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Presented at:Third Joint China/USA Chemical Engineering Conference (CUChE-3)

Beiing! China! Se"tem#er $% - $&! $'''

Modeling the Complex Chemical Reactions and Mass Transfer in a PhosphoricAcid Reactor

Paul M. Mathias , Chau-Chyun Chen and Marten Walters

Aspen Technology, Inc.Ten Canal Park

Camridge, MA !"#$#-""!#%.&.A.

'(MWorks Technology, Inc.)*") Imperial Park+ay, &uite #!)

Mulerry /0!-10"#%.&.A.

Introduction

The "rodction of "hos"horic acid # the *+et "rocess, is a com"le! large-scale indstrial o"eration in+hich "hos"hate roc. is #eneficiated! reacted +ith slfric acid and filtered to remoe the #-"rodct!

calcim slfate0 1n this "a"er +e demonstrate that ala#le insight can #e gained into this com"lechemical "rocess throgh a fndamental model im"lemented in the As"en Pls "rocess simlator0

2

Phos"horic acid is "rimaril "rodced #+hat is .no+n in the indstr asthe +et "rocess (Slac.! 45&6

Bec.er! 4&4)0 1n the +et "rocess! "hos"hate roc.! +hich incldescalcim! "hos"hate and a nm#erofim"rities! is mined! #eneficiated (concentrated) and then sometimesgrond dr or +et throgh the se

of #all mills or rod mills0 The roc. is fed into an attac. tan.or reactor and reacted (digested) +ith slfricacid (7 S8 )0 An etremel sim"le re"resentation of the digestion is asfollo+s:$ 9

Ca (P8 )3 9 $ 37 S8$ 9 57 8$ > $7 P83 9 3CaS89$7 8$ ()

The reaction "rodces"hos"horic acid (7 P8 ) and calcim slfate dihdrate or g"sm (CaS83 9 9$7 8)0$The g"sm "reci"itatesand is filtered! giing "hos"horic acid asthe "rodct0 1n a ariation of the +et

"rocess - the hemihdrate "rocess - calcim slfate hemihdrate (CaS89;7 8)is "rodced asthe$"reci"itate0 The "resent "hos"horic-acid discssion focseson the "rodction of "hos"horic acid # the

dihdrate "rocess0

The dihdrate "rocess contains sectionssch as attac. and digestion reactors! flash

coolers! filtration nits! and "hos"horic acidconcentration0 The "resent simlation

focses mainl on the slfric-acid-attac.reactor! +hich is the heart of the "rocess0

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Model Fundamentals Thermodynamics, Kinetics and Mass Transfer

Phos"hate roc. mineralog can #e highl aria#le de"ending "rimaril on the origin of the roc.(sedimentar origneos)! formation and +eathering0 >ost sedimentar de"osits contain arietiesof

car#onate-flora"atite collectiel .no+n asfrancolite! +hich containscalcim! "hos"hate! floride!car#onate and other s"eciesheld togetherin a crstal lattice0

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im"ortant to the "rocess0 =igre 3 "resents acom"arison #et+een model and data for the

sol#ilit of g"sm in "hos"horic acid soltions0The model "roidesan accrate correlation oer a

+ide range of conditions0

The "resent thermodnamic model "roides acom"rehensie descri"tion of the thermodnamic

"ro"erties of this sstem0 =rther details are"roided in >athiasand >ende?(44&)0

The .e reaction .inetics and mass transfer

"henomena in the reactor are roc. dissoltion andits tendenc to coat! "reci"itation of g"sm (#oth

normal and ncleated) and co-"reci"itation of "hos"hate com"ondsin the g"sm (Slac.! 45&6 Bec.er!4&4)0 The "lant lossesare the ndissoled or coated roc. (.no+n ascitrate-insol#le or C1 loss) and the

"hos"hate co-"reci"itation (.no+n as the citrate-sol#le or CS loss)0 These rate "henomena hae #eenca"tred +ith the follo+ing assm"tions:

0 Doc. dissoltion! assmed to #e "ro"ortional to the concentration of ndissoled roc. and inersel

"ro"ortional to the slfate concentration0$0 Doc. coating! assmed to #e "ro"ortional to the concentration of ndissoled roc.! the g"sm

"reci"itation rate and the slfate concentration030 ormal g"sm "reci"itation! assmed to #e "ro"ortional to the calcim and slfate concentrations

and the distance from eili#rim090 cleated g"sm "reci"itation! assmed to #e negligi#le " to the s"ersatrated limit! and

etremel fast once this limit is reached0%0 Co-"reci"itation losses! assmed to #e "ro"ortional to the total g"sm "reci"itation rate and ratio of

the "hos"hate and slfate concentrations0

All these effects hae #een incor"orated into a ser s#rotine forthe stirred-tan.reactor (DCSTD)inAs"en Pls0

Aspen Plus Implementation and Results

The .e #ilding #loc.s for the As"en Plssimlation model are the thermodnamic model

and the DCSTD model0 Each tan.of the reactoris an DCSTD #loc. and the other "ieces of

ei"ment in the "lant are re"resented ##loc.s sch as 7EATED! =AS7$ ($-"hase

flash)! SEP$ (t+o-otlet com"onent s"litter) etc0The filtration sstem is treated as a series of

SEP$ #loc.s0 A schematic re"resentation of a"articlar As"en Pls flo+sheet is "resented in

=igre 90 1t is assmed that most of the roc. isfed to D$! and that most of the slfric acid

and retrn acid are fed to D30 The slfric acidand retrn acid are assmed to #e "i"ed

inde"endentl! #t! as noted earlier! in crrent"ractice reccle acid and slfric acid are

generall mied "rior to addition to the reactor0 8ther "lant configrationscan easil #e modeled0

%igure 2

Solu3ility of Calcium Sulfate in Phosphoric Acid Solutions

Comparison of ASP"4 P56S Model to 7ata of Tapero#a 0.-8(1 and Tapero#a

and Shulgina 0.-8'1

!')C

,()C

8()C

*()C

'0'

'0$

'09

'05

'0&

0'

0$

09

05

' ' $' 3' 9' %' 5'

Wt/ P inSolution! '

Wt/CaS

8inS

aturatedSolution

%igure 8$ AspenPlus Simulation of the 7ihydrate Process9

Rx .

Rx -

Rx ,

Rx *:+

Rx !

Rx 2

Rx 8

Rx '

%ilter

H !"#ap$

Gypsum

ProductAc id

Gases

Water

Return

Ac id

Phosphate RockSulfuric Acid($(.

($-,

($(.

($-,

($(.

($(.

($-,

Slurry

( $(.

($(.

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The As"en Pls model hasman ses! inclding gaining insight into the "rocess! tro#le-shooting "lant"ro#lems and antici"ating the effect of design changes0 7ere +e onl #riefl descri#e three stdiesof the

effect of "lant o"eration on C1 and CS losses0

A0 Plant throgh"tB0 Slfate leel

C0 Slfric acid s"lit #et+een Deactors $ and 3 (see =igre 9)

1n all the stdies"resented #elo+! the acid strength +asheld constant at $5G P 8 0$ %

A0 Plant throgh"t0 The model "redicts that the effect of increased "rodction +ith the same ei"mentis to increase C1 losses! +hile .ee"ing the CS lossesa""roimatel constant0 This effect reslts

#ecase the reactors hae less residence time to dissole the roc.! and the reslt is in agreement+ith "lant e"erience0

B0 Slfate leel0 =igre % sho+s the effect of

increasing the slfric acid flo+ to the "lant!+hich +ill reslt in an increase in the residal

slfate leel of the "rodct acid0 The CS lossesgo throgh a +ea.maimm at a slfate leel of

a#ot $0%G0 This is a com"licated effectreslting from the direct #eneficial effects of

increasing the slfate leel (lo+er CS losses)and the negatie secondar effect +here

increased slfate leel shifts more of theg"sm "reci"itation to Com"artment H$ +here

the slfate leels are lo+er (higher CS loss)0This .ind of insight into the interaction of effects

can onl #e gained throgh a com"lete "lantmodel li.e the "resent one0 1t shold #e noted that the "resent model sggests o"erating at the

lo+est "ossi#le slfate leels in order to minimi?e the total "hos"hate loss0 7o+eer! it is +ell .no+nthat the g"sm crstals that form at lo+ slfate leels are difficlt to filter0 1n ftre ersionsof the

model +e "lan to inclde the descri"tion of the "article-si?e distri#tion of the g"sm crstals0

C0 Slfric acid s"lit0 =igre 5 "redicts the effect offeeding the slfric acid to the roc. com"artment

(D $ in =igre 9) rather than Com"artment H30The CS losses come do+n #ecase slfate leel

in the roc. com"artment rises! #t nfortnatelthe C1 losses rise faster #ecase the higher

slfate leel significantlincreases the rate ofroc. coating0

The a#oe eam"lesindicate the ale that can #e

gained from a fndamentall #ased "rocess modelincor"orated into a "rocess simlator0

Conclusions

Phos"horic acid reactors hae #een considered to #e too com"le to #e modeled on a fndamental

#asis0 This +or. demonstratesthat these .indsof chemical "rocesses can indeed #e modeled and thatthe #enefits in terms of "rocess nderstanding and rational "rocess im"roement are considera#le0

%igure '$ 5osses as a %unction of S8

(

.

!

2

8

'

*

.$' !$( !$' 2$( 2$'

Weight / S8 in Product Acid

Weight/CS

andC;5o

ssesCS5

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References

Bec.er! P0! *Phos"hatesand Phos"horic Acid0 Da+ >aterials! Technolog! and Economics ofthe arcel @e..er! 1nc0! Second Edition (.-,-)0

Bro+n! E0 706 Carlton! @0 athias! P0 >0! *Analsis and Com"arison of the @ihdrate and 7emihdrate Processes Throgh

Process >odeling!, "resented at the S>E/A1ChE/A1P/=1PD =orteenth Annal DegionalPhos"hate Conference! a.eland! =lorida! 8cto#er $-$$! .---

>athias! P0 >0! and >ende?! >0! *Simlation of Phos"horic Acid Prodction #the @ihdrateProcess!, "resented at the $$

ndClear+ater Conention on Phos"hate =ertili?er M Slfric Acid

Technolog! Clear+ater! =lorida! $$-$3 >a! .--,0Slac.! A0 I0! Ed0! *Phos"horic Acid!, Iolme ! >arcel @e..er! 1nc0 (.-*,)0

Ta"eroa! A0 A0! Shlgia! 0 >0! *Sol#ilitiesof CaS8 7dratesin Phos"horic Acid! , 7h. Prikl.9'him0! &! %$ (.-8')

Ta"eroa! A0A0! 7h. Prikl.'him0! 3! 593 (.-8(10