kristalisasi 4 rev - 3 may 2012

3 May 2012 Jurusan Teknik Kimia UKWM 1

Nani Indraswati

PROSES PEMISAHAN 1CHE223

CRYSTALLIZATION(4)


CRYSTALLIZATIONHeat balance

Heat removed to cool the feed solution from T1 (Tfeed) to T2 (Tcrystallization) without any solid phase precipitating out

+ heat liberated when crystals are formed (heat of crystallization) at T2

= heat lost through walls + latent heat of evaporation + heat removed by cooling

Using specific-heat and heat of crystallization data :


Example 12.11-2


ExampleHeat removal in crystallization cooling of lactose

Lactose syrup is concentrated to 8 g lactose per 10 g of water and then run into a crystallizing vat which contains 2500 kg of the syrup. In this vat, containing 2500 kg of syrup, it is cooled from 57°C to 10°C. Lactose crystallizes with one molecule of water of crystallization. The specific heat of the lactose solution is 3470 J/(kg)(°C). The heat of solution for lactose monohydrate is -15,500 kJ/ mol. The molecular weight of lactose monohydrate is 360 and the solubility of lactose at 10°C is 1.5 g / 10 g water. Assume that 1% of the water evaporates and that the heat loss through the vat walls is 4 x 104 kJ. Calculate the heat to be removed in the cooling process.


ADIABATIC VACUUM CRYSTALLIZER


CRYSTALLIZATION: Mass Balance

CrystallizerCrystals C kg/h

Mother liquor L kg/h(saturated solution)

Magma M kg/h

Feed F kg/h

H2OVaporV kg/h

Feed F = magma M + vapor VMagma M = crystals C + mother liquor L

CLMF CxLxMxFx balance Solid

V M V CLF

balance material Overall


Adiabatic evaporator-vacuum crystallizer: Heat balance

Adiabatic operation : → Q = 0

F = rate of feed, kg/h L = rate of mother liquor , kg/h C = rate of crystals, kg/h M = rate of magma, kg/h V = rate of vapor (superheated), kg/h H, h = enthalpy, J/kg

VMF

VCLF

VHMhFh

or

VHChLhFh

3 May 2012 Jurusan Teknik Kimia UKWM 8xL xM xC

TCryst


Enthalpy-concentration diagram

SystemMgSO4 - H2O

hF

xFxL xM xC


qp

q

M

L

qp

p

M

Cba

b

F

Vba

a

F

M

a

bp

q

q

p

L

C

b

a

V

M

F = feedL = mother liquorC = crystalsM = magmaV =vapor

V

Feed F

Vapor V

Mother liquor L

Magma M

Crystals C

Adiabatic evaporator-vacuum crystallizer

F = M + VM = L + C

3 May 2012 Jurusan Teknik Kimia UKWM 11lb crystal/lb mother liquor

Example

L

CC = 10000 lb


ExampleThe vapor leave the crystallizer at 86oF (superheated vapor → saturated at 84oF + BPR 2oF)

HV = Hsat + (Cpsuperheated vapor )( oC or oF superheat)

enthalpy of saturated vapor at TSat BPR

1.884 kJ/kg.oC


Vapor V

Magma M

Feed F

Crystal C

mother liquor L

F = M + VM = C + L

xL xM xC


vapor

magmafeed

crystals

mother liquor

isotherm 86oF

fd

fede

fe

0.2241

0.224

M

C1

0.224

L

C

ab

be

M

V

0.285 0.488

-62.4-43

1098

-149


Enthalpy-concentration diagram

SystemMgSO4 - H2O

hF

xFxL xM xC


point e

1 x 0.2850.322

d f

d f

ab

be

M

V

Basis: 1 lb mother liq L

lb 10000C1

0.224

L

C

VM

L

= F

M

M

F

V

LC

V =


vapor

magmafeed

crystals

mother liquor

isotherm 86oF

fd

fede

fe

0.2241

0.224

M

C1

0.224

L

C

ab

be

M

V

0.285 0.488

-62.4-21

1098


qp

q

M

L

qp

p

M

Cba

b

F

Vba

a

F

M

a

bp

q

q

p

L

C

b

a

V

M

F = feedL = mother liquorC = crystalsM = magmaV =vapor

V

Feed

Vapor

Mother liquor

magma

crystals




If TF is increased at the same P the point F(xF,hF) moves upward on a vertical line the point M move downward to the right ratio C/M will increase

If lower operating P is used at the same TF and xF lower operating temperature HV decreases → point V(y,H) will move downward the line LM will move downward point L will move to the left along curve A the point C moves down on the vertical line the point M shifts downward to the right ratio C/M will increase


Problem Larutan MgSO4 30% berat pada suhu

220oF dialirkan ke suatu crystallizer dengan laju 3500 lb/jam. Suhu pada crystallizer 86oFLarutan jenuh MgSO4 mempunyai kenaikan titik didih sebesar 2oF.Hitung laju kristal yang dihasilkan


Crystallization Mechanism


CRYSTALLIZATIONCrystal growth

Rate of crystal growth: the distance moved per unit time in a direction perpendicular to the face

Solute molecules A reach the face by diffusion from the super-saturation bulk solution to the surface

surface the at A)solute of fraction (mole ionconcentrat solute y

solution bulk of A)solute of fraction (mole ionconcentrat ationsupersatur y

m area, surface A

A/skmol A.solute of rate tranfer mass N

tcoefficien transfer mass k

1-12.12 ...........................

'A

A

2

A

_

y

.

frac molmskmol/ , 2

)'A

yA

(yykA

_

AN


CRYSTALLIZATIONCrystal growth

surface the at ionconcentrat solute C

solution bulk of ionconcentrat ationsupersatur C

area surface A

A/skmol A,solute of rate tranfer mass N

tcoefficien transfer mass k

'A

A

A

_

c

_

)'A

CA

(CckAA

N


CRYSTALLIZATION Crystal growth

At the surface : integration reaction of the molecules into the space lattice

surface the at A of fraction) (mole ionconcentrat solute y

Aof fraction) (mole ionconcentrat saturation y

area surface A

kmol/s A,solute of rate tranfer mass

frac) )(molkmol/(s)(m t,coefficien reaction surface k

2-12.12 ...................

'A

Ae

A

2S

_

N

)Ae

y'A

(yS

kAA

N



surface the at ionconcentrat solute C

ionconcentrat saturation C

area surface A

kmol/s A,solute of rate tranfer mass N

)/)(kmol/(s)(m t,coefficien reaction surface k

2-12.12 ...................

'A

Ae

A

2sc

3mmol

)Ae

C'A

(CsckAA

N



Solution must be super-saturated for the diffusion (12.12-1) & interfacial (12.12-2) steps to proceed



Combining eq. 12.12-1 & 12.12-2 :

ky predicted by methods for mass transfer to suspensions of small particles (sect. 7.4), mass transfer through fixed bed or fluidized bed

tcoefficien transfer overall K

3-12.12 .........).........yK(y.

k1

k1

)(y

A

NAeA

Sy

AeAA

_

y



tcoefficien transfer overall K

)C(CK.

k1

k1

)C(C

A

N

c

AeAc

SCC

AeAA

_

kc predicted by methods for mass transfer to suspensions of small particles (sect. 7.4), mass transfer through fixed bed or fluidized bed



Crystal growth is measured as the increase in length L (linear dimension of one crystal, mm)

In the same environmental conditions, L is independent of initial crystal size

t time at crystal of dimension linear D

constant rate growth G

ttGDDΔL 1212

kristalisasi 4 rev - 3 may 2012

Documents

mother liquor

mass transfer

mother liquor

crystallizationcrystal

move downward

adiabatic

surface ion

small particles