Literature review

The need for biopolymer such as polyhydroxybutyrate (PHB) is increasing due to the negative outcomes generated by the petrochemical based plastics. PHB produced by a certain type of bacteria with the presence of carbon source. The process of forming PHB proceeds under several chemical processes such as fermentation. One of the essential techniques to produce pure and good-quality PHB is drying which is the last step before the production.

Drying is one of the major units used in most industries. The main purpose of using this process is to remove most of the moisture content in a product. This is done mainly by thermal and mechanical separation methods. It is often regarded as the last step after a series of chemical processes.

The percentage of water or moisture in a sample is often referred as moisture content of a material. During the process of drying, the moisture content may increase or decrease until equilibrium is reached between the temperature and humidity levels (Coulson et al, 1991). Moreover, the fraction of moisture differs from product to product and can be found either on the surface of the solid, contained within the product such as the removal of solvent inside a polymer, or in both locations (McCabe et al. handbook).

There are variety of dryers that can be used in this process depend on the material, the moisture content and vendor specifications. Since drying is energy-consumption process, it is important to design an efficient dryer for industrial use. Also to avoid the high energy and cost associated with this unit operation, mechanical separation of liquid from solid using centrifuge for example, should be done before introducing the mixture to the dryer. The reason to do so is to ensure enough moisture removal so that drying time would be shorter (McCabe et al. handbook).

Dryers are classified in three main ways; mode of operation, mode of heat supply and degree of agitation. Some dryers can be used for a variety of materials while others can only be practically used for certain types of materials. Also some dryers are classified as continuous whilst other can be classified as batch (McCabe et al. handbook). Since this process is batch, dryers of this type will be assessed, and the best choice based on environmental, economic and technical analysis will be designed for this project.

 

TRAY DRYER

Tray dryer which is also referred as shelf or cabinet dryer, is used in a batch mode. The material to be dried is spread in a number of trays with a depth range of 10 to 100mm, and the trays are inserted in a cabinet. Hot air is introduced to the cabinet by fan and then the air is recirculated inside. Electric power is used to power the dryer. After each batch, the trays are removed and replaced by a loaded ones, and the process starts again (Geankoplis handbook). This type of dryers is used where the production rate is relatively low, and theoretically they can dry almost all materials. However, the operating cost is considered high since intensive labor is required to load and unload the trays. Also recirculating air to dry the material can be slower and so time consuming. A modification to reduce the time of drying has been suggested by using through circulation drying (McCabe et al. handbook). Another recommendation is also to use tray-truck type to reduce the time of replacing the trays in the dryer (Geankoplis handbook). Unfortunately, these suggestions cannot solve the problem of labor required for each batch. Also, the use of tray dryer in the production of PHB was not reported previously, reducing the chance of using it in this project.

Figure 1 http://www.medibalt.info/en/products/details/14

 

SPRAY DRYER

Spray dryer is a common choice in most industries such as pharmaceutics. The principle behind this type of drying is to spray the feed into hot gas stream (air mostly) injected by gas chamber. The design of the atomizer is dependent on the properties of feed, the rate of production required and the required particle size. Based on the atomizer design and the residence time of the material, the suitable spray dryer design can be chosen (Mujamdar, 2016). Four different designs were found for the atomizer; rotary, pressure nozzles, two-fluid and ultrasonic atomizers (Vehring, R. Pharm Res, 2008).

Although it has been suggested as the method of choice for drying PHB (Garcia et al), (Posada et al, 2010) and (Choi et al, 1997), it was found to be unfeasible when the feed is that of sugar-rich materials (Bhandari et al, 1997). The addition of certain agents as well as modifying the process condition could solve the problems associated with drying this kind of products. Spray dryer is less time consuming which makes it feasible for heat sensitive products. Also they can be used to achieve different particle shapes, sizes and properties, therefore, can produce good product quality.  Another advantage is that it can be used as an alternative to multiple operations such as crystallization and evaporation (McCabe et al. handbook).

Even though spray dryer has a remarkable benefits, it may have some drawbacks.
Due to the loss of high amount of heat with the hot gas, it is not considered as the most efficient dryer. Also it is not simple to operate and require relatively large area (more than 25m) (McCabe et al. handbook).

 

FLUIDIZED BED

This type is used in different applications such as drying coal and biosynthesis products. Hot air fluidized the particles using boiling bed unit (McCabe et al. handbook). According to figureX, hot air is introduced into the bottom of the dryer and passed through the particles of the wet feed. The mechanism of drying in this particular dryer works against the gravitational force (Chua & Chou, 2003). The drying process can be achieved fast using this kind of dryers, and the contact area between solid and gas is very large. Also the product can be easily removed from the dryer by gravity reducing the need for mechanical equipment.

However, the product obtained can be that of lower quality due to poor fluidization. Furthermore, cost associated with operating such a dryer is very high because the pressure drop needed may be large enough to add compressor. Corrosion and scaling are another drawback when choosing this type of dryers (Duad, 2008).

Figure 2 spray dryer

 

Rotating pulsed fluidized bed this type of dryer is a new substitute to the conventional fluidized bed dryer. It has some advantages over the previous design such as; reduction in the use of energy and gas flow. Nevertheless, enough research have not been done on this type of dryers (Ugri & Tranto, 2007).

 

PROCESS SELECTION

In table 1, a comparison has been made between the alternatives, each method is assessed based on the target production rate of PHB, the cost of construction and operation, environmental impact and safety. A score is given for each dryer from 1 to 3.
Number 1 = low
Number 2 = intermediate
Number 3 = high

Table 1 comparison between alternatives.

	Tray dryer	Spray dryer	Fluidized bed dryer
PHB purity	–	3	2
Cost	1	2	3
Environmental impact	–	–	–
Safe	3	2	1

(-) Means no data found from the literature.

Based on the comparison provided, spray dryer was found to be the most efficient alterative for this particular process. The reason was that the high purity of PHB that can be achieved (up to 99.9 wt%) (Posada et al, 2011). Tray dryer was not listed as one of the commercial methods to produce PHB even though it can theoretically be used regardless its operating cost. The cost of maintaining the fluidized bed dryer can be high as well as the operating cost due to the implementation of a compressor which can add more cost to the operation. Also, the energy required to inject the hot gas can increase the utility cost, and it is less safe to use this dryer since it uses highly pressurized hot gas. According to Posada et al (2011), the energy associated with using the spray dryer is relatively low (1.02 MJ/kg) compared to the other alternatives.

In order to design the spray dryer, the type of atomizer should be chosen to suit the process conditions. Since atomizer design will decide how efficient the spray dryer will be.
Pressure nozzle atomization is the most suitable type since it does not require high energy to operate compared to the other techniques. Additional pump is beneficial in order to provide pressurized air flow (SDS, 2014). Another design considerations must be addressed such as the temperature difference between the inlet and the outlet, the rate of evaporation and the required particle size. Once these parameters are available, the cost of the dryer can be obtained. According to SDS (2014), the cost of the dryer increases as the DT decrease, since the required area and inlet air flow will increase as well. Therefore, it is better to design for low DT not only to reduce the cost, but also to avoid thermal degradation of the product. The size of the particles was not specified by the company which gives the freedom to design for any size, however, the smaller the particles the less area required and so the capital cost. Furthermore, the flow configuration and the mixing technique are another essential factors for the design of spray dryer. Co-current, counter current and fountain flow arrangements are available with co-current flow is being the most suitable for this particular design. It is the most appropriate pattern for heat sensitive products and can be used with all atomization methods. On the other hand, mixing techniques can be divided into two categories; slow and fast. Slow mixing is preferred when the size of the particles matters as well as other variables, while fast mixing is favored when drying is the only aim of the process (SDS, 2014).

The last step is the recovery of the product. A variety of ways can be used to recover the product and control the emissions from the process. The method used can decide the quality and the yield of the product and the energy required. Two commonly used designs are cyclones and packed columns. Cyclones are cheap with a collection efficiency of 98%, but additional scrubber should be used to reduce the emissions from the product. Packed columns are used to remove impurities and to recover the heat generated from the process, and can also produce hot water to be used in different parts of the plant. Since spray dryer generates large amount of heat, packed column can be chosen for this particular case.

 

 

Equipment design – process

 

Table 2 mass flow rate of the compositions in each stream based on the feasibility report.

Stream	Composition	Mass Flow Rate (kg/h)
24	Sugar	400.95
	PHB	3877.97
	Water	3877.97
	Biomass	1584.18
	CO2	169.38
	Other	1319.29
	N2	6372.30
27	Biomass	166.84
	CO2	3116.32
	Other	1299.50
	N2	6276.71
26	PHB	3819.80
	Impurities	195.15

 

The target production rate of PHB is 4014.95 kg/h with 95 wt% purity, and the evaporation rate was found to be 12872.9 kg/hr. The feed stream to the dryer is 17602.04 kg/h.

The dryer will be designed for 2 hours drying period with a temperature of 80 C according to the feasibility report.

The following equations were used to determine the amount of heat required by the dryer.

DH = åmH_out – åmH_in
(J.M. Smith, 2004)

 

Table 3 feed data obtained from the feasibility report.

Feed	m (kg/hr)	?H (Kj/Kg)	Q (Kj/hr)	Q (KW)
Glucose	400.9505	-7061.1	-2831151.576
PHB	3877.969	146	566183.474
Water	1584.178	-15877.8	-25153261.45
Biomass	169.384	465	78763.56
CO2	3163.778	-8943.18	-28294236.13
Others	1319.294	0	0
N2	6372.298	0	0
Total			-55633702.12	-15453.80615

 

Table 4 waste stream data obtained from the feasibility report.

Waste	m (kg/hr)	?H (Kj/Kg)	Q (Kj/hr)	Q (KW)
Glucose	394.9362425	-7061.1	-2788684.302
PHB	58.169535	146	8492.75211
Water	1560.41533	-15877.8	-24775962.53
Biomass	166.84324	465	77582.1066
CO2	3116.32133	-8943.18	-27869822.59
Others	1299.50459	0	0
N2	6276.71353	0	0
Total			-55348394.56	-15374.55404

 

 

 

Table 5 product stream data obtained from the feasibility report.

Product	m (kg/hr)	?H (Kj/Kg)	Q (Kj/hr)	Q (KW)
PHB	3819.799465	146	557690.7219
Impurities	195.1482375	-471.2562	-91964.81684
Total			465725.905	129.368307

 

Q = (129.368307 + (- 15374.55404)) – (- 15453.80615) = 208.6204 KW

Therefore, the amount of heat required to operate the dryer is 208.62 kW. Since the fermentation process is exothermic with -856.321 kW of heat generated (feasibility report, 2016), some of this heat can supply the dryer to minimize the cost of energy used.

 

 

Equipment design – mechanical

 

 

Instrumentation and control

 

 

Capital cost

 

 

References

 

http://www.spraydrysys.com/spray-dryers/spray-dryer-controls.htm