Baker’s Yeast Production BEng Design ProjectDepartment of Chemical Engineering,

Loughborough UniversityM.Alwazir(Chair), M.Hardcastle, A.Mohamed, A.Saufi(Secretary), D.Zarbo

April-May 20011

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Contents

1 Summary 3

2 Production of Baker’s Yeast 32.1 Process Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Process Science 43.1 Baker’s Yeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1.1 Growth Kinetics . . . . . . . . . . . . . . . . . . . . . . . 53.1.2 Cell Requirements . . . . . . . . . . . . . . . . . . . . . . 6

3.2 Growth Medium Formulation . . . . . . . . . . . . . . . . . . . . 73.3 Oxygen Demand . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.4 Environmental Conditions . . . . . . . . . . . . . . . . . . . . . . 93.5 Sterilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Process Design 104.1 Production Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2 Modes of operation . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.2.1 Batch Culture . . . . . . . . . . . . . . . . . . . . . . . . 104.2.2 Fed-batch Culture . . . . . . . . . . . . . . . . . . . . . . 11

4.3 Process selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3.1 Nutrient preperation . . . . . . . . . . . . . . . . . . . . . 114.3.2 Innoculation . . . . . . . . . . . . . . . . . . . . . . . . . 144.3.3 Fermentaion . . . . . . . . . . . . . . . . . . . . . . . . . . 144.3.4 Seperation . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.3.5 Decanter Centrifuge . . . . . . . . . . . . . . . . . . . . . 154.3.6 Disc Stack Centrifuge . . . . . . . . . . . . . . . . . . . . 154.3.7 Tubular Bowl Centrifuge . . . . . . . . . . . . . . . . . . . 154.3.8 Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3.9 Wastewater treatment . . . . . . . . . . . . . . . . . . . . 174.3.10 Anaerobic digestion . . . . . . . . . . . . . . . . . . . . . 184.3.11 Compound Fractioning and Recovery . . . . . . . . . . . 184.3.12 Off-site treatment . . . . . . . . . . . . . . . . . . . . . . 18

4.4 Process Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . 194.5 Equipment Design . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.5.1 Steriliser . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.5.2 Fermenters . . . . . . . . . . . . . . . . . . . . . . . . . . 194.5.3 Centrifuge . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.5.4 Centrifuge Design . . . . . . . . . . . . . . . . . . . . . . 224.5.5 Fluidised Bed . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.6 Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.6.1 Fermentation . . . . . . . . . . . . . . . . . . . . . . . . . 234.6.2 Centrifuge . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.7 Piping and Instrumentation Diagram . . . . . . . . . . . . . . . . 244.8 Equipment Specification Sheets . . . . . . . . . . . . . . . . . . . 244.9 Plant Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.10 Start Up/Shut Down Procedures . . . . . . . . . . . . . . . . . . 25

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5 Safety 275.1 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.2 Staff Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.3 Material Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.4 Process Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.5 General Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.6 HAZOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6 Process Economics 296.1 Capital Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.2 Operating Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

7 Conclusion 29

8 References 29

1 Summary

The report includes a full plant specification for the annual production of 2,500tonnes of Saccharomyces cerevisiae, a single cell protein commercially known asBaker’s yeast. In specifications of the plant were based on economical, processcontrol and product quality considerations to achieve a cheap food-grade qual-ity product which could be sold for commercial and non-commercial use. Thesafety of operation was also considered, although proved not to have a signifi-cant impact on the process design.

The production begins by inocculation of a ATCC 4126 strain of Saccha-romyces cerevisiae, chosen for having the highest specific growth rate (0.54 hr−1,in the presencse of a growth medium containing glucose, nitrogen compoundsand other essential nutients. Once the cell culture reaches the required mass of3.5kg, it then passes through five fermentation stages, with a total of 9 fed-batchfermenter vessels in order to achieve a final production of 54,136kg per batch.The yeast culture is then seperated and dried in order to produce 30 percentactive dry yeast and 70 percent compressed yeast.

2 Production of Baker’s Yeast

Saccharomyces cerevisiae, or Baker’s yeast has been used for many years inthe baking industry, mainly due to it’s dough-leavening characteristics. Baker’syeast metabolises sugars, and produces CO2 which causes the dough leaveningand contributes to the flavour and crumb structure of the bread. S. cerevisiaeis a glucose-sensitive yeast, which exhibits aerobic ethanol production in thepresence of excess glucose. It is for this reason they are also used in the manu-facturing of alcoholic beverages.

There are range of types of Baker’s yeast products which can be made in-dustrially, including cream yeast, compressed yeast and active dry yeast. Com-pressed yeast, which is similar to cream yeast with most of the moisture removedtends to be used in bulk for commerical use, while active dry is sold for non-commerical baking and is the form of yeast which is available in supermarkets.

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2.1 Process Overview

2.2 Stages

Inoculation

The first stage of the process, a

Sterilisation

Fermentation

Seperation

Drying

Wastewater Treatment

2.3 Block Diagram

3 Process Science

The design requirements of a suitable fermentation process are based on thespecific needs of the living yeast culture. This includes providing the essen-tial resources as well as replicating the necessary environmental conditions toencourage cellular reproduction.

3.1 Baker’s Yeast

Members of the kingdom fungi, Baker’s yeast is the commercial name for yeaststrains of the species Saccharomyces cerevisiae, which single cell organismswhich have lost the ability of mycellial growth. Saccharomyces cerevisiae cellsreproduce by budding, when a single cell referred to as the ’mother’ cell repro-duces a second cell attached to itself. The ’daughter’ cell, when fully developed,detaches from the mother and may then reproduce its own bud. The originalmother cell may also undergo further cycles of reproduction leading to the cellculture growing at an exponential rate which is the basis of the growth kinetics.[Industrial microbiology page 19] When refering to the growth rate, this is withregard to the growth of the overall population of the culture rather than thegrowth of each individual cells.

However, when refering to the cell concentration within the growth mediumit is with regards to the mass of the entire culture. Single cell organisms undergoboth anabolic and catabolic metabolism (ie the cells use energy to grow andshrink in size) but these process will not significantly affect the overall mass ofthe culture[aspects of yeast metabolism]. In order to obtain the high mass ofyeast cells which is specified in this production an extremely large population isrequired. A high yield of cells which are actively reproducing until the desiredculture mass is reached is required by this process. Cellular reproduction shouldtherefore be encouraged as much as possible.

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3.1.1 Growth Kinetics

An understanding of the growth kinetics of a microbial culture is importantwhen designing a suitable fermentation process. Modelling the cultural growthis a nessecery step before selecting mode of operation, process set-up, equipmentselection and control philosophy.

Yeast cell are single cell organisms which multiply by first forming bubs thatenlarge until they almost equal the size of the mother cell. Nuclear divisionthen occurs forming a cross wall before the daughter cell breaks off.After the culture has been inoculated, and before the yeast culture begins togrow, there is a period of zero growth, during which time the cells adapt tothe new environment. The lag phase can slow down down production and it’slength should be reduced as much as possible by using a suitable inoculum, andchoosing suitable equipment and operating condition.

Under favourable conditions a growing unicellular organism population dou-bles at regular intervals. Each of the two daughter cells produced by a divisionhave the same potential for growth as the mother cell. At first cells tend todivide at fairly regular times known as synchrony and growth rate graduallyincreases. Then minor differences in the cell reproduction time add up andgrowth occurs in an exponential fashion. This is called the exponential growthphase, which can be descibed by the equation

dx

dt= µx

where x is the concentration of microbial mass, t is time(hours), and µ is thespecific growth rate in hours−1.

The stationary phase occurs when cells have exhausted the nutrients requiredfor growth, they utilise all the space that is available, or they may also die dueto an accumulation of toxic substance, a by product of their own activity whichhas led to their environment being inhospitable. The production of the toxicby-products may depend on the resources and conditions. In the case of yeastcells, the presence of oxygen in the growth medium will enable the cell to respireaerobically. If the medium did not have enough oxygen for the entire cell cul-ture to respire aerobically, then some cells would begin anaerobic respiration. Inyeast cells a product of anaerobic respiration is ethanol. The ethanol is toxic tothe culture and its production in the process should be limited to zero in orderto obtain the highest growth rate of yeast. Once the stationary phase of growthhas been reached the cell culture needs to be moved to a larger environmentwhich will meet the space and nutrient requirements of a larger population inorder to continue growth. If the cell culture has reached the desired mass thenit should be extracted and exploited at this point. Operating the process intothe stationary phase with no growth will be wasteful of raw materials and energy.

As the growth rate is limited by the presence of nutrients, the decrease ingrowth rate leading to the stationary phase may be modelled by experimentationwith a range of substrate concentrations. The growth may be described inrelationship to the growth limiting substrate by the following equation (Monod,

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1942):µ = µmaxs

Ks + s

where s is the residual substrate concentration, Ks is the substrate utilisationconstant(numerically equal to the substrate concentration when µ is half µmax

and is a measure of the affinity of the cell for its substrate.// The specific growthrate for Saccharomyces cerevisiae was found to be XX.XX.(Ref) where glucosewas the rate-limiting substrate.

3.1.2 Cell Requirements

The cells will only reproduce when they have the essential resources and suit-able conditions. It is important to consider in designing a suitable process set-upthat the single cells are limited at the rate at which they can reproduce. Un-derfeeding the cells will lead to a reduced growth rate, while overfeeding thecells will not increase the growth but in fact will also reduce the growth duealso. This is due to the increase in other metabolic activity which will lead tothe production of ethanol which is undesirable and will reduce the productionrate, which will be discussed further later. It is therefore essential to feed theculture with a growth medium amounts of nutrients in order to make a growthmedium that will promote optimum rate of cellular reproduction will limitingthe production of undesired products to zero.

Carbon

S.Cerevisiae cell are chemo-organotrophs which means they cannot utilise en-ergy from light and must therefore use sources of chemical energy to sustain lifeand perform the metabolic activity which leads to cellular reproduction. Thecells also require carbon as it comprises around fifty percent of the cell compo-sition. Carbohydrates are the most common source of carbon and energy in formicroogranisms.

Nitrogen

Nitrogen is requirement for growth as it is an essetial elemenent as the are avital component in amino acids and protein synthesis within the cell. Yeast cellsare able to utilise organic and inorganic sources of nitrogen such as ammoniaand urea.

Vitamins and minerals

The yeast cells also require a range of other vitamins and minerals for healthygrowth, including vitamin B, magnesium, phosphorous and potassium as wellas others which will be highlighted later. These components are used to makeup part of the cell mass and although present in small quantites from othersources, may need to be supplied to the cell as individual compenents.

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Element Percentage by dry weightCarbon 45-50

Hydrogen 7Nitrogen 7.5-11

Phosphorus 0.8-2.6Sulphur 0.01-0.24

Potassium 1.0-4.0Sodium 0.01-0.1Calcium 0.1-0.3

Magnesium 0.1-0.5Iron 0.01-0.5

Table 1: Elemental compostion of yeast[Aiba et al, 1973]

3.2 Growth Medium Formulation

The essential nutrients required for cell growth will make up the componentsof a suitable growth medium which will supplied to the cell culture in the fer-mentation tanks. The proper concentraiton of each component in the growthmedium is essential to ensure the highest growth rate and product yield.

The stoiciometry of the production of biomass mas be used to determinethe components of the growth medium, by performing a mass balance on themaximum growth of the culture. The chemical equation for the productionBaker’s yeast biomass is:

C6H12O2 + bO2 + cNH3 + (Xo

Mc)C6H10.9O3.06N1.03

+Heat→

(Xo

Mc+ d)C6H10.9O3.06N1.03 + eH2O + fCO2

However, some elements required for product formation are not required forbiomass composition and a knowledge of the elemental composition of the cellis therefore required in order to ensure the correct quantities of nutrients aresupplied. Table 1 shows the elemental composition of a typical yeast cell.

Considering the large-scale operation of the production the growth mediumwill be required in high volumes. Therefore, considerations into the cost of rawmaterials and availability should be made. The cheapest source of nutrientswhich is suitable for Baker’s yeast production and readily available all yearround should be used in this production.

Water

Water should be the main component of the growth medium used in any fer-mentation process. Yeast cells do not actively use water for cell metabolism,however an aquesous medium for growth is essential for the transport of essen-tial nutrients into and around the cell. The quantity of water used will be basedon the water also makes the solution less viscous making feeding and mixingprocesses easier. The quantity of water is based on the nesseccery dilution of

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other components of the medium.It should also be noted that that a food grade product is required and thatthe water used must be potable and free of microbial contaminants. It shouldtherefore be sterilised before being used in the fermentation process.

Carbon Source

Carbon has a role in both biosynthesis and energy generation. The carbonrequirements for the cell culture can be estimated using the cellular yield coef-ficient (Y) which is defined as:

Quantity of cell dry matter produced

Quantity of carbon substrate utlised

In the production of Baker’s yeast the highest value of Y for glucose has beenfound to 0.5 g of yeast/ g of sugars, for a growth rate between 0.1 hr−1 to 0.26−1.[deKock]Some cheap sources of sucrose which are commonly used in the industrial pro-duction of Bakers yeast are sugar beet and sugar cane molasses, which areresidual produces left after sugar scystallisation in the sugar refinement pro-cesses. They both contain around fifty percent of sucrose. Molasses providea source of simple monosacheride sugars which are easily utilised by the yeastcells which should always be available as the sugar refinement industry operatescontinuously all year round. The use of beet and cane molasses was also widelyadopted by industry because its use was encouraged by the European Unionwith a mimimum price set.The concentration of the sugars must be maintained at 0.1kg/m3 in order tomaintain the highest yield possible.[ref] The amount required will not be fed ata continuous rate, as the demand for sugar will increase as the culture grows,and therefore knowledge of the cell mass produced in each step is needed inorder to maintain the correct concentration of sugar.

Nitrogen Source

A nitrogen source is an essential part of the growth medium and is a requirementfor cells in order to produce biomass. Nitrogen makes up between 7-10 percentof a dry yeast cell mass and medium which is lacking in a nitrogen source will notbe suitable to encourage the production of biomass. Ammonia gas, ammoniumsalts and nitrates are all suitable sources of nitrogen (ref needed). However, itshould be considered that the addition of inorganic nitrogen sources may affectthe pH level of the medium. Ammonium salts will often casuse acidic conditionsas the ammoinium iion is ulitlised and the free acid is liberated. Wherease theuse ifammonia nitrates and gases will usually cause an alkialine shift as the aremetabolised ince the ammonia source has been utilised. [Martin and macmillian1954]

Minerals and Vitamins

Many elements which compose the yeast cells are essential for growth and mustbe present in a suitable growth medium. Sodium, calcium and magnesium com-proise between 0.1-0.5 percent of the dry yeast cell mass and potassium can

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range between 1-4 percent of the weight and they may therefore need to besupplemented to the medium as distinct components.

The addition of biotin, a vitamin B growth factor is required for S.Cerevisiaegrowth and may be present in the beet or cane molasses which are used as carbonsources However, if the sources are deficient the biotin may need to be addedalthough this will incur greater cost from purchasing individual componentsfrom external sources.

3.3 Oxygen Demand

Oxygen must be supplied to the cell throught the liquid growth medium there-fore the dissolved oxygen concentraion must be maintained at a certain level byaerating the growth medium at all times. The rate of oxygen uptake is indepen-dant of the concentraion of dissolved oxygen, provided that the concentrationis above a certain critical level.[Bioengineering] Below this level, the respirationrate of the culture is dependant on the dissolved oxygen concentration and willhave a negative affect on the production of biomass. It is therefore nesseseryto provide provide sufficient oxygen through the growth medium to ensure thedissolved oxygen concentration is above the critical level. The critcal concentra-tion of dissolved oxygen, for Baker’s yeast production at 30oC is approximately0.0007g/l [indstr. engng chem(1950)]

Oxygen is normally supplied to the growth medium in the form of air, withit being the cheapest source of oxygen. The oxygen transfer from air to cell isbroken into three stages; the transfer of oxygen from the air bubble to growthmedium solution, the transfer of dissolved oxygen through the solution to thecell, and the uptake of dissolved oxygen by the cell. The limiting is the masstransfer from the air bubble to the solution[Bartholemew et al, 1950]. The rateof oxygen transfer can be described by the equation:

dCL

dt= KLa(C ∗ −CL)

where CL is the concentration of dissolved oxygen in mmoles dm−3, t is timein hours, KL is the mass transfer coefficient(cm h−1), a is the gas/liquid inter-face area per liquid volume (cm2 cm−3, C* is the saturated dissolved oxygenconcentraion, in mmoles dm−3.

It is however almost impossible to practically measure the interfacial area ofeach air bubble in the system. Therefore the only determination of the oxygendissolution rate is to experimentally measure the rate of absorption by sulphateion oxidation, a method devised by Cooper et al. For air at 25oC, the aerationefficiency KLa was found to equal 5. The air supply rate can then be derivedbased on the intended cell culture mass oxygen demand, mass transfer rate tothe solution and the percentage of oxygen present in the feed stream.

3.4 Environmental Conditions

The conditions most suited to culture growth come from the natural environ-mental conditions in which the species evolved. It is for this reason that the

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conditions within the fermenter should be regulated to maintain the most de-sirable environment for the cells to reproduce. The the optimal conditions are:

• Pressure -atmospheric[ref]

• Temperature -30oC[ref]

• pH -4.5-5[ref]

3.5 Sterilisation

The plant is specified to produce food grade product. Therefore, the productmust be suitable for human consumption and sterilization of process equipmentand operation aseptic environment is absolutely essential.

Furthermore, the invasion of foreign microorganisms into the growth mediumcan have a significant negative impact on production. If contaminated, thegrowth medium would have to support both the growth of the yeast as well asthe contaminant. The contaminating microorganisms would also degrade thepurity of the final product and make extraction of the desired product moredifficult.Sterilization should be performed on all parts of the process including the thegrowth medium, fermenter vessels, ancillary equipment and asceptic conditionsshould be maintained throughtout the fermentation process in oder to ensurethe desired purity of food grade product with no decrease in production rate.

Steam is most commonly used as source of heat in order to kill microorgan-isms. Their rate of death can be described by a first order chemical reaction, asdisplayed the equation:

4 Process Design

4.1 Production Rate

The plant is specified to produce 2500 tonnes of Baker’s yeast each year. Allow-ing for an operational down-time of ten percent[Sinnott, 2005], the plant willbe operating for 325 days per year. The weekly production rate can thereforebe calculated 53191.5kg of yeast per week.

4.2 Modes of operation

There are three modes of operation which are suitable for the fermentation ofbiomass.These are batch, continuous and fed-batch processes. Each process hasits advantages but the process selection depends on the type of product andscale of production.

4.2.1 Batch Culture

A batch culture is closed culture system in which an inoculated culture is placedwith a limited amount of nutrients. The cell culture, if environmental conditions

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are suitable, will then undergo the phases of growth which were described ear-lier. After the lag phase and the phase of increasing growth rate, the cells beginto grow at a constant, maximum rate.Batch production may be used for the pro-duction of biomass, with a growth medium and environmental conditions whichsupported maximum cell population, encouraging a long exponential phase.

4.2.2 Fed-batch Culture

The term fed-batch culture was first introduced by Yoshida et al (1973) anddescribes and operation by which fresh growth medium is fed to the culture ina continuous or sequential manner while the culture remain in the vessel, henceincreasing the volume of the culture as the operation is run.

One major advantage of a fed-batch culture is that the concentration ofsubstrate can be easily controlled by the feed rate of fresh medium into thefermenter vessel. As previously stated an excess of glucose in the fermentationof bakers yeast is undesirable. It has been recognised that increasing the con-centration of glucose will lead to increased growth and the oxygen demend ofthe system will increase, resulting in anaerobic respiration if the demand of oxy-gen is not supplied and an increase in ethanol production. Bakers yeast is verysensitive to high glucose concentration, and respiratory activity was shown tobe repressed at around 5ug dm−3 (Crabtree, 1929). In a fed-batch productionof bakers yeast the medium feed rate should be under strict automatic controlbased on the detection of ethanol in the fermentation exhaust gas.

In order to achieve the highest production rate fed-batch culture is the mostsuitable mode of operation because of the scale of the operation is not so largethat is demands continuous production rate. It is easy to control the rateof substrate fed to the vessels and this means that the process can avoid theproduction of ethanol. A number of fermentation vessels will be required as thecell culture grows and must be moved to a larger environment.

4.3 Process selection

4.3.1 Nutrient preperation

Sugar beet and blackstrap cane molasses will be the principal raw materials forthe production of baker’s yeast as they are rich in fermentable sugar contentassimilable by the yeast. These molasses are produced as a waste product fromthe processing of sugar beet and sugar cane. Other than fermentable sugar (su-crose, glucose and fructose), these molasses also provide essential minerals suchas potassium, phosphorus, magnesium, zinc, iron, and copper, amino nitrogen,and vitamins such as biotin. (H.J.Peppler, 1967).

The composition of each molasses varies according to the type and geo-graphical origin. The average composition of beet molasses and blackstrap canemolasses are presented in the table below:

In general, beet molasses has higher organic nitrogen than cane but not allof this organic nitrogen can be assimilated by Saccharomyces Cerevisiae. Canemolasses on the other hand has substantially higher composition in biotin, pan-tothenic acid, magnesium and calcium. Both types of molasses are nearly equal

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Constituent Beet molasses Blackstrap cane molasses(percentage) (percentage)

Brix 84 88Water 16.5 21

Organic ConstituentsSugars:Sucrose 51 38.5Glucose 2.7Fructose 5.2

Invert Sugar(Raffinose) 2 5.6Total Sugars 53 52

Nonsugars(Nitrogenous materials,free and bound acids,

soluble gummy substances) 19 10Inorganic Constituents (Ash)

SiO2 0.1 0.5K2O 3.9 4.1CaO 0.26 0.8MgO 0.16 0.24P2O5 0.06 0.08Na2O 1.3 0.08Fe2O3 0.02 0.0014Al2O3 0.07

Soda and carbonate residue 3.5Sulfate residue (as SO3) 0.55 0.7

Chlorides 1.6 2.1Other Constituents 8

Total Ash 11.5 17

Table 2: Elemental compostion of beet and blackstrap cane molasses

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in fermentable sugar content potassium and other trace minerals.

However, yeast manufacturers usually prefer beet molasses to cane molassesbased on the overall composition. Frequently, a mixture of both molasses is usedrather than one molasses alone to ensure adequate supply of essential nutrientsfrom the molasses. The common recipe used in Baker’s yeast production is beetmolasses with at least twenty percent cane molasses mixture to ensure amplesupply of biotin. Other mineral nutrients such as phosphorus and nitrogen mustalso be supplemented in addition to the mixture in order to sustain optimumgrowth, maximum yield and quality as molasses contains only a small portionof the mineral nutrients.

To prepare the mixed molasses solution as a feed into the reactor, the so-lution must be subjected to preliminary treatment by means of dilution, pHadjustment, heating, clarification and sterilisation. The objectives of the pre-treatment are

1. Removal of suspended matter, colloids, colouring materials, volatile acids,nitrites and sulphites by clarification.

2. Reducing and elimination of microbial flora by disinfection or sterilisation(Olbrich, 1963)

It is then supplemented with additional nutrients as necessary. These steps mayadd additional operating cost but they are necessary to guarantee the safety ofthe end products for human consumption.

Preliminary Treatment

The dilution of the mixture is practised partly to dissolve the microscopic sugarcrystals in the solution but mainly to make it easier to move the solution bymeans of pumps as molasses can be very viscous. The viscosity of the mixtureprior to dilution can reach up to approximately 7.5Pa s−1.

The dilution of molasses is usually represented by using degrees brix. It is aunit representative of the dissolved solid content in a solution e.g. 1 brix corre-sponds to 1g of solid dissolved in 100g of solution. In industry, the mixture isusually diluted to 40-50 brix and adjusted to pH 4-6 upon mixing. To determinethe most suitable dilution factor, adjusted pH and heated temperature, the nextstep after mixing, which is clarification and sterilisation, are studied. In orderto maximise the efficiency of the pre-treatment, optimum physical conditionsshould be employed.

Clarification

Clarification of molasses is primarily employed to clarify colloids and non-sugarsuspended materials (mud) in the molasses. These colloids and suspended ma-terials may interfere with the assimilation of sugars during the fermentation ofthe baker’s yeast hence reducing the efficiency (A. H. E1-Refai, 1992). Treatedmolasses is expected to give maximum yeast growth productivity as a result ofbetter fermentation efficiency. The most effective way to clarify the molasses is

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by centrifugation after dilution, heating and sulphuric acid addition to assist co-agulation of the mud. The clarification method, design equation and alternativeprocesses will be reviewed.

Centrifugal Clarification

The effects of dilution, temperature, acid addition on the efficiency of ash re-moval were studied by HW Bernhardt, 1998 (Bernhardt, 1998). The optimumoperating conditions were found to be at temperature 70C, pH 4 and dilutionto 50 brix.

4.3.2 Innoculation

The process begins in a laboratory, where a pure culture of yeast is inoculatedinto a sterile flask, and grown with a growth medium in a batch process underanaerobic conditions. This is done to prevent any bacterial or viral contamina-tion of the yeast which could cause the culture to die. Once the flask can nolonger contain the population, it’s contents are transferred into a larger flaskunder the same sterile anaerobic conditions.

Various strains of S. cerevisiae have been investigated, with new strains witha faster reproduction rate being created constantly. As of right now, the maxi-mum average growth rate is 0.54 h-1, which comes from the strain ATCC 4126.(De Kock et. l, 2000) This is a great improvement from 0.4 h-1, which was thevalue in 1981 (Reed, 1981), mainly attributed to strain selection.

The strain of S. cerevisiae that will be used is CBS 8066, which has a max-imum growth rate of 0.50 hr-1, and a saturation coefficient of 40 mg L-1. TheATCC 4126 does provide a higher maximum growth rate of 0.54 h-1, howeverthat does also come with a higher saturation coefficient of 60 mg L-1, which is150 percent of the CBS 8066, and therefore a higher concentration of substrateis required to maintain the same growth rate and the same doubling time.

4.3.3 Fermentaion

After the innoculum has grown sufficiently a number of fermentation stageswill be required in order to achieve the required cell mass. As the cell cultureis transfered to the next fermentation stage,a larger vessel will be required tocontain growing cell population.

4.3.4 Seperation

Once the desired mass of yeast has been yielded from the fermentation stagethe desired product should be seperated from the fermentation broth in orderto obtain a commercially viable product. Three different centrifuges were con-sidered for the separation of the fermenter outlet. The centrifuges that wereconsidered are tubular, decanter and disc stack centrifuge. It was found in liter-ature, these centrifuges were used in different plants for the separation process.The fermenter outlet with solids concentration of 6 percent will be concentratedto 19 percent solids. The solid discharge from the centrifuge is termed yeastcream and will be further dewatered to produce compressed yeast with solids

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concentration of 30 percent. The yeast cream will also be dehydrated to 93percent solids concentration to make active dry yeast. The centrate will containmostly water, with residual unfermented sugar and traces of by products andcells.

4.3.5 Decanter Centrifuge

The decanter centrifuge consists of two rotational pieces which are the bowl andthe screw conveyor (scroll). The scroll is within the cylindrical bowl and bothare rotating at different speed. The scroll will provide a conveying motion forthe removal of solids due to the difference in speed. Feed enters the centrifugethrough a hollow tube along the centre of rotation into the liquid pond at thebowl wall. Centrifugal force generated by the rotating pieces will cause solidsto settle and accumulate at the bowl wall. Solids are conveyed along the scrolltowards the solids discharge end of the centrifuge. The liquid moves towards theadjustable weirs located on the other end of the centrifuge. The weirs dictatethe level of the liquid inside the bowl.

Towards the solids discharge end, the bowl is sloped inwards to the centre.This will cause the solids to be conveyed up towards the centre and liquid willdrain back into the liquid pond. It is common for a decanter centrifuge to have ahorizontal axis of rotation and it is most widely used in industry but centrifugewith vertical axis of rotation is also available however it is not used as widelyas the horizontal centrifuges. Around the rotating bowl is the housing. Itsprimary function is to avoid the solid discharge and centrate from mix backtogether after separation. Electrical motor generates power to rotate the bowland the scroll and a gearbox controls the speed of the conveyor.

4.3.6 Disc Stack Centrifuge

The disc stack centrifuge has several parallel disc stacked on top of one anotherinside the bowl. The discs inside the centrifuge provide a large area for par-ticle settling. Centrifugal force generated by the motor causes liquid to movetowards the axis of rotation and flows upwards through the discs. Heavy solidparticles settle on the discs and moves downwards to the bowl wall. The discsare slopped at a certain angle to provide efficient separation.

Feed enters at the centre of rotation either from the top or the bottom. Atthe end of the centrifugation process, clarified liquid will leave the centrifugethrough the opposite end of the entrance. Solids will be collected at the side ofthe bowl wall and could be discharged either periodically or continuously. Discstack centrifuges are commonly used in the separation of fine solids and liquids,and it could also be used for classification of solids.

4.3.7 Tubular Bowl Centrifuge

The tubular bowl centrifuge comprises of a rotating vertical cylindrical tube.The length of the tube is usually several times the diameter. The tube is placedin between two bearings at each end to allow the tube to be rotated by the

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motor. Feed enters at the bottom of the tube and separation between solid andliquid occurs due to the centrifugal force. Purified liquid flows through the axisof rotation and exits through the top of the tube. Solid will adhere to the side atthe tube wall. Collected solids, in time, will form a cake with a certain thickness.

Liquid is able to flow through the centrifuge continuously. However, theoperation of the centrifuge needs to be stopped for the removal of solids. Thesolids could either be manually scrapped off from the tube wall or flushed out.This centrifuge is commonly employed in the separation of immiscible liquidsand purification of fine solids.

4.3.8 Drying

The plant is designed to produce 30 percent active dry yeast and 70 percentcompressed yeast based on our market research. Drying is an important unitoperation widely used in the food industry to reduce water content and im-prove shelf life of products. The principle is based on the removal of water frommaterial by evaporation (Strumillo and Kudra 1986). During thermal drying,they may undergo some changes such as denaturation of proteins or enzymes,destruction of cell membranes or death of cells. In order to keep the adverseeffects of thermal drying to a minimal, the optimal operation of drying processis required (Adamiec, et al. 1995).

Several drying methods were looked into including spray drying, Rotoloverdryer and tunnel dryer but the Fludised bed dryer stood out as it has been widelyused under batch or continuous operations for industrial drying of Baker’s yeast(Hovmand 1995). Also, the use of fluid bed drying for granular material is nowwidely used and well established in Industry (Trkera, et al. 2006). Baker’syeast, Saccharomyces cerevisiae is a granular product and this drying operationreduces its moisture content from 70-65 percent to 6-4 percent with a varyingtime between 30 and 200 minutes. This gives us product within the desiredmoisture content for dry active yeast, as well as greatly improving its shelf life(Beker and Rapoport 1987).

Fluidised Bed

For many years, fluidised beds have been used in the food, chemical and phar-maceutical industries to carry out a variety of chemical reactions as well as unitoperations. A fluid bed is formed when a bed of particles is transformed into afluid-like state by forcing a gas through the bed (Villegas, et al. 2009).

A primary advantage of the fluidized bed is that it provides high heat andmass transfer due to the turbulence created in the bed, as well as good solidsmixing and easy transport of material. Though, it is difficult to predict thedynamics in these systems as they are known to be highly non-liner (Kunii andLevenspiel 1991).

A typical fluidized bed dryer consists of a blower, heater, fluidized bed col-umn and a gas-cleaning system. The conventional fluidized bed is formed by

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Parameter Concentration(mg l−1)

BOD5 18,300-27,200COD 52,600-88,800TS 3,774-7,307

TKN(N) 1,612-2,057pH 3.8-4.0

SO2−4 4,600-6,300

Table 3: Typical Baker’s yeast production wastewater[Lo, Liao]

passing a gas stream from the bottom of a bed of particulate solids. The fludiz-ing gas passes through a gas distributor plate, where the bed of partiles rests,and is uniformly spread across the bed. As the fluidizing gas velocity is in-creased, the pressure drop across the bed increases.

The bed is fluidized when the gas stream totally supports the weight of theentire bed which occurs at a certain gas velocity known as minimum fluidizationvelocity. The pressure drop remains the same across bed even if the gas velocityis further increased at minimum fluidization. A fluidized bed is operated athigher gas velocities than the minimum fluidization velocity, typically 2-4 umf.This fluidization velocity is normally obtained experimentally. Particles with aninitial high moisture content have a higher minimum fluidization velocity thana comparable bed of dry particles. (Handbook of Industrial Drying n.d.)

4.3.9 Wastewater treatment

Effluent from the fermentation of Bakers yeast contains high amounts of organicand inorganic matter including polyosaccharides, minerals and sulphate. Highconcentration of BOD and COD encourages bacterial growth in water and leadsto a depletion of dissolved oxygen in water which can be harmful to the ecosys-tem which it supports. Below is a table displaying a typical composition of thewaste water from an industrial bakers yeast production

Due to the high polluting potential of the process liquid effluent it is nessec-cery for the wastewater to undergo some form of treatment or alternative procc-sing so that it can be disposed of safely without significant harm to the localwater system.

There a number of options available for the treatment of wastewater from theproduction of Baker’s yeast, some of which may be more economically viable.The most important criteria for a successful wastewater treatment process is toreduce the BOD, COD and high nitrogen level to below government standards.Current British Water standards request the post-treatment effluent quality dis-played in table 4[British Water Code of Practise].

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Parameter Concentration(mg l−1)

BOD5 20Suspended Solids 30Ammonia (as N) 20

Table 4: Typical Baker’s yeast production wastewater[Lo, Liao]

4.3.10 Anaerobic digestion

The first process option is to treat the wastewater by anaerobic digestion, whichis a natural process by which microogransism break down the organic matterof the effluent stream in the absence of oxygen. In this process the organicmatter is broken down over a number of weeks, to carbon dioxide and methanegas which is formally known as biogas and is suitable as fuel for power genera-tion, which may be a potential option in electricity generation to power parts ofthe plant. The biogas produced from the anaerobic digestion can be collected,stored and used when needed. However, there are significant economic implica-tions for the installation and operation of an anerobic digestion process alongwith the technology required for biogas fueled power generation. The viabil-ity of a designated wastewater treatment process will depend on the volume ofwastewater and cost of production, operation and maintenance.

4.3.11 Compound Fractioning and Recovery

Another wastewater process option available is is to concentrate the effluentstream by evaporation or distillation leaving condensed molasses solubles (CMS),also known as vinasse, which is a by-product of sugar beet fermenation processes.Condensed molasses solubles may be used as fertiliser of as a feed for cattle, ifthe potassium concentration is reduced to a safe level. Potassium salts, anessential component of plant fertilizer, may be obtained by the addition of am-munium sulphate during an evapotation process, concentrating the vinasse todry matter content of 50-80 percent dry solids. During this process homogenousgrowth of potassium salt crystals occurs, and these crystals may be collected bya solid/liquid seperating process such as decanting[US PATENT, 1996].

4.3.12 Off-site treatment

The final option for handling wastewater from the plant is to pay an off-sitesewage works to treat the wastewater to an acceptable standard, by an ac-tivated sludge process before being desposed of. Although off-site treatmentwould reduce the production and operating costs of the plant, it would generatevolumetric charges for disposal as well as transporation of effluent to off-sitesewage works.

In the UK, the cost of wastewater treatment is broken down in five com-ponents, and factors in the strength of the effluent. The following equation is

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used to calculate the cost of biological treatment of wastewater by SouthernWater[Trade effluent charger]

UNIT CHARGE = [R+ V + Ot.B

Os+ St.S

Ss+M ]

where

• R is a fixed charge for reception and conveyance = 50.07p/cubic mtr

• V is a fixed charge for preliminary tretment = 42.37p/cubic mtr

• Ot is a measure of the organic nature of the wastewater, generally mea-sured in terms of COD

• B is the biological treatment cost per cubic metre of sewage = 48.12p/cubic mtr

• Os is a meaure of the organic nature of settled foul sewage

• St is the total suspended solids of the trade effluent mg/l

• S is the sludge treatment and disposal cost = 32.91p/cubic mtr

• Ss is the total suspended solids of crude sewage

• M is a fixed charge per cubic metre for discharge through long sea outlet= 6.37p/cubic mtr

Assuming the treatment for average values for the COD and total suspendedsolids disposal the unit charge for off-site waste water treatment would be ap-proximately 55 pounds per cubic meter (tonne) of wastewater for off-site treat-ment. This is due to the high concentration of COD and some some on sitetreatment may be nessesery to reduce the COD before being sent to a sewageworks, thereby avoiding expensive treatment costs.

Due to the high particularly high COD and nitrogen concentration of Baker’syeast effluent some form of wastewater treatment should occur on site, loweringthe organic matter content before it can be sent to local sewage works so thatthird party charges for disposal are kept to a minimum. The plant will thereforefeature a suitable anaerobic biological contact reactor in order to treat wastew-ater and produce biogas which may be used to power generation.

4.4 Process Flow Diagram

4.5 Equipment Design

4.5.1 Steriliser

4.5.2 Fermenters

Requirements of a suitable fermenter are based on the nutrient and environmen-tal requirements of the yeast culture. The fermentation vessels should meet thefollowing design criteria[Principles of fermentation]:

1. Capable of aseptic operation for a number of days

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2. Provide adequate aeration and agitation to the cell culture, while notdamaging the cells through mechanical stress

3. Provide a system of temperature control, to maintain 30oC with the vessel.

4. Provide a system of pH control, to maintain a pH of between 4.5-5.

5. Require minimal amounts of labour during operation, cleaning and main-tenance.

6. Constructed using welds to provide smooth internal surfaces.

7. Constructed using the cheapest materials which provide satisfactory op-eration.

Body Construction

A cylindrical shaped vessel with hemispherical top and bottom, in order to with-stand pressure sterilisation. For smaller vessel the body should be constructedof 7mm thick stainless steel, while larger vessels will be constructed of mild steelwith a stainless steel cladding in order to reduce capital cost of larger vessel.Stainless steel is corrosion resistant and non-toxic, and will be satisfactory formany years of continued use.

Aeration and agitation

Every cell needs to be in contact with oxygen supplied as air is suitable for thisprocess. The air is introduced to the growth medium through a sparger, a deviceused to pump gas bubbles into a liquid. There are a number of different spargertypes available all of which would be suitable for aerating the growth medium,although most modeern fermenter designs use a nozzle sparger, which shouldbe posisionted directly beneath the agitator but as far away as possible, so thatthe agitator does not become flooded with gas bubbles leading to a decrease inpower.

Agitation is desired not only to break up air bubble to increase the interfacialarea for oxygen transfer to the cell culture, but also to ensure a uniform con-centration of components within the fermentation broth. An impeller is a seriesof blades or paddles attached to a shaft which is driven by an electric motor.The blades move the fermentation broth and create axial flow patterns withinthe vessel. The Rushton turbine is the most suitable impeller for gas-dispersionand should be position above the base a distance of one-third to one-half of thevessel diameter. For the larger fermenters 2 discs will be needed in order toachieve the same dispersion. An agitation speed of 500rpm is suitable for theproduction of Baker’s yeast, slightly decreasing the lag-phase while avoidingdamage to the cells.[Ahmad, Holland]

4.5.3 Centrifuge

This production plant uses centrifuges at two different part of the process. Firstis for the separation of yeast cells from the final fermenter and the other is usedin the preparation of sugar molasses; clarifying beet and cane molasses from

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ash mud. The centrifuge inlet solids concentration, outlet solids concentration,density difference between solid and liquid and diameter of particle to be sep-arated are important parameters that were considered during the selection ofcentrifuge. Separation of fermented yeast.

Seperation of fermented yeast

After the final fermentation process, inside the reactor will consist of 94 percentwater, 6 percent yeast and traces of unreacted sugar and by products. These willthen pass through the centrifuge and concentrated to 81 percent water and 19percent yeast. The centrate flowing out of the centrifuge is assumed to containno yeast cells and only consist of water and traces of other feed to the process.Live Baker’s yeast has a density of 1100 kg/m3 (Bryan, Goranov, Amon, Man-alis, 2010) and as the process stream coming into the centrifuge only consist ofyeast cells and water, the density difference between the solid and liquid is 100kg/m3. In literature, baker’s yeast was reported to have a spherical morphologywith diameter ranging from 5 to 10 micrometers (Charinpanitkul, Soottitanta-wat, Tanthapanichakoon, 2008).

Figure 1.7 gives an indication of the appropriate centrifuge to be used ac-cording to the particle diameter to be separated. Tubular bowl, disc stack(batch, nozzle and valve), imperforated basket and decanter centrifuge fallsin the range of baker’s yeast cell diameter. However, tubular bowl centrifugeis rarely employed for separation of solution with solids concentration higherthan 1 percent (Coulson Richardson, 1983). Tubular bowl centrifuge generatescentrifugal force of around 15000g which is excessive for concentrating cell sus-pension and poses high possibility of damaging the yeast cells. Similarly, thecentrifugal force generated by disc stack centrifuge is lower than tubular cen-trifuge but is still within the range that is able to cause yeast cells to disrupt.A decanter centrifuge would provide a continuous separation where as an im-perforated basket centrifuge operates in a batch manner. Decanter centrifugewill require less space in the plant.

Based on the equipment selection guide by Lavanchy et al. (1964), the mostsuitable centrifuge was chosen. Figure 1.8 clearly depicts, for a feed volumetricflowrate of 16.2m3/h at a Q/Σ of 6.1x10−6, a scroll type or decanter centrifugeshould be used for separating the fermented yeast from its nutrient broth. Equa-tions used and calculations of these values can be found in appendix XX.

Clarification of beet and cane molasses

The beet and cane molasses mixture flowing out from the mixing tank (refer tosheet 1 of P and ID) have a solids concentration of ash mud of 7.3 percent. Thisneeds to be brought down to 2.2 percent solids. The solid discharge end of thecentrifuge contains only ash mud which mainly consists of calcium sulphate andpotassium sulphate. The molasses feed into the centrifuge flows at a flowrateof 12.24m3/h. By employing similar calculations as with the separation of fer-mented yeast, the Q/Σ was found to be 1 x 10−7.

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Figure 1.9 indicates that a disc stack centrifuge should be employed forthe clarification of the molasses mixture based on the values mentioned above.Appendix XX1 contains calculations of all the physical parameters and variablesinvolved.

4.5.4 Centrifuge Design

Decanter Centrifuge

The decanter centrifuge was designed to separate 900,000 kg of fermented bakers’yeast within 3 days. The centrifugation process runs continuously to produceconcentrated yeast cream for further dewatering and dehydrating, and centrate,consisting of water and traces of other materials into the fermentation process,which will be treated at the wastewater facilitiy before being discharge. Thegeneralised design equation for decanter centrifuge was obtained by derivingfrom basic centrifugation principles and equations (particle settling velocity,residence time and liquid volume inside the centrifuge). The equation obtainedis

Q =ω2D2

p(ρs − ρl)18µ

[πb(R2w −R2

L))(ln(Rw/RL)

]

Where ? is the rotational speed (rads/s), Dp is the diameter of the particle tobe separated (m), ?s is the density of the solid (kg/m3), ?l is the density of theliquid (kg/m3), is the viscosity of the liquid (kg/(m s)), b is the length of thecylindrical part, Rw is the radial distance from axis of rotation to the liquidpool (m) and RL is the radial distance from axis of rotation to the centrifugewall (m). The above equation could be further simplified by assuming that thedistance for a particle to settle is small in comparison to the bowl radius i.e.Rw-RL¡¡RL. The generalised design equation becomes

Q =πbR2

wω2D2

p(ρs − ρl)9µ

The simplified design equation was used to obtain the volume and dimension ofthe decanter centrifuge for this plant. The centrifuge will handle 0.001 m3/s offeed consisting of bakers’ yeast cells and water. By evaluating different cylin-drical length, b and centrifuge radius, Rw , it was found the centrifuge to havea volume 0.07 m3. The decanter centrifuge would be 0.9 m of cylindrical lengthwith 0.31 m diameter. This gives a length to diameter of 2.9. Usually for indus-trial centrifuge, the length to diameter ratio is around 4 (Records Sutherland,2001). Any value higher than this would require the centrifuge to be operatedinefficiently in terms of energy consumption. Full calculations regarding the sizeof the decanter centrifuge could be found in appendix XX2.

4.5.5 Fluidised Bed

Anaerobic Biodigester

4.6 Process Control

All stages of the production process need to be under strict control in order toobtain the desired product quality and quantity. Poor control of glucose level

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and environmental conditions during fermentation can leads to batch spoilage,while failure to control post-fermentation processing could produce productswhich are not suitable for retail. It is also nesseccery to control the volume ofsteam during sterilisation stage to ensure aseptic conditions and prevent con-tamination. The control philosophy for each processing stage will be discussed.

A control loop has three basic components:

1. Measuring element

2. Controller

3. Final control element

A measuring element monitors a property of the operation such as temper-ature, pressure or flowrate and converts this physical property into an outputsignal. This electronic signal may then be displayed on the external casing ofthe measuring instrument or displayed in a control room. A controller, eitherhuman or computer, then compares the measurement to a predetermined valuerefered to as the set-point. Any difference in the measured value and the setpoint will lead to an adjustment in the final control element in order to manip-ulate and return the measured property back to the desired set point. This maybe done by hand in the adjustment of a valve or by a computer which generatesan output signal based on the varience between measured value and desiredsetpoint which controls some device such as a valve. It is nessesery to have allthree components for successful control, wether it be manual and automatic.

4.6.1 Fermentation

Ensuring constant environmental and nutritent conditions is essential to en-sure the disered quantity of yeast production during the fermentation stage.Each fermenter vessel should have the capability of monitoring and regulat-ing environemental conditions include temperature, pH and dissolved oxygenconcentration as well as glucose concentration to ensure optimal growth.

Temperature

The temperature of the solution containing in the fermenter vessels is measuredusing a temperature indicator. This is connected to temperautre controller witha predetermined set point of 30oC.Thetemperautrecontrolleristhen

pH

Dissolved Oxygen Concentration

4.6.2 Centrifuge

There are several flow indicators connected at the streams around the centrifuge(at the inlet, solids discharge and centrate stream). These indicators provideoperators with flow measurements to ensure no blockage inside any of the pipe.It is expected not much variation in flowrate around the separation process be-cause the centrifuge was designed to handle flow fluctuations up to 10 percent.The valves at the outlets of the centrifuge will have been opened manually at

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the start up to allow a fixed flowrate and will remain throughout the time thecentrifuge operates. Hence, no flow controllers are used.

There is a possibility that the centrifuge may get too hot and stops func-tioning completely because there are a lot of rotating and moving parts in thecentrifuge which generate heat. Therefore, the temperature of the centrifugemust be monitored throughout its operation. A temperature indicator gives thetemperature of critical parts within the centrifuge and if it gets too high, analarm will be set off notifying the operating of the situation.

The moisture content of the solids discharge stream is considered the mostimportant parameter in the separation process. Hence, a quality indicator isposition at the solids discharge stream to monitor the water content inside thestream as the solids flowing out needs to be at 19 percent solids concentration. Iffor any reason, the solids concentration falls below 19 percent, the indicator willsend a signal to the actuator connected to the control valve at the centrifuge inletstream to reduce the flow of feed into the centrifuge. If the solids concentrationflowing out is too high, no action will be taken as this would be more desirable forthe further downstream processes which dewater and dehydrate the centrifugeoutlet.

4.7 Piping and Instrumentation Diagram

4.8 Equipment Specification Sheets

4.9 Plant Layout

The plant would be situated in an industrial area near other plants becausefacilities and utilities are readily available for new plants. The plant also hasto be not too far away and easily accessible. Raw materials such as molassesand nutrients for feed preparation and fermentation has to be refill every week.Therefore, easy access to the plant would ensure steady flow of raw materialswhich would prevent any disruption on the production of bakers’ yeast. Theplant also requires constant supply of utilities such as electricity, water andsteam. Situating it in the industrial area where these are all already establish,would be beneficial to the plant. The plant layout could be found in appendixAA.

The plant consists of four main parts; feed preparation zone, fermentationarea, downstream processing zone and administration. Feed preparation is lo-cated near the front of the plant. Feed storage tanks are situated near theloading dock in order to ease the tanks being refill and avoid any clutteringwhile doing so. The heat exchanger for the sterilising unit is placed a bit fur-ther away from other equipment as it is the only equipment the pose potentialharm because hot steam and stream passes through the equipment. This alsoallows the equipment to be isolated when there is a problem and any damagecould be contained.

After the feed preparation zone is the fermentation area which comprise ofnine bioreactors of increasing volume. The first two are fairly small as they are

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just an innoculum for the yeast. The reactors are could be observed from thecontrol room. The control room is located near the first four reactors. Otherreactors could easily be reached by an operator in case of any emergency. Ma-jority of the pumps used in this process could be found in the pump room atthe centre of the plant. This is to minimise the space that would be occupiedby the pumps. A laboratory is also required for this plant to conduct qualitytest on various sampling points throughout the plant and to assess the purity ofthe final yeast product. Therefore, it is placed next to the control room nearbythe bioreactors.

Six storage tanks which supplies the fermenters with the nutrients requiredfor optimum growth of yeast are located at the middle part of the loading dock.These nutrients, similarly to the molasses, must also be replenish on a weeklybasis as production of each batch will consume all the content of the tank. Ithas been found that it would be more economical to refill the tanks on a weeklybasis than to have large tanks that would store huge amounts of nutrients forproduction of several batches of bakers’ yeast. Furthermore, there would be ahigher possibility of contamination with storing large volume of nutrients at onetime.

Centrifuge, filter press, fluidised bed dryer, storage tanks and the wastew-ater treatment facility makes up the downstream processing unit of this plant.The centrifuge is placed right after the last four fermenters to decrease the pipedistance between the equipments. After the centrifuge, the concentrated yeastcream goes through the filter press and the fluidised bed dryer for further dewa-tering and dehydrating. These two equipments are parallel to each other as theyeast cream flows into both simultaneously. Storage tanks for compressed andactive dry yeast are situated next to the unloading dock which at the end of theplant. The final product could be easily transferred into vehicles for distributionand opening space for storage for the next batch. The wastewater treatmentfacility is located on the opposite site of the plant from the storage of the finalproducts. The treated waste could be discharged safely into a river nearby theplant.

4.10 Start Up/Shut Down Procedures

Steriliser

Fermenter

Before the fermentation process can begin the proper start up sequence needs tobe performed. This included checking equipment, instrumentation and prepar-ing vessels to reduce the risk of an unsuccessful fermentation. Towards the endof the sterilisation of the fermenter vessels and ancillary equipment should bechecked for leaks, which may be performed in person by a process operator.Once they are satisfied that the equipment is suitable for operation then thefirst fermentation vessel should be filled with the aquesous growth solution byopening valve V-101, measuring the flow rate in order to calculate the volume ofsolution added. Once the vessel has reached the desired level the valve is shut off.

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Once the water and nutrient solution has been added and the process op-erator is satisfied that the correct volume of solution has been used, then theagitation should begin by switching the impeller motor to the desired speed.Also, monitoring and regulation of the solution temperature and pH shouldbegin and air flow should be allowed into the vessel by opening valve-101 andswitching one pump P-101. Aeration should be allowed to occur for a few min-utes to ensure that sufficient dissolved oxygen is present before the yeast isadded. At this point, if the temperature and pH of solution are satisfactory, themolasses feed should begin to be added by opening valve V-103 and switchingon pump P-101, while simultaneously adding the innoculum to the solution byopening valve V-102 and switching on pump P-101.

The fermentation process will then run until the desired mass of yeast hasbeen produced. The feed of molasses will be stopped. The fermentation brothwill remain in the vessel, while being agitated and aerated and still under tem-perature and pH control. The broth will then be pumped to the next fermenterby following the same procedure, until it has passed through all fermentationstages. After the broth has left a fermenter, the vessel should be cleaned byopening the drain and spray hosing to ensure that no residual liquids or solidsremain in the vessel. It may then undergo sterilisation in preperation for thenext fermentation batch.

Centrifuge

All pipe connections, valves and instruments around the centrifuge and down-stream needs to be thoroughly checked at the start of the fermentation. Thecentrifuge itself must be inspected and any moving parts (bearings and gears)needs to be oiled including any parts within the motor. Connections to thepump must also be checked to ensure no leakage that would allow outside airinto the system which will contaminate the sterile system. Control valves mustbe tested to ensure it respond appropriately according to the signal sent by theactuator. Calibration test should be done on the quality and flow indicatorsso that it display the actual moisture and flowrate flowing out from the solidsdischarge end.

Once the fermentation ends, the centrifuge needs to be turned on followedby opening of the feed inlet valve to allow the fermented yeast in. Steady statewould be reached within 70 seconds. Once steady state has been reached, solidsand liquids will start to flow out, and outlet pump needs to be switched on.The control valve connected to the outlet of the pump must be opened to allowpassage for the separated yeast to further downstream processing.

Towards the end of the centrifugation, inlet valve has to be closed once allthe content of the final fermentation tank goes into the centrifuge. Once all ofthe fermented yeast has been separated, the centrifuge could be isolated andflushed with water to wash away any residual yeast left inside. Before doingthis, the outlet valve needs to be closed and outlet pump needs to be switchedoff as well. Maintenance work should be carried out straight away while thenext batch of fermentation proceeds to ensure the centrifuge will work properly.

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Dryer

5 Safety

5.1 Maintenance

The whole system has been set up to be maintained regularly and safely. Themajority of the main equipment, including the reactors, heat exchangers, pumpsand towers are all located in easy to access areas in order to make maintenanceas easy and as fast as possible. The plant will be maintained regularly forthe first few months, during the wear in period, in order to avoid any majordowntime.

5.2 Staff Safety

Staff are to wear safety goggles, hard hats and heat proof gloves whenever inthe plant.

5.3 Material Safety

The data sheets for all the materials used in the process are attached to theappendices of the report. None of the chemicals are toxic, flammable or corro-sive, and therefore no safety measures have to be taken, as the plant will not beoperating anywhere within the:

• Flash point

• Flammability Range

• Autoignition Temperature of the materials.

The materials in the plant are:

– Water– Yeast– Ammonia– Anti-Foam– Vitamins Minerals– Molasses

None of the materials are shock sensitive nor do they have an unusualphysical property.

5.4 Process Safety

Fermenters

The reaction in the fermenters is exothermic, and the heat generated hasbeen accounted for. In order to avoid raising the temperature of the fer-menters, cooling water is used in the jackets to remove the heat generated,

27

and maintain the temperature at an optimum of 30oC. Temperature in-dicators are placed on all the fermentors, and a cascade control systemis used, where the temperature of the fermenter and the entering coolingwater are measured, and the flow rate of the cooling water is adjustedaccordingly. This allows more stable control of the temperature of thereactor. A temperature high alarm has been placed on the reactors if it’stemperature is too high. An alarm has also been placed on the coolingwater stream, in case it’s temperature is too high to cool down the fer-mentors adequately. Level indicators have been placed on the fermentors,along with a level high alarm. As a batch process, the fermentor is meantto empty out after a certain amount of time and go into the next reactor.If the reactor fills up before this time, the fermentor empties out into thenext part of the process normally. The level high alarm goes off if thefermentor does not empty out after reaching the high level for a certainamount of time, and the reactor is emptied manually. An analyser is alsoused to maintain the sugar concentration in the reactor by controlling theflow rate of the incoming sugar molasses. A vent is also placed on all thefermentors in order to prevent the build up of air and CO2 in the reactor.The gases are released into the atmosphere as they are not dangerous ortoxic.

5.5 General Safety

The plant is equipped with LED lights throughout as to prevent any sparksbeing emitted from the lighting system. Even though none of the materialscan ignite, but since this is a batch plant, it may be used in the futurefor the production of other materials, which may require chemicals thatcould ignite easily. It is for this reason sprinklers, deluge systems andfire fighting equipment are also be placed throughout the plant, and allequipment is grounded. Ventilation points are provided around the wallsin the plant, in order to prevent the building up of any fumes in the plant,which can cause ignition. The roof of the plant is also a blast cap roof,where in the case of a build up of pressure due to the build up of gases,or during an explosion, the roof will rise to reduce the pressure, and inthe case of an explosion, reduces the damage done to the plant. Safetyshowers, eye baths and first aid kits are also easily located throughout theplants. Drains are also placed throughout the plant in case of needing tourgently empty out any of the equipment.

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5.6 HAZOP

6 Process Economics

6.1 Capital Costs

6.2 Operating Costs

7 Conclusion

8 References

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