Following are the main material flow patterns and problems routinely encountered by the industries in a storage hopper. The flow patterns are illustrated in the figures given below and measures to fix them with Mass Flow Hopper.
Funnel Flow: The material starts flowing from the top surface in a conical fashion representing a funnel shape and the material near the wall is stagnant. This results in a first in last out flow pattern and can also result in material segregation problems.
Core or Ratholing Flow: The material starts flowing from the top surface in the form of a cylinder at the central portion of the hopper and all the material surrounding this central hole upto the wall is stagnant. This results in a first in last out flow pattern and can also result in material segregation problems.
Bridging or Arching Flow: The material does not flow at all and forms a stable arch just above the hopper discharge. Arching may be due to the mechanical interlocking of large particles or cohesive locking of fine particles due to factors like humidity, temperature, pressure etc,.
Operators generally overcome the above problems by using sledge hammers, poking of the material using rods either from the bottom or from the holes made on the sides of the wall, use external mechanical vibrators, provide internal pneumatic lines etc., to make the powder flow. All the above methods employed are temporary solutions and further leads to other type of problems.
Mass Flow: The best solution for the above material flow problems is a mass flow pattern, where at any instant of time, all the solid particles are flowing vertically downwards towards the outlet of the hopper. This will ensure that there is no stagnation of the material inside the hopper. Mass flow ensures that the material which first filled the hopper is the first to go out of the hopper. To achieve this, the hopper walls must be sufficiently steep and smooth to reduce the friction. Check hot air dryer design for more information.
Mass Flow Hopper: To design a mass flow hoppers, the properties of the powder must be generated in a shear cell tester. Typical results of the powder tests for mass flow hopper are given below.
Bulk Density & Tap Density Graph
Effective Angle of Wall Friction Graph
Effective Angle of Internal Friction Graph
Mass flow Hoppers Function Graph
Flow Function Graph
Critical Arching Diameter
Critical Rathole Diameter
DRYING is a very versatile and important unit process for the Process Industry. The process that results in reduction of moisture content in a given material is known as DRYING.
We will now discuss how to obtain a perfect hot air dryer design.
Obtain a Perfect Hot Air Dryer Design
The simplest and hence traditional method of drying a material, specially agricultural material in a country like India, has been to merely spread it under the sun and use the thermal energy available from the sun to gradually evaporate the water. Drying under the sun or the simple solar drying, though the easiest form of drying, has enough limitations since it requires long duration of drying, large area requirement for the material to be spread , vagaries of nature, etc. This is why, the Industry that is interested in a achieving and maintaining a steady output fromthe manufacturing facility , consistent quality in terms of its moisture content, best value for money, looks for a well designed mechanised hot air dryer design. Here again, relevant expertise is needed to identify the appropriate drying machine out of the several that are available now viz. tunnel dryer, rotary dryer, fluidised bed hot air dryer design, flash hot air dryer design, vacuum hot air dryer design, impingement dryer, etc.
Every material has its own drying characteristics and it requires thorough process engineering knowledge to understand the mutual dependence of the various operating parameters involved in the drying process.
It is the job of the Process Engineer to make hot air dryer for a given material which starts with studying the drying characteristics in the Lab. While it is one option to a hot air design from the first principles of arriving at the individual transfer coefficients with respect to heat and mass transfer that takes place all through the drying process, the other and most reliable method is to extrapolate the overall coefficient from study on a prototype dryer.
Achieving energy economy during drying is an important objective that the designer has to target. This involves conceptualising and detailing the method of recycling the hot air that exhausts from the dryer. One often argues asking the question
how can the exhaust air be recycled when it contains so much of moisture ?'Against such questions my typical reply is: ‘ leave it to the expert to calculate and decide much to recycle and how much of the exhaust air to be let out in the final exhaust’. Usually, the rate of moisture removal during the initial part of drying is high and then it gradually starts dropping as the material loses more and more moisture. Supply hot air and not recycling is meaningless since the energy bill becomes drastically high with such zero recycle. This is where the control dynamics becomes important. The process control introduced in dryers plays around with the percent recycle in the pilot plant design. In the initial phases of drying the recycle percentage is maintained low and as the drying progresses, the percentage recycle is increased.
I was trying to get an appropriate definition of the term ‘
scale-up' which we we hear so often and find a very simple definition of the term to be ` an increase according to a fixed ratio’. I wish the technique of scaling up was as simple as the definition above. May I therefore write this blog on `scale-up’ , by defining scale-up to be an activity or a process which eventually results in an operation or a set of operations being carried out on a higher scale?
Requirement to scale-up a process or a machine becomes relevant when a Lab has successfully produced the end product on a small scale and now has the objective to produce the same on a higher scale. This is of great relevant when the process is novel, has many un-knowns, and is likely to require a high investment.
A process often involves more than one process variable which is known to affect the yield of the product, its purity, the batch time, temperature , pressure, agitator speed, to name a few. If one attempts to design the large scale reactor by merely multiplying all the numbers by a factor of 2 to double the production, all hell can break loose when the plant starts.
This is where a careful approach is necessary that analyses the inter-relation between the various process parameters that results in a achieving the required productivity in the larger scale plant.
We encounter challenging conditions for scaling up, for example, a jacketed reactor with agitator where an exothermic chemical reaction is carried out between two liquid reactants. The reactor is scaled-up for the required capacity through careful computation of the following parameters : heat load, flowrate of cooling medium in jacket, temperature of cooling medium at exit from jacket, heat transfer coefficients inside and outside the reactor wall, iteration to arrive at the optimum impeller diameter and RPM, optimum cooling water flowrate through jacket, etc.
The know-how for upscaling a process , an equipment or a machine helps in achieving the final result in a optimum manner at the least cost, rather than living with the compulsion to make adjustments in the scaled up plant or equipmemt that results in increased cost of operation, increased investment, high energy requirement , etc.
Process & Instrumentation Diagram usually expressed as `P & I Diagram’ is a vital tool for a Plant Designer. The simplicity or complexity of any manufacturing process is understood through a P & I Diagram
During transfer of the manufacturing technology from the Lab to the Industry, it is vital for the Process Engineer to develop the P & I Diagram that can effectively capture the knowledge imparted by the technologist and help design the manufacturing plant.
P & I Diagram is a platform which helps all ideas, be it from the technologist, the client or the consultant, to be put on paper for better understanding of the process. The precursor to a P & I Diagram is the PFD or the Process Flow Diagram which essentially is a Block Diagram describing pictorially the process flow.
The Process & Instrumentation Diagram usually captures the sequence of material and utility flow for the manufacturing plant. Apart from the process equipments like the reactors, filters, dryers, etc. accessories like columns, condensers, pumps , blowers, and instruments like temperature sensors, pressure sensors, pH sensors, etc. also get incorporated in the P & I Diagram.
As the diagram goes through more and more detailing, the pipelines get detailed with display of their size, schedule number, materials of construction with location of flanges or other relevant types of joints, valves. There are tags specific to the types of valves that are used – viz. ball, diaphragm, butterfly, gate, and so on.
The P & I Diagram, once complete, becomes the basis for preparing the Bill Of Quantities (BOQ) or the Bill Of Material (BOM) which then paves the way for estimating the investment.
Most of chemical reactions encountered in inorganic or organic chemistry involve either release of heat or supply of heat to the reactor where the chemical reaction is in progress. One does not normally come across a chemical reaction which has zero heat effect. Chemical reactions carried out in continuous flow reactors, be it a CSTR or a plug flow reactor, most often require isothermal conditions to be maintained at any given cross-section or at any given point in the reactor under steady state operating conditions in the adiabatic reactor design process.
For reactions that are exothermic, the heat released from the reaction requires to be extracted in a manner that the temperature of the reaction mass in the reactor remains constant. For endothermic reactions, heat requires to be supplied to the reactor to hold the temperature constant.
Thermal effects are also relevant during dissolution of any chemical in a liquid. Dissolution of sodium hydroxide flakes or concentrated sulphuric acid in water are two of the most common examples where considerable heat is evolved.
The first task towards quantification of thermal energy involved during a chemical reaction or dissolution is achieved on the basis of published data available with respect to standard heats of formation, bond energies, or heats of dissolution. For those reactions where data is not readily available, thermal data requires to be generated through controlled experiments on laboratory scale.
Importance of Adiabatic Reactor Design
Adiabatic reactor design is one of the core competencies of Chemical Process Engineers. The expertise available with Chemical Process Engineers starts with quantitative estimation of the thermal energy (Kcals) that is expected to be released during an exothermic reaction.
The rate of heat transfer required to be maintained across the reactor wall is then computed through computations involving quantification of heat transfer coefficients, temperature difference which is the driving force between the reaction mass temperature and temperature of the cooling medium, and area available for the heat transfer to take place. Chemical Process Engineers develops the algorithm required to solve and compute the iterative relations involved in the computations to optimize the Adiabatic Reactor design.
For the Adiabatic Reactor design of a new reactor, the most optimum design for the reactor is arrived involving the reactor dimensions, agitation parameters, viz. impeller diameter & impeller speed, dimensions of the jacket or a coil, etc. are determined and produced in the form of a General Arrangement Drawing which then becomes a basis for the equipment manufacturer.
Adiabatic Reactor design is used because it helps in taking place without transfer of heat or matter between a thermodynamic system and its surroundings.
In the case of an existing reactor, Chemical Process Engineers help the end user in determining the set of operating parameters which can then help them in improving the performance of the reactor in terms of reaction time reduction and hence increased productivity from the existing reactor design, if possible.
Manufacturing of any chemical in commercial scale involves mutli-step flow of the input material starting with reaction, neutralization, crystalization or precipitation of solids, filtration, washing and drying, as a typical example shows importance optimization of chemical process.
The first step towards optimization of chemical process requires all the processes involved above to be so tuned up that none of the processes in the chain in Optimisation of Chemical Process becomes the bottle-neck which prevents rated production capacity being achieved.
The optimization of chemical process begins by sizing the various equipment in the chain in a manner that the capacity of a given process equipment in the chain like the Dryer , for example, is so chosen that its capacity matches the output from the equipment upstream, filter in this case.
Sizing the Dryer larger than the optimum will result in more idle time for itself and hence loss of capacity with a higher investment, of course. Sizing the Dryer smaller than the optimum will make equipment on the upstream, viz. the Filter unit holding its output till the Dryer is ready to receive the material.
Any mismatch in the plant either during design or during operation, will result in loss of productivity and under-utilzation of the Plant.
Very often in the chemical industry, even if the Plant has been designed optimally and set up, it fails to deliver the targeted quantity and quality merely due to the utilities not matching up with the requirement by process.
For example, Optimisation of Chemical Process of an exothermic reaction may take longer than what it should ideally take because the circulation flowrate of cooling water through the jacket is not enough to remove the heat at the same rate at which the heat is generated during the reaction.
How does Optimization of Chemical Process help ?
Chemical Process Engineers help the end user in determining the set of operating parameters of Optimisation of Chemical Process which can then help them in improving the performance of the reactor in terms of reaction time reduction and hence increased productivity from the existing reactor design, if possible.
At Chemical Process Engineers, we specialize in optimization of chemical process in a manner that Optimisation of Chemical Process bottle-necks in the plant are avoided by properly designing and / or selecting the process equipment and utilities, most often through a calculated approach using principles of fluid flow, thermodynamics, heat and mass transfer.
Also, learn the specific knowledge on how the P and I Diagram works with the chemical reactors.
For a more specific understanding of our approach with regard to design of a batch type chemical reactor, read through our blog on optimization of batch chemical reactors.