering yields.  Lower temperatures can be achieved by vacuum evaporation conditions, but such a process is energy intensive and costly. 
The separation of concentrated hydrochloric acid and glucose is not generally new to the chemical process industry.  An early method was the Bergius-Rheinau process for recovering concentrated hydrochloric acid from a composition containing glucose and hydrochloric acid.  The fundamental idea behind this sugar-acid separation was evaporation under reduced pressure.  Although this vacuum evaporation suppressed sugar degradation, the lack of suitable construction materials made the evaporation difficult.  As acid resistant materials were developed, this process was modified by use of a series of vacuum evaporators.  The principal drawback of such a system, however, was that the system was not economical and was quite large and complicated.  Further, in this process, the recovery was limited as a result of the azeotropic relationship between hydrochloric acid and water. 
More recently, a process for separating and recovering concentrated hydrochloric acid from a hydrolyzate solution was the subject of U.S. Pat.  No.  4,237,110.  The principal idea behind the process described in this patent was to recover concentrated hydrochloric acid by solvent extraction using a C.sub.5 -C.sub.9 alcohol as the extracting solvent.  In this process, the solvent is brought into contact with the hydrolyzate resulting in the formation of two phases, a hydrochloric acid enriched solvent phase and an acid depleted hydrolyzate phase.  Concentrated hydrochloric acid was then recovered from the solvent phase by distillation.  While this method is effective under certain conditions for the recovery of hydrochloric acid, it does have certain disadvantages.  One disadvantage is that the alcohols chemically react with the hydrochloric acid during extraction, thus requiring a further step to regenerate the hydrochloric acid and alcohol components before proceeding with further separation. 
Accordingly, there is a need in the art for a method of recovering concentrated hydrochloric acid from a sugar/hydrochloric acid product obtained from the acid hydrolysis of biomass or other cellulose based materials.  A further need is for a method to recover concentrated hydrochloric acid and still permit subsequent recovery of the sugars.