INST212D Additional topics

1. INST212D Additional topics Note the items in this presentation are supplementary to the presentations covered in class.

2. REFORMER REFORMER

3. INTRODUCTION • The Steam reforming produces various syng-ases for the chemical and petrochemical industries, such as Ammonia, Methanol, Hydrogen. • Steam reforming is a well-established catalytic process that convert natural gas or light hydrocarbons in a mixture containing a major portion of Hydrogen. • Hydrogen is an important product for the refinery desulphurization and hydrocracking process units. • The furnace may “stand alone”, or operate in conjunction with a pre-reformer, post-reformer or other schemes. • In the furnace, the reforming of steam-hydrocarbon mixtures is accomplished in catalyst-filled tubes. • In hydrogen plants, in-tube fluid pressures are typically 25 to 30 kg/cm2 with outlet temperatures up to 860°C (and even higher) depending on the process requirements. • The reformer reaction process is endothermic, requiring high level heat input. • A variety of catalyst (nickel-based) are available for a given feed and product requirement. • Safe, reliable and efficient operation is needed to meet the user’s product demands.

4. Process parameters that must be controlled • Feed Gas: • Pressure • Flow • Reformer outlet Temperature • Combustion Air Flow • Reformer furnace pressure • Fuel gas: • Pressure • Flow

5. Centrifugal Compressors Theory of operation Imagine a simple case where flow passes through a straight pipe to enter centrifugal compressor. The simple flow is straight, uniform and has no vorticity. As illustrated below α1=0 deg. As the flow continues to pass into and through the centrifugal impeller, the impeller forces the flow to spin faster and faster. According to a form of Euler's fluid dynamics equation, known as "pump and turbine equation," the energy input to the fluid is proportional to the flow's local spinning velocity multiplied by the local impeller tangential velocity. In many cases the flow leaving centrifugal impeller is near the speed of sound (340 metres/second). The flow then typically flows through a stationary compressor causing it to decelerate. As described in Bernoulli's principle, this reduction in velocity causes the pressure to rise leading to a compressed fluid.

6. Compressor Surge • Surge - is the point at which the compressor cannot add enough energy to overcome the system resistance or backpressure • This causes a rapid flow reversal (i.e. surge). As a result, high vibration, temperature increases, and rapid changes in axial thrust can occur. These occurrences can damage the rotor seals, rotor bearings, the compressor driver and cycle operation. Most turbomachines are designed to easily withstand occasional surging. However, if the machine is forced to surge repeatedly for a long period of time, or if it is poorly designed, repeated surges can result in a catastrophic failure. Of particular interest, is that while turbomachines may be very durable, the cycles/processes that they are used within can be far less robust.

8. Anti-Surge Compressor Anti-surge systems measure the air flow rate and the outlet pressure. If the compressor system approaches the surge line the anti surge controller will open the anti-surge valve to reduce the resistance to forward flow thereby creating forward flow through the compressor. Other process conditions such as outlet air temperature and inlet pressure may be measured to feed into the approach to surge line

9. Industrial Boilers • Boilers are used to provide steam to be used as feed or for power transmission • Boilers are normally built with Intrinsic controls but will generally be able to be connected to general plant controls • Process parameters that must be monitored are • Liquid Level • Boiler pressure • Fuel Pressure • Fuel Flow • Combustion air flow

10. GENERAL BLCOK DIAGRAM OF BOILER DRUM

11. BLOCK DIAGRAM DESCRIPTION The block diagram of boiler control is shown in above figure the output from the boiler i.e, the steam outputs and the level of water is given to transmitters. The output of transmitter is given to the controller which act as level indicator controller and flow indicator controller. If there is any error corresponding to desired set point, the signal from controller is given to the converter which will open or close the valve and the water will be drained out or filled according to required steam. The major loops in boiler control are 1) Combustion control 2) Feed water control

12. COMBUSTION CONTROL • A combustion control system is broken down into (a) fuel control and (b) combustion air control subsystems. • The interrelationship between these two subsystems necessitate the use of fuel air ration controls. • The primary boiler fuels are coal, oil and gas. The control of gas and oil fuels requires simplest controls- i.e a control valve in the fuel line. • The steam drum pressure is an indication of balance between the inflow and outflow of heat. Therefore by controlling the steam supply one can establish balance between the demand for steam (process load) and supply of water.

13. HARDWARES USED IN COMBUSTION CONTROL • ON/OFF controls: Are still used in many industries but are generally used in small water tube boilers. When the pressure drops to a present value, fuel & air are automatically fed into the boiler at predetermined rate until pressure has risen to its upper limit. • Positioning systems: Respond to changes in header pressure by simultaneously positioning the forced draft damper and fuel valve to a predetermined alignment. This is not used in liquid , gaseous fuel – fired boilers.

14. Metering control system: In this system control is regulated in accordance with the measured fuel and air flows. This maintains combustion efficiency over a wide load ranges & over long period of time. • Both metering & positioning control systems use steam header pressure as their primary measured variable & as a basis for firing rate demand. A master pressure controller responds to changes on header pressure & positions the dampers to control air flow and fuel valve to regulate fuel supply.

15. FEEDWATER CONTROL • Feedwater control is the regulation of water to the boiler drum. It provide a mass accounting system for steam leading and feedwater entering the boiler. • Proper boiler operation requires that the level of water in the steam drum should be maintained within certain band. • A decrease in this level may uncover boiler tubes, allowing them to become overheated. • An increase in the level of water may interfere with the internal operation of internal devices in the boiler drum. • It is important to made that the water level in the boiler drum must be above 50% all the time.

16. As system for feedwater control must be designed to maintain the mass balance over expected boiler load changes so that the level in the steam drum remains within the required limits for safe and efficient operation. • Control system complexity is based on number of measured variables used to initiate control action and include single element ,two element,3 – element and advanced control schemes to improve accuracy of final control action.

17. SINGLE ELEMENT CONTROL SYSTEMS • For small boilers having relatively high storage volumes and slow changing loads ,single element control system is used. • It controls feed water flow based on drum level. • Response is very slow because a change in feedwater flow takes a long time to show up the level change. • As a result the steam drum causes water to increase and decrease in volume, resulting in false measurements.

18. TWO ELEMENT CONTROL SYSTEMS • The two element system overcome these inadequacies by using steam flow changes as a feed forward signal. • This control is used in intermediate boilers as well as large boilers. • Here the flow and level transmitters are summed by a computing relay and will be the set point for feedwater. • Here the response is faster.

19. THREE ELEMENT CONTROL • Boilers that experiences wide and rapid load changes require three element control. • Three element control is similar to two element system except that the water flow loop is closed rather than open. • The level and steam flow signals are summed and used as an index or set point to the feedwater flow. The feedwater flow measurement provides corrective action for variation in feedwater pressure.

20. THREE ELEMENT BOILER CONTROL

21. Pumps and pressure vessels

22. Pumps Pumps can be classified into three major groups according to the method they use to move the fluid: • Centrifugal Pump • Displacement • Reciprocating Pumps

23. Centrifugal Pump A centrifugal pump converts the input power to kinetic energy in the liquid by accelerating the liquid by a revolving device - an impeller.

24. Centrifugal Pump

25. Operation of Centrifugal Pump Fluid enters the pump through the eye of the impeller which rotates at high speed. The fluid is accelerated radically outward from the pump chasing. A vacuum is created at the impellers eye that continuously draws more fluid into the pump.

26. Operation of Centrifugal Pump

27. Questions • What does NPSHA and NPSHR mean? • What is cavitation? • How is NPSHA and NPSHR related to cavitation?

28. Cylindrical pressure vessels • Cylinders are widely used for storage due to their being less expensive to produce than spheres. However, cylinders are not as strong as spheres due to the weak point at each end.

29. Cylindrical pressure vessels

30. Spherical pressure vessels • Spheres however, are much more costly to manufacture than cylindrical or rectangular vessels. This type of storage vessel is preferred for storage of high pressure fluids. A sphere is a very strong structure.

31. Distillation What is distillation? Distillation is a widely used method for separating mixtures based on differences in the conditions required to change the phase of components of the mixture.

32. How mixtures are separated To separate a mixture of liquids, the liquid can be heated to force components, which have different boiling points, into the gas phase. The gas is then condensed back into liquid form and collected. Repeating the process on the collected liquid to improve the purity of the product is called double distillation Although the term is most commonly applied to liquids, the reverse process can be used to separate gases by liquefying components using changes in temperature and/or pressure.

33. What Distillation is used for? • Distillation is used for many commercial processes, • such as production of: • gasoline • distilled water • xylene • alcohol • paraffin • kerosene

34. Diagram Showing: Products obtained from different levels of a distillation process

35. Types of distillation • Types of distillation include: • simple distillation (), • fractional distillation • (different volatile 'fractions' are collected as they are produced), • destructive distillation • (a material is heated so that it decomposes into compounds for collection).

36. Types of Distillation Columns There are two types of distillation columns: Batch Columns Continuous Columns

37. Batch Columns Batch distillationrefers to the use of distillation in batches, meaning that a mixture is distilled to separate it into its component fractions before the distillation still is again charged with more mixture and the process is repeated.

38. Continuous Columns Continuous distillation, a form of distillation, is an on going separation in which a mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams.

39. Main Components of Distillation Columns Distillation columns are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation contains several major components: • A vertical shell where the separation of liquid components is carried out. • Column internals such as trays/plates and/or packingswhich are used to enhance component separations. • A reboiler to provide the necessary vaporisation for the distillation process. • A condenser to cool and condense the vapour leaving the top of the column. • A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column.

40. The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. A typical distillation unit with a single feed and two product streams is shown below:

41. Operation and Terminology The liquid mixture that is to be processed is known as the feed and this is near the middle of the column to a tray known as the feed tray. The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler.

42. Operation and Terminology ( Continued ) Heat is supplied to the reboiler to generate vapour. The source of heat input can be any suitable fluid. In most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns. The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms product. The vapour moves up the column and as it exits the top of the unit it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product.

43. Operation and Terminology ( Continued )

44. COLUMN INTERNALS Trays and Plates The terms trays and plates are used interchangeably. There are many types of tray designs, the most common ones are : Bubble cap trays A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapour. Vapour rises through the chimney and is directed downward by the cap, Then it discharging through slots in the cap and finally it bubbles through the liquid on the tray.

45. Diagram Showing Bubble Cap Trays

46. Valve trays In valve trays perforations are covered by lift-able caps. Vapour flows lifts the caps thus self creating a flow area for the passage of vapour. The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than sieve trays.

47. Sieve trays Sieve trays are simply metal plates with holes in them. Vapour passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters. Because of its efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the bubble cap trays in many applications

48. Liquid and Vapour Flows in a Tray Column The diagrams below shows the direction of vapour and liquid flow across a tray and across a column

49. Liquid and Vapour Flows in a Tray Column ( Continued ) Each tray has 2 conduits, one on each side, called ‘downcomers’. Liquid falls through the downcomers by gravity from one tray to the one below it. A weir on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the holdup is at a suitable height, e.g. such that the bubble caps are covered by liquid. Vapour flows up the column and is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapour on each tray is called the active tray area. The hotter vapour passes through the liquid on the tray above, it transfers heat to the liquid. This causes, some of the vapour condenses adding to the liquid on the tray.

50. Liquid and Vapour Flows in a Tray Column ( Continued ) The condensate is richer in the less volatile components than is in the vapour. Additionally, because of the heat input from the vapour, the liquid on the tray boils, generating more vapour. This vapour, which moves up to the next tray in the column, is richer in the more volatile components. This continuous contacting between vapour and liquid occurs on each tray in the column and brings about the separation between low boiling point components and those with higher boiling points.