DRYING AS A UNIT OPERATION IN DOWNSTREAM PROCESSING

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DRYING AS A UNIT OPERATION IN DOWNSTREAM
PROCESSING

• Prof. Kehinde Taiwo
• Dept of Food Sci Tech, OAU, Ile-Ife, Nigeria
• 0803 582 9554, kehindetaiwo3_at_yahoo.com

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3rd International Conference on Bioprocess
Engineering

• Double Tree by Hilton
• Baltimore, Maryland, USA
• 14 15 Sept. 2015
• organised by OMICS Group Conferences

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Introduction

• All foods and biomaterials need some form of
  preservation to
• Reduce or stop spoilage
• Make them available throughout the year
• Maintain desired levels of nutritional and
  bioactive properties for the longest possible
  time span and
• Produce value added products

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Downstream processing

• Refers to the recovery of biomolecules from
  natural sources such as animal or plant tissues
  besides fermentation broth
• It is an essential step which determines final
  cost of the product in the manufacture of
  biomolecules e.g.
• Antibiotics, vaccines, antibodies,
• Hormones (e.g. Insulin and human growth hormone)
• Antibodies (e.g. Infliximab and abciximab),
  enzymes, and
• Natural fragrance and flavor compounds

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Drying

• Air-drying is an ancient preservation method
• Foods are exposed to a continuously flowing
  stream of hot air
• It involves simultaneous mass and heat transport
• Moisture availability has a great impact on the
  transfer of heat to microorganisms
• Consumer demand has increased for processed
  products that keep more of their original
  characteristics

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Drying Methods

• This requires the development of operations that
  minimize the adverse effects of processing
• There have been various advances in the drying of
  foods with respect to quality, rehydration, and
  energy minimization
• Some of the improvements and advancements made
  leading to the new developments in drying are
  discussed

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Intermittent batch drying

• By varying the operating conditions of a drying
  process
• Airflow rate
• Temperature
• Humidity or
• Operating pressure
• It can be monitored in order to reduce the
  operating cost e.g. thermal input and power input

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Intermittent batch drying

• The objective is to obtain high energy efficiency
  without subjecting the product beyond its
  permissible temperature limit and stress limit
  while maintaining high moisture removal rate

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Hybrid drying techniques

• May include either use of
• More than one dryer for drying of a particular
  product (multi-stage drying)
• More than one mode of heat transfer
• Various ways of heat transfer or
• Multiprocessing dryers

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Hybrid drying techniques

• For particulate drying
• Variants of fluid bed or
• Fluid bed with some other techniques can be used
  in series to achieve faster drying
• For liquid feedstock
• Generally spray drying is followed by the fluid
  bed dryer
• To reduce moisture content to an acceptable level
  which is not possible by spray dryer alone

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Modified atmosphere drying

• The presence of oxygen results in various
  unwanted characteristics in dried food materials
• oxidation of the drying material
• destruction of its bioactive compounds
• browning
• O2 can be replaced by N2 or CO2
• In addition, it increases the effective moisture
  diffusivities of some food products

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Superheated steam drying

• Superheated steam does not contain oxygen, hence
  oxidative or combustion reactions are avoided
• It also eliminates the risk of fire and explosion
  hazard
• It allows pasteurization, sterilization and
  deodorization of food and bio-products
• Net energy consumption can be minimized if the
  exhaust (also superheated steam) can be utilized
  elsewhere in the plant and hence is not charged
  to the dryer

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Impinging stream drying

• In Impinging stream dryers
• The intensive collision of opposed streams create
  a zone that offers very high heat, mass and
  momentum transfers
• Hence rapid removal of moisture from surface
• Other advantages - low foot prints and high
  robustness due to absence of moving parts
• Effective alternatives to flash dryers for
  particulate materials with very high drying loads

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Contact sorption drying

• The contact-sorption drying can be achieved by
• 1) contacting a wet material with heated inert
  particles, thereby removing the moisture as a
  result of heat exchange or
• 2) contacting of wet material with heated sorbent
  particles where the moisture is transferred from
  wet solids to the sorbent particles

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Contact sorption drying

• A typical contact-sorption drying technique
  involves good mixing of wet solid particles with
  the sorbent particles to achieve the heat and
  mass transfer and then separation of these two
  media
• The sorbent particles are regenerated and
  returned back to the dryer
• The typical inert sorbent particles (also called
  a carrier) are molecular sieves, zeolites,
  activated carbon, silica gel, etc.

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Heat pump-assisted drying

• Heat pump dryers use low temperature dehumidified
  air as the convective drying medium
• It incorporates a dehumidification cycle, where
  condensation of water allows the removal of water
  from the closed system of drying air circulation
• The heat pump recovers the sensible as well as
  latent heat by condensing moisture from the
  drying air
• An auxiliary heater is generally added for better
  control of the temperature at dryer inlet

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Radio frequency drying

• Dielectric heating is the use of either microwave
  or radio frequency (RF) technologies to heat
  materials
• Microwave and RF interact with individual
  molecules to quickly generate heat within a
  product
• This is in contrast to conventional heating where
  heat is applied externally
• A wet product submitted to a RF field absorbs the
  electromagnetic energy, so that its internal
  temperature increases

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Radio frequency drying

• If sufficient amount of energy is supplied, the
  water is converted into steam, which leaves the
  product and gets dried
• The amount of heat generated in the product is
  determined by the
• Frequency
• Square of the applied voltage
• Dimensions of the product and
• The dielectric "loss factor" of the material
  which is essentially a measure of the ease with
  which the material can be heated by this method

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Microwave drying

• Microwave oven has ability to heat food products
  rapidly, conveniently and economically in a
  compact space
• The primary drawback is its inability to heat
  materials in a predictable and uniform manner
  leading to
• -hot spots that damage the item being heated
• - cold spots - under heated or under processed,
  thereby compromising product quality and
  repeatability
• Microwave heating in combination with vacuum has
  been used extensively for drying in
  pharmaceutical processing

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Drying in Downstream Processing

• Process industries manufacture different products
  from a variety of raw materials
• The raw materials are pretreated and conversion
  takes place in a reactor and separation of
  product of interest and its purification takes
  place in subsequent steps
• All the steps that are prior to the reactor form
  upstream processing
• All the steps after the reactor form downstream
  processing

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Drying in Downstream Processing

• In all the unit operations involved in downstream
  and upstream processing only physical changes
  occur and do not involve chemical changes
• Unit operations for separation and purification
  during downstream processing include
• distillation, absorption,
• extraction, crystallization,
• drying, mixing,
• evaporation

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Downstream Processing Vs Analytical Bioseparation

• Both refer to the separation or purification of
  biological products, but at different scales of
  operation and for different purposes
• Downstream processing implies manufacture of a
  purified product for a specific use in marketable
  quantities
• Analytical bioseparation refers to purification
  for the sole purpose of measuring a component or
  components of a mixture, and may deal with sample
  sizes as small as a single cell

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Complexity Of Downstream Processing

• Two factors
• 1) the desired product is generally present in
  low concentrations and
• 2) it is present along with several impurities or
  undesired components
• The economics of downstream processes are
  determined by the required purity of the product
  which in turn depends on the applications of the
  product.
• As a result downstream processing mostly
  contributes 40-90 of total cost

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Applications in Downstream Processing

• Thermal drying is more expensive than mechanical
  dewatering
• For dehydration of the biomass after harvest
• Thermal drying should be preceded by a mechanical
  dewatering step such as filtration or
  centrifugation
• Harvesting generally results in a 50 to 200-fold
  concentration of biomass
• The harvested biomass slurry (515 dry solids)
  must be processed rapidly, or it can spoil within
  a few hours in a hot climate
• The specific postharvest processing necessary
  depends strongly on the desired product

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Applications in Downstream Processing

• Membrane processes such as microfiltration,
  ultrafiltration and reverse osmosis
• the recovery and concentration of microbial
  cells/biomolecules
• enable volume reduction of slurry/solution
  before downstream processing operations
  (chromatography, electrophoresis, freezing or
  freeze-drying )
• Drying methods include spray drying, drum drying,
  freeze-drying and sun drying

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Additives/Carriers/Transporters

• Use of additives offer protection to
  microorganisms during drying
• The choice of an appropriate carrier is important
  to increase their survival rates during
  dehydration and subsequent storage
• Differences exhibited are related to their
  water-binding capacity and prevention of
  intracellular and extracellular ice crystal
  formation
• Additive materials increase the glass transition
  temperature and result in a dried product with
  increased stability and less hygroscopicity
• The characteristics of the transporters involved
  in sugar uptake lead to differences in their
  performance

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Carriers/transporters

• Protein (whey protein, skim milk)
• Mrs-broth-based protectants
• Sugars (e.g. maltodextrin, glucose, fructose,
  lactose, mannose and sucrose)
• Sugar alcohols (e.g. sorbitol and inositol)
• Non-reducing sugars (e.g. trehalose)
• Disaccharides give better viabilities after
  freeze-drying than monosaccharides

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Fig 1. Schematic diagram of a spray-drying process
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Spray Drying - Advantages

• Used to dry thermo-sensitive bioactive compounds
  and probiotics
• Increases surface to volume ratio of the liquid
  particles and consequently enhance the heat and
  mass transfer during the drying process
• Continuous operation
• Short time of contact with hot air
• Drying taking place at wet bulb temperature
• Process larger volumes and operate at higher
  energy efficiency

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Spray Drying -

• Allows preparation of stable and functional
  powder products
• Can be implemented for large scale throughputs
• Main disadvantages
• High installation costs
• Removal of aromatic volatiles
• Prone to damaging heat sensitive components such
  as enzymes and probiotic bacteria

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Process conditions in spray drying

• Air inlet temperature
• Feed flow rate
• Feed formulation
• Out let air temperature and
• Nozzle pressure
• Affect
• Retention of activity of bioactive compounds
• Survivability of microorganisms

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Process conditions

• Low outlet temperature, lower residence time, low
  nozzle pressure - good enzyme activity retention
  and survivability of microorganisms has been
  observed
• However, too low out let air temperature may
  result in higher residual moisture content
  leading to loss of viability and enzyme activity
  retention during storage

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Selection of Dryers

• Drying technologies have become more diverse and
  complex
• Dryer selection has become an increasingly
  difficult task
• The need to meet
• Stricter quality specifications
• Higher production rates
• Higher energy costs and
• Stringent environmental regulations

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Selection of Dryers

• Characteristics of different dryer types should
  be recognized when selecting dryers
• Changes in operating conditions of the same dryer
  can affect the quality of the product
• The dryer type right operating conditions for
  optimal quality and cost of thermal dehydration

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Selection of dryers

• Drying of products require adherence to Good
  Manufacturing Practice and hygienic equipment
  design and operation
• Drying kinetics play a significant role
• Location of the moisture (whether near surface or
  distributed in the material)
• Nature of moisture (free or strongly bound to
  solid)
• Mechanisms of moisture transfer (rate limiting
  step)

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Selection of dryers

• Physical size of product
• Conditions of drying medium (temperature,
  humidity, flow rate of hot air for a convective
  dryer)
• Pressure in dryer (low for heat-sensitive
  products)
• Demands on product quality may not always permit
  one to select the least expensive option based
  solely on heat and mass transfer considerations
• In the drying of non-aqueous (organic) solvent or
  a mixture of water (pharmaceutical products) with
  a solvent, care is needed to recover the solvent
  and to avoid potential danger of fire and
  explosion

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Classification of dryers

• Mode of operation
• Heat input-type
• State of material in dryer
• Operating pressure
• Drying medium
• Drying temperature
• Relative motion between drying medium and drying
  solids
• Number of stages

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Table 1 - Classification of dryers
Criterion Types
Mode of operation Batch Continuous
Heat input-type Convection Conduction Radiation Electromagnetic fields Combination of heat transfer modes Intermittent or continuous Adiabatic or non-adiabatic
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Table 1 - Classification of dryers
Criterion Types
State of material in dryer Stationary Moving, agitated, dispersed
Operating pressure Vacuum Atmospheric
Drying medium (convection) Air Superheated steam Flue gases
Drying temperature Below boiling temperature Above boiling temperature Below freezing point
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Table 1 - Classification of dryers
Drying temperature Below boiling temperature Above boiling temperature Below freezing point
Relative motion between drying medium and drying solids Co-current Counter-current Mixed flow
Number of stages Single Multi-stage Residence time Short (lt 1 minute) Medium (1 60 minutes) Long (gt 60 minutes)
Most common in practice Most common in practice
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Drying system includes

• Pre-drying stages
• Post-drying stages

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Drying system - Pre-drying stages

• E.g. -
• Evaporation Mechanical dewatering
• Dilution Pelletization
• Feeding Size reduction
• Flaking Extrusion
• Pre-conditioning of feed by solids back-mixing
  with dry product

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Drying system - post-drying stages

• Exhaust gas cleaning
• Product collection
• Partial recirculation of exhausts
• Cooling of product
• Coating of product
• Agglomeration, etc.
• The optimal cost-effective choice of dryer will
  depend on these stages

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Over-drying

• Increases the energy consumption
• Increases drying time
• Can be avoided by
• Reducing the feed liquid content by less
  expensive operations such as
• Filtration
• Centrifugation and
• Evaporation

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Future Potentials and Challenges

• Downstream processing of biological products has
  been affected by
• The growth of the biopharmaceutical industry
• Drastically changing purity expectations
• Processing volume
• Production flexibility to accommodate new
  products

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Future Potentials and Challenges

• A volume-reduction step should achieve high cell
  concentration, with minimal product loss or
  change in product quality even at large scale
• Such high cell concentrations can be achieved
  with appropriately sized systems and
  consideration of system hold-up volume

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Future Potentials and Challenges

• Detailed knowledge on protein stability i.e.
  understanding of structural changes of
  biomolecules as a result of environmental
  influences can help in process design
• The product bioavailability challenge is more
  related to improving solubility which may play an
  important role as it may promote super saturation
  
• In the scale-up process control over particle
  size is a priority in spray drying

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Conclusions

• Drying of heat labile biological materials
  preserves activity of enzymes/cells during
  storage and stabilize the bulk product until it
  can be formulated
• Drying becomes expensive unless the product is of
  high value and low volume
• Suitable drying methods need to be selected
  depending on the value of the product

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• Thank you