Purification Unit Design: Classical Method for Sugar Beet Processing

This project focuses on the design and analysis of a purification unit for a sugar beet processing plant, specifically utilizing the classical purification method. The assignment provides a detailed overview of the purification process, including the theory behind it, the objectives of the unit, and the characteristics of the resulting thin juice. It delves into the two main methods of purification, classical and defeco, justifying the selection of the classical method based on a comparative study. The project then details the classical purification steps, including pre-liming, main-liming, carbonation, and clarification, with a focus on the pre-limer stage. The pre-limer design involves vessel sizing, material balance, and energy balance calculations, along with control strategies. The document also covers the mainlimer stage, including its function, design considerations, and material balance. Appendices provide additional details on reactions, stoichiometry, and design aspects. The overall goal is to produce purified thin juice suitable for downstream processes like evaporation and fermentation.

Purification Unit
This part will cover the purification unit of the plant. This segment will provide details
of different phases of purification unit and technology chosen. This section will also explain the
purpose, theory and detailed process description including design details.
The primary objective of purification station is to purify raw juice required for evaporation and
fermentation units. Raw sugar beets juice contains 2.5% by mass containments that contains of
various minerals, acids, colloids and nitrogenous compounds that can impact the operation of
downstream units including evaporators and fermenters. The objectives of this unit are
underneath.1
 Elimination of every insoluble substance.
 Elimination of specific soluble matter, for example, non-sugars.
 Creation of thermostable nature juice with that of very low hardness.
Theory of Purification.
The product of the unit is thin juice and Calcium Carbonate is the by-product.
To precipitate and destabilize the non-sugars Milk of lime (MOL) is mixed with juice.
Carbonation gas, which is CO2 and air mixture is fed in 2 steps, which absorbs impurities and
colors. Lime is precipitated as calcium carbonate, CaCO3 a byproduct. It absorbs the color bodies
and impurities from the liquid by inclusion within the carbonate crystals2. The clarifier separates
the precipitated calcium carbonate. The overflow thin juice is carbonated in the second
carbonator reactor, and filtered before sending to evaporation unit of the refinery.
1 Beet Sugar Handbook Mosen Asadi page 214
Condensed Phase Thermochemistry Data. Water 24/02/2015] http://webbook.nist.gov/cgi/cbook.cgi?
ID=C7732185&Mask=2#Thermo-Condensed.
2 Beet Sugar Handbook Mosen Asadi page 234

Following reaction takes place.
Ca (OH )2 +CO2 yields
→
CaCO3 ↓+ H2 O
Thin Juice has following characteristics.3
 Enhanced Purity
 Less color
 Reduced hardness
 5ree of insoluble solids
 Suitable settling and filterability
 Improved thermal stability
Method of Purification
Two methods are widely used in industry Classical Purification and Defeco Purification.
Defeco purification includes consolidating more than two of the liming and carbonation cycle to
minimize required vessels, nevertheless this process requires a significant amount recycle (up to
700 times of incoming juice)4 of suspended solids, which causes additional capital costs.
Appendix 4.1 consists of detailed study of both methods and comparison sheet. Based on study
in appendix, Classical purification method is selected for this project.
Classical Purification
In this process Milk of Lime (MOL), as aqueous Calcium dihydroxide Ca (OH )2 is
added. The lime addition helps to precipitate, neutralize and coagulate. This objective is achieved
in Preliming and Mainlining. In carbonation reactors carbon dioxide gas takes place in the form
3 Beet Sugar Handbook Mosen Asadi page 215
4 Beet Sugar Handbook Mosen Asadi page 241

of bubble, this bubbles get through the limed mixtures in order to remove any high Ca (OH )2 by
the drizzle of calcium carbonate. The formation of calcium-carbonate results as a adsorbent for
various specific impurities to bind. Clarifier removes precipitates physically and overflow juice
is carbonated further before sending to downstream sections.
Classical Purification steps consist of:
 Prelimer
 Mainlimer
 First Carbonation
 Clarifier
 2d carbonation
 Ultrafiltration
Overall Purification section will be able to produce 59411 Kg/Hr purified thin juice to
downstream evaporation Section.

Stream Number 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416
Temperature ( C)⁰ 20 40 25 40 25 85 60 85 85 85 85 63.5 92 92 88 88
Pressure (bar) 1 1 1 1 1 1 2 1 2 1 1 2 2 1 1 1
Component flows (kg/h)
Water 391247 391247 293.24 391351 2625 393976 0 394692 0 22694 371998 0 0 372123 353517 18643
Sucrose 59410.65 59410.65 0 59410.7 0 59410.65 0 59410.65 0 0 59410.65 0 0 59410.65 59410.65 0
Calcium Hydroxide 0 0 410.54 174.02 3675 2481.768 0 736.26 0 0 736.26 0 0 220.9 0 202.5
Impurities 1523.65 1523.65 0 1110.0 0 1231.43 0 1231.43 0 0 1231.43 0 0 1231 596.13 635
Suspended Solids 0 0 0 398 0 397.63 0 397.63 0 377.75 19.88 0 0 19.88 0 655.18

CCC 0 0 0 0 0 9071 0 9071 0 9071 0 0 0 0 0 0
Carbon Dioxide 0 0 0 0 0 0 2618.62 872.78 0 0 524.3 220.9 0 0 0
Air 0 0 0 0 0 0 15541 15541 0 0 3093 3093 0 0 0
Calcium Carbonate 0 0 0 0 0 0 0 3968.64 0 3770.208 198.43 0 0 198.43 0 198.43
Total 452181.3 452181.3 703.78 452443.67 6300 466568.5 18159.62 469507.61 16413.78 35912.96 433594.65 3617.3 3313.9 433204.29 413523.8 20335
Purification Unit: Prepared and Reviewed : Inam Ullah
Khan

Process Description
1. Prelimer
This section will explain the details of design and vessel sizing, material and energy balance.
Agitation design will along with process control strategy will be elaborated.
Preliming stage is foundation step for downstream stages of purification, due to complex and
challenging role of Preliming which contains number of chemical reactions, Prelimer is selected
for detailed design.
Prelimer main roles include:5
 Neutralization of juice
 Precipitation of nonsugars
 Destabilization of the colloids
Theory
The addition of the lime causes precipitation and neutralization. In Preliming, very less quantity
of lime (about 0.2 to 0.7% on juice) is added. The Prelimer operating parameters include, pH 8.5,
temperature 40°C6. Residence time is about 10 to 15 minutes.
Six-second order reactions occur in the Prelimer. Three Precipitation reactions and three
neutralization reactions take place. The precipitation takes place among Ca (OH )2 , and oxalate
minerals of sodium, potassium and magnesium with each forming calcium oxalate precipitate
5 Beet Sugar Handbook Mosen Asadi page 228
6 Beet Sugar Handbook Mosen Asadi page 241

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 Pre Liming, 4 processes are taking place and include precipitation, neutralization, coagulation
and decomposition. Ca (OH )2 Solution reacts with various impurities. Precipitation reduces
mineral content and neutralization reduces acidity in the juice. The mount of diffusive juice is
very complex. On the other hand, the process is made into simpler form to six major reactions
that occur in the Prelimer.7 Appendix 4.2 explains the details of reactions and their
stoichiometry.
Design
Horizontal and vertical both nature of prelimers are used within the industry. Vertical
type is selected for the process as vertical prelimers required low space and the process of mixing
is also very effective. Juice fed from the top and flows down due to gravity and milk of lime is
fed from the bottom. Lime is spurred towards the juice by influence of baffles. It comprises of
the seven compartments that includes a shaft mounted towards the stirring paddles within each of
the compartment for effective mixing8.
7 Beet Sugar Handbook Mosen Asadi page 231
8 Beet Sugar Handbook Mosen Asadi page 233

Size of the Prelimer
Component Value
Type CSTR
Volume 88.5mᶾ
Diameter 3.9 m

Height 7.7 m
The Vessel is provided with agitator, the design of agitator is explained in Appendix 4.2
Material Balance
Extracted Juice from beets contains sucrose and impurities with flow rate=60934 Kg/Hr.
It contains 2.5% impurities9. Hence Impurities composition flow is1523.35Kg/Hr.
Assumptions
Based upon the above table the flowing assumptions is being made in order to simplify
design:
 Oxalic, Citric and Tartaric acid are non-nitrogenous and present in equal quantities.
 Oxalates compounds that of the Sodium, Potassium, and that of the Magnesium are
considered as minerals and are present within the equal mass.
Impurities Composition
 The nitrogenous compounds and unknowns are considered as long chained colloids and
flow calculated above is 792.14Kg/Hr.
 Minerals Total flow=182.8 Kg/Hr, containing each 60.9Kg/Hr.
 Non nitrogenous components flow=548.41Kg/Hr containing each 182.8 Kg/Hr.
The extent of reaction of precipitation and neutralization reactions composed of about 35%
and 65% by moles respectively.10 Detailed composition of impurities, Material Balance and
energy Balance is explained in Appendix 4.2
9 Beet Sugar Handbook Mosen Asadi page 105
10 Beet Sugar Handbook Mosen Asadi page 216

Lime Requirement.
Based on Stoichiometry Ca (OH )2 requirement is 410.54Kg/Hr.
According to solubility data (0.14/100g)11 water required=293.24 Kg/Hr
Ca(OH)2 Required 1.539 mol/s 410.54 Kg/Hr
water required 293.24 Kg/Hr
Lime flow rate 703.75 Kg/Hr
Unreacted Ca (OH )2 = 173.93 Kg/hr.
Summarized Material Balance Inlet
Total inlet Flow=452885 Kg
Hr
Total outlet flow=452885 Kg
Hr
Energy Balance:
Prelimer Energy Balance calculated. The diffusion juice from raw juice tank enters 25°C.
The Milk obtained from that of the Lime solution proceeds within the reactor at ambient
temperature such as that of the 25°C. The reaction takes place isothermally within the 40°C.
Thus, the product stream also takes place within the 40°C. Impurities considered energetically
equal with specific heat capacity of 1.0 kJ/kgK as this is averaged over the known impurities.
The milk that is obtained from that of the lime heat capacity if predicted as a function of pure
11 https://www.lime.org/documents/lime_basics/lime-physical-chemical.pdf

component of the heat capacities and that of the mass fractions:
Cp of Ca( OH )2 Solution= ( 410.54
703.75∗1.18 KJ
KgK ) + ( 293.24
703.75 ∗4.18 KJ
K ) =2.32 KJ /kgK
The sucrose and water solution have heat capacity for the sucrose solution of 3.75 kJ/kgK.12
Assume reference heat temperature of 25°C for the sole purpose of the enthalpy calculations.
Q=H 2−H 1=704−7050=−2 KW
It is obvious that Heat in and out is almost balance. Vessel needs insulation to keep it
isothermal at 40 °C.
Control Strategy:
R-01 is composed with that of the robust basic control strategy in order to control and monitor
the critical parameters. A PH controller is provided that manages the lime flow supply to monitor
the constant PH. Level controller is provided to keep level steady state. For detailed description
refer to appendix 4.2
2 – Mainlimer R-02
Introduction
This section will cover summary of the function of the Main liming Stage. This section will
explain the design vessel sizing. The mass and energy balances will be calculated. The vessel
needs to get heat with that of the steam in order to ensure isothermal operation with that of the
steam jacket.
12 Beet Sugar Handbook Mosen Asadi page 787