Ammonia process flowsheet

Fig, 2. Ammonia process case study. Reprinted with permission from Comp. Chem. Eng., 11, 143-152, Y.-D. Lang and L. T. Biegler, A Unified Algorithm for flowsheet optimization, Copyright 1987, Pergamon Press PLC. 

Figure 2.3. Process flowsheet of a plant making 47 tons/day of ammonia from available hydrogen and hydrogen made from natural gas (The C. W. Nofsinger Co.). 

The process flowsheet inside the battery limits (IBL) is at this stage unknown. However, the recycle of reactant may be examined. The patent reveals that the catalyst ensures very fast reaction rate. Conversion above 98% may be achieved in a fluid-bed reactor for residence time of seconds. Thus, recycling propylene is not economical. The same conclusion results for ammonia. The small ammonia excess used is to be neutralized with sulfuric acid (30% solution) giving ammonium sulfate. Oxygen supplied as air is consumed in the main reaction, as well as in the other undesired combustion reactions. 

Fig. 1. Flowsheet of the process for recycling water and ammonia from condensates in ammonia processing...

Figure 1. Process flowsheet for conversion of methane to ammonia...

In all of these alternatives, the design team selects acceptable temperature levels and flow rates of the recirculating fluids. These are usually limited by the rates of reaction, and especially the need to avoid thermal runaway or catalyst deterioration, as well as the materials (rf construction and the temperature levels of the available cold process streams and utilities, such as cooling water. It is common to assign temperatures on the basis of these factors earily in process synthesis. However, as optimization strategies are perfected, temperature levels are varied within bounds. See Chapters 10 and 18 for discussions of the use of optimization in process synthesis and optimization of process flowsheets, as well as Example 6.3 to see how constrained optimization is applied to design an ammonia cold-shot converter. 

Typically, a chemical plant must provide its own refrigeration, either offsite, or more typically onsite. Petroleum refineries use light-hydrocarbon refrigerants, while other plants may consider ammonia and R-134a, unless very low temperatures (<—30°F) are required, where cascade refrigeration systems are often used, as discussed in Examples 10.19 and 10.21. These systems are often included with the equipment in the process flowsheet. 

As we saw in the ammonia process, the reactor is usually the key unit. The reactor will often dictate whether a chemical process is possible. Separators are secondary only to the reactor in most processes the performance of the separator(s) will often determine if a process is profitable. Examples of separators include dryers, filters, absorbers, adsorbers, and centrifuges. The method used to separate a mixture - distillation, condensation, or filtering - is usually indicated by a specific symbol on the flowsheet. We will introduce these specific units later, as needed. 

M. W. Kellogg has played a major role in the development of the modern ammonia industry (see Sect. 6.5.3.2.1). Their most recent Low Energy Process [37,419,766-768,960] represents a further development of the classical process maintaining basically the same process scheme, but introducing updated technology and energy saving features in the various process steps. A simplified process flowsheet is shown in Fig. 6.29 from ([419]). Key features of the process are ... 

We first considered applications of this approach within process engineering. Steady-state flowsheeting or simulation tools are the workhorse for most process design studies the application of simultaneous optimization strategies has allowed optimization of these designs to be performed within an order of magnitude of the effort required for the simulation problem. An application of this strategy to an ammonia synthesis process was presented. Currently, flowsheet optimization is widely available commercially and has also been installed on the FLOWTRAN simulator for academic use. 

A simplified flowsheet for an ammonia plant that processes natural gas via steam reforming is shown in Figure 6.7. A block diagram of this same plant is shown in Figure 6.8. This diagram lists typical stream compositions, typical operating conditions, catalyst types (recommended by Synetix) and catalyst volumes82. 

Figure E5.5a shows a simplified flowsheet. All the units except the separator and lines are adiabatic. The liquid ammonia product is essentially free of Nz, Hz, and A, and assume that the purge gas is free of NH3. Treat the process as four separate units for a degree-of-freedom analysis, and then remove redundant variables and add redundant constraints to obtain the degrees of freedom for the overall process. The fraction conversion in the reactor is 25%.

FIGURE 7.2 Flowsheet for the ammonia-soda (Solvay) process to provide sodium carbonate. Units numbered I and 2 are ammonia stills, the first being used to recover free ammonia and the second to recover that produced by the addition of slaked lime (Eq. 7.16). 

The simplified flowsheet of the coupled synthesis gas and ammonia plant is shown in Fig. 1. The streams are numbered from 1 to UU and are listed in Table I and Table II Table I refers to the reactant and product streams especially. The units of the process may be identified by Table III. 

The flowsheet of the process for descaling sea water and simultaneously producing a high analysis fertilizer is shown in Figure 1. Wet process phosphoric acid (or sodium phosphates obtained from reaction of wet process phosphoric acid and sodium hydroxide or soda ash) and anhydrous ammonia are added continuously to the raw sea water to precipitate the scale-forming elements as metal ammonium phosphates and other phosphates. The precipitated solids are removed from the sea water by settling and the descaled sea water is pumped to the saline water conversion plant. 

Figure S. Flowsheet for alkanol/ammonia/water-hexane extraction processing of canola.

In many cases, the catalytic reactor model is used as a stand-alone unit in the design, simulation, and optimization of catalytic reactors. There are some typical cases in the petrochemical industry where the catalytic reactors dominate the production lines (e.g., the ammonia production line usually contains about six catalytic reactors representing almost 90% of the production line). In these cases, an evaluation has to be made in order to decide whether the reactor modules should be added to the flowsheet simulator or the few noncatalytic processes should be borrowed from the flowsheet simulator and added to a specially formulated flowsheet simulator of catalytic reactors forming the production line. 

A process for producing ammonia from air, water, and methane was outlined in this chapter. Draw the flowsheet for the entire process. Label the units and list the components in each stream. Where appropriate, use the proper symbol to identify a unit operation. Finally, improve the energy efficiency of the process by adding heat exchangers before the air condenser and the ammonia reactor. 

Since the first commercial application of the Haber process for ammonia production in 1913, a wide variety of synthesis loop designs have been developed. A history of the early developments is given in Chapter 1, and has also been reviewed elsewhere. However, by the 1950s and early 1960s, a broad consensus about the optimum design conditions for an ammonia synthesis loop had been reached. A typical design from this period will be described. This will be used to demonstrate how the elements of the synthesis loop are applied to produce a practical design, and will also serve as a base case for the discussion of modern developments. A flowsheet for this type of synthesis loop is shown in Fig. 7.4. 

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