vacuum systems in the edible oil industries · deodorization bleaching fatty acid soapstock...

Körting Hannover AG

VACUUM SYSTEMS

IN THE

EDIBLE OIL INDUSTRIES

VACUUM SYSTEMS

IN THE

EDIBLE OIL INDUSTRIES

Körting Hannover AG • Division SBadenstedter Str. 56 • D-30453 Hannover • GermanyPhone ++49 511 2129 - 0 • Fax ++49 511 2129 - 223 • E-mail [email protected]

page : 2


STEAM JET VACUUM EJECTORSIN THE EDIBLE OIL AND RELATED INDUSTRIES

The basic idea for the design of STEAM JET EJECTORS has to be considered

in that way that the available raw materials and available energy (in which ever form that

they take) should be converted with the least possible waste and therefore the best possible

efficiency.

For the present and immediate future one has to live with the fact that only part

of this general requirement can be fulfilled . In all transformation processes not only there

is a limitation by the physical constraints but also financial aspects have to be observed.

In this respect every material processing plant and every machine has to represent a compromise

between that which is desired, that which is possible and that which makes financial sense.

This statement is also true for the vacuum ejector systems that will be presented in this paper.

With regard to this topic the application of these systems in the edible oil industry will be shown.

However, that which is true for this branch of industry is also true for related industries,

e. g. fats and soap production, and indeed for all processes in which products are manufactured

for human consumption.

In the past steam ejectors were - and sometimes they are still today - reputed of being "steam guzzlers".

This may be true in the case where ejectors are misdesigned or misoperated. But if they are well designed

and properly operated, ejectors are a simple and robust equipment being capable to convey large

mass flows from low pressure to a higher pressure level.

page : 3


Venturi Tube / Ejector

PART I In its simplest form an ejector is a Venturi Tube. In the converging section an acceleration withpressure drop takes place, and in the diverging section a retardation with pressure increase.If the acceleration and, consequently, the pressure drop is large enough, it is possible,for example, to achieve a lower pressure than the atmospheric pressure in the smallest section.

PART II At this point anything can be entrained.

PART III If the Venturi tube is cut at this point and a chamber is laid round the cutting, we have alreadythe simplest form of a jet ejector, for example, a water jet ejector.

PART IV When these features are transferred to gaseous media such as steam, and when a Laval Nozzleis used, it is possible to convert the adiabatic difference into high velocities. Now, we have asteam jet ejector operating with sonic or supersonic velocity in the smallest mixing section.

III

pc

2´

pc

trtr

1 22´´´

2´´

ss

pc

43

B

IVA

A

supersonicB

subsonic

mixmix

4´

cp d

d

5

C

C

A

II

A

B

I1 2 3

C

C

4 5

page : 4


Characteristic Curves of Pressure and Velocity

1

trc

sonic

2

c s

3

2´´2 ´

2´´´

veloci ty

sp

4 4 ´

c

5

d

dp

pressure

p tr

tr

tr

trpc

m

suct ionf low

Bpc s

s

m s

mot ive

Aflow

Cmixedf low

c d

m d

dp

The motive flow enters the motive nozzle with a fixed pressure and velocity. It is not possibleto transfer the high energy of the motive flow directly to the suction flow.

The LAVAL NOZZLE converts the pressure energy of the motive flow (steam) into kinetic energy, until the nozzleoutlet the velocity will be increased up to supersonic velocity. At this point of the lowest pressure, the suction flowwill be entrained, then accelerated and mixed with the motive flow. In the following converging supersonicdiffuser, the mixing duct and the diverging subsonic diffuser velocity decreases with increasing pressure.

This finally results the discharge pressure of the ejector.

page : 5


Standard terms for Ejectors

spec. mot ive s team consumpt ionB

p s

m s

6Expansion rat io

compress ion rat io

1 steam chest with strainer

2

A

trptrm

1

2 mot ive nozzle3 head

4 inlet dif fuser

6 gauge connect ion G 1/2"5 outlet di f fuser

3 54

µ=mtr/ms = f (pd/ps, ptr /ps)

ptr /ps

pd/ps

p dd

Cm

B suct ion f low ms at pressure ps

A mot ive steam mtr wi th pressure ptr

C mixed f low md = ms + mtr at pressure pd

The compression attainable from the suction nozzle to the outlet nozzle is, dependent onthe motive pressure, the specific motive steam consumption, and finally on the internal geometryof the ejector.

page : 6


Vacuum in Vegetable Oil Processes

neutralisationhydrogenation fractionation

deodorization

bleaching

fatty acidsoapstock

soapstockdecomposition

winterizing

fatty acid

deacidification anddistillative

deodorization

hydrogenated fats

edible oilsedible fats

bleachingdegumming

degummingelectrolyser

vegetable oils animals fatsfish oil

This picture shows the various process stages in the processing of edible oil.The red fields show the stages of the process which are carried out under vacuum,

– separation,– bleaching, drying,– deodorising following alkaline deacidification,– destillative deacidification with deodorization.

page : 7


Design Parameters

vapour = suction flow

refined oil

tp

mt

strippingsteam

mheating fluid

tm

t col tp col p

p col mtp

m

distallation column

crude oil mt

tw out

waste gas

condensate

coolant m,

vacuum pump system

P

coolant

elt. energymotive steam

m, tw inm, p, t

el

In all of these process stages similar requirements must be fulfilled;

i.e. relatively large mass flows (mainly stripping steam and some non condensable gases)must be sucked from a vessel (column, dryer, reaction vessel) at a low absolute pressureand be compressed to atmospheric pressure.

In general atmospheric pressure is 1.000 mbar (at sea level).

Working pressure in a distillation column is generally these days taken to be 2,5...4 mbar(sometimes it can be lower).

I.e. ejector systems in such columns must be able to achieve compression ratiosfrom 400..250 (max. compression ratio of a single stage is 12 ....14).

That is why multi-stage ejector systems are used. Those systems consist of ejectors with after connectedcondensers where all condensable vapours are liquefied.The following ejector stage has only to compress the non condensable gases, which reduces the total motivesteam consumption of a multi-stage system to a minimum.

page : 8


Multi-Stage Ejector Vacuum System

Today nearly exclusively those ejector systems are used in the edible oil industry world wide.

The advantages are obvious:

- large mass flows can be moved within acceptable plant sizesand sensible steam requirement figures (300 kg/h stripping steamat 2,5 mbar still represent a volumetric flow of 150 000 m³/h,that can be conveyed by 1450 kg/h motive steam = 0,4 MW therm.)

- low investment costs.

- low maintenance costs, easy to install and to operate.

page : 9


Multistage Ejector Vacuum System withLiquid Ring Vacuum Pump

3

D

1

2 5

6

AB

C

motive steam

C to ejector vacuum system4

E plate heat exchangerF cooling tower

1-3 boosters / ejector

5-6 direct contact condensers7 liquid ring vacuum pump

4 stand by ejector

8

EF

8 separator

7

D hotwell

A process vapour from columnB scrubber

It is sensible in the case of multi-stage compressions to combine the advantages of steam ejectors andwater ring pumps.

That means up to the first possible condensation stage the stripping steam is exclusively compressed byejectors (so called boosters); after the first condenser a combination of steam ejectors and water ring pumpscan be implemented.

The available coolant and its temperature have a decisive influenceon the design of the first inter condenser, the number of ejector stages and on the total motive steamconsumption for the plant.

page : 10


Cooling water temperature

Cooling tower water

River water

Dec.Apr.Jan.0

5

Mar.Feb.

10

15

Aug.Jul.May Jun. Nov.Sep. Oct.

25

20

30

[°C]

[months]

The possibility of condensing the water vapour (suction- and motive flow), depends on the temperature of thecoolant, which is mainly cooling water.

The graph shows the cooling water temperatures through a year. The corresponding saturation (condensation)pressure gives the limitation for any condenser (surface or direct contact type).

An ejector system has to be designed for the most unfavourable operating conditions,i. e. for the max. stripping steam flow and the highest cooling water temperature.

If -during operation- the cooling water temperature changes to a lower value and the system is not adaptable,energy will be wasted.

As far as the motive steam consumption is concerned, this problem can be overcome by adapting the motivesteam consumption to the actual counter- or discharge pressure in the condenser which is varying with thecooling water temperature.

page : 11


With decreasing water temperature, this can be done reducing the motive steam pressure or, in a moreeconomic way, by reducing the nozzle throat diameter with a movable nozzle needle as shown below.

steam jet ejectornozzle spindle

motor e lement

supply of air

ent ra inment

mot ive nozzle

wir ing d iagram

Electr.power

manipulated var iab le

mot ive s team

transmit ter

pressuremeasurement po in t

mix ing water

var iat ion of d ischargepressure

cool ing water

gas out let

page : 12


Vacuum Pump Systems, where Stripping SteamCondensate is strictly partitioned from Coolant

loop

condensat ioncondensate

directcontact

cond

ensa

tion

a lkal ine

4

2,5

3,2

6,3

2

4

5

8

dist

i l lat

ion

1113

5

20

12,5

31,5

16

810

25

20

14

107

21

18

25 loop

process step

p mbar

70

*40

**

29

t °C

condensat ionsur face

condensat ioncontactdirect

directcontactcondensat ionbr ineloop

condensat ioncontact

alkal ineloop

directcontactcondensat ioncondensateloopC C F

direct

condensat ionice

In the realisation of this separation numerous systems have been developed which are shown in thepicture above:

on the left a pressure scale can be seen,

next to it a temperature scale (this shows the corresponding water boiling or condensation temperatures),

the green areas show at which temperature the first condensation takes place.

Furthermore the following can be seen:

with "normal", cooling water temperatures, (cooling tower / river water), atwo-stage pre compression with steam ejectors is unavoidable.

By using chilled water sets, i. e. water temperatures less than 10°C only onesteam jet compression stage is sufficient.

Only with coolant temperatures far below 0°C it is possible to work without a steam ejector for precompression; condensation is then done either by a salt solution in direct contact condensers or in so calledice-condensers with indirect condensation (shell and tube condensers).

Some of these systems are shown schematically on the following pages.

page : 13


Alkaline System (ACL) Normal Cooling WaterTemperature

4

HP condensate, highly pollutedCW cooling water

EX exhaust gas

2 5

5-6 direct contact condensers

A process vapour from column

C to ejector vacuum system

E plate heat exchangerF to/from cooling tower

D fat separator

H circulation pumpG ph-control unit

1-4 boosters / ejectors

B scrubber

heat ing

NaOH

D

TICEX

AB

C

13

6

motive steam

HP

E

PH

LC H

G

FCW

When using the proven mixing condenser, the mixture of condensed motive steam andstripping steam stays in a closed loop and this circulating water is cooled against "normal" cooling waterin plate heat exchangers, i. e. cooling water at ambient temperature. Sodium hydroxide solution is addedto the circulation fluid to avoid too fast fouling in the heat exchanges from crystallising fatty acids.Using either the usual seal box or an especially developed separator and removing only the portion of the mixedmotive steam and stripping steam condensate from the circulation leads to extremely waste water amount.

The benefits of these systems are:

- small amount of waste water,- low investment costs,- lowest maintenance costs.

Alkaline System (ACL) Low Cooling Water Temperature

page : 14


EX

B scrubber

I water ring vacuum pump

3 direct contact condenser

L heat exchangerK separator

4 surface condenser

1-2 booster / ejector

C to ejector vacuum system

H circulation pump

D fat separatorE plate heat exchangerF refrigeratorG ph-control unit

NaOH

D

TIC

heatingheating

A process vapour from column

AB

1

3

2

motive steam

C

HLC

CWG

PH

E

CW

HP

L SPK

F

HP condensate highly pollutedSP condensate slightly polluted

I

4CW

EX

CW cooling waterEX exhaust gas

With the help of a refrigeration system the circulating water is cooled to about 4...6 °C and is warmedby about 3...4 °C in the mixing condenser

As a result of this the pressure in the mixing condenser is no longer in the region of 50...70 mbarbut about 11...15 mbar.

The necessary compression ratio before the condenser is no longer 20...30 but is now only 5...7;as a result of this one booster stage with a correspondingly low steam consumption may be used.

The compression of noncondensable gases does however require a somewhat higher motive steamconsumption but the total costs for energy in the form of motive steam and electrical power are lower than for the„Normal Cooling Water Temperature“ system.

Due to the higher cost for the refrigeration system the whole system is more expensive.

page : 15


Comparison figures for Alkaline System (ACL)with chilled cooling water

design data :

Stripping steam rate : 200 kg/h water vapour+ 6 kg/h air+ 10 kg/h FFA

Suction temperature : 120 °CSuction pressure : 2.66mbarMotive steam pressure : 8 bar g./sat.

305323

345358

373388

404

424

157155153151148145140 143

0

50

100

150

200

250

300

350

400

450

motive steam (kg/h)

required power (kW)

4 5 6 7 8 9 10 11

cooling water inlet temperature (°C)

el. p

ow

er (

kW)

m

oti

ve s

team

(kg

/h)

page : 16


Ice Condensation Vacuum System

waste water

suction flowfrom deodorizer

TC

M M

1A

M

2

M

3

10M

1B

M M

OF OverflowFW Fresh water

PIC

CW cooling water

11

M

12M

OF

FW

motivesteam

8 separator

exhaust gas

11 coolant compressor

9 heat exchanger10 coolant separator

7M

waste water

CW

98

6

5

4

CW

7 water ring vacuum pump

1A ice condenser

4 steam jet ejector5 steam jet ejector6 surface condenser

1B ice condenser2 melting vessel3 condensate pump

Following the idea to condense the stripping steam at the lowest possible temperature andpressure leads to a system where the stripping steam can be condensed at the processworking pressure of the column. In case this pressure is below 6 mbar, water vapour condensesinto the ice phase.

This can be done:- by direct connection between column and surface condenser which is cooled

by an evaporating refrigerant (i.e. NH3) having a temperature of less then- 20 °C.

That means:- water vapour freezes outside the tube bundle- at a certain load which can be chosen in appropriate limitations, the condenser has to

be isolated from the column and the ice has to be molten by vapour generated in the melting vessel.

- meanwhile a second condenser has to be connected to the column for continuos operation.

This ice condensation system combines the smallest possible quantity of waste liquid with the lowest energyconsumption when operating a vacuum system at a pressure below 6 mbar.Even at the relatively high investment cost, it is worthwhile to invest in such systems because of steadyincreasing costs for energy and waste water treatment.

page : 17


The picture shows a section of an ICE-condensation plant.

vacuum systems in the edible oil industries · deodorization bleaching fatty acid soapstock...

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