Preliminary design of multipurpose pilot plant for the recycling of polyurethane waste

.

,

F.V.O. Nr: 2959

Vakgroep Chemische Procestechnologie

Verslag behorende

bij het fabrieksvoorontwerp

van

" C.M.B. Hoogeveen M.A.T. Bisschops

onderw

er

p

:

prelirninary desigtl of a lDultipurpose

,

pilot plant for tbe recyc1itlg of polyurethane waste " Aart

v.d.

Leeuwlaang 189 2624 PP Delft Oude Delft 123 2611 BE Delft

opdrachtdatum:

verslagdatum:

maart mei 1992

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PRELIMINARY DESIGN OF A

MUL TI

PURPOSE PILOT PLANT

FOR THE RECYCLING OF POLYURETHANE WASTE

March - May 1992

University of Technology Delft,

Department of Chemical Engineering

M.A.T. Bisschops

C.M.B. Hoogeveen

- J

- - - p u

recycling---SUMMARY

00

This report covers the preliminary design of a multipurpose pilot plant for the recycling of polyurethane (PU) waste. The assignment was carried out in cooperation with ICI Holland B.V ..

The preliminary design of on industrial plant is part of the study Chemical Engineering at Delft University of Technology (course ST44).

The recycling con take place as glycolysis or aminolysis (in mono-ethyleneglycol MEG and mono-ethanolamine MElA resp.). At temperatures up to 200°C and pressures up to 3 bar, a conversion of 100% can be achieved.

This multipurpose pilot plant consists of a feed, reaction and work up section.

In the feed section, the PU is cut, washed and fed to the reactor. Two possible routes of feeding are feasible: as a dispersion or as solid particles.

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In the work up section the reaction produets (polyols, rigids and

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are purified by means of liquid-liquid extraction and evaporation. To meet the ICI quality specifications these processes have shown rwt be sufficient so further purification is necessary.

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To maximize the flexibility of the pilot plant the process is carried oot ~9kch-wisa and

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equipment is combined as much as possible, as can be found in plant layout design. )

During plant operation special care should be taken with MEG vapour (explosive, toxic),

MELA (flammable), DADPM (caneer inducing) and the wash solvent (e.g. methanol:

flammable and explosive). Solutions have been proposed to minimize the risks for operators.

The cost engineering of the process yields a total investment of ~/131,6oq Htl or 3,5l~, ~ Hfl when a rotary drum dryer is not included. The total costs per kg PU are 50 Hfl or 25 Hfl respectively. It has not been possible to make the choice whether to use a rotary drum dryer or not. ,-,)

Taking the current prices into account, the process will not be profitable. This pilot plant,

however. will not be built to make money, but to do research and explore consumers

interests.

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-CONCLUSIONS

In this report a description has been given of the design of a multipurpose pilot plant. As not all information is available at the moment. this design leaves some questions

unanswered. This pilot plant may be an excellent method for obtaining these data. The plant consists of three sections, being the feed, reaction and work up section.

The feed section contains a cutter, a dilution vesseL a high shear mixer, a centrifuge and a dryer. Because the performance of the centrifuge is unknown a dryer might not be needed at all. Cost calculations show that this option is worth considering: the dryer determines

6~

'10

of the investments.

The solid feed system can not operate against a 2 bar pressure difference. Two solutions have been proposed: feeding the reactor while it has a pressure of 1 bar and feeding the reactor from a hopper, which is at 3 bar as weil. The latter is the best solution.

The dispersion feed system can be operated in three ways: cold circulation, hot circulation and stepwise circulation. The latter seems to be the best sOlution, but, to make sure, more research has to be done.

In the work up section, only two apparatus are used: a RDC extractor and an agitated film evaporator. Both are chosen because of their flexibility ond possibility to handle viscous substances.

If it is possible to leave the drum dryer out of the design, the investment costs are relatively low. The operational costs however, are very high (1.28 Hfl/kg PU). Usually batch processes have higher energy consumption and more manpower is needed. If the process is scaled up to an industrial scale continuous plant, 0 huge reduction of operational costs can be

,_J PU recyCling

CONTENTS

1.

Introduction

1

2.

Assignment description

2

3.

Process scheme

3

3

.

1.

Feed section

3

3.1.1.

Preparation of the PU particles

3

3.1.2

.

Removal of the solvent

4

3.1.3

.

Feeding the PU particles by a dispersion to the reactor

4

3.1.4.

Feeding the PU particles as dry solids to the reactor

5

3.2.

Reaction section

5

3.2.1.

Glycolysis

5

3

.

2.2.

Aminolysis

6

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_3.3.

_{Work up section}

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3.3.1

.

Glycolysis of flexible PU

6

3.3.1.1.

Phase separation

6

3.3

.

1

.

2.

Liquid liquid extraction of polyol phase

6

3.3.1.3

.

Removal of the MEG from the polyol

6

3.3.1.4

.

Polyol purification up to

20

ppm DADPM content

6

3.3.1.5.

Removal of the MEG from heavy phase

7

3.3.2

.

Glycolysis of rigid PU

7

3.3.2

.

Aminolysis

7

3.3.2.1

.

Phase separation

7

3.3.2

.

2.

Work up of the polyol phase

7

3

.

3

.

2.3.

Work up of the DADPM phase

8

3.4

Time schedule

8

4.

Balances

9

4.1

.

Mass balances

9

4.2.

Energy balances

9

5

.

Equipment 10

5.1.

Feed section

10

~ J

5

.

1.1

.

Cutter

1

10

,

5.1.2.

Cutter

2

10

5.1.3.

High shear mixer

10

5

.

1.4.

Dilution Vessel

10

5.1.5.

Centrifuge

11

5

.

1.6.

Dryer

11

5.2

.

Reaction section

11

5.3.

Work up section

12

5.3.1.

Liquid liquid extraction equipment

12

5.3.2.

Removal of volatile solvents

13

5.3.3.

Batch distillation

13

5.4.

Storage section

14

5.5.

Auxiliary equipment

14

5.5.1.

Heat exchangers

14

5.5.2.

Pumps and transport

15

5.5.3.

Vent system

16

6.

Plant layout

17

7.

Safety, health and environment

18

7.1.

Risk of explosion

18

7.2.

Risk of fire

18

7.3.

Toxicity

19

7

.

4.

Noise

19

7.5.

Loss of containment and waste streams

19

,

-. _1

- - - PU recycling

-8. Cost engineering 8.1. Investments

8.1.1 . "Factor" methods

8.1.2. Calculation of offsite facilities 8.2. Varia bie cost

8.3. Cost of labour 8.4. Total costs 8.5. Economie criteria 9. Recommendations 10. list of symbols Chemical Engineering Cost Engineering 11 . Literature Acknowledgement

APPENDICES:

O.

1 . 2. 3. 4. 5. 6. 7. 8. 9. 10 . Quote

Description of Renory equipment Structure formulas

Mass & heat balance sheets

Calculation of distribution coefficients McCabe thiele diagrams

Isothermal flash calculations Safety sheets

Physical properties

Hydrodynamic design of a RDC extractor list of companies 20 20 21 21 22

23

24 24

25

26 26 27 28 29

- - - PU recycling

-1.

INTRODUCTION

This report covers a preliminary design of a multipurpose pilot plant. for the recycling of polyurethane (PU) waste. The aimes of this pilot plant are:

to obtain data on process scale up of PU recycling,

'*

to supply customers with products for evaluation.

All assumptions made with regard to this design are described in the assignment description (chapter 2).

Polyurethane is a copolymer of MDI and polyol. It is used as insulation mate riaL shoe soles, car bumpers, car chairs and double bed mattresses.

The disposal of plastic waste products is presently of considerable concern. The presently used disposal methods are landfill and incineration. Neither method is acceptable because long range ecological goals dictate zero pollution and conservation of raw materiais. Therefore emission free recycling processes are required.

POlyurethane foam con not be remelted and blended with virgin material for reuse.

Therefore chemical recycling is the right solution to this recycling problem.

The automotive industry is a great consumer of PU. The expectation is that in future the car manufacturers will have to take back their own products for recycling. As a service towards its customers, ICI is designing a recycling process for their PU waste.

D

- - - PU

recycllng---2.

ASSIGNMENT DESCRIPTION

In cooperation with ICI Holland B.V. a multipurpose batch pilot plant for the recycling of pOlyurethane was to be designed. Three different chemical ways were proposed to carry out thls process:

aminolysis. gtycolysis. hydrolysis.

As no information about hydrolysis at moderate temperature is avaiable, this route was excluded from this study.

The design is based on the following assumptions. postulated by ICI Holland B.V.:

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Renory equipment should be used, including a 1.5 m3 _{reactor as main item}

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As many combinations of equipment\'6s possible. ~ . ~

Different kinds of polyurethane waste have to be recycled, e.g. flexible and rigid PU. The size of the incoming PU waste may vary from shoe soles to double bed

mattresses.

Two ways of feeding the PU to the reactor have to be taken in accoun~being

dispersion and solid feed.

The polyol is purified by liquid-liquid extraction. No other methods have to be discussed.

The design should also include:

a flowsheet of the main plant items, a layout of the plant, including storages,

mass and energy balances over the entire process, material specifications,

safety, health and environmental (SHE) aspects, cost calculation and comparison.

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- - - p u

recycling---3.

PROCESS SCHEME

The entire process scheme can be divided info three secfions:

1 . Eeed sectjon

Preparation of the PU and feeding into the reactor. Two ways are possible:

a. dispersion feed,

b. solid particles feed.

2. Reactionsection

Solvation and reaction of the PU. Two different chemical reactions were chosen:

a. glycolysis,

b. aminolysis.

3. Work up sectjon

Work up of the products. The resulting products of the two chemical reactions are different. so the work up will follow different routes as weil.

An overview of the design can be found in figure 1. Process flow diagrams are shown in figures 2 to 7.

3.1.

Eeed section

The feed section can be subdivided into three parts:

1. preparation of the PU particles.

2. feeding the PU particles as a dispersion to the reactor.

3. feeding the PU particles as dry solids to the reactor.

3.1.1. Preparation of the PU particles

The process is designed to recycle PU waste of any size. There are several reasons why the PU has to be cut in small particles:

washing and drying efficiencies increase as the particle size decreases.

the solvation rate in MEG or MELA increases as the particle size decreases.

cut PU has a higher density (Iower porosity) 50 smaller storage volumes are needed.

To achieve high solvation rates. it is necessary to cut the PU in particles into 0.5 mm or less [1]. It is not possible to grind the PU because of its high shear stresses. These will cause a considerable local temperature rise. which will degrade the PU and form unwanted products.

Therefore the PU is first shredded (M4) to a size of approximately 5 cm. This shredded material is fed to a stirred tank (V13). filled with wash solvent. like methanol or xylene. The suspension is circulated through a high shear mixer (M6). where the particles are cut to the desired size. An additional advantage of the use of wet high shear mixing is the washing of the particles. Additives and water. which would cause an undesired reaction during glycolysis. are removed from the PU.

The washing process can be carried out in two ways:

1. Washing with 2.8 m3 _{solvent that is constantly refreshed with solvent from the storage} tank (contents 6 m3). The refreshing is necessary because not all solvent is removed

in the centrifuge. The polluted solvent is pumped back into the storage tank without being cleaned first. In this way the solvent is polluted with 1 kg additives and water per batch. When the pollution is substantia!. the solvent can be cleaned by batch distillation.

2. Washing with only 1 m3 solvent.

Advantages are increased flexibility and smaller solvent consumption. Main disadvantage is hich pollution concentration.

In the design the first option was chosen.

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-Af ter high shear mixing, a part of the suspension is recycled to the dilution vessel and the remaining part is separated into wash solvent and PU particles. Because the PU still is very porous and holds the solvent tight, this cannot be achieved by a simple filter or screen and a centrifuge (M3) has to be used. It is assumed that this process will densify the suspension to SOOk solids content by mass, though it moght be possible to achieve drier suspensions. Because the wash solvent might be explosive or flammable, the centrifuge has to be blanketed with nitrogen and an explosion protected design has to be used.

Af ter centrifuging, the PU is put in a drier (M1), that is also used as intermediate storage. This means, th at it should be able to contain a whole batch load at once (850 kg or 10 m3).

The remaining solvent is evaporated and regained by condensation.

3.1.2. Rerooyal of the solyent

The wash solvent can be removed by:

1. Drying in a dryer. Disadvantage is that even the dispersion feed has to be dried first before it is dissolved in MEG or MELA.

2. Drying in a centrifuge, this is only possible when the centrifuge dries so much that no further dryer is needed. It is only possible to say something about this when tests have been done with the centrifuge.

3. Evaporation of the solvent in a reactor. This is possible to the amount of 15%

solvent. for more removal the temperature must be increased above 200°C. At this temperature the PU might dissolve in the MEG or MELA.

4. Evaporation of the solvent during round pumping of the slurry. This is also only possible till 15% unless the temperature is increased.

In this design is chosen for the first option, because it gives the securest results.

3.1.3. Feeding the PU particles

by

a dispersion to the reactor

When the PU particles are fed to the reactor by means of a dispersion, they are mixed with the solvent (MEG or MELA) in the dilution vessel (V13) and pumped into the reactor (24). Because pumping of the suspension is only possible when the solids content is less then 15%, the solvent is circulated through the reactor and dilution vessel. The solid particles are retained in the reactor by a filter or sieve, which is placed in the outlet.

The volume of the reactor is 1.5 m3• therefore the PU has to be pressed into the reactor.

Pressures up to 3 bar are allowed. which is assumed to be enough for the feeding of 10m3 PU.

It is important to blanket both vessels with nitrogen because the solvents are explosive and flammable.

It is possible to heat the solvent (MEG or MELA) during circulation. This has the following advantages:

1. When the PU is not dried before feeding to the reactor. the wash solvent can be flashed off in the dilution vessel. This is only possible when volatile wash solvents are used. e.g. methanol.

2. Because of the insulating properties of PU. it is difficult to heat the batch reactor when it is entirely filled with the dense suspension. Heat transfer rates in a 10% suspension will be considerably higher.

Hot circulation also has disadvantages:

1. More MEG or MELA will be unnecessarily heated.

2. Solvation of PU might occur while circulating the hot MEG or MELA. This leads to loss of PU and contamination of MEG or MELA.

A special case of dispersion feed is achieved when a 10% dispersion is fed to the reactor and heated to 200°C. As soon as the PU is dissolved, this mixture is used to disperse fresh PU

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M.A. T. Bisschops C.M.8. Hoogeveen

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- - - PU recycling

-in the vessel. Because the dilution vessel can not withstand any temperatures over 175°C, the mixture has to be cooled before making a new dispersion. The main disadvantages of this procedure are the number of actions it takes and the toxic hazards.

3.1.4. Feeding the PU particles as dry solids to the reactor

Af ter drying the particles, it is also possible to feed them directly to the reactor. Before the PU is added to the reactor, it has to be filled with hot MEG or MELA and blanketed with nitrogen. When these solvents are heated to the desired temperature, pressures up to 3 bar might develop.

It is rather difficult to design a solid feed system (M20, M21) th at con overcome a pressure ditterence of 2 bar. The following options might otter a solution:

1. Limit solvent heating to about 120°C, most solvents than have a saturation pressure of about 1 bar. A disadvantage of this system is th at it's difficult to dissolve the PU in 120°C. If the solution is heated after the PU is pushed into the reactor, the insulating properties of the PU will make this very energy intensive.

2. Put the hopper under a nitrogen blanket of 3 bar and push the solids into the reactor by increasing the pressure in the hopper a little bit. This system requires high quantities of nitrogen what makes it expensive .

3.2

.

Reaction section

The two chemical routes to break down PU described here are aminolysis and glycolysis. The main ditterences between the two routes are the type of solvent and catalyst used. The glycolysis uses MEG and KAc as catalyst and for the aminolysis MELA is used as solvent and NaOH as reactant (it is not a catalyst. because it is converted itself).

3.2.1. Glycolysis

The PU is dissolved in MEG with catalyst (KAe) at 200°C. The resulting mixture will reaet for about 3 hours. The reaction is represented by [1. 2]:

PU + MEG - Rigids + Polyols ( I )

The conversion of the reaetion is assumed to be 100% [1]. When water is present. there is on unwanted side reaetion:

PU + HzO - Polyols + DADPM + CO₂

(11)

The structure formulas of these eompounds are given in appendix 2.

The only way to prevent the formation of DADPM is to<@ducfYthe amount of water in the PU by~ashingjt. If flexible PU is used as raw materia!. the reaction products are divided in two layers: a light phase containing MEG, polyol and DADPM, and a heavy phase

containing MEG, rigids DADPM and KAc. If rigid PU is reeycled, there will be only one phase.

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PROCESS SCHEME OF MULTIPURPOSE

PILOT PLANT FOR PU RECYCLING

M.A. T. Bisschops

C.M.B. Hoogeveen

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Stream index FVO nr 29159 MAY 1992

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- - - PU recycling

-3,2,2, Aminolysis

The reaction of PU with MELA and NaOH [3]:

PU + MELA + 2 NaOH .... DADPM + Polyols + Na₂C0₃ + MELA (111)

The structure formulas are given in appendix 2.

Besides DADPM and polyols, also some urea derivatives are formed. Little is known about them but they should be taken into account in mass balances.

Solvation and reaction occurs at temperatures of about 160°C.

The reaction products are divided in two layers: a light phase containing pOlyols, MELA and a smal! amount of DADPM and a heavy phase containing DADPM, urea, Na₂C0₃ and MELA. The ratio between heavy and light phase depends on the kind of PU used. Separation and purification of these layers is discussed in the work up section.

3,3,

Work up section

In this paragraph the work up of the reaction products is described.

3,3,], Glycolysis of flexible PU

3.3,1.1. Phase separation

Af ter flexible PU has reacted, the two liquid phases have to be separated. Calculations done on the two-phase system [1] show that settling velocities of several millimetres per second can be expected. This means that, to overcome 1 meter height, a residence time of several minutes is required. When the settling of the phases is allowed overnight, the separation is assumed to be ideal.

3.3.1.2. Liguid liguid extraction of polyol phase

The amount of DADPM in the polyollayer is assumed to be approximately 1%, though good washing in the preparation of the PU probably wililimit DADPM formation. Because DADPM is a contamination of the polyol and because it is toxic, it has to be removed. Dutch government restrietions dictate the DADPM content to be less than 1000

ppm. ICI quality specifications are 20 ppm.

The first step in DADPM removal is an extraction process (T38) with hot MEG. This process should be able to reduce the DADPM content from 1% to SOO ppm.

3.3,1.3. Removal ot the MEG trom the polyol

Af ter the extraction process, the polyol contains 15% of MEG. This solvent has to be

removed by evaporation (M37).

3.3.1.4. polyol purification up to 20 ppm DADPM content

Purification of polyol is necessary to decrease the amount of DADPM from SOO ppm to 20

ppm. There are several ways to achieve this:

1. By zeolite. Especially mole sieve Y should be able to absorb DADPM in its large holes. The angle between the two benzene rings in DADPM will probably cause a favourable equilibrium. It makes absorption slightly more difficult. but desorption will be restricted to a minimum. The zeolite are recovered af ter use by burning oft the DADPM [4].

H5

Vent

O P 2

(]

M4

~

r

-I

W-OR-K-U-P-H-EA-V-Y-P-H-A-s-"EI

~

AMINOLYSIS

streams

42

and

38

are

processed subsequently

P7

o

,...- ~ I"

Vl1

V14

V16

OP12 OP15 OP17

M8

~

[pnt

@

o

P18

o

PlO

(]

c

r--

~ ~

P30

1

I

~OOling

~~t··1

0 0

r= -

~

:",ater ...

(

V19

V23

V25

V29

V31

[r-t

Vent

M20

-..,

\/27

()

P26

heavy phase

\/28

lBC

S.H.Steam

_•

S.H.Steam

S.H.Steam

M37

POLYOL DADPM, UREA

Q

H40 Cooling

water

T38

H41

37 ~---~( 4')~---~

PROCESS SCHEME OF MULTIPURPOSE

PILOT PLANT FOR PU RECYCLING

M.A. T. Bisschops

C.M.8. Hoogeveen

®

Stream index FVO nr 29!111 MAY 1992

c

~ ') Cl ( '

H5

~

I I Vent ( j P 2

(]

M4

~

.---,8

WORK UP LIGHT PHASE AMINOLYSIS

M6

P7

o

,,- ~ ~ ---- ~ MELA

I

V11 V14 V

1~

V OP12 19 OP15 P17

IIM8

_[Yent

_I

/.'-@

o

P18

o

P10

-...

--

---V23 Vent Vent

IM20

V2

light phase .,---

---I

polluted

I

OP30

MELA V29

0

I

t=- __

::...:::l

Af"""> ,

V25 V31

0

P26 .9 P32

-

r-1

ru~,.AI

I

S.H.Steamil 11 M37

•

S.H.Steam

o

polyols H40 T38 Cooling water H41 r---~---~~ ~ water heavy phase

Vl8

PROCESS SCHEME OF MULTIPURPOSE PILOT PLANT FOR PU RECYCLING

M.A. T. Bisschops C.M.B. Hoogeveen

@

Stream index FVO nr 29S9 MAY 1992

ü

C C ('

c

') 'I ( '

- - - PU recycling

-2. Actlve carbon. ICI experiments have shown that it is possible to remove DADPM this

way. The major disadvantages of this process are the (relative) high costs of carbon. It can not be recovered and has to be burnt after use [5].

3. By an ion exchange process [1

J.

Recovery of the resin Is done by washing with hydrochlorlc acid and water. Major disadvantages are the high volumes of water and acid needed for regeneration.

4. Adsorption on acid clay. The polluted acid clay usually is not recovered but burnt. Because the acid clay is not very expensive. it may be an attractlve process for DADPM removal [6. 7].

5. By membrane filtration. The difference in molecular sizes between polyol and

DADPM Is very large. 50 filtration techniques con be used. Major problem is the foet. th at DADPM. which is not the continuum should pass the filter. Usually the continuum passes the filter and the contamination is held back [8].

6. By formation of DADPM-salts. Although this method is not really developed vet. it might give favourable results.

Because very little is known about the processes mentioned. it is recommended to do some experimental work. As mentioned in the assignment description (chapter 2) the choice of this purification step is left bevond the scope of this study.

3.3.1.5. Remoyal of the MEG trom heavy phase

The bottom product of the liquid liquid extraction is added to the heavy phase. because it contains some DADPM. Before this is further processed. a considerable amount of MEG will have to be removed. This can be done by distillation, flashing or evaporation (M37). The bottom prOduct of this process will have to be separated into three fractions: rigids. MEG and a solution of KAc in MEG (50010 KAc by mass). This can be done by liquid liquid extraction with water. since rigids and DADPM are not soluble in contrast with water and MELA and KAc.

The DADPM will be esterificated or propoxylated to rigids. Because little is known about these processes. this is left bevond the scope of this study.

3.3.2. Glycolysis of rigid PU

Glycolysis of PU results in only one phase. The work up of this phase follows the same route as mentioned above. The only difference is the absence of the phase separation. Liquid liquid extraction (T38) is carried out with the complete product mixture. The bottom product

is processed further analogue to the heavy phase mentioned above. The top prOduct is

the pOlyol phase and is treated as the light phase.

'3

3.3.1.

Aminolysis

3,3ll. phase separation

This process has already been discussed in the work up section of the glycolysis products. Again the separation is assumed to be ideal. After the aminolysis three phases are formed

I

*-[2]. The two heavy phases are kept together.

3

3,312, Work up of the polyol phase

The work up of the polyol phase, obtained from aminolysis, is analogue to the route

discussed at glycolysis (paragraphs 3.1.2. to 3.1.4.). An alternative route. using a centrifuge. has been reported [3], but no information is available.

7

.

7

_ c~ ,!<:".v,' C.

,

')

' .... CO v. po '-'V î

-.

----~

-Table 3.1. Time schedule

Process nme (h)

Preparatlon and reactlon

Cutting 2

Washing and centrifuging 5

Drying (overnight) 14

Dispersion feed

_-

2

Solid feed 2

Solvation and reaction 5

Work up of glycolysis

Settling of the two phases 14 .~.

Phase separation 1

LL -extraction 3

Distillation of pOlyol 5

Distillation of heavy phase 5

Polyol purification

~I

Work up of aminolysis

Settling of the phases 14

Phase separation 1

LL-extraction of light phase 5

Distillation of polyol 3

LL-extraction of heavy phase 5

Distillation of heavy phase Polyol purification

-.J'

- - - PU

recycling---3

3.3,%'3. Work up of the DADPM pbase

The extract of the extraction of the light phase with MELA is added to the heavy phase. This phase contains DADPM, MELA and sodium carbonate. Thls salt can easily be removed by extraction with water (T38). DADPM is not soluble in water and the extract will only contain water, MELA and all sodium carbonate. The MELA can be recovered by evaporation: first water is distilled off and subsequently MELA is evaporated and condensed (M37).

The raffinate of the water-extraction is separated by evaporation of MELA. The DADPM is a raw material for MDI production, which is a monomer of polyurethane.

3.4

TIME SCHEDULE

The time schedule is represented in table 3.1. Certain processes mentioned in this table can be carried out at the same moment. A scenario of the entire process is given below.

Day

1.

Cutting and shredding the PU

Washing, HS mixing and centrifuging Drying takes pi ace overnight

Day 2. Feeding of the reactor Reaction

Overnight: settling of the phases. Day 3. Draining of the phases

Extraction of polyol phase Distillotion of polyols

Day 4. Distillation of heavy phase (Glyc.) or Extraction of heavy phase (Amin.)

Distillation of heavy phase (Amin.)

Day 5. Work up of pOlluted solvents

2 hours 5 hours 2 hours 1 hour 3 hours' 5 hours' 5 hours' 5 hours'

. Two processes are carried out at the same moment.

5 hours

According to this schedule, two operators and one supervisor wil! be needed to run the pilot plant, The nature of the process does not require continuous operation (24 hours a day),

Table 4.1.

n

Overall mass balances

IN PU MEG Total amount PU MEG Total amount PU NaOH Total amount PU NaOH Total amount OUT Glycolysis of flexible P U

850

POlyol

130

Rigids

DADPM

980

Total amount

Glycolysis of rigid PU

850

Polyol

190

Rigids DADPM

1040

Total amount Amnolysis of flexible PU

850

POlyol

115

DADPM Urea Na₂C0₃

965

Total amount

Aminolysis of rigid PU

850

Polyol

185

DADPM Urea Na₂C0₃

1035

Total amount

\

lAh~tS ()..-,~

",-"\~~

',,,,,-

~,

640

320

20

980

510

510

20

1040

640

115

50

170

965

510

180

75

270

1035

- - - p u

recvcling---4.

BALANCES

In this chapter the balances are discussed.

4.1.

Mass balances

From the mass balance sheets (appendix 3) one can find that the overall mass balances over all processes are right. Because th ere are no recycle streams. the balances are quite uncomplicated. The conversions in the reactor for the different chemical routes are calculated accordlng to the reactlon mentioned in chapter 3.2.

Because the magnitude of the streams mentioned are just rough guidellnes. the equipment is designed to process the largest stream they are related to.

Nitrogen consumption. to blanket equipment. is neglected in the mass balance. The overall mass balance is shown in table 4.1.

4.2.

Energy balanees

A5 not all processes involve enthalpy ditferences, the energy balances are just shown tor the reactor. dryer. heat exchangers and evaporator (appendix 3). To obtain a process as flexible as possible, the product-streams are not used to heat the reactants. Moreover the possibilities to do 50 are limited since the process is carried out batchwise.

Heat exchanger duties are calculated in chapter 5.5.1.

Fig. 8. Shredder (UNTHA)

1

(

~'\

il\.

J

1

\.~

Fig. 9. Detail of cut section

RECIRCULEREN EN KONTINU TOEVOEGEN

•

Fig. 10. In-line High Shear Mixer (Silverson)

- - - p u

recycling

-5.

EQUIPMENT

All the equipment necessary for the process as described in chapter 3 is specified here. This chapter is divided In a feed, a reactlon, a work-up and a storage section. Also auxiliary equipment Is described.

5.1.

Feed section

The feed section consists of a description of two cutters, a high shear mixer, a dilution vesseL a centrifuge and a dryer.

5.1.1. Cutter 1

Since different sizes of PU can be recycled in the pilot plant the PU has to be cut in

workable parts, e.g. 1.5 m, before it can enter the shredder. This can be accomplished by avertical bandknife from Mica machinery. The bandknife has to be manned by two operators (for safety reasons) because it is not a fully automatic device. It is advisable to cut the PU as soon as it comes in, so it will not be a bottie neck in the process scheme.

5.1.2. Cutter 2

The rigid and the flexible PU are cut from pieces of 1.5 m into pieces of 5 cm in the shredder from Untha (fig. 8). The PU can be fed to the cutter either by hand or by

emptying a small container with a fork lift truck. Rotating knives in the machine pull the PU into the cutter (fig.9). At the bottom ot the cutter a sieve plate with holes ot 5 cm is

installed. This plate prevents particles bigger than 5 cm trom leaving the cutter. Though the throughput ot the cutter is 1000 kg/hr, the cutting might take longer because the volume ot the flexible PU is about 30 m3•

5.1.3. High shear mixer

The particles are ted to the vessel with a solvent (methanol). An In-Line High shear mixer trom Silverson pumps the suspension several times round and reduces the size of the partieles to 5 mm (fig. la). The size of the vessel is 2.8 mJ• The vessel is part of the Renory

equipment. The rigids as weil as the tlexibles can be cut in the high shear mixer by simply

changing the disintegration head.

-n

When a partiele passes the high shear mixer, it is assumed to be cut in two. To decrease ).

~

I

"o~

I

the size ot a partiele trom 5 cm to 0.5 mm, it should pass the high shear mixer at least 7

times. To be quite on the safe side, the process is designed with la passes. This means that n :

1-af ter leaving the high shear mixer 90% of the dispersion is recycled to the dilution vessel and lOOk is centrifuged.

5.1.4. Dllution Vessel

The in-line high shear mixer needs a vessel. A dilution vessel trom the Renory equipment can be used. This vessel has to be equipped with:

entrance tor solid particles (hopper with discharging device and vent system), entrance tor liquids: wash solvent, MEG and MELA trom storage tanks and MEG and MELA trom the reactor.

entrance tor dispersion (mixing), entrance for nitrogen,

vent system with reflux condenser.

control instruments for level. pressure and temperature,

pressure relief,

exit tor dispersion (mixing and filling the reactor). 10

--.

Fig. 11. Horizontal D-Canter centrifuge (Sharpies. Alfa Laval)

LS LP

Fig. 12. Rotating Drum Dryer (Ohl)

J

- - - p u

recycling

-A description of the dilutlon vessel is found in appendix 1.

5.1.5. Centrifuge

When the PU leaves the high shear mixer, it is dispersed in poliuted wash solvent. To remove

the solvent a centrifuge is needed. Because the solvent might be flammable and explosive,

the centrifuge should be sealed and blanketed with nitrogen. These high specifications are

'~ met by Sharpies super D-canter centrifuges from Alfa Laval. Both horizontal and vertical

orientated centrifuges are avoilabie. The latter is especially designed for operation under extreme conditions with regard to pressure and temperature. For this process the horizontal centrifuge (fig.11) wilt do.

\, J " I ' c\ <f ( ,

::J..--

5 1

6 Drver

,

.

.' ,)

\)

~

,

\,

~'

' ,,('

.

I' " .~ .. , \ ~ \, _I '\ '\ I

V O~ ~,i(

The cenlrifuge is assumed to remove 90% of the solvent trom the PU. Probably better results

c!J

~€)JI:;be achieved, but experiments wilt have to prove this.

X

It is possible to replace the hopper of the solid feed system by the dryer. The advantages

-'

7

are: - - _ _ _

1. It saves the building and maintenance of an additional apparatus (rotary drum / '

,-,I dryer). _ '

2. 1f saves transportation equipment.

The disadvantages are:

1. Less flexible system because it's more difficult to make dispersion feed. 2. The formation is higher because the hopper has to be taller to get a good

distribution of nitrogen.

~-' 3. The drying is less efficient.

Because it is more difficult to make dispersion feed if the hopper and the dryer are combined, the pilot plant is designed with a separate hopper and dryer.

A rotary drum dryer was selected because high heat transfer rates are achieved in this apparatus. This is a point of concern because PU is an insulator. The ~rum dryer can remove nearly all the solvent (Iess than 1%

remai~

.,~

Vol

~t.vt

. \,)"

\''"1,

---~

~

v The maximum degree of load of the dryer is 70%, but to guarantee the flexibility of the

entire process a 20 m3 was chosen. A good example of a drum dryer is the Ohl tumbling drum dryer (Taumeltrockner) (fig.12). Special features are:

J

can be operated under reduced pressure, explosion protected design available,

delivered with vacuum station and condenser.

5.2.

Reaction section

The reaction takes place in a 1.5 m3 reactor from the Renory equipment. To operate

adequately it must be equipped with:

7

exit with sieve (cireulotion of MEG/MELA at dispersion teed), .

t!j

exit with peephole (for phase separation), - .

vent system with reflux condenser,

control instruments tor temperature, pressure and level. pressure reliet,

inline/online sampling,

entrance solid feed, entrance dispersion, entrance solvent,

entrance nitrogen,

heating jacket.

stirrer.

A description of the reactor can be found in appendix 1.

I

0

~

t

e

p

lw>t:

spld;

*'

l~ J.r'1~ry

re.p

r~.se'-\

.

,t--1 ~

ll

vev-y

lO""'f~e

r

CLd:

01

i\.t\;~.)f""",~

GO.$f:s.

Wnt

~ot ~0~'L

cotLCuJ..CII

.

.JlC1\..V)

Ct?

t(~~eS5 \À~~f..vt,

-Table 5.1. Results of the R DC -design

Solvent S/F ratio 1 Eq.stages

N,

14.34 drop size (mm) dp 1 E.input (W/kg) E 0.264 P.input (W) Po 0.095 Area (m2) A 0.0169 Diameter (m) 0 0.146 Rotor d. (m) R 0.088 Stator d. (m) S 0.103 Comp. h. (m) H 0.022 Rotor sp. (S-I)

N

8.52 H. stage (m) HETS 0.240 H. column (m) H,o, 3.442

v=:

0.0

1

-)t

'·4 ::

0·;'3

~

V::

SJ 00

*

11 . 1 1 ::.

~

DO

lc..t

1Yt'MPo-"'l<;;" -:::

~~O

MEG MELA 2 1 2 3.70 14.34 3.70 1 1 1 0.264 0.256 0.256 0.139 0.095 0.139 0.0219 0.0180 0.0234 0.167 0.152 0.173 0.100 0.091 0.104 0.117 0.106 0.121 0.025 0.023 0.026 7.82 8.25 7.57 0.488 0.166 0.274 1.676 2.380 0.941

-

.I

. ...)

- - - PU

recycllng---5.3.

Work up section

The work up sectlon can be divided in the glycolysis and the amlnolysis. For the work up of the produets of both phases a liquid-liquid extraction column and a device to re move volatiIe solvents are necessary. Because the work up of both reactions has to take place in the same equipment, no subdivision is made. For polyol purification either a fixed bed

reactor can be chosen or the purification can simply take place in the reactor. For .

regeneration of the polluted solvent a batch distillation column is the most suitable ; "I

'- - { ~1(.l"""V(tt~.,,~1

solution.[18, 19] f . • . t'\,.C\" J/ect\~,,"\"

" 0 v\-·_ \ , "t '.) ' . ' \) \

5,3.1.

Llguid-liguld extraction eguipment

(

c.a

"'\..t

i

"'-U.o

~s

ó

?~'c"C'--t

\

0"1)

There are several different extraction devices. In the entire process three extraction processes are required. Because they have to be carried out in the same column, the extractor has to be very versatiIe and flexible with regard to its capacity.

To fulfil these requirements a rotating-disc contractor (!mC) extractor was chosen. It has several advantages, apart fr om its versatility and flexibility:

~

t

~

aJ.'( \

t

q

te

1. Control of droplet size is possible by varying the SR

2. High efficiencies can be achieved. so size of equipment is limited. Smaller columns need less time to reach abilit' 0 less prOduct is wasted or has to be

refluxed. Besides that. small columns are easier cleaned. which is an advantage in batch processes as weil.

The main disadvantage is the high energy consumption.

The theoretical number of stages was calculated by the Kremser equation [9]:

f =

S -

1

SN+1 - 1

Where f = part of DADPM not separated (x,lx,) xr = fraction DADPM in feed

x, = fraction DADPM in raffinate S

=

separation factor (<l>/K.<l>r)

K = distribution coëfficiënt <l>1 = amount of solvent (m3/s) <l>r = amount of feed (m3/s) N

=

number of equilibrium stages

\ __ \J

(1)

The distribution coëfficiënt for the extraction of DADPM with MEG was calculated from ICI experiments at 100°C (appendix 4). Because no data were known about extraction with

MElA. the distribution coëfficiënts were assumed to be equal.

The liquid-liquid extraction column was designed according to the course "Advaneed Separation Technology" by Prof. R. Krishna (1991) [10]. Physical properties were evaluated at 100°C, because the distribution coëfficiënt was known at that temperature. The most important data of the column and the calculated efficiencies are shown in table 5.1. Extended results and the procedure are shown in appendix 9.

- - ---.:

It should be noted th at the liquid-liquid extraction column should be/stabilize . Until this situation is reached. it must be operated with total reflux to decrease mount of waste streams.

Table 5.2. T (0C) 150 175 200 ' 225 250

-Solvent contents in liquid phase atter flashing; Polyol contents in vapour phase is negligible

«

0.1%).

Mixture MEG/Polyol Mixture MELA/Polyol

P (bar) P (bar) 0.100 0.050 0.025 0.100 0.050 0.025 0.500 0.250 0.126 0.195 0.097 0.049 0.200 0.100 0.051 0.086 0.043 0.021 0.090 0.046 0.023 0.042 0.021 0.010 0.044 0.022 0.011 0.022 0.011 0.006 0.023 0.012 0.006 0.013 0.006 0.003 - I

~i

- - - p u

recycling

-5.3.2. Removal of volatiIe solvents

There are three ways to remove volatiIe solvents from DADPM, rigids and polyols:

1. Distillation. The high difference in vapour pressures results in a high relative volatility. A McCabe Thiele diagram has been drawn (appendix 5) which shows that the number of required equilibrium stages is very low.

Disadvantages of distillation are:

If it is done continuously, it takes time, energy and prOduct before stabilisation is achieved.

If it is done batchwise, the polyol can not be as pure as in continuous processes, because a batch distillation only has a rectifying and no stripping section.

Contact times of polyol with heated surface are quite large, which can lead to degradation of the pOlyol.

Even at high temperatures, polyol is very viscous. This will result in fouling of the sieve trays or packing.

2. Flash. Isothermal flash calculations have been done (appendix 6). This shows reasonable purity of both phases. Results are shown in table 5.2.

3. Evaporation. Because polyol is very viscous, a film evaporator might be an elegant solution. The advantages of film evaporation are [11]:

Short contact times with heated surfaces.

It is a flexible process with regard to composition and quantity of feed. Very pure bottom products can be achieved.

The process is suitable for high viscous and even solid products.

The equipment is very easy to clean.

There are three kinds of evaporators [11]:

1. Short tube evaporators.

2. Long tube evaporators. A subdivision into falling film, rising film and forced circulation can be made.

3. Agitated film evaporators. Higher heat transfer rates at viscous products [11]. This makes this type particularly effective with viscous heat sensitive products.

Disadvantages are its high costs and maintenance.

A wiped film (or agitated film) evaporator was chosen to separate volatile solvents fr om polyols, DADPM. and rigids.

Heat duties are calculated by the following equation:

Where:

Q

=

<Pm,

vap !1 vap H +

<Pm

,

f Cp !1 T

mass flow of vapour. kg/s mass flow of feed, kg/s heat of vaporisation, J/kg

specific heat capacity of the mixture, J/kg.K

difference between feed and boiling temperature, K

(2)

When viscous liquids (100 cP) are processed, the heat transfer coefficient is approximately \ \ )

1700 W/m2K. This results in an necessary area of about 10 m2. --___________ \....

O"~

(cvt

(

tJ

[,-y

\~.

5.3.3. Batch distIlIation

The easiest way to recover the polluted solvent is by using a batch distillation process. Because the degree of pollution is expected to be low and because the distillate is the desired product, a batch column is the most suitable solution.

Batch columns usually give higher purities for the top product. but lower for the bottom

'J

I

- - - p u

recycling -product [18]. Because the top -product is the solvent, thls is ideal: the bottom product (containing little solvent) is not purified any further and is burnt.

5.4.

Storage sectlon

For this pilot plant at least six storage tanks are needed. Three to store the clean MEG, MELA and solvent and three more to store the pOlluted liquids.

When experiments with other solvents than mentloned are deslred (e.g. DPG, DEG as reacting solvents and xylene as wash solvent), even more tanks can be added.

All other products, catalysts etc. are stored in intermediate bulk containers, drums or in the apparatus.

The storage tanks should be able to contain 6 m3• Normally a tank is only filled for 800k [1]

so the volumes of these tanks have to be 7.5 m3• Since the volumes of all tanks are the

same, their dimensions will be equal as weil. The dimensions of the tank are calculated by using the following equations [12]:

v

= 'lt X D2 X H

4

H

₌

1.3

D

When the volume of the tank is 7.5 m3 the diameter of the tank is 1.95 m.

(3)

(4)

To prevent evaporation of the solvents the tanks are floating roof types or they should be blanketed with nitrogen (appendix 0).

For storage of intermediate mixtures and produets intermediate bulk containers (lBC) and drums can be used. They have the big advantage of being moveable (which means

flexibility will be increased). The major disadvantage is the risk of leakage. Containers up to 1 mJ _{are transportable by a simp Ie fork lift truck.}

5.5.

Auxiliary eguipment

The auxiliary equipment consists of heat exchangers, pumps and the vent system.

5.5.1.

Heat exchangers

The heat exchangers are designed by calculating the duty and the necessary area to

warm each stream th at passes the exchanger. Then t~fH~~a is chosen to be the

design area. The formulas used for the calculations ar . hd awn rom Coulson and Richardson [12]. For calculating the area and duty of the condenser the same calculation route was followed.

where

Q = U A AT

Q = heat transferred per unit time, W,

U

=

the overall heat transfer coefficient. W /m2°C A = heat-transfer area, m2

M = the mean temperature differences,

o

e

.

14

-

,-Table 5.3. Heat exchanger duties

on

d

arees

Heat exchanger AREA (m2) DUTY (kW)

H5 1.25 28.20 H34 0.32 18.90 H35 0.16 9.34 H36 3.00 100.00 H40 0.90 16.73 H41 2.20 41.34

Table 5.4. Pump duties

Pump ~ (I/s) ~P (kPa) 11 (%) P (W)

P2 0.70 40 35 80 P7 0.77 55 40 105 PlO 5.20 33 40 430 P12 0.70 24 35 53 P15 1.26 33 40 104 P17 1.26 33 40 104 P18 5.20 200 40 2600 P26 0.17 200 35 95 P30 0.05 50 30 10 P32 0.09 200 30 400

... )

- - - p u

recycling

-In this procedure counter current flow is assumed.

The value of U is derived trom literature [12]. The mean temperature is calculated by using the following equation:

Tl -

tz ) - (

T

_{z - t}

_l )

In

(Tl -

tz )

( T

_{z -}

t_l )

where LlTln = log mean temperature difference,

Tl = inlet shell-side fluid temperature, T2 = outlet shell-side fluid temperature, tl = inlet tube-side temperature,

t2

=

outlet tube-side temperature.

The necessary heat (Q) is calculated by:

Q = Cp <I> m

11

T

where Cp = heat capacity kJ/kgOC.

The results of these calculations are shown in table 5.3.

5.5.2. Pumps and transport

(6)

(7)

All pumps used are centrifugal pumps. Centrifugal pumps are the most common pumps,

being very flexible with regard to the amount and the nature (viscosity, dispersions) ot the fluid. Pump duties are calculated according to Coulson & Richardson [12] .

p

Where P = Power requirement of pump, W

LlP = Pressure difference, Pa

<l>v = Throughput, m3 Is

11 = Estimated pump efficiency.

The results are shown in table 5.4. Two pumps (10 & 18) are used for pumping a 10% dispersion.

(8)

Transportation of the solid PU particles is necessary several times during the process. Various transportation methods can be used e.g.:

pneumatic transport through pipes, rotating spirals in pipes,

screw conveying through pipes.

Untortunately little intormation on these systems is availobie. More research wil! be necessary betore a decision con be made.

A guaranteed and flexible method is transport by a fork lift truck, but that demands much attention of the operators.

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Both the reactor and the dilution vessel are equipped with a vent system to collect all evaporated solvent. It consists of a reflux column, a condenser. a condenser receiver. a demister and an active carbon filter (2]. It is recommended to equip the hopper of the solid feed system with a vent system as weil.

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6.

PLANT LAYOUT

The basic setup of the pilot plant requires easy access to all equipment. This demands a spacious arrangement of the different apparatus. To diminish the influence of

meteorologie al effects on machines and operators. it is advised to place all equipment indoors.

A centraliane separates all process equipment from the storage section. A fork lift truck should be able to manoeuvre freely around in the plant. because it must move IBeis and drums. Pumps and other equipment th at need maintenance must be installed near this lane. for easy access ..

Emergency exits must especially be placed near the process equipment because the operators will work at this part of the plant most of the time.

To enable future extensions. the space might be designed larger than strictly necessary. Use of other solvents and experiments with other equipment should be possible.

Figure 13 presents the proposed layout of the pilot plant with minimum requirements. To increase flexiblity. not all piping should be fixed. The piping system would be very extended, since the equipment is combined frequently.

There is no fixed connection between the feed & reaction section. extraction column and evaporator. The fork lift truc will transport the drums and containers with intermediate products.

Flexible pipes connect the equipment to the tap. Special care should be taken to make sure th at the right pipes are connected. Therefore good labelling of pipes and taps is necessary.

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