INSTRUMENTATION

Introduction:- 

                               A chemical plant is an arrangement of processing units (reactors, heat exchanger, pumps, distillation column, absorber, evaporators, tanks, etc), integrated with one another in a systematic manner. The plant’s overall objective is to convert certain raw materials into desired products using available sources of energy, in the most economical way.

During its operation, a chemical plant must satisfy several requirement imposed by its designers and general technical, economic, and social conditions in the presence of ever-changing external influences (disturbances). Among such requirements are the following:-

(1)             Safety:-

              The safe operation of a chemical process is a primary requirements foe the well-being of the people in the plant. Thus the operating pressures, temperatures, concentrations of chemicals, and so on, should always be within allowable limits.

(2)             Production specifications:-

                                                           A plant should produce the desired amounts and quality of the final products. For example, we may require the production of 2 million pounds of ethylene per day, of 99% purity. Therefore, a control system is needed to ensure that the production level (2 million pounds per day) and purity specifications (99.5% ethylene) are satisfied.

(3)   Environmental regulations:-

                                                                  Various federal and state laws may specify that the temperatures, concentrations of chemicals, and flow rates of the effluent from a plant be within certain limits. Such regulations exist, for example, on the amounts of SO2 that a plant can reject to the atmosphere, and on the quality of water returned to a river or a lake.

(4)   Operational constraints:-

                                                          The various types of equipment used in a chemical plant have constraints inherent to their operation. Such constraints should be satisfied throughout the operation of plant. Foe example, pumps should maintain a certain net positive suction heads; distillation columns should not flooded; the temperature in a catalytic reactor should not exceed an upper limit since the catalyst will be destroyed.                                                             

(5)   Economics:-

                                The operation of plant must confirm with the market conditions, that is, the availability of raw material and the demand of final products. Thus it is required that the operating conditions are controlled at given optimum levels of minimum operating cost, maximum profit, and so on.

All the requirements listed above dictate the need for the continuous monitoring of the operation of chemical plant and external intervention (control) to guarantee the satisfaction of operational objectives. This is accomplished of a rational arrangement of equipment (measuring devices, valves, controllers, computers) and human intervention (plant designers, plant operators), which together constitute control system.

Control Over Continuous Stir Tank Reactor:

Temperature Control:-

                                              For temperature control we employed cascade control configuration. In a cascade control configuration we have one manipulated variable and more than one measurement.

The reaction is endothermic and heat is supplied by dowtherm, which flows in the jacket around the tank. The control objective is to keep the temperature of the reacting mixture, T, constant at the desired value. Possible disturbances to the reactor include the feed temperature T_f. and the dowtherm temperature T_h. The only manipulated variable is the dowtherm flow rate F_h.

Configuration:

We control the reaction temperature by measuring T_h and taking control action before its effect has been felt by the reacting mixture. Thus if T_h goes down, increase the flow rate of dowtherm to give the same amount of heat. Decrease the flow rate when T_h increases. Disturbances arising within the secondary loop are corrected by the secondary controller before they can affect the value of the primary controlled output.

CONTROL OVER DISTILLATION COLUMN

Cascade control is usually employed to regulate the temperature (and consequently the concentration) at the bottom or top of a distillation column

CONTROL OVER HEAT EXCHANGER

The control objective is to keep the exit temperature at 110 ^oC for our shell and tube heat exchanger.

The possible disturbances are:

(1)   Offset of temperature value from its desired value of 110^oC.

(2)    Variation in temperature of dowtherm used as a coolant media.

DISTILLATION COLUMN

Introduction:

       The separation of liquid mixtures into their various components is one of the major operations in the process industries, and distillation, the most widely used method of achieving this end, is the key operations in any oil refinery. In processing the demand for purer products, coupled with the need for greater efficiency, has promoted continues research into techniques of distillation. This process of getting pure products is accomplished by partial vaporization and subsequent condensation.

Distillation:

           “Process in which a liquid or vapor mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat”

TYPES OF DISTILLATION COLUMNS;

                  There are basically two types of distillation columns used in industries.

·        Batch columns

·        Continuous columns

There selection criteria depends upon total number of stages and reflux ratio. As it is shown that when a large number of plates are used, then continuous distillation has the lowest reflux requirements and hence operating costs. If a smaller number of plates are used and high purity product is not required, then batch distillation is probably more attractive.

Batch Columns:

                           In batch distillation the more volatile component is evaporated from the still which therefore becomes progressively richer in the less volatile constituent. Distillation is continued, either until the residue of the still contains a material with an acceptably low content of the volatile material, or until the distillate is no longer sufficiently pure in respect of volatile content. In batch operation, the feed to the column is introduced batch-wise. That is, the column is charged with a 'batch' and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced. Most distillation processes operate in a continuous fashion, but there is a growing interest in batch distillation, particularly in the food, pharmaceutical, and biotechnology industries. The advantage of this separation process is that the distillation unit can be used repeatedly, after cleaning, to separate a variety of products. The unit generally is quite simple, but because concentration are continuously changing, the process becomes more difficult to control. 

Continuous Distillation:

                                        In contrast to batch columns, a continuous feed is given to the column. No interruptions occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs and are the more common used. I will put light only on this type of distillation column.

CHOICE BETWEEN PLATE AND PACKED COLUMN

The choice between use of tray column or a packed column for a given mass transfer operation should, theoretically, be based on a detail cost analysis for the two types of contactors. However, the decision can be made on the basis of a qualitative analysis of relative advantages and disadvantages, eliminating the need for a detailed cost comparison.

Which are:

1.     Because of liquid dispersion difficulties in packed columns, the design of tray column is considerably more reliable.

2.     Tray columns can be designed to handle wide ranges liquid rates without flooding.

3.     If the operation involves liquids that contain dispersed solids, use of a tray column is preferred because the plates are more accessible for cleaning.

4.     For non-foaming systems the plate column is preferred.

5.     If periodic cleaning is required, man holes        will be provided for cleaning. In packed columns packing must be removed before cleaning.

6.     For large column heights, weight of the packed column is more than plate column.

7.     Design information for plate column is more readily available and more reliable than that for packed column.

8.     Inter stage cooling can be provided to remove heat of reaction or solution in plate column.

9.     When temperature change is involved, packing may be damaged.

10.  Random-packed columns generally are not designed with diameters larger than 1.5 m, and diameters of commercial tray column are seldom less than 0.67m.

As my system is non foaming and diameter calculated is larger than 1.5m so I am going to use tray column.

Also as average temperature calculated for my distillation column is higher that is approximately equal to 98oc. So I prefer Tray column.

PLATE CONTACTORS:

                                             Cross flow plate are the most commonly used plate contactor in distillation. In which liquid flows downward and vapours flow upward. The liquid move from plate to plate via down comer. A certain level of liquid is maintained on the plates by weir. Other types of plate are used which have no down comer (non-cross flow) the liquid showering down the column through large opening in the plates (called shower plates). Used when low pressure drop is required.

Three basic types of cross flow trays used are

(1)               Sieve Plate (Perforated Plate)

(2)               Bubble Cap Plates

(3)               Valve plates (floating cap plates)

I prefer sieve plate because:

(1) Their fundamentals are well established, entailing low risk.

(2) The trays are low in cost relative to many other types of trays.

(3) They can easily handle wide variations in flow rates.

(4) They are lighter in weight. It is easier and cheaper to install.

(5) Pressure drop is low as compared to bubble cap trays.

(6) Peak efficiency is generally high.

(7) Maintenance cost is reduced due to the ease of cleaning.

FACTORS AFFECTING SELECTION OF TRAYS

 Relative Cost of plate will depend upon material of construction used.

   For mild steel, the ratio of cost between plates is

        

Sieve plate     :       valve plate    :        bubble-cap plate

                                   3.0           :            1.5           :              1.0

·        There is little difference in Capacity Rating of the three types (the column diameter required for a given flow rate).

Sieve tray     >      valve tray    >      bubble-cap tray

Operating Range means the range of liquid and vapour flow rates which must be above the weeping conditions and below the flooding conditions. Operating range flexibility comparison is.

Bubble cape tray >   Valve tray   >   Sieve tray

       Sieve plate depends on the vapours flow through the holes to hold the liquid on the plate, and cannot operate at very low vapour flow rates. But with good design, sieve plate gives satisfactory operating range.

The Plate pressure drop will depends on the detailed design of plate but, in general, sieve plate gives the lowest pressure drop, followed by valves, with bubble-caps giving the highest.

Operation of Typical distillation Column:

                                                                        The operation of typical distillation column may by followed by figure. The column consists of a cylindrical structure divided into sections by a series of perforated trays which permit the upward flow of vapour. The liquid reflux flows across each tray, over a weir and down a down comer to the tray below. The vapour rising from the top tray passes to condenser and then through an accumulator or reflux drum and a reflux divider, where part is withdrawn as the overhead product D and the remainder is returned to the top tray as reflux R.

In the bottom there is reboiler which is used to give heat to the system. Liquid from the bottom of distillation column is fed to the reboiler which vaporises the in coming liquid. These vapours in turn move towards the bottom plate interact with the liquid over that plate. Due to which partial condensation of vapours occur. Also partial vaporization of liquid occurs too. That is less volatile component condensed first and more volatile component vaporizes first. This phenomenon occurs on each plate. Causing enrichment on each plate.

A schematic of a typical distillation unit with a single feed and two product streams is shown below.

FACTORS AFFECTING DISTILLATION COLUMN OPERATION

Vapour Flow Conditions

Adverse vapour flow conditions can cause:

• Foaming
• Entrainment
• Weeping/dumping
• Flooding

                    

è Foaming

Foaming refers to the expansion of liquid due to passage of vapour or gas. Although it provides high interfacial liquid-vapour contact, excessive foaming often leads to liquid build-up on trays. In some cases, foaming may be so bad that the foam mixes with liquid on the tray above. Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but is sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always reduced.

è  Entrainment

Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by high vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate. Excessive entrainment can lead to flooding.

è  Weeping/Dumping

This phenomenon is caused by low vapour flow. The pressure exerted by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency.

è  Flooding

Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The increased pressure from excessive vapour also backs up the liquid in the down comer, causing an increase in liquid hold-up on the plate above.  Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.

Reflux Conditions:

                                       Minimum trays are required under total reflux conditions, i.e. there is no withdrawal of distillate. On the other hand, as reflux is decreased, more and more trays are required.

Feed Conditions:

                                   The state of the feed mixture and feed composition affects the operating lines and hence the number of stages required for separation. It also affects the location of feed tray.

State of Trays:

                                Remember that the actual number of trays required for a particular separation duty is determined by the efficiency of the plate. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the column. Tray efficiencies are affected by fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus appropriate materials should be specified for tray construction.

Column Diameter:

                                     Vapour flow velocity is dependent on column diameter. Weeping determines the minimum vapour flow required while flooding determines the maximum vapour flow allowed, hence column capacity. Thus, if the column diameter is not sized properly, the column will not perform well.

DESIGNING STEPS OF DISTILLATION COLUMN

•          Calculation of Minimum number of stages.N_min
• ·        Calculation of Minimum Reflux Ratio R_m.
• ·        Calculation of Actual Reflux Ratio.
• ·        Calculation of theoretical number of stages.
• ·        Calculation of actual number of stages.
• ·        Calculation of diameter of the column.
• ·        Calculation of weeping point.
• ·        Calculation of pressure drop.
• ·        Calculation of thickness of the shell.
•  ·        Calculation of the height of the column.

POWER REQUIRED BY PUMP

Diameter of pipe = 1 ft = 0.3048m

Cross sectional area of pipe = πD²/4

                                            = π/4 (0.3048)²

                                            = 0.07297 m²

Viscosity of Methanol = 0.5478 Kg / m.sec

Density of Methanol = 787.225 Kg/m³

Mass flow rate = 0.686 Kg/sec

Volumetric flow rate = 1.4594 m³/sec

Velocity, U = Volumetric flow rate /Area

                        = 1.4594 / 0.07297

                        = 20.0 m/sec

Reynolds number = ρud/µ

                             = 787.225*0.3048*20.0/0.5478

                            = 8760.36

If      e = 0.05

      e/d = 0.002

 R/ρu² = 0.0278

Head losses due to friction:

      

h_f = -∆P_f/ρg = 8(R/ρu²)(l/d)(u²/2g)

   

 = 8*0.0278*(30/0.3048)(20²/2*9.81)

     = 446.60m

∆Z = 25m

from Table 3.2, 0.8 velocity heads are lost through each 90o bend so that the loss through two bends is 1.6 velocity head.

                = 1.6*20²/(2*9.81)

                    = 32.62m

Total head = 446.60 + 25 + 32.62

                = 504.62m

Mass flow rate = 3.132 Kg/sec

So,    Theoretical Power required = Mass flow rate*Total head * g

                                          = 3.132*504.62*9.81

                                          = 15504 W

 or                                      =  15.5 KW

Net Positive Suction Head (NPSH):

From an energy balance equation, the head at the suction point of the pump is:

hi = (P_o/ρ_h) + x – (u2/2g) – h_f

the losses in the suction pipe = 1.5 m , at (u²/2g) = h_f = 1.5

the net positive suction head (NPSH) is:

NPSH = hi – (Pv/ρg)

Where

 Pv is the vapor pressure of the liquid being pumped.

Po is the vessel pressure above the liquid.

NPSH = (P_o/ρh) + x – (u²/2g)  –  (P_v/ρg)

P_o = 760-640 = 120mmHg = 16,000 N/m²

P_v = 760-710 = 50 mmHg = 6670 N/m²

NPSH = 3.5 -1.5 + (16,000 +6670)/(787.225*9.81)

NPSH = 4.94 m

FLASH TANK

If we say that Chemical Engineering is nothing but the combination of art and science to design and control the separation equipment, it won’t be a lie. In a chemical industry, more than the 70% of total capital investment is incurred on separation and purification equipment. These stats might highlight the importance of separation equipment in chemical industry.

Defining the problem:-

                  

In Cativa Process, one of the product streams is coming out from the reactor. This stream contains the Acetic Acid; which is our sole product, and the Iridium Catalyst Complex. We have to maintain some liquid level in the reactor as well so that we might use this liquid as the solvent for the incoming feed stream. The catalyst has to be recycled back to reactor for further utilization. So we need equipment that might separate out the product (not essentially all of it) and recycle back some fraction of Acetic Acid along with the catalyst. A little amount of water should also be maintained in the reactor as this is the requirement of the technology (Cativa Process) we are using. So up till now, we have successfully defined our problem. Let’s look for a solution to it.

Looking for the solution:-

                  

                   Now there are a number of equipments that are available to us for this purpose. We need to have a look at the physical conditions of the stream. All the components are in liquid state at 110 ^oC and 27 atm pressure.

We need to recycle some of the Acetic Acid and the catalyst back to reactor. Both of these are required to be there for further conversion. The feed mixture is in homogenous phase. This makes our choice quite simple. We can eliminate the possibility of a phase separator. One thing that must be kept in mind is that the solution has to be economical and quite effective. If we have a look at various industries; we find that most industries generate a second phase from this feed and recycle successfully some of the desired components in liquid state. This is quite an energy efficient process. Now let’s have a look at the possible choices that we have at our hand.

Possible Choices Available:-

                       

We have our feed in liquid state in which catalyst is homogenously dissolved. We want some of the Acetic Acid, little amount of water and the catalyst recycled back to reactor. We’ll make use of equipment that can generate the vapor phase without expenditure of much of external energy and then successfully recycle the desired components back to reactor. One choice looks obvious. It’s the Flash Drum. There are other possible alternatives available to us, likewise Knockout Drum, Horizontal Flash Drum or the spherical one. All have their own characteristics and are used in specific situations. We’ll make use of Vertical Flash Drum.

Construction of a Flash Drum:-

                When feed is flashed in a Flash Drum, vapor and liquid mixture is generated. As this mixture enters the drum, the surface area is increased, due to which pressure drop is generated. Right at eh entrance of the feed, there’s a splash plate in the drum. This splash plate directs the vapor and liquid flow downwards. This way the effect of gravity is enhanced. The liquid settles down at the bottom while the vapors with little momentum, change their path and rise up the vessel. At the top of the vessel, there’s a mist eliminator. Actually when vapors rise up the vessel, small liquid droplets also accompany them. The phenomenon of splashing is avoided by the use of splash plate. So our splash plate is serving two major purposes. First it helps us to avoid the splashing of liquid. Secondly, it directs the vapor liquid mixture downwards which in turn enhances the effect of gravity. Due to this effect, liquid is separated out of vapor. One thing should be kept in mind is that most of the impaction process takes place at the splash plate. So it has to be mechanically sound so that it can handle all the impact. Now there are two kinds of mist eliminators.

• Vane type Mist Eliminator
• Mesh Eliminators

Vane type mist eliminator consists of metallic plates arranged closely to each other. Vapors with small liquid droplets rise. The plates are arranged in such a manner that they provide a zigzag path to the incoming vapor and liquid droplets. Droplets due to inertia and large momentum strike the plates and are captured at the surface while the vapors change their path accordingly and escape the eliminator. The phenomenon is referred to as Impaction and the size increase of droplets is called as Coalescence. Hence vapors are collected at the top of the vessel.

Now in mesh mist eliminators, a metallic or plastic wire mesh with a diameter ranging 0.006 to 0.011 in is used. The phenomenon is the same; impaction on the wire and then captured. Mist escapes the wire while droplets are captured at the surface where they coalesce and fall down as large drops.

There’s a radial vane vortex breaker shown at the bottom of the vessel. The purpose of this vortex breaker is to avoid the phenomenon of Vortex Formation. There are a couple of causes that induce the vortex formation in the drum. The first one is the earth’s rotational speed. Due to the earth’s rotational speed, anticlockwise vortex is observed in Northern Hemisphere while a clockwise motion is observed in Southern Hemisphere. Second reason is the introduction of feed in the vessel tangentially. Whenever feed is entered tangentially, vortexes are formed. Third reason is the vapors. Whenever there’s a two phase mixture and they differ in their velocity; then the fluid with lesser velocity and high density would start the rotational motion (Vortex Formation). In our case, we are handling a vapor-liquid mixture. Vapors are at a higher speed in the vessel while the liquid are a bit slower due to the impaction with the splash plate. So the vapors would induce the vortex to the liquid. The formation of vortexes brings some disadvantages to the system. Our system with vortex formed, experiences:

• Loss of valuable vapors
• Downstream equipment damage
• Loss of flow
• Erroneous liquid level readings resulting in poor control
• Vibrations caused by unsteady two phase flow.

The formation of vortexes is shown in the following figure:

To avoid the vortex formation, we should avoid the usage of a tangential feed line. Secondly, we can use a vortex breaker to get rid of vortexes. Following types of vortex breaker are usually used in the industry:

• Flat plate vortex breaker
• Crosses
• Radial vane or gratings

We are using a Radial Vane Vortex Breaker. A vortex breaker is stationary and it doesn’t move. If it starts the motion with the vortex then it wouldn’t break the vortex rather it would just weaken it. To break the vortex and get rid of it, we’ll have to fix the vortex breaker and make it stationary.

Why use Vertical Flash Drum?

               

Let’s carry out the process of elimination to justify our choice. We can simply rub aside the choice of Knockout Drum as it is used wherever there’s gas in the feed stream. In our stream there are no gases. We have only liquid phase. So we will not go for the Knockout Drum. Now we are left with Horizontal, Spherical and Vertical Flash Drums. Horizontal Drums are used when we have to handle a large liquid flow rate. But in our case we’ll see that the liquid flow rates wouldn’t be that huge. Instead we’ll have to deal with a high amount of vapor flow rate. Also Walas carried out a survey and in his book “Chemical Process Equipment Selection and Design” writes that out of every ten chemical industries; seven are making use of Vertical Flash Drums. The choice is made due to the economy and the ease with which we can handle the flow rates. A design engineer is required to start designing a Vertical Flash Drum by default and then after the design is complete we have a look at the L/D (length to Diameter Ratio) to decide which configuration to use. So we’ll follow the same procedure. We’ll design a Vertical Flash Tank and then would analyze the L/D ratio obtained to determine which configuration to use. Just remember one rule of thumb; for large liquid flow rates, we’ll use Horizontal Flash Drum and for small liquid flow rate, you’ll go for a vertical configuration. You can start designing any one of these and then the final decision would rest upon the L/D ratio of the drum. So don’t bother. Just start your computer software and begin designing any configuration. Let’s start the design of Vertical Flash Drum. Before the process of designing, we’ll see what exactly flashing is.

Throttling:-

               

When a fluid (liquid or a liq/vapor mixture) at high temperature and high pressure experiences sudden reduction in pressure, then some of the liquid is vaporized and the phenomenon is referred to as Throttling. During the process the temperature of the feed stream doesn’t change that much and in such a case the process is called as Adiabatic Flashing. Actually for an ideal gas or a fluid behaving likewise an ideal gas, there’s no temperature drop. But in real fluids, little temperature drops have been observed. These temperature drops are due to the Joule-Thomson Effect and the frictional loss. Since there’s no appreciable change in the kinetic and potential energy; and also there’s no shaft work or heat transferred, therefore the eq:        

Δ (H + u²/2 + gz) = Q + W_s reduces to ΔH = 0.

                        We know that the enthalpy depends upon the temperature of fluids. Since there’s no change in enthalpy so theoretically there will be no change in the temperature of the fluid stream. Usually for real fluids, a very little temperature drop is observed. In our case, the feed is at 110 ^oC and the pressure is 27 atm. We’ll suddenly reduce the pressure of the liquid stream and this would ultimately generate a vapor phase without the expenditure of any external energy. There will be ignorable temperature drop. All the beauty of equipment lies in this phenomenon. We are generating a second phase without expanding any external energy. But we know that energy is always conserved. We have generated the vapors on the expense of the pressure of the incoming feed So although the process of throttling makes us lose some of the energy contents of the feed stream, yet we get more benefits. Now the problem comes out to be the selection of the valve.

Selection of Valve:-

No ordinary valve would be used for this purpose. We need such a valve that would handle a feed stream with such a high temperature and pressure and allow it to expand suddenly. The valve would allow only one sided flow of the stream. There are a number of options open to us. Globe Valve, Gate Valve, Butterfly Valve, Ball Valve etc are all at our disposal. But none of these is manufactured for the purpose of throttling. As we look for the best choice, we come to know that there’s a valve that is manufactured keeping in mind the sole idea of throttling. This is Lever sealed Plug Cock. The valve operates up to a temperature of 260 ^oC. It has plastic lining that makes it corrosion resistant. It has a tapered plug that is moved by a lever. The plug contains perforations just like a ball valve. As the feed stream passes through it, pressure drops from 27 atm to 1.4 atm. The temperature change is negligible. So after getting flashed, vapors are generated. The temperature of the stream remains more or less the same.