Drying and gas or vapor contact with solids – Process – With fluid current conveying or suspension of treated material
Reexamination Certificate
1999-05-04
2001-07-03
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
With fluid current conveying or suspension of treated material
C034S366000, C034S373000, C034S405000
Reexamination Certificate
active
06253463
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to spray drying as applied within a broad range of industries, e.g. the pharmaceutical, chemical, dairy, food, ceramic and powder metallurgical industries.
More specifically, the invention deals with improvements in spray drying where an amorphous product is desired, as is often true in the pharmaceutical industry and/or where a high-bulk density of the resulting powder is desired and/or where increase of the production capacity of a spray drying device is desired.
BACKGROUND OF THE INVENTION
A lot of different spray drying processes and equipment therefore have been developed during the last many decades. A standard textbook on this technology is Masters, Keath: Spray Drying Handbook, 5th edition, Longman Scientific & Technical (1991), incorporated herein by reference.
It is conventional to select the spray dryer design and configuration and also the process parameters in consideration of the type of product to be dried and the desired characteristics of the final product, e.g. agglomeration, particle size, density etc.
Some of the issues hitherto considered in this respect are drying chamber design as to shape and dimensions; integration of a fluidized bed in the chamber bottom; integration of filters for separating product from the drying gas; selection of type of atomizer for the feed —rotary atomizer or nozzles, pressure nozzles or 2-fluid nozzles; type of gas disperser; drying gas temperature and velocity; feed-spray and gas flow directions; feed formulation and properties etc.
Other means for influencing product characteristics comprise separation of the total drying process into two or more steps in which the temperatures are controlled individually, recirculation of fine particles as well as control of several other parameters.
However, in spite of the fact that numerous measures are thus conventional for influencing product characteristics there is still room for improvements within certain areas of spray drying technology.
Thus, spray drying of some products involves creation of large vacuols in the droplets during drying thereof which results in blowing-up “balloon” particles, having thin walls which may break down before the drying process is terminated. Such breaking down of the particles results in a low-density and dusty product implying disadvantages in handling, transport and use, e.g. as pharmaceuticals.
Certain pharmaceuticals are preferably administered in formulations in which they are present in amorphous state. This is due, e.g. to the fact that the solubility rate for these pharmaceuticals is higher for the amorphous form than for crystalline forms thereof. Several modern pharmaceuticals have such low solubility rates in crystalline form that their bioavailability after administration is impeded thereby. Therefore, there is a need for preparing such pharmaceuticals with a structure wherein the amorphous state is more dominating than in the structure obtained by the conventional spray drying methods. The preference of pharmaceuticals in amorphous form is described inter alia in WO 98/57967 A, U.S. Pat. Nos. 5,612,367 and 5,641,745.
The amorphous form may be the preferred one in several forms of pharmaceutical preparations intended for various routes of administration.
The difficulties in obtaining a dominating amorphous structure when spray drying certain products are to some extent connected to the creation of thin-walled easily breaking particles since the surfaces exposed by breaking of said walls may initiate or accelerate crystallization processes.
Apart from the above described problems connected to the obtainment of a low-density product consisting to a large degree of fractured particle walls and the problems relating to the production of a powder having amorphous particle structure, it is a problem that in conventional spray drying processes the possibility for increasing the drying rate and, thus, the capacity of a certain apparatus without impairing heat economy and product quality is very limited.
SUMMARY OF THE INVENTION
It has now turned out that the above problems may be solved and further advantages obtained by conducting the spray drying in a pressurized atmosphere not below 1.25 bar absolute.
Thus, the invention deals with a method of spray drying a liquid medium comprising an evaporable liquid in which material is dispersed, able to form particles when said medium being spray dried, by atomizing said liquid medium as droplets into a drying chamber, maintaining in said chamber conditions causing evaporation of said evaporable liquid from said droplets to form particles containing said material, and recovering said particles from said chamber, which method is characterized in maintaining in the chamber a pressure not below 1.25 bar absolute. selected or experimentally determined pressure not below 1.25 bar absolute.
The liquid medium to be spray dried comprises an evaporable liquid in which a material to be recovered as powder is dispersed. The material may be dissolved in or suspended as solid particles in the evaporable liquid or it may be emulsified as droplets therein, provided that by the spray drying, possibly by the influence of adjuvants, it forms particles.
The actual pressure to be the most optimal for a certain drying process obviously depends on the material to be dried and the desired characteristics thereof and is selected within the range from 1.25 bar to the maximum pressure which the equipment is designed for. Said optimal value is either pre-selected on basis of previous experiences or is determined by simple initial experiments using the same equipment and materials as intended for the actual production.
Based on present experiences, it is assumed that the pressure shall preferably be from 1.5 to 75 bar, more preferably from 2 to 15 bar. For certain products, the pressure shall most preferably be from 5 to 15 bar, and for other products more preferably from 2 to 10 bar.
As to inter alia the bulk-density increasing aspect, the method according to the invention is characterized in that the pressure in the drying chamber is selected or determined to suppress or reduce formation of vacuols in the droplets, which vacuols could otherwise result in thin, easily breaking particle walls. Thereby a product of higher bulk-density and better flowability is obtained than if only atmospheric pressure had existed in the drying chamber and, consequently, a larger proportion of broken particle walls would be in the product.
Especially, when the solution or suspension to be spray dried comprises film-forming and/or binding materials, e.g. polymers added with the intended use of the spray dried material in pharmaceutical preparations, the problem caused by formation of vacuols in the drying droplets exists.
Examples of such film forming and/or binding additives comprise the following:
Film Forming Polymers (Both Water Soluble and Insoluble)
Cellulose derivatives
Acrylic polymers and copolymers
Vinyl polymers and other high molecular polymer derivatives
Synthetic Polymers
Methylcellulose
Hydroxypropylcellulose
Hydroxypropylmethylcellulose
Ethylcellulose
Cellulose acetate
Polyvinyl pyrrolidone
Polyvinyl pyrrolidone acetate
Polyvinyl acetate
Polyvinylmethacrylates
Ethylene-vinyl acetate copolymer
Materials for Improving the Properties of Film Forming Polymers Plasticizers
Phthalic acid esters
Triacetin
dibutylsebacate
Monoglycerides
Citric acid esters
Polyethyleneglycols
Anti Adhesives
Talc
Metal stearates
Diffusion—Accelerators
Diffusion—Retarders
Functional Coats that are pH Sensitive
Cellulose acetate timellitate (CAT)
Hydroxypropylmethyl cellulose phthalate (HPMCP)
Polyvinyl acetate phthalate (PVAP)
Cellulose acetate phthalate (CAP)
Hydroxypropyl methylcellulose acetate succinate (HPMCAS)
Carboxymethyl ethylcellulose (CMEC) Shellac
Other Functional Coating Materials
Methylmethacrylates or copolymers of methacrylic acid and methylmethacrylate
Eudragit polymers
Eudragits L, S, “L and S” and LD are anionic copolymers of methacrylic acid and methylmethacrylate.
A very important aspect o
Niro A/S
Sughrue Mion Zinn Macpeak & Seas, PLLC
Wilson Pamela
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