Transesterification For Fossil Oils Into Biodiesel: Process, Design And Equipment

Section 1

Discuss about the Transesterification for Fossil Oils into Biodiesel?

Transesterification is the process to convert the fossil oils into Biodiesel. Biodiesel and fossil diesel are fall into same category of fuel. It is produced from straight vegetable oil, waste cooking oil , animal fats and tallow.

Biodiesel transesterification represents the heart of any biodiesel manufacturing plant. It involves various complex set of chemical actions and heat transfer mechanism. There are several reactors which performs these operations. Each of the factors has their own advantages and disadvantages.

In manufacturing unit the continuous transesterification is more preferable than batch processes. Because, the contineus processes give product quality, low capital and  low operating costs per unit. The continuous stirred-tank reactors are the most popular continuous flow reactors in the manufacturing sectors.

The primary objective of this report is to design and prototype a transesterification reactor to convert canola oil to biodiesel Furthermore, this report design a heat exchanger, storage tank and pump including the technical design as well as decision on the controls to use and instrumentation.

Process description: Once sodium hydroxide dissolved in methanol to make methoxideand react with canola oilthat pumped from the storage tank passes through heat exchanger into the reactor as the transesterification reaction occurs at condition 60 and 4 bar.

In the process of transesterification the reaction of lipid and alcohol forms esters and a byproduct namely glycerol. On the other hand the process displaces one alcohol from an ester. This process is called alcoholysis. This reaction is reversible and as a result of this an excess of alcohol forces the equilibrium in product side. The ratio of alcohol to lipid in the equilibrium is 3:1. However, this is increased to 6:1 to increase the product yield. The transesterification procedure has of three phases. The first step is the conversion of triglycerides to diglycerides, the next phase represents the transform of diglycerides into monoglycerides, and finally the last phase puts monoglycerides into glycerol. Each step yields one ester molecule from each glyceride. The reactions are reversible. The equilibrium helps to produce glycerol and fatty acid esters.

Storage Tank: The storage tank is used to store pure conola oil. The tank loads various discharged material by the transporters.Since moist increase spoilage risk tank must be protected from atmospheric moisture as moisture raises enzymatic and biological activity that leads to mold growth(Canada, 2003). The tank must be heated enough to prevent freezing.

Section 2

Flow diagram

Construction material: Schweitzer, 1991recommends the carbon steel must be used in 25C and canona fluid must be added in the process. Because, the corrosion of these materials are less.

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Sizing: The size of the oil storage tank is determined using the above mentioned relations and is as represented in the table (E.1)

Cost of the tank

Heat exchanger: The heat exchanger used in this section of biodiesel process will be selected here. In choosing the heat exchangers, various factors should be considered for  each application. These factors include operating temperatures of the exchanger, fouling, pressure and pressure drop of the exchanger, utility stream characteristics, process stream and capital cost of the operation. These factors play important role in selecting the best heat exchanger.

Table (6.18) represents the applicability of different heat exchangers [Van Den Berg , 2000].

Type	Advantages	Disadvantages	Operating ranges
Tube and Shell	Flexible procedure, robust and easy to maintain	Sometimes not economical  for low pressure	Tube-side 1400 bar Shell side 300 bar -270C to 1600C
Air cooler	Larger heat loads, Free utility ( air )	Economical for process fluid at low temperature	470 bar, process fluid up to 500c
Double pipe	High pressures possible , cheap construction, easy to maintain	Poor heat transfer coefficient expensive for large duties	Tube-side 1400 bar and 100-160 c
Frame and Plate	Lower areas needed, High effectiveness factor, compact	Not appropriate for carbon stell, limited to below 30 bar	30 bar and -40 to 170c

The above table shows that the tube and shell exchanger is the most suitable in this context.

The next decision is to be taken after the selection of shell and tube heat exchangers is the necessary fluid required on tube side or on shell side. The table shown below represents few measures for the fluid allocation.

Factors	Solution
Fouling	When the  fouling fluid on tube side, the fouling is reduced by higher available velocity
Temperature	On the tube side hottest liquid is placed – on the tube construction more expensive material is placed
Operating pressure	The fluid with high-pressure is placed on tube side – high pressure shell is more expensive than high pressure tube
Pressure drop	Fluid with lowest variation in temperature should be allocated to tube side where it gets highest heat transfer coefficient.

Construction material

Carbon steel

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Control of heat exchanger: The most equipment configuration of heat exchangers are shown above. The utility stream flow rate controls the product outlet stream temperature. So, by controlling utility flow rate the output temperature can be monitored. The above table shows how the free forward control is applied considering the flow to the exchanger at cascade outlet temperature

Design Procedure : The amount of  heat transfer across a surface is given by the below mentioned equation:

Determining  the surface area required for the specified duty with the available temperature difference is the primary importance of  the design of the exchanger. The heat coefficient is measured by the equation as mentioned below:

The dimensions of the reactor vessel are determined in regards of the above mentioned equation, and the results are shown the table below:

Pumps : Pumps can be classified into two categories. These are dynamic pumps and positive displacement pumps. The example of dynamic pump is centrifugal pump whereas, the example of positive displacement pump is reciprocating and diaphragm pumps.

The major benefits of centrifugal pumps are as mentioned below (Peters and Timmerhaus, 1991):

-  The pumps are cheap and have simple constructional structure

-  The pumps are useful for liquid with large amount of solids 


Section 3

-  costs of Maintenance of these pumps are much  lower than those of other pumps.

Miscellaneous Pressure Losses: The turbulence is generated by obstruction in the flow. So, the process of pipe fitting drops the pressure of pipeline the process creates turbulence by bends, eklbows , reduction or enlargement of sections etc.

Construction material

Flow diagram

Control of Pumps: The rate of flow of the pumped streams are regulated to get a specific amount of pressure of streams. This stream enters into the reactor and exists from the reactor in the process of pumping.

The pumps are designed to be operated under constant speed. There are pumps of various variable speeds. But, these pumps are very expensive. Apart from this, the pump control system ranges from single hand-operated pumps to various highly advanced pumps like automatic flow control, pump speed control etc. The factors important for control system choice are pump type and drive type. Such as in centrifugal pump by changing either the speed or the valve setting the output can be controlled.

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Pump sizing

Reactor Design: A general reactor design procedure is outlined below:

Collect the thermodynamic and kinetic data based on the side reactions and desired reaction.

Collect the data of physical property which are required for the design

Identify the predominant mechanism for rate controlling: Kinetic, mass or heat transfer.

Make an initial selection of the conditions of reactor to give the desired yield and conversion.

Estimate its performance after sizing the reactor.

Select material suitable for construction

create preliminary mechanical design for the reactor.

Cost the proposed design, capital and operating.

Transesterification Process:

The final step of obtaining Fatty Acid Methyl Ester (FAME) by reacting with alcohol such as methanol and Ethanol with triglyceride molecules is knows as transesterification. For transesterification process , an alkaline catalyst NaOH ( sodium hydroxide) to elevate the reaction at STP and to produce FAME and glycerol after step by step conversion as shown below. [7]

Figure 1:  Transesterification Process

Transesterification term represents the important class of organic reactions whereas, an ester of represent the transfer into another through interchange of the alkoxy moiety. The transesterification process is called alcoholysis when the original ester does chemical reaction with an alcohol,.

Various industrial process use the Transesterification Process. This reaction happens by the mixture of reactants and it is an equilibrium reaction. The catalyst accelerates the reaction and increases the temperature. To obtain the high amount of esters the alcohol ahs to be used in excess amount. In the presence of alcohol and a catalyst with glycerol, the reaction transforms the transforming triglyceride into fatty acid alkyl esters.

Section 4

The stoichiometric ratio of the reaction is 6:1. The reaction is inherently endothermic. The overall reaction scheme can be shown below:

Three are batch reactors, semi continuous flow reactors and continuous flow reactors are three types of reactors are used in biodiesel production. The batch process is cost effective. It requires much less initial capital and investment in infrastructure. The process is flexible. Variations in feedstock type, composition and quantity are available in this process. Low productivity, more intensive labor, larger variation in product quality and energy requirements are major drawbacks of this process.

The batch process and semi continuous process are similar in most of the aspects except in the one case. In this process the producer starts the reaction in lesser volume than the vessel can hold and then continues to add ingredients until it is full.

Continuous-stirred Tank Reactor (CSTR) will be used in this purpose. In CSTR, any materials is easy and free to enter and exit. The conditions do not change with time which will make our process conditions constant and not always changing which is easier to handle and control. The products are removed continuously and the reactants are entered into the reactor continuously.The conditions of entering and exit streams are the same. CSTR are well mixed, the contents have uniform and equal properties such as the temperature, density and pressure. The volume of CSTR are usually at constant volume. There are stirrers called agitators inside CSTR to mix well the reactants and catalyst. A CSTR can act as loop reactor when heated. The pressurized fluid is inserted into the system to facilitate the stirring or the agitators for higher heat and mass transfer rates.

The Kinetics

The Table 2.6 represents the set of transesterification reactions considering the simulation of the primary transesterification reactor. The In the Table 2.8 it is showing the values of  rate constant and the equilibrium constant. The two factors limit the reactor temperature and controls it to not go beyond a certain limit. These are the instability of the evaporation and the increased rate of byproducts formation.

Configuration of Reactor: For the transesterification reaction a jacketed continuously stirred tank reactor is appropriate.  It is required to operate it continuously for 8150 hours per day. The reactor helps to operate the transesterification reactions and helps  the suspension of finely suspended solids. This process creates suspension by agitation. As recommended by Bassel, (1990), the ratio between the height of the liquid and the diameter of the reactor is 2:1. The reactor must be 90% full. The heating efficiency of the reactors must be adjusted properly to attain reaction times and short heating. The care should in the whole operation as the jacket is joined with the heating tube across its length.

Construction material: The constructional material consists of three portions. These are the vessel of the reactor, Jacket of the reactor and reactor agitator.

The vessel of Reactor: The reactor vessel in the process of transesterification process must be kept under 60c temperature and moderate pressure. Schweitzer, 1991, and Hermann Ludewig, 1971 has recommended to use the stainless steel 316  for construction of this vessel

Jacket of Reactor: According to Hermann Ludewig, 1971,  the use of carbon steel must be used when the  water heating is required.

Reactor agitator : According to Hermann Ludewig, 1971, high-grade steel must be used as some corrosive reactant. Stainless steel 316 is best for this purpose.

Size of the reactor: The mixing intensity represents the amount and proportion of mixing of alcohol and triglyceride (TG) in the reaction. The phases of the mixing are very important. In most of the cases the mechanical mixing procedure is followed to fasten the reaction process and increase the contact between the particles. This process increases the transfer arte of the mass. This procedure has its origin in the eighties. Thus the variation in mixing intensities in the reaction can change the kinetics of the transesterification reaction.The Rqynold’s number helps to access the mixing intensity. In the fluid dynamics, the ratio of inertial forces to viscous forces is represented by NRe (dimensionless number) in fluid dynamics. The following equation represents the NRe.

Here  n represents the rotational speed of the impeller, Da represents  the impeller diameter, and ρ and μ represent the fluid density and viscosity. Noureddini and his team investigated on the effect of Reynolds number on the transesterification reactions.

The reactor is sized using the previous relations, and the Table (E.1) summarizes the results of the sizing of the reactor.

Mixer design: The mixer design represents the type of mixer to be selected. The selection is done according to the guidelines specified of Ulrich, 1984. The measurements of the agitation system should be as specification provided in the guidelines by Geankopolis, 1993,pp. 144.

Jacket selection

The factors considered to select the type of jacket are as mentioned below:

-  The Cost 


-  The heat transfer rate 


-  The Pressure 


The table below represents various jacket types and their characteristics considering these above mentioned three factors, Coulson and Richardson, 1999. 


Type	Remarks
Spirally and baffled jackets	High heat transfer rate.
Dimple jacket	Used up to 20 bar.
Simple and not baffled	Lowest cost and used up to 10 bar.
Half pipe jacket	High heat transfer rate, and used for high pressures up to 70 bar.

The jacket is sized by the some relations, and the results of jacket summarizes in Table (E.1) summarizes          

Vessel support design: The vessel support method requires to consider below mentioned factors:

The shape, size of the vessel and weight of the vessel, 


To design the temperature and pressure of the vessel.


Vessel location and arrangement 


The fittings of internal and external portion of the vessel and attachments.


The table shown below represents the various  supports and their characteristics, Coulson and Richardson, 1999.

Support types	Remarks
Skirt	Appropriate for vertical columns
Saddle	Appropriate  for horizontal vessels

A skirt, in cylindrical shape, made of plain carbon steel is used to support the reactor and the highest dead weight load occurs when the vessel is full.

Control of reactor: Because the process is continuing. The feed of the reactor needs to be constant within certain limits. The feed consists of two main components: oil and methanolxide. The methnoaloxide stream is kept constant and is measured by a flow control acting on a valve in the feed stream. A flow constant is attached and acts on the input of the amount of oil. The amount of the methanoxide and oil are now fed in the right amounts to get the desired ratio in the reactor. Also, the reaction is endothermic and quite an amount of heat is needed but the temperature must be continuously kept at 60c. the amount of heat needed is controlled by a temperature controller.

The reactor controlled by a level controller which acts on a valve attached to the outgoing stream. This done to ensure the residence time in the reactor.

Startup and shutdown

Considering the above mentioned factors , the Transesterification reactor start-up needs to consider three things. These are as getting ambient temperature to steady operating temperature. Setting the process flow rates from zero to their steady state values and controlling the operation to reach the desired equilibrium state rate. Similarly the shutting down involves to stop the flow and to let the ambient temperature to cool down. But, sometimes stopping the reactant flow into the system, without controlling other important aspects, can lead to the explosion. So, the shut down process as well as the start up process requires detailed thought, planning and analysis.

Cost of the reactor

Utilities: The biodiesel plant makes use of different utilities such us steam, water and electricity. Most of the steam is low pressure steam.

Reason for selection and other options: The heating processes uses steam and the cooling is carried out by air and cooling water. All electricity is produced outside battery lirnits, no steam is used in the design to generate power. Further it isn't necessary to use a refrigerant to cool or condense any one of the streams.

Major users

The pieces of equipment using a lot of electricity are  reactor, heat exchanger and pump. These three instruments together use almost 23% of the total electricity consumption of the plant. The major users of steam are heat exchangers and users of water is jacket of the reactor.

Possibilities of future reduction: Heat integration is a part of the design which isn't applied. There are great opportunities to reduce the consumption of utilities by heat integration.

Hazard and Operability Analysis (HAZOP):

The Hazard and Operability Analysis (HAZOP) identifies  hazard and operability problems of the industry. It investigates the deviation of the operation of the plants from the intended design. The HAZOP result represents the inevitable problems occur in the course of production. The main aspect of HAZOP problem is to identify the problems and the firm should not dwell on the problems which are not apparent in the analysis. HAZOP is concerned about the safely of the operation and hazards represents the operational problems which degrades the performance of the plant. It also affects the product quality, production rate nad profit. Hence, the insights of individual engineers who work in the plant is very important  to make the overall efficiency of the plant high.

In the present discussion, the main trancesterification reaction was chosen as the subject of HAZOP’s study. The reason for is the importance of the reaction in the Biodiesel plant. To measure the intensity and probability of hazardous operation a Hazard Matrix was created. To analyze the hazardous effects of unit operation few factors have been chosen. These factors are flows, pressure, and temperature of the reactor. These factors determine the possible deviation which could lead to definitive hazards the possible causes, consequences for each deviation has been identified and the researcher also provided the recommendation for each aspect. The matrix is shown below.

Hazard Matrix

Process: Transesterification Reaction

STREAM	PROCESS PARAMETERS	DEVIATIONS	POSSIBLE CAUSES	CONSEQUENCES	ACTION REQUIRED
Feed       (ST1) Reactant (ST2) Catalyst  (ST3) Catalyst  (ST4) Reactant (ST5)	Flow	High Low No	Valve failure or fully open Valve failure or  closed Failure of  Flow Failure of  Operator Failure of flow control Pipe breakage control sensor Plugged pipe Empty storage tank	Possible upset in downstream Overflow Increased reaction rate Downstream process backed-up Pipe damage Reaction rate Reduced No reaction Pump cavitation	Install HA Install LA check valve should be  Installed Regular  calibration  and maintenance Train the operator Implementation of absorbing avoid leaks to ground Inspection  must be done prior to startup
Reactant (ST2) Catalyst  (ST3) Catalyst  (ST4) Reactant (ST5)	Pressure	Low	Valve failure or fully open Failure of operator Leaked pipe Plugging in pipe Failure of Pumps	partial MeOH vaporized Breakage of pipe Possible upset in downstream Damage of pipe	differential pressure should be checked across valve routinely maintenance Fail-closed mechanism Inspection should be  done prior to startup
Feed       (ST1) Reactant (ST2) Catalyst  (ST3) Catalyst  (ST4) Reactant (ST5)	Temperature	High Low	High heating Failure of operator Control of temperature Failed temperature sensor Not sufficient heating Cooling occurs	I Increased pressure CH3OH boil WCO boil Pipe melt Pump damage Reduced pressure WCO solidify upon cooling Viscosity increase	Install HA + thermo couples Operator training Install throttle Install LA + thermo couples installed Regular maintenance

PROCESS: TRANSESTERIFICATION REACTION