Processes are the main concern of chemical engineers. A process is a system that converts feedstocks of lower intrinsic value to products of higher value. For instance, the block diagram of a process to manufacture ammonia from natural gas is shown in Figure II-1. This process has several sections, each one of which carries out a specific task and is, in effect, a mini-process Natural gas, steam and air are fed to the reformer Section that converts these feeds into a mixture of H2 co H20 CHA CH4 Water gas Methane CO2 Steam Shift Reformer Removal Converter steam SY GAS CH4 Ammonia Product Methanation L Fig 1-1. Block Diagram for Ammonia Process C02. N2. and H20. The overall chemical reactions involved are CH4+H20→C0+3H2 CH4+O2→CO+2H2 CH4+202→CO2+2H2O Since H2 is the desired raw material from which to make ammonia, this gas mixture is sent to the Co Shift Section where additional steam is added to improve conversion by the water gas 13-
-13- Processes are the main concern of chemical engineers. A process is a system that converts feedstocks of lower intrinsic value to products of higher value. For instance, the block diagram of a process to manufacture ammonia from natural gas is shown in Figure II-1. This process has several sections, each one of which carries out a specific task and is, in effect, a mini-process. Natural gas, steam and air are fed to the Reformer Section that converts these feeds into a mixture of H2, CO, Fig. II-1. Block Diagram for Ammonia Process CO2, N2, and H2O. The overall chemical reactions involved are: CH4 + H2O ‡ CO + 3 H2 CH4 + O2 ‡ CO + 2 H2 CH4 + 2 O2 ‡ CO2 + 2 H2O. Since H2 is the desired raw material from which to make ammonia, this gas mixture is sent to the CO Shift Section where additional steam is added to improve conversion by the water gas shift reaction: CH4 Steam Air Steam Methane Reformer Water Gas Shift Converter CO2 Removal CO2 H2O Argon Purge Methanation CO CO2 H2 H2O N2 CH4 Ar CO H2 N2 CH4 Ar SYN GAS H2 N2 CH4 Ar NH3 Product Ammonia Synthesis Loop
CO+H20→H2+CO2 The CO2 and h20 present in the gas mixture leaving the Co Shift Section are removed in the CO2 Removal Section. The gas mixture leaving the CO2 Removal Section contains primarily a 3/1 mixture of H2 and N2(the N2 coming from the air fed to the Reformer Section). It also contains small amounts of Co(the water gas shift reaction is equilibrium limited) as well as argon from the air feed to the reformer section The Co must be removed from this mixture because it will deactivate the catalyst used in the ammonia converter This is done in the methanation section via the reaction CO+3H2→CH4+H20 The gas mixture leaving the Methanation Section contains a 3/1 mixture of H2 and N2 and trace amounts of CH4 and Ar. This is sent to the NH3 Synthesis Loop where the ammonia is made via the well-known reaction N2+3H2→2NH3 Ammonia Synthesis Loop in more detail. A process flow diagram(PFD)is shown in Fig. the So far each section of the process has been considered as a black box. Let us look The ammonia synthesis reaction is equilibrium limited to the point where it must be run at rather high pressures(3000 PSIA or 20 mPa)in order to achieve a reasonable conversion of Syn Gas to ammonia across the Converter. Since the front end of the process is best run at much lower pressures, a Feed Compressor is needed to compress the Syn gas to the operating pressure of the Synthesis Loop Due to the unfavorable reaction equilibrium, only part of the Syn gas is converted to ammonia on a single pass through the Converter. Since the unconverted Syn Gas is valuable, the majority of it is recycled back to the Converter. Due to pressure drop through the Synthesis Loop equipment Recycle Compressor is required to make up this pressure drop. The recycled Syn Gas is mixed with fresh Syn Gas to provide the feed to the Synthesis Converter The effluent from the Synthesis Converter contains product ammonia as well as unreacted Syn Gas. These must be separated. At the pressure of the synthesis loop ammonia can be con- densed at reasonable temperatures. This is done in the Ammonia Condenser where the Syn Gas is cooled to approximately ambient temperature using cooling water(CW). The liquid ammonia is the
-14- CO + H2O ‡ H2 + CO2. The CO2 and H2O present in the gas mixture leaving the CO Shift Section are removed in the CO2 Removal Section. The gas mixture leaving the CO2 Removal Section contains primarily a 3/1 mixture of H2 and N2 (the N2 coming from the air fed to the Reformer Section). It also contains small amounts of CO (the water gas shift reaction is equilibrium limited) as well as argon from the air feed to the Reformer Section. The CO must be removed from this mixture because it will deactivate the catalyst used in the ammonia converter. This is done in the Methanation Section via the reaction CO + 3 H2 ‡ CH4 + H20. The gas mixture leaving the Methanation Section contains a 3/1 mixture of H2 and N2 and trace amounts of CH4 and Ar. This is sent to the NH3 Synthesis Loop where the ammonia is made via the well-known reaction N2 + 3 H2 ‡ 2 NH3. So far each section of the process has been considered as a black box. Let us look at the Ammonia Synthesis Loop in more detail. A process flow diagram (PFD) is shown in Fig. II-2. The ammonia synthesis reaction is equilibrium limited to the point where it must be run at rather high pressures (3000 PSIA or 20 mPa) in order to achieve a reasonable conversion of Syn Gas to ammonia across the Converter. Since the front end of the process is best run at much lower pressures, a Feed Compressor is needed to compress the Syn Gas to the operating pressure of the Synthesis Loop. Due to the unfavorable reaction equilibrium, only part of the Syn Gas is converted to ammonia on a single pass through the Converter. Since the unconverted Syn Gas is valuable, the majority of it is recycled back to the Converter. Due to pressure drop through the Synthesis Loop equipment, a Recycle Compressor is required to make up this pressure drop. The recycled Syn Gas is mixed with fresh Syn Gas to provide the feed to the Synthesis Converter. The effluent from the Synthesis Converter contains product ammonia as well as unreacted Syn Gas. These must be separated. At the pressure of the synthesis loop ammonia can be condensed at reasonable temperatures. This is done in the Ammonia Condenser where the Syn Gas is cooled to approximately ambient temperature using cooling water (CW). The liquid ammonia is then
P-l STIo gon Syn S12 ST9 Gas ST3 sT8 M Mix o Compressor R-1 Ammonia Converter ST4 KO Drum E-1 Condenser ST7 Fig 1-2. Ammonia Synthesis Loop process Flow diagram separated from the cycle gas in the Ammonia Knockout(KO)Drum. The liquid ammonia is removed from the bottom of the drum and the cycle gas leaves the top Some of the cycle gas must be purged from the Synthesis Loop. Otherwise, the argon that enters the loop in the Syn Gas has no way to leave and will build up in concentration. This will reduce the rate of the ammonia synthesis reaction to an unacceptable level. To prevent this from appening, a small amount of the cycle gas must be purged, the amount being determined by the amount of argon in the feed and its acceptable level in the Synthesis Converter feed(generally about 10 mol % This description of the Ammonia Synthesis Loop covers only the most important Modern ammonia plants are much more complex due to attention paid to maximizing th of ammonia produced per mol of Syn Gas fed to the loop. However, the process shown Il-2 is typical of many chemical processes 15-
-15- Fig. II-2. Ammonia Synthesis Loop Process Flow Diagram . separated from the cycle gas in the Ammonia Knockout (KO) Drum. The liquid ammonia is removed from the bottom of the drum and the cycle gas leaves the top. Some of the cycle gas must be purged from the Synthesis Loop. Otherwise, the argon that enters the loop in the Syn Gas has no way to leave and will build up in concentration. This will reduce the rate of the ammonia synthesis reaction to an unacceptable level. To prevent this from happening, a small amount of the cycle gas must be purged, the amount being determined by the amount of argon in the feed and its acceptable level in the Synthesis Converter feed (generally about 10 mol %). This description of the Ammonia Synthesis Loop covers only the most important aspects. Modern ammonia plants are much more complex due to attention paid to maximizing the amount of ammonia produced per mol of Syn Gas fed to the loop. However, the process shown in Figure II-2 is typical of many chemical processes. C-1 Feed Compressor M-1 Feed Mixer R-1 Synthesis Converter E-1 Ammonia Condenser F-1 Ammonia KO Drum P-1 Argon Purge C-2 Recycle Compressor ST1 Syn Gas ST2 ST3 ST4 ST5 ST6 ST7 Liquid Ammonia ST8 Purge ST10 ST9
B. Basic processing functions Several basic processing activities are required by the Ammonia Synthesis Loop in order to convert the hydrogen and nitrogen to ammonia product. These activities are common to almost all chemical processes; the functionality of each can be considered independently of any process There are five of these processing activities that are of major interest in chemical engineering 1)Chemical Reaction (2)Mixing ()Separation (4)Materials Transfer(Fluid flow) (5)Energy(Heat) Transfer Of these the first three are involved in the process material balance. The last two are necessary adjuncts to operation of chemical processes. Material must be transported from one piece of equipment to another. For the large number of chemical plants processing only liquids and vapors, this involves fluid flow. Also, streams must be heated to or cooled to specified temperatures as dictated by the needs of the process. For example, reactors in general are operated at temperatures higher than those that are acceptable for most separation operations Thus, streams must be heated to reaction temperature and then cooled back down for subsequent processing 1. Chemical reaction What distinguishes the chemical process industries from almost all others is the use of chemical reactions to convert less valuable raw materials to more valuable products. In other words, chemical reaction is the heart and soul of almost all processes In the Ammonia Synthesis Loop example, R-1, the Synthesis Converter is where the ammonia synthesis reaction takes place Aring Many chemical reactions involve two or more reactants. In order for these reactants to react. these must be brought into contact at the molecular level. i. e. mixed, before the desired reactions can proceed properly Mixing is also required if several substances are to be blended to create a product mixture with the desired properties
-16- B. Basic Processing Functions Several basic processing activities are required by the Ammonia Synthesis Loop in order to convert the hydrogen and nitrogen to ammonia product. These activities are common to almost all chemical processes; the functionality of each can be considered independently of any specific process. There are five of these processing activities that are of major interest in chemical engineering: (1) Chemical Reaction (2) Mixing (3) Separation (4) Materials Transfer (Fluid flow) (5) Energy (Heat) Transfer Of these the first three are involved in the process material balance. The last two are necessary adjuncts to operation of chemical processes. Material must be transported from one piece of equipment to another. For the large number of chemical plants processing only liquids and vapors, this involves fluid flow. Also, streams must be heated to or cooled to specified temperatures as dictated by the needs of the process. For example, reactors in general are operated at temperatures higher than those that are acceptable for most separation operations. Thus, streams must be heated to reaction temperature and then cooled back down for subsequent processing. 1. Chemical Reaction What distinguishes the chemical process industries from almost all others is the use of chemical reactions to convert less valuable raw materials to more valuable products. In other words, chemical reaction is the heart and soul of almost all processes. In the Ammonia Synthesis Loop example, R-1, the Synthesis Converter is where the ammonia synthesis reaction takes place. 2. Mixing Many chemical reactions involve two or more reactants. In order for these reactants to react, these must be brought into contact at the molecular level, i.e., mixed, before the desired reactions can proceed properly. Mixing is also required if several substances are to be blended to create a product mixture with the desired properties
For our example process, mixing of fresh synthesis gas and recycled synthesis gas takes place in the Feed Mixer, M-1, before being sent to the Synthesis Converter 3. Separation In an ideal chemical process, exactly the right amounts of reactants would be mixed and reacted completely to the desired product. Unfortunately, this is seldom the case. Many reactions cannot be carried to completion for various reasons. Seldom do the reactants react only to the desired product. Unwanted byproducts are formed in addition to the target product. Finally, the reactants are seldom 100% pure, again for many reasons The result is that the material leaving the reactor is a mixture containing the desired product, by-products, unreacted raw materials, and impurities. It may also contain other compo- nents deliberately introduced for one reason or another. The use of a homogeneous catalyst is just one example of this This mixture must separated into its various constituents. The product must be separated from almost everything else and brought to an acceptable level of purity such that it can be sold The reactants, being valuable, must be recovered and recycled back to the reactor. The impurities and by-products must be separated out for disposal in a suitable manner Thus, the activity of separation is fundamental to the operation of almost any process Some separation systems are relatively simple. Others constitute the major part of the process As will be seen, separation takes on many forms Separation of ammonia from cycle gas takes place in the Ammonia KO Drum, F-1. The cycle gas leaving the Ammonia Condenser, E-1, contains droplets of liquid ammonia. This mix- ture enters the middle of the KO Drum. The vapor, being less dense, flows upward while the liquid ammonia falls to the bottom of the drum 4. Materials Transfer The Ammonia Synthesis Loop(Figure II-2)consists of several items of equipment, each of which has material flowing in and material flowing out. These flows take places though process piping connecting the various items of equipment. If the pressure in the upstream item of equipment is sufficiently higher than that in the downstream item, then material will flow from the upstream equipment to that downstream without the need for any additional equipment. This pressure difference is necessary to overcome the friction due to fluid flow through the piping As a result, the pressure will decrease in the direction of flow through the process Thus, as synthesis gas flows from the inlet to the reactor (Stream 3) through the reactor condenser and knockout drum, the pressure decreases significantly. In order to be able to recycle the unreacted hydrogen and nitrogen back to the reactor, some means is required to increase the
-17- For our example process, mixing of fresh synthesis gas and recycled synthesis gas takes place in the Feed Mixer, M-1, before being sent to the Synthesis Converter. 3. Separation In an ideal chemical process, exactly the right amounts of reactants would be mixed and reacted completely to the desired product. Unfortunately, this is seldom the case. Many reactions cannot be carried to completion for various reasons. Seldom do the reactants react only to the desired product. Unwanted byproducts are formed in addition to the target product. Finally, the reactants are seldom 100% pure, again for many reasons. The result is that the material leaving the reactor is a mixture containing the desired product, by-products, unreacted raw materials, and impurities. It may also contain other components deliberately introduced for one reason or another. The use of a homogeneous catalyst is just one example of this. This mixture must separated into its various constituents. The product must be separated from almost everything else and brought to an acceptable level of purity such that it can be sold. The reactants, being valuable, must be recovered and recycled back to the reactor. The impurities and by-products must be separated out for disposal in a suitable manner. Thus, the activity of separation is fundamental to the operation of almost any process. Some separation systems are relatively simple. Others constitute the major part of the process. As will be seen, separation takes on many forms. Separation of ammonia from cycle gas takes place in the Ammonia KO Drum, F-1. The cycle gas leaving the Ammonia Condenser, E-1, contains droplets of liquid ammonia. This mixture enters the middle of the KO Drum. The vapor, being less dense, flows upward while the liquid ammonia falls to the bottom of the drum. 4. Materials Transfer The Ammonia Synthesis Loop (Figure II-2) consists of several items of equipment, each of which has material flowing in and material flowing out. These flows take places though process piping connecting the various items of equipment. If the pressure in the upstream item of equipment is sufficiently higher than that in the downstream item, then material will flow from the upstream equipment to that downstream without the need for any additional equipment. This pressure difference is necessary to overcome the friction due to fluid flow through the piping. As a result, the pressure will decrease in the direction of flow through the process. Thus, as synthesis gas flows from the inlet to the reactor (Stream 3) through the reactor, condenser and knockout drum, the pressure decreases significantly. In order to be able to recycle the unreacted hydrogen and nitrogen back to the reactor, some means is required to increase the