Please answer the questions in a Word document, maximum 2 pages (include answer only). APA style is preferred.1) What is quantity theory of money? Please elaborate in details.2) What are the factors t
5. Control
This section will emphasize significance of process control. Brief information on plant wide control will be followed by detailed discussion of control of the dryer under consideration.
5.2. Control philosophy for the whole plant
The overall process proposed for Titanium Dioxide (pigment) production contains multivariable interactions within and between indicated nodes. Poor control of a process variable in one of the nodes may lead to non-uniform production rate and/or formation of products of poor quality. Consequently, this would result in loss of profit and reputation (Seborg, Edgar, & Mellichamp, 2004). Most importantly, bad control may put under risk the safety of operation and cause catastrophic disaster with casualties, thus economic loss; both of which are not desirable.
To maintain good plant-wide control, hierarchy of information systems, presented in table 20, will be used (Green & Perry, 2007).
Factors and aims:
Measurement and actuation: Utilization of measurement devices, indicators and actuators
Safety and Environmental Protection: Combination of human action and equipment grouping to maintain system at desired operation conditions
Regulatory control: Implementation of automatic control at level 1 to maintain targets set by company authority
Real-time optimization: Analysis of individual units comprising of:
a. Investigation of effects of variable conditions on
efficiencies, product quality and quantity.
b. Evaluation of optimum operation condition to meet required product quality and quantity at minimum cost
Corporate Information Systems: Communication of plant requirements to maintain steady operation (e.g. raw material demand for near future).
Control of the drying process
The objective of rotary dryer is to reduce moisture content of the raw material before sending it to the process. When designing the RD101, temperature and inlet moisture content of the feed was assumed constant based on average of the bulk. In practice, however, the feed entering the dryer may have fluctuating temperature and moisture content. They are uncontrollable disturbance variables. Subsequently, the product stream from the dryer would be either over-dried or under- dried; and it would require either higher or lower air inlet temperature (i.e. higher or lower firing rate of the burner.
It must be noted that over-drying would not be an issue for the downstream process efficiency. However, as it is not a requirement from the downstream, under-drying means excess usage of energy, thus higher operational costs, hence is undesirable.
The fact that there is no apparatus to measure (directly and instantly) the product moisture content makes control of drying operation complex. To arrive at an alternative control strategy to maintain the product stream at constant moisture, following considerations are made:
By measuring product outlet temperature, moisture content of the product may be indirectly determined/estimated on-line (Green & Perry, 2007).
However, that is not reliable, when the particle residence time (in the dryer) is large. Sole feedback loop between particle outlet temperature (as controlled variable) and firing rate of the burner (as manipulated variable) would only act against disturbance s which had occurred in the past (Perry & Chilton , 1973).
In the case of designed rotary dryer, a disturbance (to particle condition) in the inlet would only be determined after 50.4 minutes at outlet.
The air residence, on the other hand, is within range of seconds.
Dry- and wet-bulb temperature of the exhaust air may be determined through measurement systems on-line; and their difference (as a measure of the rate of evaporation) used for control (Perry & Chilton , 1973).
Therefore, cascade control strategy will be used to maintain product stream at its desired moisture content. The strategy will have following steps:
Solids outlet temperature will be measured (as a representative of its moisture content);
Dry- and wet-bulb temperature difference of the exhaust air will be measured and controlled;
Set point for the controlled temperature difference (as a representative of the rate of evaporation) will be based on solids outlet temperature (i.e. moisture content);
Manipulated variable for the control will be flowrate of the natural gas to the combustion chamber.
Other control systems around the dryer
Pressure of the system will be controlled by measuring the pressure at dryer outlet air stream and manipulating a control valve at the clean air exit stream from the bag filter. Amount of particles entrained in the air stream may also fluctuate depending on the amount of undersized particles in the feed. Hence, weight of the material in screw conveyor, conveying dryer product to the ball mill will be controlled to assure required mass is being provided via manipulation of speed of screw conveyor feeding the dryer. This will consequently affect the stock material in the hopper, thus a level controller will be utilised on it. To maintain constant amount of material in the hopper, speed of belt conveyor feeding it (from the stockpiles) will be manipulated. Finally, instability in the dusting rate will also mean that particles collected in the cyclones and bag filter will vary. Therefore, level controllers will be installed in their collection sections and to keep the levels constant, associated screw conveyor speed (i.e. motor power) will be manipulated. The instruments and control systems specified above are illustrated in the piping and instrumentation diagram (P&ID) presented in figure 28 at the end of this section (section 5)
5.4. Operation procedure of the rotary dryer (RD101)
5.4.1. Commissioning
To arrive at first start-up of the dryer following tasks must be completed:
1) Design review and discussion of readiness.
2) Review of detailed start-up, shutdown and emergency shut down procedures. It is essential to assure no source of i gnition is present around the dryer. Fineness and volume of particle handled may explode even with small spark.
3) On-site inspection of feedstock, process equipment, instruments, and emergency kits.
4) Assessment of site-operators knowledge on the process and procedures.
5.4.2. Start-up
With rapid increase in the dryer shell temperature, excessive differential expansion may occur between dryer shell and the tyres (at the two ends) around it. This could gradually dislocate the tyres after number of start-ups and shut downs. Rapid temperature changes may negatively affect the other pieces of equipment as well, reducing their service life. The start-up procedure, with consideration to avoid thermal shock along the hot air flow line, is detailed below:
1) Start the dryer rotation (at 1rpm).
2) Open all of the valves the on air flow stream.
3) Start the air blower operation.
4) Start the combustion chamber operation initially at low heat duty.
5) Gradually and simultaneously increase the rotation of the dryer and heat input of the burner (to the air).
6) Wait for the desired operation values to be reached and the thermocouples placed on the equipment indicate them being at operating temperature.
7) Operate the feed screw conveyor to introduce material to the dryer. After 25 minutes (half of the particle residence time) start operation of the dryer product screw conveyors.
8) Let the automatic control to regulate the process and observe the operation.
5.4.3. Shutdown
As in start-up, gradual temperature changes will be maintained during shutdown. The procedure to follow is as below:
1) Stop solids feed rate to the dryer (i.e. motor power of the feed screw conveyor).
2) Wait until no material is leaving the dryer (i.e. it is empty).
3) Gradually and simultaneously decrease the rotation of the dryer and heat input from the burner (to the air).
4) Keep the rotation of the dryer at 1 rpm.
5) Wait until the equipment are at around 40oC and then shut down the burner.
6) Continue blowing air through the system until the equipment temperature are close to ambient.
7) Shut down the air blower.
8) Stop dryer rotation when it is at about ambient temperature.
A note on emergency shutdown
If some process conditions reach dangerous levels or in case of failure of a critical piece of equipment, emergency shutdown may be inevitable. The steps of such shutdown is as next:
1) Stop screw conveyor feeding the dryer, operation of combustion chamber and blower simultaneously.
2) Keep the dryer rotating to convey the material inside, out.
3) Stop dryer product screw conveyor.
4) If possible, allow the dryer to continue rotating while being cooled naturally.
7. Summary
To conclude, the design and estimation approaches in this report is accompanied by reality checks and two different solution methodologies, where relevant and possible. Hence, the proposed values may be taken reliable. Following sections will briefly summarize the design matters covered in this report.
7.1.Summary of theRD101design
After designing the dryer based on specified requirements via two different methods, most sensible one was chosen. As per condition, the designed dryer will process 13.214 tonnes of titanium dioxide slag per hour, reducing its moisture content from 4% to 0.6%.
Dryer design will have the following characteristics:
Length: 9.16 metre
Diameter (inner): 2.1 metre
Rotation per minute: 5
Particle residence time: 50.4 minute
Loaded mass: 15.7 tonnes To enhance evaporation rate and energy usage, flights are to be used to shower solids through the hot air stream. Suitable number of flights is 21, with angular distance being about 17o. Pressure drop through the dryer is found negligible, about 3 Pa. Finally, total power required by the dryer (including the motive and drive) is calculated equal to 14.2 kW.
7.2. Summary of the auxiliary equipment
The designed auxiliary equipment of the dryer and their key aspects as follows: Transport:
Initial solids transport system includes utilization of belt and screw conveyors. Overall power consumption of these are estimated to be 5.2 kW.
For air streams, acknowledging the water content evaporated from the moist solids, stainless steel pipes are chosen, with optimum diameter calculated as 0.56 metre. Pressure drop through the pipe is about 2620 Pa.
Cyclone:
2 reverse-flow high throughput stairmand cyclones are utilised in parallel to clean the dryer exhaust air from entrained dust particles.
Characteristic diameter is calculated as 0.515 metre with allowable pressure drop of 5327 Pa Bag Filter:
Following cyclones, air is further cleaned from extremely fine particles with aid of bag filter.
1 bag filter compartment with 55 bags of 2.51 m2 area is found enough. The related pressure drop is estimated as 750 Pa. Combustion chamber:
The required chamber length and diameter are calculated as 0.3 and 1.35 metres respectively.
Natural gas is chosen as burning fuel to provide the required 2916 MJ of energy per hour.
Air Blower:
The blower will be installed attached to the combustion chamber and provide 13,800 m 3 of air per hour (accounting for the excess air for fuel burning).
Based on the pressure drops specified above a suitable blower was chosen from potential retailer. The power consumption of chosen blower is about 6.4 kW.
7.3. Summary of the process control
Before choosing an automatic control strategy for the section under consideration, process variables that need control and potential disturbances to them were identified, hence variables chosen for manipulation. The strategy chosen was to apply cascade control as below:
1 Measure dried solids temperature leaving the dryer (to estimate existing moisture content).
2 Control difference between wet- and dry-bulb temperatures of the exhaust air (as representative of rate of evaporation) based on set point evaluated from the solids outlet temperature.
3 Manipulate flowrate of the natural gas for the control.
7.4. Summary of economic estimation
For economic analysis contributions only from the designed equipment was considered. Comparing the designed part of node 1 to untouched, it may be assumed that the overall cost of the node will be three times the value of the section designed (which is part of TiO2 processing line). The figure 29 illustrates contribution of various costs on the overall cost. However, as the capital and operating costs should not preferably be mixed, this is useful just for general comparison. Table 26 is presenting the value breakdown for the evaluated costs separately (for capital and operating).
Firstly, the designed section of node 1 is developed with emphasis on its feasibility in reality. However, the theories and correlations used throughout the design are not always specific to the given case. For example for drying time calculation data for fine sand is used. Therefore, it is strongly recommended to invest in pilot plant to assess the proposed design and make suitable adjustments. Alternatively, appropriate simulation packages may be used as a check.
Also, the given process is designed to continuously and solely heat ambient air to the dryer air inlet temperature. However, the air leaving the dryer at 115oC may be recirculated to the combustion chamber after dust removal. This would reduce both the operational costs and impact on environment.
Finally, the particles entrained in air stream and collected through cyclones and bag filter may be re - processed in a separate line and fed into the chlorinators. On long run, this would show a significant reduction in raw material cost.
Following preliminary process design of titanium dioxide pigment production plant, the engineers of Titanium Dioxide Development shave produced detailed design of the proposed plant in six separate documents. This paper is the first of those documents looking at design of node 1 of the proposed plant.
First section of this paper re-introduces the plant design specifications and summarizes the overall process steps developed previously. In the second section a renewed process flow diagram of node 1 is presented and pre-processing equipment discussed.
The main body of this report covers design of co-current direct heat rotary dryer utilised in TiO2
processing line of node 1. The required dryer capacity is 13.2 tonnes of TiO2 slag per hour, reducing
its moisture content from 4% to 0.6%. To arrive at a reliable design, two different methodologies are
covered and compared. Obtained final main design criteria are the following: (note:a detailed specification is provided in section3.8.9)
Dryer length: 9.2 metre Dryer Diameter: 2.1 metre Power requirement:14.2kW
Design of the dryer is followed by specification of its auxiliary units, namely the transport systems, combustion chamber, blower, exhaust air cyclone and bag filter. The overall power consumption of these units equal 11.6kW, with the chamber consuming 52.5 tonnes of natural gas per hour.
Discussing various control strategies for the dryer and its auxiliary, in section 5, the best approach is found to be cascade control. This section also covers commissioning, start-up and shutdown of the processing line.
Finally, an economic evaluation is covered in section 6, giving the total capital investment as £1,652,000 and annual operating cost as £900,000. Note that, these values are representative of only the dryer and specified auxiliary units. Cost for the whole node 1 may be estimated as 3 times these values.
