In Thermal Process Unit Operations Pilot Plants, How Can Htst Equipment Be Modified? Match Batch Profiles

To make an HTST pilot plant behave like a batch pasteurizer, modify the holding section and heating profile so the continuous process delivers the same time–temperature history—or, at minimum, the same integrated thermal lethality. The principal adjustments are holding-tube length, product flow rate, heat-exchanger temperature zones, and heating and cooling ramp rates. For high-solid or heat-sensitive products, the target should include both microbial safety and the batch process’s sensory and structural effects, such as flavor development and protein denaturation.

An HTST system can approximate batch pasteurization by using a longer or adjustable holding tube, lower product velocity, and staged heat-exchanger temperatures to reproduce the batch process’s thermal dose and temperature trajectory. Matching only the final temperature may be insufficient when product quality depends on the complete heating, holding, and cooling history.

Why Batch and HTST Processes Behave Differently

Batch pasteurization exposes the product gradually

A batch system typically heats a fixed volume in a jacketed vessel, holds it at an elevated temperature, and then cools it. The product therefore experiences substantial heating and cooling periods in addition to the nominal holding period.

For formulations such as high-fat, low-overrun ice cream mixes, this extended exposure can influence protein denaturation, flavor development, viscosity, and body.

HTST concentrates the thermal treatment

An HTST system rapidly heats the product in a heat exchanger, transports it through a holding tube, and rapidly cools it. Its residence time is controlled primarily by the flow rate and holding-tube volume.

This produces a different thermal history even when the inlet and outlet temperatures are similar to those of the batch process.

Thermal dose is the first comparison point

Microbial destruction depends strongly on both temperature and time. The process can therefore be compared using an integrated lethality or F-value, calculated from the product’s temperature history rather than from the holding temperature alone.

However, equivalent lethality does not automatically guarantee equivalent flavor, texture, or protein functionality. Quality reactions may respond differently to the shape of the temperature curve.

How to Modify the HTST Pilot Plant

Install an adjustable holding tube

The most direct modification is to make the holding section modular. Operators can add or remove tube sections to change the residence volume and therefore the exposure time at the target temperature.

For a given flow rate:

[ t_{\text{hold}} \approx \frac{V_{\text{tube}}}{Q} ]

where (V_{\text{tube}}) is the internal tube volume and (Q) is the volumetric flow rate.

To approximate a longer batch-style exposure, engineers can increase holding-tube length or reduce flow rate. A modular design allows the same pilot plant to represent both conventional HTST and extended batch-equivalent treatments.

Add flow-rate control

A variable-speed feed pump, together with accurate flow measurement, allows the residence time to be adjusted without physically rebuilding the system.

Lowering flow increases the holding time, while increasing flow reduces it. The selected flow must remain compatible with stable heat transfer, acceptable pressure drop, and the desired production capacity.

Use staged heat-exchanger temperature zones

Instead of heating the product abruptly to one high temperature, divide the heating section into independently controlled zones. These zones can reproduce a more gradual batch-like temperature rise.

A useful arrangement may include:

Regenerative preheating using heat from the treated stream.
Controlled final heating using steam, hot water, or another heating medium.
An extended holding section at the selected process temperature.
Controlled cooling through regeneration followed by cooling water or chilled fluid.

The zone temperatures should be selected from measured batch data, not simply copied from nominal equipment settings.

Adjust the heating and cooling ramps

If the objective is to reproduce the batch process’s thermal history, the HTST system should be tuned to approximate the batch heating and cooling slopes.

This may require adjusting heat-transfer-medium temperatures, heat-transfer area, flow arrangement, or the number of exchanger stages. Rapid cooling should be retained where product quality or safety requires it, but the cooling profile can be modified when the batch process’s cooling period contributes meaningfully to the observed product characteristics.

Select a suitable exchanger geometry

For high-solid or viscous formulations, a tubular heat exchanger may be more appropriate than a narrow-channel plate exchanger because it can provide a larger flow path and reduce blockage risk.

The selection must consider viscosity, particle or solid content, fouling tendency, cleanability, pressure drop, and heat-transfer coefficient. A configuration that provides the correct residence time but causes excessive fouling will not be a practical pilot-scale solution.

How to Match the Batch Thermal Profile

First measure the batch reference profile

Record the product temperature continuously during the batch process, including:

Heating from the initial temperature.
Time spent near each elevated temperature.
The nominal holding period.
Cooling to the target exit or storage temperature.

The reference should be measured in the product itself where practical, rather than inferred only from the jacket temperature.

Convert the profile into a target lethality

Use the recorded temperature history to calculate the batch process’s integrated thermal lethality. The continuous process can then be adjusted until its calculated lethality is equivalent.

The comparison should include the full HTST path, including preheating, final heating, holding, and cooling—not only the nominal holding tube.

Compare the complete temperature trajectory

For products whose quality depends on cooking effects, compare more than F-value. Examine:

Time required to reach the process temperature.
Time spent above critical quality-reaction temperatures.
Maximum product temperature.
Cooling rate.
Residence-time distribution.
Product properties after processing and storage.

This distinction is important because two processes may have the same microbial lethality but produce different viscosity, flavor, color, or protein functionality.

Use staged residence zones when necessary

A single holding tube creates an approximately constant-temperature section, whereas a batch vessel experiences a continuous temperature change. If the batch profile must be closely reproduced, the HTST pilot plant can use several heated and insulated tube sections with separate temperature control.

Each section then represents a segment of the batch heating or holding curve. This approach provides better control than relying on one long tube operated at a single temperature.

Verify Residence-Time and Thermal Uniformity

Confirm actual residence time

The calculated residence time assumes ideal plug flow, but real systems may have axial dispersion, wall effects, dead zones, or bypassing.

Conduct residence-time-distribution testing, using an appropriate tracer or validated measurement method, to determine whether all product elements receive comparable exposure.

Prevent underprocessing and overprocessing

A broad residence-time distribution creates two risks:

Underprocessed material may leave before receiving the required treatment.
Overprocessed material may remain in the system long enough to experience unnecessary thermal damage.

The holding tube should therefore be designed and operated to approximate stable, well-characterized flow. Avoid sudden expansions, stagnant regions, and poorly drained sections.

Monitor the holding-tube exit

Install a reliable temperature sensor at the holding-tube outlet or another validated control location. A flow-diversion valve should automatically return product to the feed side when the temperature or flow conditions fall below the defined process limits.

This protects the product during start-up, shutdown, disturbances, and loss of heating capacity.

Preserve Pilot-Plant Safety and Energy Performance

Maintain the clean-side pressure advantage

When regeneration is used, the processed stream should remain at a higher pressure than the raw stream across the relevant exchanger section.

A booster pump or homogenizer can establish this pressure gradient. If a heat-exchanger leak occurs, the pressure direction should favor flow from the treated side toward the untreated side rather than allowing raw product to contaminate the processed stream.

Retain regeneration, but control its influence

Thermal regeneration can substantially reduce heating and cooling demand by transferring heat from the treated product to the incoming feed.

However, regeneration changes the temperature profile and can complicate efforts to reproduce a batch reference. Its performance should therefore be measured and incorporated into the thermal model rather than treated as a fixed assumption.

Provide accurate instrumentation

A profile-matching pilot plant should measure, at minimum:

Product inlet and outlet temperature.
Temperatures between exchanger zones.
Holding-tube exit temperature.
Product flow rate.
Pressure before and after critical exchanger sections.
Heating and cooling-medium temperatures.

Data should be recorded at a sufficient frequency to capture start-up, steady-state, and shutdown behavior.

Understanding the Trade-offs

Equivalent lethality is not identical product quality

Matching the integrated F-value is essential for comparing microbial treatment, but it does not ensure identical sensory or functional results.

Protein denaturation, flavor generation, browning, pigment changes, and nutrient degradation may have different temperature sensitivities. The process should therefore be validated using both lethality calculations and product-quality measurements.

A longer hold can reduce throughput

Increasing tube length or reducing flow rate improves the ability to reproduce a prolonged batch treatment, but it reduces capacity and can increase pressure drop, equipment footprint, and cleaning requirements.

The practical design is usually a compromise between profile fidelity and pilot-plant throughput.

Excessive heating can increase fouling

High-fat and high-solid formulations may foul heat-transfer surfaces, especially when wall temperatures are substantially higher than product temperatures.

Fouling reduces heat-transfer performance and can alter the actual thermal profile over a production run. Monitor pressure drop and temperature response, and include cleaning validation in the pilot protocol.

Plate exchangers may not suit every formulation

Plate exchangers are compact and efficient, but narrow passages and high-solid formulations can create flow or cleaning challenges.

A tubular or wider-channel design may provide more reliable operation, although it can require more heat-transfer area or a larger equipment footprint.

Regeneration can complicate profile matching

High regeneration improves energy efficiency, but it makes the product’s inlet temperature dependent on the performance of the entire system.

For experimental comparisons, regeneration should be adjustable or bypassable so researchers can distinguish the effect of the thermal profile from the effect of energy-recovery configuration.

How to Apply This to Your Pilot Plant

Begin by measuring the batch product-temperature curve and calculating its integrated lethality. Then configure the HTST system iteratively using holding-tube volume, flow rate, exchanger-zone temperatures, and cooling conditions.

If your primary focus is microbial equivalence: Match the integrated lethality using validated temperature measurements, controlled flow, and a properly sized holding tube.
If your primary focus is batch-like flavor and protein functionality: Match the complete heating, maximum-temperature, holding, and cooling profile rather than relying on F-value alone.
If your primary focus is high-solid or viscous formulations: Use a suitable tubular or wide-channel exchanger and verify fouling, pressure drop, and cleanability.
If your primary focus is energy efficiency: Retain regenerative heating, but measure its effect on the product-temperature profile and make regeneration adjustable during trials.
If your primary focus is process validation: Combine RTD testing, temperature mapping, flow-diversion control, and post-process quality analysis.

With adjustable residence time, staged temperature control, verified flow behavior, and profile-based validation, an HTST pilot plant can reproduce the relevant thermal effects of batch pasteurization while retaining the advantages of continuous processing.

Summary Table:

Key area	Recommended approach	Main purpose
Holding time	Use modular holding tubes and adjustable flow rates	Reproduce batch exposure time
Heating profile	Install independently controlled exchanger zones	Match gradual batch heating
Cooling profile	Control cooling stages and ramp rates	Reproduce the complete thermal history
Thermal validation	Compare temperature curves and integrated F-value	Confirm microbial equivalence
Product quality	Measure viscosity, flavor, color, and protein functionality	Verify batch-like results
Flow behavior	Conduct residence-time-distribution testing	Prevent underprocessing and overprocessing
Equipment selection	Use tubular or wide-channel exchangers for viscous or high-solid products	Reduce fouling and blockage risks
Safety and control	Add temperature monitoring, pressure control, and flow diversion	Protect product safety and process reliability

Need to reproduce batch pasteurization results in a continuous pilot plant? Contact LABPARK to discuss your requirements. LABPARK provides Educational and Vocational Unit Operations Pilot Plants for chemical engineering, bioprocess and biotech, and environmental and water treatment applications. Our systems can be configured with adjustable holding tubes, controlled flow rates, staged heat-exchanger zones, regenerative heating, cooling control, accurate instrumentation, and validation features. These solutions help universities, research institutes, and enterprises compare thermal lethality, optimize product quality, evaluate high-solid or viscous formulations, reduce fouling risks, and develop reliable scale-up processes.