Conversion per pass

The propylene-based process developed by Sohio was able to displace all other commercial production technologies because of its substantial advantage in overall production costs, primarily due to lower raw material costs. Raw material costs less by-product credits account for about 60% of the total acrylonitrile production cost for a world-scale plant. The process has remained economically advantaged over other process technologies since the first commercial plant in 1960 because of the higher acrylonitrile yields resulting from the introduction of improved commercial catalysts. Reported per-pass conversions of propylene to acrylonitrile have increased from about 65% to over 80% (28,68—70). 

For a given dehydrogenation system, i.e., operating temperature and pressure, thermodynamic theory provides a limit to the per pass conversion that can he achieved. A general formula is... 

One major problem of the industrial process of phenol methylation is the low yield with respect to methanol, due to its decomposition consequently, a large excess of methanol is usually fed in order to reach an acceptable per-pass conversion of phenol. This aspect, however, is often forgotten in scientific literature, and only... 

Continuous flow of both phases in upflow with recycling of liquid-phase For packed bubble columns and trickle beds, under the assumption of complete recycling of the liquid-phase, the solutions are the same as in sluny bubble columns and slurry CSTR with batch and reacting liquid-phase (see Section 3.5.1) (Ramachandran and Chaudhari, 1980). In this operation, VL/VR is greater than unity. Recycling is useful when the per-pass conversion of the liquid phase is very small. 

All of the pilot plant tests were made at 60 liquid volume percent per pass conversion below the recycle cut point. Temperatures were adjusted as necessary to maintain this conversion. Total pressure and recycle gas rate were held constant for all of the runs with a given feed. 

YIELDS AND PRODUCT PROPERTIES PROM HYDROCRACKING OP CALIFORNIA GAS OIL 60% PER PASS CONVERSION BELOW 550°F... 

A primary compressor increases the pressure of the entering ethylene gas (and propylene gas, which is added as a molecular weight control agent) from between 5 and 15 bar to about 250 bar. The secondary compressor further increases the gas pressure from 250 bar to the desired reactor pressure (approximately 2500 bar). An initiator is added to the gas as it enters the reactor. The reactor is operated to ensure a per-pass conversion of 15%-35% and is a wall-cooled reactor where the cooling water can be used to produce steam. The reaction mixture then enters the HP separator (-250 bar), where the mixture is flashed to produce two distinct phases a PE-rich melt phase and an ethylene-rich gas phase. The separated gas then enters the recycle loop. The ethylene gas is cooled before entering the secondary compressor. The PE enters the low-pressure separator. This low-pressure separator, also referred to as a hopper, performs the final degassing step. The separated ethylene gas is cooled and some components are removed. This step takes place... 

It is obvious that the way to design this reactor is to keep the concentrations of C and B small so as to keep the second reaction rate small. This implies that a large excess of reactant A should be used in the reactor, which will dilute the C and B concentrations. It will also help to drive the first reaction because of the large value of CA despite the small value of CB. With this design, the per-pass conversion of A will be small, but the yield of C per mole of A reacted will be large. 

As an alternative to multiple CSTRs we might consider the use of a single CSTR followed by a distillation column. The per-pass conversion of reactant can be low, giving a reactor effluent with considerable reactant. Then this mixture is separated in a distillation column that recycles the unreacted component back to the reactor. 

An alternative to the series of CSTRs is to have one reactor followed by a distillation column that recycles reactant back to the reactor. The flowsheet of this process is shown in Figure 2.59. The reactor can be small because the concentration of the reactant in the reactor can be large. The per-pass conversion of reactant is not equal to... 

Remember this is for a fixed per-pass conversion and inlet composition and flowrate. If the desired conversion is decreased, the inlet temperature can be increased and the reactor size reduced. Of course, this means that more unreacted material in the reactor effluent must be recovered and recycled. Keep in mind the difference between the per pass conversion, which is a crucial design optimization variable, and the overall conversion of the entire process, which is typically quite close to 100% because the cost of raw materials imposes a severe economic penalty on a process with low overall conversion. A lower per-pass conversion translates into a higher recycle flow with its associated higher energy and capital costs. But chemical process economics usually dictate that capital investment can be justified to save energy, and both energy and capital can be expended to reduce losses of raw materials and/or products. 

Typical adiabatic reactor design involves finding the best values of recycle concentration, inlet temperature, reactor size, recycle flowrate, and per-pass conversion. Of course, not all of these parameters are independent. When the entire process is studied... 

In this chapter we have discussed the several types of tubular reactors and some important aspects of their steady-state design. In all tubular reactors, the inlet temperature plays a significant role in the design of the reactor system. Higher inlet temperatures result in smaller reactors for the same per-pass conversion, but also result in higher exit temperatures. 

The reaction considered is the gas-phase, irreversible, exothermic reaction A + B — C occurring in a packed tubular reactor. The reactor and the heat exchanger are both distributed systems, which are rigorously modeled by partial differential equations. Lumped-model approximations are used in this study, which capture the important dynamics with a minimum of programming complexity. There are no sharp temperature or composition gradients in the reactor because of the low per-pass conversion and high recycle flowrate. 

The new kinetic parameters drastically increase the sensitivity of the reactor to inlet temperature. The sensitivity to inlet temperature occurs because of the high activation energy and heat of reaction and because of the high reactant concentrations (low per-pass conversion). Remember that the feed to the reactor is a 50/50 molar mixture of pure reactants. There are large amounts of reactants available to fuel the reaction runaway. 

Per-pass conversions vary from a low of 5 to a high of 70 percent for the gas-phase reaction. 

All reactive stripping experiments showed that reducing the water content level (due to better stripping performance) increases the per-pass conversions, but has a negative effect on selectivity in the chosen model reaction system. Nonetheless, the water contents are the result of a balance between stripping efficiency and catalyst hold-up. As a consequence, the space-time yield was highest for katapak-S , whereas in DX -packings, the excellent separation efficiency optimized the use of catalyst, but decreased the selectivity. For industrial applications, the choice will always depend on the balance between mass transfer performance, the kinetics, the activity of the catalyst, and the process economics. 

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