Abstract
This paper focuses on investigating the influence of varying barrel fill levels on the mean residence time, granule properties (median size, size distribution, and shape), and tensile strength of tablets. Specific feed load (SFL) (powder feed rate divided by screw speed) and powder feed number (PFN) (i.e., powder mass flow rate divided by the product of screw speed, screw diameter, and the material density in the denominator) were considered as surrogates for the barrel fill level. Two type of powders (lactose and microcrystalline cellulose (MCC)) were granulated separately at varying fill levels at different liquid-to-solid ratios (L/S). It was observed that by controlling the barrel fill level, the granule size, shape, and tablet tensile strength can be maintained at specific L/S. It was also noticed that the mean residence time decreased with increasing fill levels in the case of both lactose and MCC powder. However, it was only found to be related to the change in granule size in case of granulating microcrystalline cellulose at varying fill levels. At very high fill levels, granule size decreased, owing to a limited interaction between MCC powder and liquid at high throughput force and short residence time.
Highlights
Continuous wet granulation using a co-rotating twin screw granulator (TSG) has become a technique of choice in the pharmaceutical industry
The focus of the present work is on understanding the effect of increasing screw speed at low and high powder feed rates, which is further linked to finding the balance between these two process variables to achieve certain desired product attributes
It was shown that the specific feed load or powder feed number at a given liquid-to-solid ratios (L/S) can act as a surrogate for the barrel fill level, which can be used to control and maintain the granule size and shape
Summary
Continuous wet granulation using a co-rotating twin screw granulator (TSG) has become a technique of choice in the pharmaceutical industry. The screws of TSGs can consist of various types of elements (conveying, kneading, distributive, combing etc.) arranged in a specific configuration around the screw shafts. The type of screw elements and the geometry is specific to the TSG manufacturer. There are limited companies that produce twin screw granulators for pharmaceutical applications—namely, Leistritz Extrusionstechnik GmbH–NANO 16; Thermo Scientific–Pharma 11, 16, and TSG and GEA Pharma Systems–ConsiGmaTM-1 (standalone TSG) and systems (TSG as part of GEA’s continuous tableting line). Each manufacturer has copyrighted screw elements and shaft geometry, length to diameter ratio (L/D), screw-barrel gap and barrel arrangement (fixed or closed shell, clam shell etc.). Several papers have been published around TSGs manufactured by Thermo Scientific and GEA Pharma Systems
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