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Reactions and Separations

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Act Like Novice
A About

Reaction Engineering
Jonathan H. Worstell, Shell Chemical Co.

New graduates should ask four key questions when given an industrial reaction engineering project.
heart of a chemical process. Running it at optimum conditions cuts operating costs not just for the reactor itself, but for post-reactor separation, as well. It also mayconserve capital by negating the need to expand an existing separation unit, thus achieving a “virtual” increase in capacity. Likewise, properly designing a new reactor reduces capital investment in it and post-reactor separation units. A newly graduated chemical engineer involved in a reactor project can contribute to its financial success by asking, then answering, four key questions: 1. What isthe reaction mechanism for product and byproduct formation? 2. For a homogeneous process, is the reactor heat-transfer or reaction-rate limited? 3. For a heterogeneous process, is the reactor mass-transfer or reaction-rate limited? 4. For a heterogeneous process, is catalyst deactivation spatially or temporally dependent?

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arket globalization is forcing all manufacturing industries toincrease productivity. This is particularly true for mature sectors, such as commodity chemicals, within the chemical process industries (CPI). This drive for increased productivity has changed the attitude that most CPI companies take toward the newly graduated chemical engineer. In the past, such an inexperienced person was assigned a mentor, and, essentially, enrolled in a 2- or 3-yrapprenticeship. These apprenticeships no longer exist. The newly graduated chemical engineer now is expected to contribute to the financial success of the company almost upon arrival. So, to succeed, that person right from the start must have an industrial mindset — that is, a determination to reduce operating costs and conserve capital. This mindset certainly is critical when dealing with reactors. After all,the reactor is the

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www.aiche.org/cep/

March 2001

CEP

Table 1. Feeding procedures to maximize product formation for homogeneous reactions. Reaction Type Generalized Reaction Mechanism Rate Equations Preferred Feed Procedure Preferred Reactor

Simultaneous

R1 + R2 R1 + R 2 R1 + R2 R1 + R 2 R1 + R2 R1 + R 2

P B P B P B

r1 = k1(R1)1.5R2 r2 = k2R1(R2)0.5 r1 = k1R1(R2)0.5 r2= k2(R1)1.5R2 r1 = k1(R1)1.5R2 r2 = k2R1R2

Keep R1 and R2 concentrations as high as possible; feed all of R1 and R2 at inlet. Keep R1 and R2 concentrations as low as possible; feed R1 and R2 continuously. Keep R1 concentration high to favor product formation; keep R2 concentration high to maximize reaction rate; feed all of R1 and R2 at inlet. Keep R1 concentration low to favor productformation; keep R2 concentration high to maximize reaction rate; stage R1 feed. Keep R1 concentration low and R2 concentration high.

Series CSTR, plug-flow batch Single CSTR

Simultaneous

Simultaneous

Series CSTR, plug-flow batch

Simultaneous

R1 + R2 R1 + R 2

P B

r1 = k1R1R2 r2 = k2(R1)1.5R2

Plug flow, series CSTR

Parallel, Competitive Parallel, Competitive Consecutive

R1+ R2 R1 + R 1 R1 + R2 R1 + X R1 + R2 P + R2

P B P B P B

r1 = k1R1R2 r2 = k2(R1)2 r1 = k1R1R2 r2 = k2R1X r1 = k1R1R2 r2 = k2(P )1.5R2

Semi-batch, staged plug flow Semi-batch, staged plug flow Semi-batch, staged plug flow

Keep R1 concentration low to minimize byproduct formation; keep R2 concentration high to maximize reaction rate. Keep R2 concentration low.

Note: R = reactant, P =product, B = byproduct; CSTR = continuously stirred tank reactor. Adapted from H. Rase, "Chemical Reactor Design for Process Plants," Wiley, New York (1977).

The purpose of this article is to highlight these questions and outline methods for acquiring answers to them.

Reaction mechanism and capital conservation Newly graduated chemical engineers generally start their careers with little...