5Glycerine

Keith Schroeder

CC Engineering, Ltd.

1. INTRODUCTION

1.1. Chemistry

Glycerine (sometimes called ‘‘glycerin’’) is the name of the commercial product

consisting of glycerol and a small amount of water. Glycerol is actually trihydric

alcohol C2H5(OH)3, which is more accurately named 1,2,3-propanetriol. Its chemi-

cal structure is shown in the formulas given below.

Much of the naturally produced (as opposed to synthetic) glycerine is a copro-

duct of saponification of fats using caustic soda. The reaction is (1)

3NaOH CH

H2C

H2C

OHHO

OHCH2

CH2

CR2

O

CHC

O

R3

O O

R1

H2C

CO

Na

O

R2

H2C

CO

Na

O

R3

H2C

CO

Na

O

+ +

Triacylglycerol Cautic Glycerol Soap

O

CR1 O

ð1Þ

Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.

191

The resulting coproduct stream, called spent lye, typically contains 10–25% gly-

cerol (depending on the production process) as well as contaminants such as salt

(NaCl), water, and various organics usually known as matter organic nonglycerol

(MONG). Probably the more abundant source of glycerol is the hydrolysis (splitt-

ing) of fats and oils, which produces fatty acid and glycerol. This reaction is (1)

CH2

CHCH2

OC

O

R3

OC

R2

O

CH

H2C

H2C

HO OH

R1

H2C

COH

O

R2

H2C

COH

O

R3

H2C

COH

O

OHH

Triacylglycerol Water Glycerol Fatty Acid

+ +3O

CR1 O

OH

ð2Þ

The concentration of glycerol in the resulting sweetwater is 10–18% and gets its

name from the characteristic sweet taste imparted by the glycerine (2). Glycerin is

also recovered as the coproduct of methyl esters (from fats and oils) and fatty alka-

nolamides; the amount produced from these sources has historically been relatively

small but has increased significantly due to worldwide construction of fatty alcohol

plants and, more recently, methyl ester biodiesel plants. There are some other inter-

esting sources of glycerol, which will be discussed below. This Chapter concen-

trates on the recovery of glycerol from soap lyes (water and glycerol from soap

making) and sweetwaters, as they represent the majority of the recovery applica-

tions. Synthetic glycerine will not be covered, as it is not derived from fats and oils.

1.2. History of Glycerine Processing

Glycerine was first identified in 1770 by Scheele, who produced it by heating olive

oil and litharge. In 1784, he observed that the same substance could be produced

from other vegetable oils and animal fats, such as lard and butter. He called this new

substance ‘‘the sweet principle of fats’’ due to glycerine’s characteristic sweet taste

(3). In 1811, Chevreul, while studying Sheele’s Sweet, as it had come to be called,

coined the modern name glycerine from the Greek word glyceros, meaning

‘‘sweet.’’ After closely studying glycerol, he was awarded the first patent relating

to its manufacture in 1823. Chevreul also distinguished himself by doing some

important early research work on fats and soaps. By 1836, Pelouze had determined

the formula for glycerol, and finally Berthlot and Luce published the structural

formula in 1883 (4).

Nitroglycerine was discovered in 1847 by Sobrero. This compound is danger-

ously unstable, which limited its potential for commercial applications. In 1863,

192 GLYCERINE

Alfred Nobel demonstrated nitroglycerine’s explosive capabilities, and in 1866

he invented dynamite. He followed that discovery by the invention of blasting gela-

tin, a mixture of nitroglycerine and nitrocellulose, in 1875. Nobel’s commercial

success and humanitarian efforts are well known, but equally important was the

vital role his inventions played in advancing the Industrial Revolution. The

demand for his explosive products helped to form a large and growing demand

for glycerine.

The history of glycerine is closely related to the history of soap making because

one of the earliest commercial sources of glycerine was recovery from soap lyes,

and soap lyes continue to be a common feed stock for glycerine recovery today. In

the early 1870’s, the first U.S. patent ‘‘for the recovery of glycerine from soap lyes

by distillation’’ was issued. The process was further developed by Runcorn in 1883.

In the decades following, the soap industry began recovering glycerine from the

‘‘waste streams’’ of their soap-making operations on a relatively large scale, thus

making glycerine a readily available commodity.

A prominent source of glycerine is from the sweetwaters of fat splitting, which

initially came from the manufacture of stearine for candle making (3). The famous

Twitchell process for fat splitting was developed at the turn of the century. Twitch-

ell developed a process for splitting fat using a catalyst and dilute sulfuric acid that

produced an acceptable product (5). This was followed by high-pressure autoclave

splitting, which relied on high-pressure steam for hydrolysis of fat and produced a

superior product. Today’s modern fat-splitting plants, using stainless steel columns

with counter current flow of fatty acid and sweetwater, are the latest development in

the splitting process. The high-quality sweetwaters obtained allow efficient refining

into the high-purity grades of glycerine used today.

1.3. Economics

The consumption of glycerine has remained strong over the years, and trends indi-

cate that this is not likely to end soon. Although glycerine is a mature product with

a multitude of uses that continue to increase, the Soap and Detergent Association is

trying to develop new uses for the product. In 2003, the market price of natural USP

glycerine in the United States was at $0.55–0.75/lb., but has been as low as $0.25/lb.

and as high as $1.00/lb. throughout the 1990’s. The U.S. production of glycerine

is approximately 600 million pounds (270 million kg) per year and grows 2–5%

annually. Imports add about 10% more supply, while only a small amount is

exported. The world glycerine production is well over one billion pounds (450 mil-

lion kg) per year (6). Many new fatty acid splitting facilities have come online,

especially in Asia, where palm and coconut plantations are plentiful. China con-

tinues to add capacity as more products come into demand.

Synthetic glycerine is produced in the United States by a single supplier. There

are only three producers worldwide. The strong supply of natural glycerine and the

rising cost of feedstock (propylene) are likely to pressure producers to revise their

production. Also, some epichlorohydrin, an intermediate product, can be diverted

into other products of potentially high profitability. Synthetic glycerine may contain

INTRODUCTION 193

small quantities of chlorinated hydrocarbons, which has caused some concern in the

health care applications.

2. PROCESSING PRINCIPALS AND DETAILS

2.1. Sources of Glycerine

Natural glycerine is essentially a coproduct of certain processes performed on

animal or vegetable fats and oils:

1. Fat splitting under high pressure and temperature, in the presence of water, to

obtain fatty acids and sweetwater glycerine. The fat-splitting process provides

the majority of glycerine in the United States. Processing of splitter ‘‘sweet-

water’’ requires less intensive processing and lower equipment costs because

salt content is not high and common stainless steels can be used as the

material of construction.

2. Saponification of fats with sodium hydroxide (caustic soda, NaOH), yielding

spent lyes containing glycerol, water, sodium chloride (NaCl), and other

impurities; depending on the process for saponification and soap washing, the

concentration of the glycerol varies greatly. The presence of high salt levels

requires higher cost metallurgies as is discussed later in this chapter.

The chemical reactions for these processes were given earlier in formulas 1 and 2.

3. Transesterification of fats, typically with methanol, using sodium methoxide

as a catalyst, to produce methyl esters and glycerol (7).

The chemical reaction for this process is as follows:

CH

H2C

H2C

HO OHO

HH3CNaOCH3

CH2

CHCH2

OC

O

R3

OC

R2

O

3RCOOCH3+ +3

Triacylglycerol Methanol GlycerolCatalyst Methyl Ester

O

CR1 O

OH

ð3Þ

Although the initial chemical reactions for the above processes yield high

theoretical concentrations of glycerol, subsequent washing steps dilute the glycerol

to typical values:

Fat Splitting 10–18%

Saponification, kettle 10–15%

Saponification, continuous 20–25%

Transesterification 25–30%

Synthetic glycerine is produced (Dow process) by chlorinating propylene to allyl

chloride, converting this to dichlorohydrin, which is then converted to glycerine

194 GLYCERINE

through the addition of small amounts of dilute sodium hydroxide and sodium

carbonate (8).

2.2. Pretreatment Techniques

The purpose for treatment of spent soap lyes or sweetwater is to prepare the feed-

stock for glycerine recovery by removing impurities that cause operating problems

in subsequent processing steps, resulting in a poor-quality product. In the handling

of dilute glycerol feedstocks, care must be taken to prevent fermentation, which can

cause serious loss of glycerine due to the formation of trimethylene glycol (TMG),

(1,3-propyleneglycol), gases, and acids during decomposition. Fermentation can

occur in glycerol solutions of 25% by wt. held at temperatures of 125�F or less.

Glycerol solutions above 25% prevent bacterial growth as does the presence of

salt or caustic. Furthermore, these gases produced by the bacteria may cause pro-

blems in evaporation of the lyes, and any TMG can be difficult to separate from the

glycerine by distillation due to their similar vapor-pressure characteristics. Excess

impurities in soap lye crudes can act as nucleation sites for crystal growth in the

evaporator, and aid the growth of larger, more easily separated salt crystals.

Spent soap lyes, as they are drawn from the continuous saponification area or

soap kettles, consist primarily of glycerine, sodium chloride, and water as well

as small quantities of sodium hydroxide, sodium sulfate, sodium carbonate, soap,

and fatty acids, and also some albuminous and oleaginous matter. To remove these

impurities, the soap lye is generally treated in batches.

First, a cooling and settling step is used to separate the soaps and fatty acids. The

fatty acids are less soluble at lower temperatures and rise to the surface, where they

can be removed by skimming. Other heavy materials will settle to the bottom and

remain in the tank until it is cleaned periodically. This skimming of the fatty acid is

best done in the soap plant before the lyes are transferred to the treatment plant

(Figure 1).

AcidAlum

Causticsoda

Carbon frombleaching columns

Air

Airagitator

Airagitator

Secondtreatment

tank

Firsttreatment

tank

Soap lye fromskimming tanks

Filtratetank

First filterpress

Second filterpress

Cake Cake

Figure 1. Batch soap lye treatment flowsheet. Courtesy of Crown Iron Works Co. (9).

PROCESSING PRINCIPALS AND DETAILS 195

Second, the skimmed and settled lye is transferred to the first treatment tank. It is

good practice to heat the lye to 50–60�C during the transfer pumping or in the treat-

ment tank; the treatment process is most efficient at this temperature. In the first

tank, the lye is acidulated to pH 4.6–5.0 with sulfuric or hydrochloric acid. Hydro-

chloric acid is recommended to avoid the formation of sulfates. These sulfates can

build up in the recovered salt, which is returned to the soap-making area where sul-

fates are not desirable. This acidulation step liberates fatty acids, which may be

skimmed, as most are insoluble in water.

Third, a coagulant may be added to cause the residual fatty acids to form inso-

luble soaps, which coagulate. The pH is adjusted to 4.5. At the same time, com-

pressed air is bubbled into the tank and is used for mixing and to ensure

oxidation. The coagulant is usually either aluminum sulfate or ferric chloride. If

ferric chloride is used, the air agitation is particularly needed to oxidize the iron

to its ferric state (10). Although ferric chloride is more difficult to handle than alu-

minum sulfate, it is strongly corrosive to iron, and is more expensive, but is usually

preferred because the salts formed using aluminum sulfate introduce additional sul-

fates. The precipitates of either aluminum hydroxide or ferric hydroxide will adsorb

other impurities such as fatty acids or proteinaceous matter. The chemical reactions

for these two precipitants neutralizing the excess sodium hydroxide are described

below.

It takes 1 kg of ferric chloride to neutralize approximately 0.44 kg of sodium

hydroxide, according to the following formula:

FeCl3 � 6H2Oþ 3NaOH! FeðOHÞ3 þ 3NaClþ 6H2O ð4Þ

Exactly 0.36 kg of sodium hydroxide is required to neutralize 1 kg of aluminum

sulfate according to the following reaction:

Al2ðSO4Þ3 � 18H2Oþ 6NaOH!2AlðOHÞ3 þ 3Na2SO4 þ 3NaSO4 þ 18H2O ð5Þ

The composition of the first treatment can be checked by filtering a sample in the

laboratory and then adding a small amount of dilute ferric chloride or aluminum

sulfate to the filtrate. Any precipitate indicates that the reaction has not been taken

to completion (11–13). The contents of the first treatment tank are then filtered,

typically in an open plate and frame filter. The filter cake is discharged periodically

for disposal. Filter aid can be added, if required.

Finally, the filtrate from the first press flows to the second treatment tank, where

sodium hydroxide is used to adjust the pH to approximately 9.0. Compressed air is

again used to agitate and oxidize the batch, before filtration through a second press.

A smaller amount of precipitate is produced in this step, and therefore the filter can

be sized for less area. In addition, spent bleaching carbon from the glycerine dis-

tillation plant can be added at this point, with the goal of recovering some residual

glycerine into the treated lye.

196 GLYCERINE

Treatment can also be carried out using a continuous or semicontinuous opera-

tion. In each case, accurate, in-line pH measurement is needed to control the addi-

tion of the treatment chemicals. Filtration can be accomplished on a

semicontinuous basis using plate and frame filters by sizing the treatment tanks

to allow surge capacity for filter cleanout intervals, or a continuous vacuum drum

filter can be used. These processes lend themselves to the large capacity plants.

Treatment of sweetwaters is less involved than the treatment of soap lyes as

described above. The purpose of treating sweetwaters is to remove any unreacted

fats and to prevent bacteriological growth in the glycerine. Typically, the sweet-

water is settled for several hours to allow any free fats and fatty acids to rise to

the surface, where they are skimmed off the top. Then, using air agitation, an excess

of lime is added to neutralize the fatty acids still present. Excess lime is calculated

as 0.23% of calcium oxide, and the batch must be adjusted appropriately to obtain

this excess (1). The sweetwater is then filtered. Sodium carbonate is sometimes

used to remove the calcium excess as carbonate. Some processes treat using barium

chloride and sulfuric acid, which is a more effective and expensive process. Also, it

has been shown that addition of a small amount of polyelectrolyte, such as used in

water treatment, will significantly improve the quality of the crude glycerine (10).

An alternative method for treating sweetwater, implemented in many facilities,

is to settle the sweetwater for approximately 24 h at 80–90�C and a pH of 4–5.

Phosphoric acid is sometimes used to help break any emulsion, but this is not

always required. The fats and fatty acids are decanted from the top of the sweet-

water and returned to the splitter feed for recycle.

The settled sweetwater is then sent to the evaporators for concentration. This

alternative method requires two tanks, one for settling and one for collecting the

sweetwater. The tanks are alternated every 24 h to run continuously.

2.3. Evaporation

2.3.1. Overview Following treatment, the soap lyes or sweetwaters are concen-

trated to 80–88% glycerol through an evaporation process. This is an energy-

intensive process and, depending on the plants capacity and energy costs, can have

either a single-, double-, or even triple-effect evaporator. The primary design considera-

tion is the amount of water to be evaporated. With soap lyes, the secondary considera-

tion is the method for handling the salt that precipitates as the water is evaporated.

The amount of water to be evaporated is calculated from the amount of

liquor (soap lye or sweetwater) and its glycerol concentration. Table 1 shows the

TABLE 1. Example of Concentrations in Soap Lye and Crude Glycerine.

Substance Percent Weight (kg) Percent Weight (kg) Change

Glycerol 10 100 80 100 None

Salt 11 110 9 14 96 kg Salt

Water 79 790 11 16 774 kg Water

PROCESSING PRINCIPALS AND DETAILS 197

concentrations that occur, starting with a dilute soap lye and concentrating to 80%

crude glycerine. The table does not include losses that occur.

Two general types of evaporators are used, and their names refer to the type of

circulation used to transfer heat to the liquor for evaporating the water. Natural cir-

culation evaporators rely on a thermosiphon to circulate liquors while forced circu-

lation units use a pump to achieve the required circulation. The heating tubes may

be inside or outside the evaporator body, but most designs, especially the older

calandria style evaporators, use internal tubes for heating (Figure 2).

Forced circulation heating is used for most new evaporation plants for several

reasons. Primarily, pumping the liquor allows the selection of a higher flowrate

and velocity than can be achieved by natural convection, which improves heat

transfer, thus reducing the surface area required for heating and evaporation of

water. Higher velocities in the tubes also reduce fouling and scaling of the tube sur-

faces. One drawback to forced circulation is the increase in energy consumption

from pump motors. In addition, if the velocity is too high, there can be problems

with crystallization and crystal growth when processing soap lye.

Two stageair

ejectorvacuumpump

Barometric condenser

Flick centrifugalcatchall and entrainment

separator

Sight glass

Effective baffles

Upward flow of vaporsthrough tubes

Outside steamconnection

Downwardflow of

liquor inlargerdown-comer

Feed line

Improvedcatchallreturn

Outsidecondensation

drain

Saltextractor

Hot well

Figure 2. Calandria-style single-effect evaportor with flick separator. Courtesy of Crown Iron

Works Co. (9).

198 GLYCERINE

Evaporation can be either single effect or multiple effect, depending on the num-

ber of evaporation stages used. Single-effect evaporation offers the simplest process

and minimum equipment investment. In a single-effect evaporator, a batch of dilute

glycerine can be evaporated to a half crude state with a concentration of 40–50%

glycerol and stored. When sufficient quantities of half crude are available, it is pro-

cessed to full crude (80% concentration). The steam economy for a single-effect

evaporator is 0.8–1.0 kg of water evaporated for each kilogram of steam used.

Single-effect evaporators, operating as a semicontinuous unit, as described above,

have been used with some frequency in the past, but are now primarily used for

small plants because of the economy and convenience of operation inherent in

continuous multieffect plants.

Multiple-effect evaporation is accomplished by joining two or more evaporators

in series using the heat in the waste vapor from one effect to heat the subsequent

effect. Live steam is added in the first effect only. The difference in the working

pressure in each effect (each effect working at a lower pressure than the previous

one) allows the use of lower pressure heating steam in the following effects. This

pressure difference also allows transfer of liquor from one effect to the other.

As a rough guideline, it can be assumed that approximately 0.8 kg of water can

be evaporated per kilogram of steam for each evaporator effect. Thus, one effect

will evaporate 0.8 kg of water per kilogram of steam, two effects will evaporate

1.6 kg, and three effects will evaporate 2.4 kg (14).

Multiple-effect evaporators are most often used in glycerine recovery plants of

medium to large size, with the typical plant having a two-effect evaporator. The

user must calculate and compare the savings in steam use for operating additional

effects against the additional equipment and maintenance costs as well as space and

operational complexity for additional effects.

Additional economy in steam consumption can be obtained through implemen-

tation of thermal vapor recompression (TVR). Water vapor from the evaporator is

captured and recompressed in a thermocompressor, using high-pressure motive

steam, so the vapor can be condensed in the first effect’s heat exchanger. The pres-

sure after the thermocompressor is between that of the water vapor and motive

steam. Only a portion of the vapor is recompressed in the thermocompressor,

with the remaining vapor used to heat the subsequent effect. The thermocompressor

can be used on a single-effect evaporator or on the first effect of multieffect units.

This equipment favors low �T in the heat exchanger to minimize the compression

ratio. The thermocompression effect will add additional economy equivalent to at

least half of an additional evaporation effect at a modest extra cost, typically just a

fraction of the cost of an additional effect. Recovered condensate can be econo-

mized by using it to preheat incoming liquor (15).

The operation of a two-effect, continuous evaporator is shown in Figure 3. Trea-

ted lye or sweetwater is fed continuously and regeneratively heated with conden-

sates coming from the heaters. Water is evaporated in the first evaporation chamber,

which is kept at a pressure slightly above atmospheric pressure (4–6 psig), using

heat provided by the thermocompressor. The thermocompressor takes part of the

vapor from the first-effect heater and recompresses it to increase its temperature

PROCESSING PRINCIPALS AND DETAILS 199

and pressure. The remaining vapors go to heat the second effect. In the first

evaporator, about 60% of the water is evaporated (In a three-effect evaporator,

the first effect evaporates about 45–50% of the water, with the remaining two

effects roughly splitting the amount of water evaporated).

The liquor from the first effect is then fed by gravity and by pressure difference

to the second effect, which is under a slight vacuum (115 mm Hg absolute). There,

it is concentrated to 80% for soap lyes or up to 88% for sweetwater crude (contain-

ing no salt) and then pumped to crude glycerine storage.

As soap lyes are concentrated, the concentration increases until the dissolved

salts begin to come out of solution and form crystals. These crystals grow and settle

in the bottom of the evaporator, where they are removed as described below. Some

salt crystals will drop out in the first effect, but most of the salt will drop in

the second effect. The proportions are determined by several factors, including the

operation of the evaporators, the characteristics of the soap lye, and the concentra-

tions reached in each effect. The solubility of sodium chloride in glycerol is

relatively unaffected by temperature, however.

2.3.2. Entrainment Separation Some kind of entrainment separation is neces-

sary to reduce losses of glycerine entrained in the vapor stream from each evapora-

tor. Control of the vapor velocity by proper evaporator and duct sizing will

minimize the amount of entrainment. An entrainment separator should be installed

at the outlet of each evaporator body, and the glycerol recovered should be returned

ThermocompressorH.P. steam

H.P. steamC.W.

C.W.

Vacuumsystem

First effectevaporator

Liquor feedEconomizer

Condensate

Condensatesystem

Crudeglycerine Hotwell

Second effectevaporator

Salt

Saltextractor

Saltreceiver

Ste

am

Ste

am

Air A

ir

Hot

wat

er

Hot

wat

er

Saltextractor

Saltreceiver

Figure 3. Double-effect evaporator with thermocompressor flowsheet. Courtesy of Crown Iron

Works Co. (9).

200 GLYCERINE

to its evaporator. The separator is usually of the cyclonic type, which can be

equipped with internal impingement baffles (Flick type). Mesh- or baffle-type

separators are possible, but care must be taken to prevent any buildup on the inter-

nal components, which would cause excessive pressure drop. Figure 2 shows a

Flick separator installed.

As the glycerol concentration increases during evaporation of soap lyes, salt

crystals begin to form as the liquor saturates. These salt crystals, as they grow,

will tend to sink to the cone in the bottom of the evaporator. Proper treatment

and filtration of the soap lye is important to ensure proper crystal growth in this

stage because impurities in the soap lye can hamper the growth of crystals. There

are several methods to recover the salt from the evaporators, and all of the methods

address the need to wash as much glycerol from the salt as practical. Washing is

done with water and sometimes lyes, with the purpose of recovering glycerol

that is removed with the salt. The wash liquor is subsequently returned to the treat-

ment plant. Obviously, any water that is added must be evaporated, so an economic

balance must be achieved between glycerol loss and water to be evaporated.

2.3.3. Salt Removal All salt removal systems start with the salt crystals (and

some glycerol) being removed from the evaporator, through an open valve, and col-

lected in a salt receiver or directly into a salt extractor, depending on the configura-

tion of the plant. If a salt receiver is used, the salt level is allowed to build up to a

certain level, determined by observation through a sight glass or by time interval.

Then the valve above the receiver is closed, the one below it is opened, and the salt

slurry is blown to a salt extractor using compressed air or steam. Salt extractors are

usually provided in pairs and used alternatively. The valves are then returned to

their original positions for recovering salt. If salt receivers are not used, the salt

is dropped directly into the salt extractor, in a similar manner as described above.

Salt receivers are placed below each evaporator and under a salt settling tank

(Figure 4).

The salt extractor is fitted with a screen to catch the salt and allow the liquor to

pass out of the extractor by being returned to the salt receiver from which it origi-

nated. Several batches of salt from the receivers are accumulated in a salt extractor

before the salt is removed. The salt is then washed to remove glycerol that is con-

tained with the salt. The wash fluid can be soap lye for the first wash and water for

subsequent washes. The wash water is returned to the treatment plant.

Some plants remove the salt manually by shoveling it out through a door in the

salt extractor. Before this salt is removed, it is dried in the extractor by steaming.

Removal of dried salt can be a labor-intensive practice. The salt is usually returned

to the soap plant, where it is dissolved into brine to be used again in the soap-

making operation.

Other plants will dissolve the salt in the salt extractor. There, after the final wash,

water is introduced into the extractor, the salt is dissolved, and the brine is pumped

back to the soap plant.

Centrifugal separation can be used to extract and wash the salt obtained in the

evaporation process. The centrifuge used for this process can be either a continuous

PROCESSING PRINCIPALS AND DETAILS 201

or a batch operation model. The continuous model typically offers higher capacity

and efficiency, but at a higher cost. The salt slurry is fed to the centrifuge in a simi-

lar fashion as described above for feeding a salt extractor, but there is typically a

slurry tank ahead of the centrifuge.

A batch centrifuge has a basket with a fine screen (or cloth bag), which catches

the salt while the liquor is drained back to the evaporation plant. The centrifuge is

fed with salt slurry from the evaporators and is spun until the basket is full of salt.

Then the salt is washed with treated lye or water to remove glycerol contained with

the salt, and then the salt is removed. The salt can be removed by mechanical means

or dissolved for reuse in the saponification plant. In the case of the cloth-bag type

centrifuge, the entire bag of salt is lifted out of the basket, and a new bag is

installed. The salt is then dissolved from the bag in a separate tank, freeing up

the centrifuge for salt extraction. Batch centrifuges require a relatively high amount

of operator attention.

The continuous centrifuge is fed with salt slurry from the evaporators. Figure 5

shows the main components and operating principles for this type of unit. The salt

slurry from the evaporators is continuously fed from the top inlet where it falls

between the helicoidal blades of the extractor and the conical basket. The basket

Saltreceivers

Second effectevaporator

First effectevaporator

Barometric condenser

Flickseparator

Flickseparator

Figure 4. Double-effect evaporation with forced circulation and external heating. Courtesy of

Crown Iron Works Co. (9).

202 GLYCERINE

has a specially designed, slotted screen installed over it to contain the salt crystals.

The slurry experiences a centrifugal load of 300–500 g, due to the spinning of the

basket. The liquid passes through the screen and is collected in the discharge chan-

nel. The extractor rotates at a positive differential speed relative to the screen,

which pushes the salt that has settled toward the bottom of the screen where it is

discharged out the bottom of the machine’s frame. The salt can be continuously

washed while it is on the screen. Some machines have more than one washing

zone, allowing the use of different wash liquor concentrations for more efficient

washing. The efficiency of salt removal depends on crystal size. For best results,

the crystals should be at least 100 mg in size (17).

2.3.4. Fat Settling and Saponification For sweetwaters that have been pre-

treated by settling alone, the concentrated crude glycerine will require an additional

step before the refining step. In this step, any excess fat that has been forced out of

the solution during evaporation requires decanting and saponification.

In the crude glycerine settling step, the crude is fed to one of two (or more)

tanks. The crude glycerine is maintained at 80–90�C and the fat and fatty acids

are allowed to rise to the top of the crude glycerine for a 24-h period. As with

the sweetwater settling step, alternate tanks are used in parallel. One tank is for

collecting the concentrated crude glycerine and one tank is used for settling the

crude glycerine. After settling, the crude glycerine is drawn off to another set of

tanks for saponification.

Figure 5. Continuous centrifuge operation diagram. 1, Frame; 2, discharge channel; 3, conical

basket; 4, screen; 5, helicoidal blades; 6, reduction gear unit; 7, electric motor. Courtesy of

Guinard Centrifugation. (16).

PROCESSING PRINCIPALS AND DETAILS 203

The saponification step involves reacting any fats and fatty acids remaining in

the crude concentrated sweetwater with sodium hydroxide. The resulting sodium

soap will be removed with the foots, or pitch, that make up the still bottoms.

A sample of crude glycerine is analyzed for saponification (SAP) value using the

AOCS Official Method Cd 3-25 analytical test (18). The SAP value is then con-

verted to a sodium hydroxide basis. This number represents the pounds (or kg)

of pure sodium hydroxide that must be added to 1000 pounds (or kg) of the crude

glycerine. Typically, an excess of sodium hydroxide is added based on the batch

size. This excess is usually in the range of 0.1–0.25%. This excess must be mini-

mized because excess sodium hydroxide will generate polyglycerols in the distilla-

tion step. Empirical evaluations have shown approximately seven pounds (or kg) of

glycerine are lost as polyglycerols for every pound (or kg) of excess sodium hydro-

xide present in the crude glycerine.

The saponification reaction is carried out at a temperature of 90�C with agita-

tion. This process is typically accomplished on a batch basis using alternate tanks

to simultaneously carry out the reaction in one tank and feed the distillation system

with reacted crude glycerine from a second tank. The system can be run on a con-

tinuous basis with this two (or more) tank system.

2.3.5. Vacuum System The vacuum system for the evaporation plant plays a

dual role by condensing the water evaporated and providing the required vacuum

of approximately 50 mm Hg absolute pressure. The vapor-condensing function and

initial vacuum device is either a barometric condenser, using water spray and a

barometric leg discharging into a hotwell, or a water-cooled surface condenser.

Despite a somewhat higher equipment cost, the surface condenser system offers

several significant advantages over a barometric condenser: (1) Any glycerine in

the vapor that is condensed is recovered and is not lost into the hotwell water,

and, (2) The surface condenser is cooled with clean water, typically circulated

from a cooling tower. In a barometric system, the spray water becomes contami-

nated, requiring a separate cooling tower system from the ‘‘clean’’ cooling water

used elsewhere in the plant. The ‘‘dirty’’ cooling tower water must be periodically

cleaned or replaced to prevent excess buildup and fouling in the cooling system.

Following the condensing section, and depending on the utility situation at the

plant site, either a liquid ring vacuum pump or a two-stage steam jet with an inter-

condenser is used. A booster ejector is not required to maintain the vacuum level

required for evaporation.

2.4. Refining of Glycerin

2.4.1. Distillation Distillation of glycerine is accomplished through distillation

with steam, under high vacuum, and at elevated temperatures. The vapor pressure

of glycerine at atmospheric pressure is 760 mm Hg at 290�C, and because glycerol

will begin to polymerize at approximately 200�C, distillation must occur at low

pressure. When distilling with steam, the partial pressure of the glycerol is reduced,

204 GLYCERINE

while maintaining the same total pressure, as related by the following well-known

equation:

Weight of glycerine vapor

Weight of water vapor¼ Partial pressure of glycerine vapor

Partial Pressure of water vapor

�MW of glycerine

MW of water

Glycerine distillation plants typically operate at 5–6 mm Hg absolute pressure

and at about 165�C. Certain undesirable chemical reactions can occur in crude

glycerol at distillation temperatures:

1. Formation of nitrogen compounds from proteinaceous matter present in the

crude glycerine (not removed in the treatment process) by thermal break-

down; these, along with volatile decomposition products, form impurities in

the refined glycerin; therefore, it is important to limit the time the glycerol is

at high temperature as well as the maximum temperature it is exposed to.

2. Formation of volatile glycerol esters by reaction with soaps (low molecular

weights) by the following equilibrium reaction:

C2H5ðOHÞ3 þ R COONa ! C3H5ðOHÞ2 O CO Rþ NaOH ð6Þ

The formation of ester is reduced in the presence of alkali.

3. Formation of polyglycerols, which occur in the presence of NaOH; thus, it

can be seen that it is important to control the alkalinity of the crude glycerol

to an optimum level.

4. Formation of acrolein (CH2¼ CHCHO), which is an odor constituent that is

difficult to eliminate.

The amount of total stripping steam for distillation is about 20% of the amount

of glycerol processed. This amount is greater with poorer quality feedstocks. How-

ever, not all of this steam is injected, as the water in the incoming crude glycerine

(80%) flashes to steam and provides a significant portion of the stripping steam

requirement. It may be undesirable to produce crude glycerine at a higher concen-

tration in the evaporation stage, as additional steam would be needed.

2.4.2. Residue Recovery and Disposal After the glycerine has been distilled

from the crude glycerine, a residue remains that is continuously removed from the

still. The residue contains some glycerol, polymerized glycerol, aldehyde resins,

organic products of decomposition, and salt, which must not be allowed to become

too concentrated as buildup of residue will cause quality and capacity problems in

the plant (19). If the residue is allowed to concentrate in the still, it displaces

volume for incoming crude glycerol, thus reducing the still’s capacity. At least

two methods of removing the residue are used:

PROCESSING PRINCIPALS AND DETAILS 205

1. A residue receiver vessel placed below the still accumulates residue, which is

periodically discharged to the treated lye tank for reprocessing.

2. Residue is removed continuously from the still and is redistilled to recover

the remaining glycerine; the concentrated residue from the bottom of the

foots still is discharged to a drum for disposal, typically to land fill as solid

waste.

The details of glycerine refining via the Crown Iron Works Co. process, which is

representative of the continuous distillation process used by a large portion of sup-

pliers for soap lye crude glycerine, are detailed here (Figure 6).

The crude glycerine is preheated regeneratively by hot distilled glycerine. The

liquor then enters the still heater where it is further heated to about 165�C and cir-

culated by means of a circulation pump. The circulated liquor is partially vaporized

through the aid of vacuum (6 mm Hg) and sparging steam in the flashing chamber.

The vapor rises through an entrainment separating section and then enters the con-

densing section. Here, the glycerine vapors are condensed in a layer of packing,

wetted by recirculated, cooled, distilled glycerine. The remaining vapors enter

the scavenging condenser where remaining traces of glycerine are condensed and

recovered as 80–90% substandard glycerine, which is sent to intermediate storage.

The substandard glycerine is re-refined after sufficient quantities have been col-

lected for a 2- to 3-day run every month. Typically, the substandard glycerine is

processed into a lower grade of glycerine, such as high gravity or dynamite grade.

The residue in the bottom of the still is continuously discharged from the crude

still to the foots still. The residue should be kept rich in glycerol (>25%) to

improve handling and distillation characteristics. A small amount (0.5–1%) of

C.W.

Vacuumsystem

To hotwellsubstandard

receiverTosubstandard

glycerinestorage

Steam

Steam

Crudestill

Vaporscrubber

H.P. steam

Scavenging condenserCondenser

Phosphoric acid

Deodorizer

Bleachingcolumns

C.W.

C.P.glycerine

CoolerCooler Polishingfilters

ResidueEconomizer/cooler

C.W. C.W.Crude

glycerine

Footsstill

Figure 6. Flowsheet of Wurster & Sanger glycerine refining plant. Courtesy of Crown Iron Works

Co. (9).

206 GLYCERINE

phosphoric acid is added to keep the residue soft by lowering the pH to retard the

formation of polyglycerols. Here, it is reheated by recirculation through an external

heater to about 175�C and partially vaporized, under vacuum and with about 25%

stripping steam. Most of the vapors are condensed in the foot still’s condenser, and

the condensate is returned to the crude still. Any remaining glycerine is recovered

where the vapors pass through the scavenging condenser. The residue at the bottom

of the foots still is discharged to a drum for disposal.

The distilled glycerine from the crude still is re-evaporated in the deodorizer at

about 130–140�C, again in the presence of high vacuum and stripping steam and

with external heating, to ensure optimal removal of odoriferous materials and resi-

dual moisture. The vapor passes through a packed section where the incoming feed

condenses the vaporized glycerine. Proper reflux rates are needed to ensure removal

of close boiling impurities. Residual volatiles are condensed in the scavenging

condenser.

An alternate method for distilling glycerine, especially sodium-hydroxide-

treated sweetwater crude glycerine, has been implemented in many facilities and

differs from the above system in many features (Figure 7). Most notably is the

use of a fractional (or partial) condenser for the glycerine product followed by a

final condenser to capture a ‘‘yellow’’ glycerine stream. A wiped film evaporator

is used for recovering glycerine from the foot’s, or pitch, stream.

Figure 7. Hydrolyzer crude refining system process flow diagram. Courtesy of CC Engineering,

Ltd. (20).

PROCESSING PRINCIPALS AND DETAILS 207

The still for this system is a simple flash still with a forced circulation reboiler.

The saponified, crude glycerine stream is fed into the recirculation pump on the

suction side. The recirculation rate is in the range of 40–50 times of the crude gly-

cerine feed rate. The top of the still vessel is equipped with a pad style entrainment

separator to eliminate carryover.

The still bottoms are drawn off and sent to a second distillation step to remove

as much of the remaining glycerine as possible. This system can be a small still,

similar to a foot’s still, or a wiped film evaporator. The foots still bottoms, or

pitch, from this step is comprised mostly of sodium soaps, polyglycerols, salts,

and minor quantities of organic compounds formed in the treatment and distillation

steps.

The glycerine and water vapors leave the still and enter the partial condenser.

This condenser is a vertical, shell and tube heat exchanger that condenses the gly-

cerine on the shell side. The tube side of the heat exchanger is configured to allow

water to boil in the tubes. The tube side outlet is equipped with a valve to control

the pressure of the steam leaving, thereby controlling the temperature at which the

water boils and glycerine vapors are condensed. Typical condensing temperatures

are 115–135�C. A balance must be reached with the temperature of condensation so

that odor and color impurities are at a minimum, yet the glycerine yield is econom-

ically sufficient.

The remaining glycerine and water vapor leaves the partial condenser and enters

the full condenser. This condenser is a shell and tube type condenser that condenses

the remaining glycerine and a small amount of the water. This glycerine stream is a

low-gravity glycerine and is very yellow. This stream is collected and can be run as

a separate recycle batch as mentioned above. The full condenser uses cooling tower

water on the tube side. Water vapor leaves the full condenser shell side and goes

through the vacuum system where it is condensed.

2.4.3. Wiped Film Evaporator (WFE) An alternate method of distilling glycer-

ine is by using a wiped film evaporator system (Figure 8). Pfaudler Corp. describes

the process as follows.

Feed material enters the inlet [2] and flows onto a distributor plate [5], which is

part of the rotor assembly. The initiation of feed to the WFE occurs at the same time

that the drive and motor [1] are started. As the rotor unit turns, centrifugal force

spreads the feed from the distributor plate onto the heated wall of the WFE. Volatile

components, such as glycerine, are rapidly evaporated. Slotted wiper blades [8]

connected to the rotor evenly distribute the feed material into a uniform, agitated,

thin film and continuously move material down the heated wall, including highly

viscous materials.

The vaporized glycerine stream passes through the entrainment separator [10]

and condenses on the internal U-tube condenser [9]. Droplets of material entrained

with the vapor stream impinge on the entrainment separator and flow back to

the heated wall through centrifugal force of the rotating assembly. Distillate flows

out the distillate outlet [11] and noncondensables flow out through the vacuum

outlet [13].

208 GLYCERINE

The bottoms, or residue, continues to move down the heated wall into the bot-

toms collector and out through the residue/bottoms outlet [12]. Optional extruder

blades [16] mounted on the bottom of the rotor assembly act to push highly viscous

material out through the residue/bottoms outlet.

The heating temperature is chosen based on the vapor pressure of glycerine. The

WFE is typically under a high vacuum (3–5 mm Hg). Noncondensables and water

vapor are removed by the vacuum system.

One option used in many facilities is to use a WFE that is not equipped with an

internal condenser. The vapor stream is then taken from the WFE to the partial

condenser inlet for fractional condensation previously discussed.

The advantages of this type of continuous process over batch or continuous dis-

tillation in a ‘‘still pot’’ include low residence time (1–2 minutes) and a small dif-

ference between the vapor temperature and the liquid film temperature. The

temperature of the glycerine can be tightly controlled, thereby reducing the forma-

tion of odor, color, and other impurities. This system does not require stripping

Figure 8. Wiped film evaporator with internal condenser components. Courtesy of Pfaudler

Corporation. (21).

PROCESSING PRINCIPALS AND DETAILS 209

steam, which reduces the capital and operating costs of the vacuum system and

overall energy consumption.

Wiped film evaporators are very effective for processing sweetwater crudes.

Yields of glycerine are very good and the quality of finished product is equal to

glycerine from other distillation systems discussed previously. Salt crude’s can

be distilled in these systems, but special modifications are required to handle the

salt that precipitates out during distillation.

2.4.4. Vacuum System The vacuum system for the distillation plant is typically

a single system for the still, deodorizer, and foots still (if supplied). To achieve a

vacuum of 2–10 mm Hg required in the distillation equipment, the vacuum system

is usually designed for 1–8 mm Hg absolute, to allow for equipment pressure

differentials, ducting losses, leaks, etc. Most systems have a surface condenser or

a barometric condenser, followed by a booster jet or multiple-stage vacuum pump.

The cooling and condensing water going to the hotwell is typically of good quality,

as virtually all condensables are removed by the condensing sections.

2.4.5. Carbon Adsorption Distilled glycerine requires one final processing step

prior to final storage and shipment. The final step is processing through activated

carbon columns. The carbon adsorption process step removes any final traces

of odor and color from the distilled glycerine and enhances stability to provide a

glycerine product that will not degrade in the final storage tanks (22).

Typical activated carbon usage is in the range of 0.5–1.0%. This can vary

depending on the amount of odor and color impurities in the distilled glycerine.

The typical carbon column configuration (Figure 9) consists of three carbon col-

umns in series with a standby carbon column filled with fresh carbon. The column

Figure 9. Carbon adsorption process flow diagram. Courtesy of CC Engineering, Ltd. (20).

210 GLYCERINE

is filled with dry, activated carbon using any of several methods, including bag

dumping, supersacks, or pneumatic conveying. The column is then filled with water

or with heated (75–85�C) final glycerine product to saturate the carbon and displace

air from the carbon granules. The filled column needs to settle while vented to the

atmosphere. This ensures the carbon bed is thoroughly saturated, all air has been

expelled, and channeling of glycerine through the carbon bed is prevented. Chan-

neling in the carbon bed leads to increased carbon usage and poor-quality glycerine

that does not meet required specifications.

Heated (75–85�C) glycerine is pumped to the carbon column system from the

distilled glycerine storage tank. It is critical to the adsorption process to avoid

‘‘bumping’’ the carbon beds with a sudden surge of glycerine because this disrupts

the beds and can cause odor and color quality degradation in the finished glycerine.

The ‘‘bumping’’ of the carbon bed can sometimes occur with a centrifugal pump.

A positive displacement pump, preferably equipped with a variable frequency drive,

is recommended to feed the glycerine at a constant flow rate.

A filter is used after the carbon columns to remove any carbon fines from the

glycerine. The filter is typically a bag type using polypropylene or PTFE cloth

bags with a pore size of 5–10 micron. The bleached and deodorized glycerine

product is then sent to final storage for shipment.

2.5. Storage and Stability

Dilute and crude glycerine contain certain amounts of suspended material (precipi-

tates, salt, etc.) that tend to settle out during storage. Therefore, to avoid introducing

these materials into the process when liquor is drawn, it is recommended to have

discharge nozzles be approximately six inches above the bottom of the tank. It is

necessary to empty and clean the storage tank(s) periodically.

Dilute solutions of glycerol (<50%) are subject to fermentation, which reduce

yields and introduce breakdown products that degrade the glycerol. Holding the

glycerol above 70�C or higher concentrations will alleviate this problem. Concen-

trated glycerine is difficult to pump at lower temperatures because of its high vis-

cosity. It is recommended that glycerine be pumped at 40–50�C; lower temperatures

make pumping difficult and higher temperatures can affect the color. If heating coils

or steam tracing are used, it is important to use low-pressure steam so as not to

overheat the glycerol and cause breakdown of products.

Stainless steel vessels or stainless-steel-lined vessels are recommended to pre-

vent the formation of color complexes, especially if moisture or residual fatty acids

are present in a carbon steel tank. Since glycerine is hygroscopic, care should

be taken to exclude moisture from the refined glycerine storage tank. Glycerine

subjected to heat should not be stored in vessels containing copper or tin, as copper

or iron salts will catalyze oxidation of glycerine under those conditions (23).

2.6. Odor and Color

As noted, color and odor problems can be mostly avoided by using a high-quality

raw material, treating and storing crude glycerol properly, and avoiding high

PROCESSING PRINCIPALS AND DETAILS 211

temperatures for an extended time period. Impurities in crude glycerine, especially

matter organic nonglycerol (MONG), affect the quality and quantity of distilled

glycerine. If the MONG content is high (3–5%), odor, taste, and color reversion

problems may exist in the final product. Tremethylene glycol, which is present in

MONG, can affect the color of glycerine and lead to problems in storage.

A properly designed deodorizer or ‘‘stripper’’ vessel, located after the distillation

column and operating at high vacuum and with open or ‘‘sparging’’ steam, will

remove close boiling impurities and most odor bodies. Remaining color bodies

are removed by adsorption with activated carbon.

Acrolein may be formed in the presence of iron, iron salts, acid, or neutral

salts:

C3H5ðOHÞ3 ¼ C3H4Oþ 2H2O: ð7Þ

Acrolein odor is distinctive and is detectable to the human nose in small quan-

tities (ppm).

It has been found that glycerine left inside processing vessels, especially carbon

columns, can degrade and develop hard-to-remove odors. Therefore, process equip-

ment should be cleaned completely after shutdown, and carbon columns blown

down with air and washed with water. Also, fresh carbon can sometimes contain

odor-causing impurities. The carbon can be washed with distilled glycerine that

can be subsequently redistilled or sold as a lower grade.

3. PROCESSING PLANTS

3.1. Layout Considerations

Plant layout considerations for a glycerine recovery plant are similar to those for

other processing plants: adequate space must be allowed for equipment installation,

operation, and maintenance, as well as operator safety. As some of the operation

requires material handling, consideration must be given to these specific require-

ments. These operations include batch feeding of treatment chemicals (most of

which are hazardous), disposal of filter cake, and charging of the bleaching columns

with carbon. It generally makes sense to arrange the equipment in order of process

flow, as piping and other connections can be made much more conveniently. In

addition, if process tanks are located to allow gravity feeds, some pumps can be

eliminated. Locating equipment on several levels also helps in reducing the overall

footprint, or floor space requirements.

All areas should be equipped with wash-down equipment, and all levels must

have adequate floor drains with the floor surfaces properly pitched. Care must

also be exercised when selecting flooring materials, as glycerine is slippery; this

is a real concern, because spills and leaks inevitably occur. Provisions must be

made in the plant for efficient cleaning procedures for the equipment, piping, and

building.

212 GLYCERINE

3.2. Material and Construction

3.2.1. Overview Careful selection of the materials of construction is important

to ensure adequate life of equipment in the glycerine processing plant and to ensure

a high-quality finished product. This is especially true when recovering glycerine

from soap lyes, where the presence of sodium chloride, in crystal and solution

forms, greatly increases the corrosion potential. The reader is encouraged to consult

textbooks and manufacturer’s data sheets regarding corrosion for the compounds

and materials in the glycerine plant.

3.2.2. Pretreatment Plant When treating sweetwater glycerine, ductile iron

(DI) can generally be used for pumps, although lined DI and stainless steel pumps

can be used with an expectation of longer life.

More care must be used when selecting material for treating spent lye. Treatment

vessels can be either lined carbon steel or stainless steel. Fiberglass-reinforced

polyester (FRP) is a good choice for both tanks and piping in the treatment plant,

but care must be taken to ensure that the temperature range of the FRP material

is observed and that the pipelines are correctly supported. Also, steam blowing

of pipes should not be allowed as the high temperature can damage the FRP

piping.

The open plate and frame filter presses used typically have cast iron frames with

polypropylene plates that give excellent life. Filter clothes are usually constructed

of synthetic materials such as polypropylene felt, rather than natural fibers such

as cotton. Plastic tools must be used to clean cake from the clothes to avoid

damage.

A plate and frame heat exchanger is sometimes used to preheat incoming soap

lye, as treatment is more effective with warm lye. The plates can be of 316L stain-

less steel, or even titanium for effective corrosion resistance.

3.2.3. Evaporation Plant Material selection for the evaporation plant is the

greatest challenge in the glycerine recovery plant, again, especially if there is

salt present. For evaporating splitter sweetwater, 304L stainless steel may be

used. The corrosive nature of the brine-glycerol solution makes carbon steel and

most grades of stainless steel a poor choice for evaporators handling soap lye.

The heavy-walled, cast-iron callandria that have been used extensively for soap

lye evaporation in the past have long lives, however, economic considerations dis-

courage new evaporators of that style from being built. The material of choice for

the past 25 years for welded plate evaporators (and related components handling

salt or brine) is copper-nickel alloy. The two principal alloys to consider are

9010 (90% copper, 10% nickel), which has the best corrosion resistance to the

glycerol-brine solution, and 7030, which has a higher cost (24).

An interesting class of materials that should be considered for evaporator service

are the duplex stainless steel alloys. These high-performance stainless steel alloys

have a ‘‘duplex’’ structure. The alloy contains both the austenitic and ferritic phases

PROCESSING PLANTS 213

in the grain structure at the same time. These materials have shown high resistance

to stress corrosion cracking in chloride-bearing environments and have the charac-

teristics of high corrosion and erosion resistance. (A description of the corrosion

protection mechanism is outside the scope of this chapter; the reader is referred

to corrosion handbooks and manufacturers data sheets for more information.)

Two alloys that show promise for this application are Sandvik’s SAF 2205 and

SAF 2507 (20, 25). Other, more expensive alloys, such as Ferralium 225, Hastelloy,

and Monel, have been used in various applications.

Pumps for the evaporation plant must stand up under the hot glycerol-brine solu-

tion and are also generally of the centrifugal design. Cast stainless steels (CF8M

alloy) are often used for this application, but a superior material for pumps is a

chromium-nickel-copper-molybdenum stainless steel known as CD-4M, which

should be considered when choosing the pump material. The extra cost is usually

not significant.

Seals for pumps are generally mechanical. If the fluid contains salt crystals,

which are abrasive, it is recommended that water be used to flush the seal. Both

seal faces on the pump side should be of a hard material; using carbon for one

of the seal faces has not proven to be successful. The reader is again urged to con-

tact seal manufacturers for their recommendation.

3.2.4. Refining Plant Materials for the refining plant can be selected based on

the impurity content of the glycerol being handled. The initial distillation vessel or

residue still, if it is handling crude glycerine with salt in it, should be a high grade of

stainless steel. The L grades generally exhibit slightly better corrosion resistance

than their base versions. The 316 and 316L stainless steels are recommended

for stills handling soap lye crude glycerine, while 304 and 304L stainless steel is

adequate for sweetwater stills, vapor scrubbers, condensers, deodorizers, and

bleaching equipment where salt is not present. The 316L is especially recom-

mended for foot stills where salt is present but water is not.

In some cases, more corrosion-resistant materials have been used; the Ittner

refining stills in use at some Colgate-Palmolive plants were fabricated from pure

nickel. The life of such equipment is exceptionally long (26).

Pumps for handling glycerine can be centrifugal as long as the temperature and

viscosity of the product is satisfactory for the pump being used. For lower tempera-

tures and pumping residue, a positive displacement pump with a pressure relief loop

(internal) should be used.

3.3. Instruments and Controls

Control techniques for glycerine plants have not changed much over the years, as it

is necessary to control pH, levels, temperatures, and vacuums in the various por-

tions of the plant. Most operations can be accomplished manually, and traditionally,

manual operation of these plants have been the norm. However, the development

214 GLYCERINE

and refinement of digital single-loop controllers and associated sensing equipment

have increased the ease and accuracy of control. Reliable inline pH monitoring has

made treatment plants easier to operate.

A caution must be given, however, to select instrumentation that can handle the

sometimes severe conditions in the glycerine recovery plant. The crude glycerine in

the evaporation and crude refining sections contains both dissolved and suspended

salt, while the residue in the refining plant contains salt as well as other impurities

that make the viscosity of the residue high. Vane-type flow meters have been used

successfully for measurement of difficult flows such as residue from the crude still.

Rotameters often will be plugged by suspended solids and are recommended

only for flows that are ‘‘clean’’ and will maintain a constant viscosity over time.

Mass flow meters (Corealis effect), such as those manufactured by Micromotion,

have proven successful in many areas of the treatment and refining plants. These

flow meters have no moving parts and can handle a wide range of viscosities and

densities.

Materials of construction for wetted portions of the instrumentation must be

selected using similar considerations to those used for the equipment itself. In addi-

tion, plastic diaphragm seals are available to cover and protect the pressure-sensing

faces of immersed level transmitters. This allows the use of stainless steel in appli-

cations where, if unprotected, the corrosion rate would be unacceptably high on the

thin face of the instrument. A diaphragm seal is recommended for pressure instru-

ments in the presence of suspended or entrained solids.

3.4. Piping Considerations

In addition to the few suggestions on material selection given earlier, other issues

need to be considered when designing or evaluating the piping for a glycerine

recovery plant. As in any piping system, the piping must be properly supported,

be given correct slopes for proper flow and self drainage, accommodate thermal

expansion, and be designed with adequate bypasses for equipment maintenance

and proper operational control. In addition, consideration should be given for sam-

pling points, drains, and for blowout-wash-down connections. Fluid velocities

should be maintained within standard design limits. The above information can

be obtained in reference books dealing with industrial piping design. Another

area of concern may be the possible problem of electrolysis as a result of dissimilar

materials used together in the piping system.

4. PROPERTIES OF GLYCEINE

Glycerine has a unique set of physical properties that allow glycerine to be used in a

variety of industries. Some of these properties are summarized in the following

tables:

PROPERTIES OF GLYCEINE 215

5. QUALITY AND TESTING

5.1. Glycerine Grades

Several grades of glycerine have been established or are in common use in

North America. U.S. Pharmacopoeia (USP) or chemically pure (CP) glycerine stan-

dards have been established for the highest grades of glycerine, water (white in col-

or), and with glycerol contents of not less than 95%. It conforms to standards given

in U.S. Pharmacopeia. It is used by the food and pharmaceutical industries because

of the high purity. Both natural and synthetic glycerine meet these specifications

(Table 5).

TABLE 2. Physical Properties of Glycerine.

Molecular Weight 92.09

Boiling Point 290�C (760 mm Hg)

Melting Point 18.17�CFreeze Point (eutectic) (66.7% glycerol solution) 46.5�CSpecific Heat 0.5795 cal/gm�C (26�C)

Refractive Index (Nd20) 1.47399

Flash Point (99% glycerol) 177�CFire Point (99% glycerol) 204�CAutoignition Point (on platinum) 523�C

(on glass) 429�CHeat of Combustion 397.0 Kcal per gram

Surface Tension 63.4 dynes cm (20�C)

58.6 dynes cm (90�C)

51.9 dynes cm (150�C)

Coefficient of Thermal Expansion 0.0006115 (15–25�C Temp. Interval)

0.000610 (20–25�C Temp. Interval)

Thermal Conductivity 0.000691 cal cm deg/sec (0�C)

Heat of Formation 159.8 Kcal/mol (25�C)

Heat of Fusion 47.5 cal/gram

Heat of Vaporization 21,060 cal/mol (55�C)

19,300 cal/mol (105�C)

18,610 cal/mol (175�C)

TABLE 3. Vapor Pressure of Pure Glycerine.

mmHg 1 5 10 20 40 60 100 200 400 760

�C 125.5 153.8 167.2 182.2 198.0 208.0 220.1 240.0 263.0 290.0

TABLE 4. Viscosity of Glycerine-Centipoise.

�C 80 90 100 110 120 130 140 150 158 167

cp 32.18 21.2 14.60 10.48 7.797 5.986 4.726 3.823 3.282 2.806

216 GLYCERINE

High-gravity glycerine is a commercial grade of glycerine, near white in color, and

with a high glycerol content (>98.7% with specific gravity minimum of 1.2583 at

25/25�C). It conforms to Federal specification O-G-491C and to ASTM D1257.

Dynamite glycerine meets all high-gravity specifications, except that a darker color

is allowed.

Hydrolyzer (88%) and soap lye (80%) crude’s are crude, unrefined grades of gly-

cerine offered for sale to glycerine refiners. Hydrolyzer crude is concentrated

sweetwater from fat splitting, while soap lye crude comes from soap making and

contains some salt.

For salt crude glycerine, the typical concentration for sale is 80%. Adjustments

in price can be made for concentrations above or below 80%, however, these adjust-

ments are negotiated by buyers and sellers. The standard for ash content in salt

crude is 10% maximum. The standard for water is 10% maximum. Limits on minor

constituents include a 0.5% limit for trimethylene glycol (TMG), 2.5% limit for

MONG, and a limit of 2 ppm for arsenic.

Hydrolyzer (sweetwater) crude glycerine is traded at a standard 88% glycerol.

Limits on minor constituents include ash at 1% maximum, MONG at 1.5%

maximum, and TMG at 0.5% maximum. The limit on arsenic is 2 ppm maximum.

5.2. Test Methods

The American Oil Chemists’ Society (AOCS) has published several methods for

sampling and testing of glycerine (18). Included are procedures for sampling crude

glycerine and test methods for glycerol, apparent specific gravity at 25/25�C,

TABLE 5. Refined Glycerine Specifications.

Characteristic 99.7% USP 99.5% USP 99.0% USP

Specific Gravity,

25/25�C1.26092 1.26073 1.25945

Color 25 hazen maximum,

APHA 10 maximum

25 hazen maximum,

APHA 10 maximum

25 hazen maximum,

APHA 10 maximum

Ash 0.01% maximum 0.01% maximum 0.01% maximum

Chlorides 10 ppm maximum 10 ppm maximum 10 ppm maximum

Sulfate 20 ppm maximum 20 ppm maximum 20 ppm maximum

Arsenic 1.5 ppm maximum 1.5 ppm maximum 1.5 ppm maximum

Heavy metals 5 ppm maximum 5 ppm maximum 5 ppm maximum

Readily

carbonizable

— Not darker than

matching fluid H

Not darker than

matching fluid H

Chlorinated

Compounds

30 ppm maximum 30 ppm maximum 30 ppm maximum

Acrolein, glucose

and ammonia

compounds

Not yellow, no

ammonia odor

Not yellow, no

ammonia odor

Not yellow, no

ammonia odor

Fatty acids and

esters (FA & E)

1.0 milliequivalent/

100 gm MAX.

1.0 milliequivalent/

100 gm MAX.

1.0 milliequivalent/

100 gm MAX.

PH Neutral Neutral Neutral

QUALITY AND TESTING 217

moisture, and color (Table 6). Tests are usually conducted to analyze the amount of

ash, alkalinity, salt, and organic residue. In addition, the refractive index of glycer-

ine can be used to estimate its concentration (13, 25). Color is determined using the

APHA scale. APHA color is determined using AOCS Method Ea 9-65, which

measures ‘‘color by comparison with artificial empirical standards’’ (23).

6. PROCESSING LOSSES

Losses are a natural part of any processing plant. The control and reduction of the

losses will, in part, determine the economic effectiveness of the plant’s operation.

All areas of the glycerine recovery plant can produce losses. In storage, fermenta-

tion can cause serious losses of glycerine due to the formation of trimethylene

glycol, gases, and acids during decomposition.

In the evaporation plant especially, glycerine can be carried out with the vapor

during evaporation and lost in the vacuum system’s condensing of water. The

amount of carryover can be minimized in several ways:

1. Entrainment separators, or ‘‘catchalls’’, and mist eliminator pads can be

installed where the vapor is removed from the vessel by the vacuum system;

the recovered product can be returned to the process at an appropriate point.

2. Control the vapor velocity and maintain an adequate vapor space at the top of

the vessel; it becomes important to control the liquid level to maintain the

required vapor space.

3. Use a surface condenser to recover any additional condensable material that

escapes into the vacuum system.

The treatment process should be done carefully to control any tendency to foam.

Other sources of glycerine losses include the following:

1. Glycerine is carried out with salt coming from soap lyes; proper washing of

the salt is required to minimize this loss.

2. The filter cake and skimming from the treatment plant contain glycerine.

3. Spent bleaching carbon contains glycerine of the highest concentration

produced in the plant. It should be allowed to drain and then the columns

should be blown down with air or nitrogen to extract as much free glycerine

as possible. Then, the spent carbon can be added to the first treatment

tank, where additional glycerine will be washed from the carbon. The car-

bon is separated in the filter press and disposed of with the rest of the filter

cake.

4. Residue removed from the distillation process contains a certain percentage

of glycerine; much of the glycerine can be recovered from this residue by a

separate distillation step in a wiped film evaporator, or foots still.

5. Glycerine leaks into heat exchangers.

218 GLYCERINE

6. There is spillage from all sources; careful plant design and operation are

needed to minimize spills from overfilling tanks, equipment malfunctions,

and other operational errors.

7. Opening of equipment for maintenance can result in loss of product. Where

possible, the liquid should be blown into the previous or next process vessel

or drained back into a bucket for recovery. Any product that is recovered that

is not badly contaminated can be returned to a separate storage tank upstream

in the process. If a ‘‘substandard’’ or ‘‘off-spec’’ glycerine tank is provided in

the refining section, it can be equipped with an easily removable cover.

7. WASTE MANAGEMENT

The three primary waste streams from a glycerine recovery plant are skimming and

filter cake from the treatment plant, contaminants in the vacuum system condensing

water, and residue (foots) from the glycerine refining plant. Filter cake discharge is

typically sent to a solid landfill. The concentrated residue from a foots still, when

allowed to cool, will typically solidify and must be disposed of as required by the

local environmental authority.

Contaminants that get into the vacuum system’s condensing water supply are a

dual problem. There is glycerol and other condensables that are carried over from

the vapor stream from the evaporation plant. First, the glycerol represents a loss

of product. Second, these contaminants will foul the condensing water supply,

which is typically recirculated through a cooling tower. A certain amount of

the condensing water is generally bled away and replaced with makeup water to

maintain an acceptable water-quality level. An important alternative here is the

inclusion of a closed-loop system that incorporates a surface condenser cooled

with clean water recirculated from a cooling tower. The surface condenser, usually

a horizontal shell and tube heat exchanger, replaces the barometric condenser in the

vacuum system and, while using clean cooling water, allows any condensables to be

recovered.

8. USES, APPLICATIONS, AND ECONOMICS

The number of uses of glycerine is truly phenomenal (Figure 10). Depending on

the publication surveyed, up to 1700 uses have been identified. Its wide range of

applications is, in part, related to a few of its key properties:

1. Glycerine is a natural product, nontoxic, and generally recognized as safe

(GRAS) for human consumption.

2. It is an excellent humectant, emulsifier, and plasticizer.

3. It is compatible with a wide variety of materials and mixes well.

4. It possesses antioxidant properties.

USES, APPLICATIONS, AND ECONOMICS 219

Below is a list of some of the more prominent or interesting uses for glycerine

(4, 23).

� Adhesives, used for plasticizing and penetrating properties.

� Agriculture, used in sprays, dips, and washes.

� Antifreeze: properties include low freezing point and excellent compatibility.

� Cleaners and polishes, used in a wide range for the home and automotive

markets.

� Corrosion prevention, used in gums and resins for surface-coating metals.

Cosmetics, acting as a bodying agent, emollient, humectant, lubricant, and

solvent; present in skin creams and lotions, shampoos and hair conditioners, soaps,

and detergents.

Dental creams, up to 50% of typical dental creams, used as a humectant, to

ensure good dispersion and to serve as a vehicle for dyes and flavorings.

Explosives, a large amount is consumed in the manufacture of nitroglycerine-

based explosives.

Food and beverages, used in a wide variety of applications; serves as a solvent,

carrier, emulsifier, conditioner, freeze preventer, and coating; used in wine,

liqueurs, chewing gum base, confectioneries, and chewy bars; kosher glycerine is

used in kosher foods.

Leather, used in tanning and finishing processes.

Metal processing, widely used for pickling, quenching, stripping, electroplating,

galvanizing, and soldering.

Paper, acts as a humectant, plasticizer, softening agent, and barrier against

grease and solvents; used for greaseproof and glassine papers.

Pharmaceuticals, used in salves and dressings, antibiotic preparations, capsules,

and suppositories; used as a vasodialator for angina pectoris (nitroglycerine).

Photography, properties include wetting and plasticizing.

Resins, includes ester gums, phthalic acid and malic acid resins, polyurethanes,

and epoxies.

Textiles, facilitates printing and dying, lubricating and snagproofing; used for

treatments for antistatic, antishrink, anticrease, waterproofing, and flameproofing.

Tobacco, used as a humectant, softening agent, and flavor enhancer.

Alkyd resins (36%)

Tobacco products (16%) Food/beverages (10%)

Urethane uses (6%)Explosives (2%)

Cosmetics/pharmaceuticals (30%)

Figure 10. Glycerine uses.

220 GLYCERINE

9. FUTURE CONSIDERATIONS

9.1. Technology

Future technologies that appear most likely to impact the production of natural gly-

cerine center around recovery of glycerine from nontraditional sources. As demand

for biodiesel fuel from vegetable oils grows, large amounts of high-quality glycer-

ine will become available. Other technologies that show promise for use in glycer-

ine recovery include ultrafiltration and reverse osmosis. As processes become more

effective and efficient and as economic conditions change, some of these processes

may come into more common use.

An interesting source of glycerol is through recovery from ethanol fermentation

still bottoms by chromatographic separation and ion exchange process. In a process

proposed by IWT/US Filter, raw stillage is first clarified by filtration through a

membrane, concentrated (by evaporation), and then fed to the ADSEP chromato-

graphy process where the glycerol is separated. The glycerol is then further purified

by ion exchange, followed by traditional concentration and distillation (27).

9.2. Future Economics

The future economics of glycerine worldwide will be sure to change as fundamental

changes on the supply side occur. Animal fat is a significant raw material for gly-

cerine production, both for fat splitting and saponification. Recent health trends

emphasizing lower fat content in meats has caused animal producers to develop lea-

ner meats, reducing the supply of animal fats. Other uses of fat add additional price

pressures. However, the soap and detergent industry is expected to increase its use

of glycerine as nonionic surfactants.

Additional glycerine production is being added from several growing and signif-

icant sources. The Asian fatty alcohol and fatty acid industry is expected to add an

additional 30,000–60,000 tons to the world market in the next few years. There is a

similar amount expected to arrive in the market, primarily from the European bio-

diesel (methyl ester production from vegetable oil) industry. Other developing

sources of glycerine, such as glycerine recovery from ethanol stillage, may add

further stocks to the world supply.

The purification of glycerine from these sources is obtained using the traditional

methods outlined in this chapter. However, the economics of production of the

crude glycerol will determine how these new sources affect the world market.

REFERENCES

1. K. P. Radhakrishnan, in L. Spitz, ed., Soap Technology for the 1990’s—Glycerine

Processing from Spent Lye and Sweet Water, AOCS, Champaign, Illinois, 1990, p. 128.

2. W. I. Rowell, FatSplitting and Glycerine Recovery, AOCS, Champaign, Illinois,

1987.

REFERENCES 221

3. L. L. Lamborn, Modern Soaps, Candles, and Glycerin, Van Nostrand Co., New York,

1906, p. 542.

4. A. A. Neuman, Glycerol, CRC Press, Cleveland, Ohio, 1968.

5. A. S. Langmuir, J. Soc. Chem. Ind., 36, 180 (1917).

6. J. Hoffman, Chem. Market. Reporter, 243, 5 (1993).

7. B. Freedman, Methyl Esters—Manufacture and Utilization, USDA.

8. A. A. Neuman, The History of Glycerol, CRC Press, Cleveland, Ohio, 1968, ch. 1.

9. Crown Iron Works Co.

10. E. Woollatt, The Manufacture of Soaps, Othe Detergents and Glycerine, Elliss Horwood

Ltd., Chichester, United Kingdom, 1985.

11. O. H. Wurster, Oil Soap, 13, 246–253 (1936).

12. O. H. Wurster, Oil Soap, 13, 283–286 (1936).

13. C. S. Miner and N. N. Dalton, eds., Glycerol, Reinhold Publishing Co., New York, 1953,

ch. 6.

14. Bulletin. SW-200R, Swenson Evaporators.

15. P. E. Minton, Handbook of Evaporation Technology, Noyes Publications, Park Ridge,

New Jersey, 1986.

16. Guinard Centrifugation.

17. Centrifuge Type C, brochure, Guinard Centrifugation.

18. American Oil and Chemists Society.

19. J. W. Lawrie, Glycerol and the Glyecrols, Chemical Catalog Co., New York, 1928.

20. CC Engineering, Ltd.

21. Pfaudler Corp.

22. Glycerine Purification, brochure, CC Engineering.

23. Glycerine: An Overview, Soap and Detergent Assoc., New York, 1990.

24. APV Evaporator Handbook, 3rd ed., APV Crepaco, Inc.

25. L. F. Hoyt, Oil Soap, 10, 43–47 (1933).

26. M. H. Ittner (to Colgate Palmolive), U. S. Patent 2,164,274, June 27, 1939.

27. B. D. Burris, Recovery of Glycerol from Still Bottoms, Illinois Water Treatment, 1987.

222 GLYCERINE