Purification Process in Desklib

Report
Purification
Introduction
This section will cover the purification segment of the process. This segment will initially provide
an outline and different phases of purification for the method chosen. The strategy, assumptions
and techniques of the design of each unit operation will be described. The major results from
the design will be evaluated and the design methods will be elaborated. Departure
Purpose of Purification
The primary objective of purification is to prepare the raw juice for evaporation and fermentation
by increasing the purity of the solution. In this case, the diffusion juice is purified from 87.5% to
93% by dry mass. Untreated sugar beets are composed of 2.5% by mass impurities that are
comprised of a variety of minerals, acids colloids and nitrogenous compounds. Because of
pre- treatment and diffusion, these impurities are released into the juice, and their presence can
affect the operation of the evaporators and fermenters. The purification process removes
 suspended solids,
 decreases the non-sugar levels,
 reduces pH
 Reduces the hardness of the solution.1
Theory of Purification
As raw juice consists of certain impurities such as non-sugars, suspended solids and collides.
Milk of lime (MOL) is added to the heated raw diffusion juice in a few steps to precipitate and
destabilize the non sugars. Carbonation gas, CO2 is also added in two steps, which absorbs the
impurities and colors to precipitate the lime as calcium carbonate (CaCO3), a byproduct. It traps
the color bodies and impurities from the liquor by inclusion within the carbonate crystals.
The precipitated calcium carbonate is separated from the juice in the clarifier2.Thin juice is the
product of this section and Calcium Carbonate is the byproduct. Thin recovered thin is
sent to downstream sections for further processing
Following Reaction takes place
Ca (OH )2 +CO2 yields
→
CaCO3 + H2 O
Thin Juice obtained has following characteristics.
 Low color
1 Asadi, M., Beet-Sugar Handbook, 2007, John Wiley and Sons, Page 190
Technology, N.I.o.S.a. Carbon Dioxide. Thermochemistry Data for Carbon Dioxide]. Available
from: http://webbook.nist.gov/cgi/cbook.cgi?ID=C124389&Units=SI%2F&Mask=1#ThermoGas
2 Asadi, M., Beet-Sugar Handbook, 2007, John Wiley and Sons, Page 213

 High purity
 Minimal hardness
 Free of insoluble solids
 adequate settling and filterability
 adequate thermal stability (resistance against heating without alteration)
Method of Purification
The process of purification involves the addition of lime and carbon dioxide gas in a series of
stages. Two methods are widely used in industry Classical Purification and Defeco Purification.
Defeco purification involves combining two or more of the liming and carbonation stages to
reduce the number of vessels required, however requires a very large recycle of suspended
solids, which causes additional costs.
Appendix consists of detailed study of both methods and comparison sheet. Classical
purification method is selected for this project. Classical Purification steps include:
 Prelimar,
 Mainlimer,
 First Carbonation,
 Clarifier
 second carbonation
 Ultrafiltration
The ancillary operation is a rotary drum filter that dries the underflows solids from the clarifier.
Overall Purification section will be able to produce 59716 Kg/Hr purified thin juice to
downstream evaporation Section.

Process Description
1. Prelimer
This section will provide an overview of the function of the Preliming step. This section will detail
the design process for the preliming reaction starting with the reaction kinetics and vessel
sizing. The energy and mass balances over the prelimer will be demonstrated. The reaction
vessel requires cooling to maintain isothermal operation and cooling jacket and coil design will
be discussed.
Agitation design will then be detailed. Finally, process control arrangements shown on the
schematic below will be discussed. A mechanical design specification sheet will also be
produced
Preliming has these main functions:
 To neutralize the acidity of the diffusion juice
 To precipitate certain nonsugars
 To destabilize the colloids

The lime is added in the form of Ca (OH )2 solution which causes precipitation and neutralization
reactions. In Preliming, small amount of lime (about 0.2 to 0.7% on juice) is added. The
prelimer operates at pH 8.5, atmospheric pressure and 40°C3. The liming time of the juice in
this step is about 10 to 15 minutes.
Six-second order reactions occur in the Prelimar. Three Precipitation reactions and three
neutralization reactions occur, although, the neutralization reactions do also produce
precipitates. The precipitation reactions occur between Ca(OH)2 and oxalate minerals of
sodium, potassium and magnesium with each forming calcium oxalate precipitate and the
corresponding metal hydroxide. The lime causes the neutralization of oxalic, citric and tartaric
acid which each produce a calcium precipitate and water.
In theory there are 4 processes happening in the liming stages and these include precipitation,
neutralization, destabilization and decomposition. The milk of lime in the form of Ca(OH)2
solutions reacts with a variety of impurity components. In Preliming, the two main processes are
precipitation to reduce mineral content and neutralization to reduce acidity in the juice. The
content of diffusive juice is variable and complex. Therefore, the Preliming process is simplified
to six major reactions that are most likely to occur during the Preliming process.4
Design Method
3 Asadi, M., Beet-Sugar Handbook, 2007, John Wiley and Sons page 232
4 Asadi, M., Beet-Sugar Handbook, 2007, John Wiley and Sons page 122