3.3 Recycle, Bypass, and Purge Systems

3.3 Recycle, Bypass, and Purge Systems

Quick Answer: Recycle returns material to an upstream unit; bypass routes feed around a unit; purge bleeds a fraction of a recycle loop to stop inert buildup. Always separate single-pass (reactor) conversion from overall plant conversion, and write balances on fresh-feed, mixed-feed, and product boundaries as needed.

Industrial chemical plants in Qatar’s energy and downstream sector—and the UPDA Chemical exam—rarely stop at a single once-through reactor. Recycle, bypass, and purge appear in Domain A process calculations and connect to reaction engineering and process control themes elsewhere in the blueprint.

Why Recycle?

Recycle does not violate mass conservation. It changes how many times a molecule may pass through a unit before leaving as product or purge.

Fresh Feed vs Mixed Feed

Draw the classic loop:

1. Fresh feed (F) enters and combines with recycle (R).
2. Mixed feed (M = F + R) (mass) enters the reactor.
3. Reactor effluent goes to a separator.
4. Separator sends product (and sometimes a purge) out and recycle back.

Critical definitions:

• Single-pass (reactor) conversion uses the reactant in the mixed feed to the reactor and the reactant leaving the reactor.
• Overall conversion uses reactant in fresh feed and reactant leaving the entire process in products/purge.

Overall conversion is almost always higher than single-pass conversion when unreacted reactant is recycled (with no reactant loss in purge, overall can approach 100% even if single-pass is low).

Recycle Ratio

A common definition:

[ \text{Recycle ratio} = \frac{\text{recycle flow}}{\text{fresh feed flow}} ]

Some problems use recycle / mixed feed or recycle / product. Read the stem. On a 25-question UPDA exam you cannot afford to invent the ratio definition—underline the words “based on fresh feed” if present.

Purge for Inert Buildup

If an inert enters with fresh feed (N₂ with air, CH₄ impurity, argon) and never reacts or leaves in the product, a closed recycle loop would accumulate inert forever. A purge stream removes a fraction of the recycle (or of a separator overhead) so inert out = inert in at steady state.

Steady inert balance around the whole plant:

[ \dot{m}{\text{inert, fresh}} = \dot{m}{\text{inert, purge}} + \dot{m}_{\text{inert, product}} ]

If product is essentially inert-free, purge carries essentially all inert out. Higher purge ⇒ lower inert concentration in the loop but more loss of valuable reactant dissolved in the purge—economics trade off against compressor load and reactor volume.

Bypass for Composition Control

Bypass sends part of a feed around a unit (reactor, absorber, heat exchanger) and recombines downstream:

• Soften a product composition that would otherwise be “too converted” or “too dry”
• Control final temperature by blending hot effluent with cold bypass
• Meet a downstream purity without oversizing a unit

Bypass streams have the upstream composition (like a splitter branch) until they remix. Do not assign them the reactor outlet composition.

Degree-of-Freedom Tips for Loops

• Balance around the entire process first: overall conversion, net product, fresh feed—recycle cancels as an internal stream.
• Balance around the reactor only to use single-pass conversion.
• Balance around the mixing point to relate F, R, and M compositions.
• Balance around the separator to split reactor effluent into product, recycle, and purge.

Internal streams cancel in overall balances—that is a feature, not a bug. Use overall balances to find net production; use unit balances to find recycle rate and compositions.

Classic Exam Traps

1. Using overall conversion inside the reactor equation (or vice versa).
2. Omitting purge when the stem mentions inerts in air or feed impurities.
3. Assuming recycle composition equals fresh feed (it usually is reactant-richer or inert-richer after separation).
4. Splitter logic on a separator (product and recycle rarely share composition).
5. Mass vs mole recycle ratios mixed mid-solution.
6. Forgetting that single-pass conversion applies to mixed feed, so high recycle means more absolute reactant through the reactor even at modest (X).

Worked Multi-Unit Sketch

Process. Isomerization of pure A to product B (isomers; mole-conserving):

[ A \rightarrow B ]

Flowsheet: fresh A → mix with recycle → reactor → perfect separator → pure B product; unreacted A recycled. No inerts, so no purge required.

Data:

• Fresh feed: (100,\mathrm{mol/h}) pure A
• Single-pass conversion of A: 60% (based on mixed reactor feed)
• Separator: all B to product; all unreacted A to recycle (ideal)

Goal: Find recycle rate, mixed feed rate, and overall conversion.

Step 1 — Overall balance (whole plant).

At steady state, every mole of A fed eventually leaves as B (no purge, complete recovery of B). So product B = 100 mol/h, overall conversion of A = 100%.

Step 2 — Reactor with unknown mixed feed.

Let mixed feed of A be (M) mol/h (pure A into reactor, because recycle is pure A and fresh is pure A).

A reacted per pass = (0.60 M).

A leaving reactor = (0.40 M).

B leaving reactor = (0.60 M).

Step 3 — Separator and recycle.

Product B = (0.60 M = 100) mol/h (must match overall product).

[ M = \frac{100}{0.60} = 166.67,\mathrm{mol/h} ]

Recycle R = unreacted A returned = (0.40 M = 66.67,\mathrm{mol/h}).

Check mix: (F + R = 100 + 66.67 = 166.67 = M) ✓

Recycle ratio (R/F) = (66.67/100 = 0.667).

Interpretation: Single-pass conversion is only 60%, but overall conversion is 100% because unreacted A never leaves the process. That contrast is one of the highest-yield conceptual points on process-calculation exams.

Variant with Purge (Inert Present)

Suppose fresh feed is 100 mol/h with 98% A and 2% inert I, single-pass conversion of A is still 60% on A in mixed feed, separator sends all B to product, and recycle + purge share the same A+I composition from the separator bottoms. At steady state, all inert that enters must leave in purge (if product is pure B). Then:

• Inert in = (0.02 \times 100 = 2) mol/h = inert in purge.
• If purge composition is 20 mol% I (example analytical result), purge total flow = (2 / 0.20 = 10) mol/h.
• Valuable A lost in purge = (0.80 \times 10 = 8) mol/h, which lowers overall conversion below 100%.

You would then solve the full set: mix balances, reactor conversion on A only, separator specs, and inert overall balance. The algebra is longer, but the logic is the same as the pure-A sketch.

UPDA Exam Strategy

1. Sketch F, R, M, reactor, separator, P, and B (purge) even if the stem is text-only.
2. Label which conversion is single-pass vs overall.
3. Prefer an overall system balance before diving into the loop.
4. Add purge as soon as inerts or “once-through air” appear.
5. Sanity-check: overall conversion ≤ 100%; recycle ratio ≥ 0; inert out = inert in.

Mastering recycle/bypass/purge completes the Domain A material-balance triad: nonreactive units (3.1), reactive extents (3.2), and multi-unit flowsheets (3.3). Energy balances in Chapter 4 will reuse the same boundary discipline.